Go with the flow:

Go with the flow: investigating bouncy fluids and other strange materials

The mission of SEP is to enhance and enrich science education and training

www.sep.org.uk

Science Enhancement Programme

GO WITH THE FLOW: INVESTIGATING BOUNCY FLUIDS AND OTHER STRANGE MATERIALS

The Science Enhancement Programme is developing curriculum resources to support effective learning in science, and providing courses and professional development opportunities for science teachers. This booklet is part of the series ‘Innovations in practical work’, exploring ways in which low-cost and novel resources can be used in secondary science.


First published 2005 by the Science Enhancement Programme Gatsby Technical Education Projects Allington House (First Floor) 150 Victoria Street London SW1A 5AE © Science Enhancement Programme 2005 Author: Peter Hollamby Editor: Richard Boohan Booklet design: From-the-Hip Cover design: Aukett Media

The materials in this booklet may be reproduced for teaching purposes in schools and colleges provided that the copyright of the Science Enhancement Programme is acknowledged. No copies may be sold for gain. ISBN 1 901351 432

ACKNOWLEDGEMENTS We should like to thank all those people who have provided help and information. In particular we are indebted to the University of Wales Institute of Non-Newtonian Fluid Mechanics, particularly Professor Howard Barnes and Professor Rhodri Williams for their support and permission to reproduce material on rheology. We are grateful also to Liz Coppard and Bryan Jackson for contributing the results of their practical trials, which have helped in the preparation of some of the experimental details summarised in this booklet. We should like to thank Dow Corning for providing information on silicone polymers and technical details on some of their products. HEALTH AND SAFETY For practical activities, the Science Enhancement Programme has tried to ensure that the experiments are healthy and safe to use in schools and colleges, and that any recognised hazards have been indicated together with appropriate control measures (safety precautions). It is assumed that these experiments will be undertaken in suitable laboratories or work areas and that good laboratory practices will be observed. Teachers should consult their employers’ risk assessments for each practical before use, and consider whether any modification is necessary for the particular circumstances of their own class/school. If necessary, CLEAPSS members can obtain further advice by contacting the Helpline on 01895 251496 or e-mail [email protected].

Foreword Teachers spend a lot of their time teaching students about the behaviour of solids, liquids and gases and about the models that underpin this. However there are many examples of materials that have surprising properties. This resource serves as a very useful background reader to inform teachers about these strange materials. It will allow them to relate the properties of materials that students come across in everyday life to their structures, and will support their teaching of the revised science curricula being introduced in 2006. Dr Colin Osborne CSci CChem FRSC Education Manager, Schools and Colleges Royal Society of Chemistry

CONTENTS Introduction

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Bouncy fluids and other strange materials

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Slime: a polymer with unusual properties

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Making the solutions Practical activities in the classroom Safety and handling Curriculum links Background science

Polymorph: a mouldable polymer

8

Practical activities in the classroom Safety and handling Curriculum links Background science

Ferrofluid: a magnetic material that flows

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Silicone polymers and Silly Putty

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Further information on rheology

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Flow properties of fluids Examples of Newtonian and non-Newtonian fluids Applications of rheology Rheology of food products

Glossary

24

References and further reading

26

Books and articles Websites

SCIENCE ENHANCEMENT PROGRAMME

INTRODUCTION This booklet introduces a range of materials that have unusual and interesting properties, and provides suggestions for how they can be used in practical work in school science. There are a variety of practical activities outlined in this booklet that can be used across the attainment range in KS3 and KS4. The activities are enjoyable to do and offer a fresh approach to learning about a number of scientific concepts in the physical sciences. The materials are mostly polymers, such as slime, liquid and solid polymorph, silicone gum and putty. There is also the magnetic liquid, Ferrofluid. All of these materials show interesting flow properties. For example, silicone putty has both liquid-like and solid-like properties – when left on a table is spreads out, but when hit with a hammer it shatters. Ferrofluid has a viscosity that increases in a magnetic field. As well as being of interest in themselves, these kinds of ‘smart materials’ also find applications in industry and consumer goods. The booklet aims to provide teachers of the physical sciences at KS3 and KS4 with suggestions and resources for introducing these unusual materials into their lessons. There is a section in the booklet about each of the materials. Each section includes the following: • • • • •

Introduction Practical activities in the classroom Safety and handling Curriculum links Background science

‘Practical activities in the classroom’ includes key ideas about the science at a level appropriate for KS3 and KS4 pupils. ‘Background science’ provides further information for those teachers who wish to know more, and may also be of use for A level students. In addition, useful references and websites are listed at the back of the booklet. All of the materials described in this booklet can be obtained from Middlesex University Teaching Resources (see back cover for details). These packs of materials are accompanied by information leaflets that give more detailed practical instructions.

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BOUNCY FLUIDS AND OTHER STRANGE MATERIALS Ketchup can be difficult to get out of the bottle. But give it a good shake and it will flow out more easily. Emulsion paint shows a similar kind of behaviour. In the tin it is thick and doesn’t flow very well, but when it is stirred it becomes more runny. After it has been put on a wall, it stays there and doesn’t flow downwards, but when being applied by a paintbrush it spreads easily. The reason for this behaviour is that if you exert a force on these materials they become more runny (i.e. their viscosity decreases).

Ketchup needs a good ‘bash’ on the bottle before it will flow

Slime is a popular material as a toy for children (and this booklet has a section on how to make it and test its properties). It shows the opposite behaviour to ketchup and paint - if you exert a force on slime, its viscosity increases (i.e. it gets thicker or flows less readily). So, if you push on it slowly with your fingers, they will sink into the material, but hit it quickly and it behaves more like an elastic solid. Wet sand can behave similarly – if you stand on it, you may sink into it, but not when you run across it.

The area of science that is concerned with exploring these kinds of behaviour is called rheology. The term comes from the Greek work rheos meaning ‘to flow’. Rheology is the study of the flow and deformation of matter and describes the properties of fluid and semi-solid materials. Though it can be a complex area, with the accurate measurement of rheological properties requiring expensive equipment, there is much that can be done at a simpler level in school. The practical activities suggested in this booklet focus on some novel and interesting materials, and will help pupils to appreciate the contribution made by rheologists to everyday products such as foodstuffs, toiletries and cosmetics. After all, what would we do if our toothpaste did not stay firmly on the brush and then coat our teeth efficiently during the brushing action, or if our hand cream required hours of endless rubbing before it was absorbed into our skin? What would you say if your expensive shampoo suddenly flooded all over your hands, or if you dripped emulsion paint onto the new carpet? Advances in our understanding of rheology have enabled us to design these products.

Slime – a bouncy fluid

‘Soft matter’ is a term coined by physicists to describe the ‘in-betweens’: those materials that are neither solids nor liquids. Ask a pupil to squeeze some toothpaste carefully onto a brush and then gently turn it upside down. Then ask: Does it fall off? Does it keep its shape? Is it a solid or a liquid? Toothpaste stays firmly on a brush until you start brushing

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SLIME: A POLYMER WITH UNUSUAL PROPERTIES A favourite for students young and old, slime provides an excellent intermolecular attraction. It is produced when solutions of polyvinyl alcohol (PVA) and sodium tetraborate (borax) are mixed together. Making and testing slime offers opportunities to complete a whole science investigation, and to understand simple polymer chemistry and a little acid-base theory. In addition, examining the PVA Borax physical properties of slime allows pupils to + = Slime Solution Solution appreciate the importance of flow characteristics in materials such as oils, gels and creams. PVA consists of long chain molecules that are free to move around in solution. When the borax solution is added, links are made between the long chains which become more fixed, and the slimy material produced behaves more like a solid. These crosslinks between PVA molecules and the [B(OH)4]- ions in the borax solution are formed by hydrogen bonding. There are a number of unusual properties of slime. If you pull the material slowly it will stretch, but if you pull it sharply it will break. The material is also elastic – if you drop a ball of it onto a hard surface it will bounce, and similarly a small object such as a marble will also bounce if dropped onto the surface of slime. MAKING THE SOLUTIONS Pupils can make their own slime by mixing solutions of PVA and borax; however, these solutions should be prepared before the lesson, since the PVA takes a little time to dissolve This contains PVA with a suitable molar mass for making slime successfully – if you are ordering from a commercial catalogue then an average molar mass of 115 000 g mol-1 is recommended. Note also that PVA glue is not generally suitable for making slime as such glue contains polyvinyl acetate (though see the section on Silicone polymer and Silly Putty on page 18, which contains instructions for making homemade bouncing putty from PVA glue). PVA solution Sprinkle 40 g of PVA into 1 dm3 of water at 50oC gradually with stirring (best done using a magnetic stirrer and a hot plate). Gradually heat the solution to 90oC with continual stirring but do not exceed this temperature. The solution should appear colourless and clear at this point (there should be very little undissolved solid). Now cool the solution, covering the beaker (or flask) with aluminium foil, and leave it to stand for 24 hours if possible. The solution will keep for some weeks if stored in plastic bottles. Borax solution Dissolve 10 g of borax (sodium tetraborate decahydrate) in 250 cm3 of water.

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PRACTICAL ACTIVITIES IN THE CLASSROOM It is best if the solutions of PVA and borax are made up before the lesson by the teacher or laboratory technician. Pupils can then make the slime themselves by mixing the solutions in plastic beakers and stirring vigorously until gelling is complete (about 30 seconds). The slime should then be removed and kneaded in the hands to remove excess water and some air bubbles. It is then ready to be explored and tested by the pupils. To achieve the best results, the recommended ratio of PVA:borax solutions is 5:1 by volume, but other concentrations and ratios can be tried out by pupils as part of their investigations. If desired, the slime can be coloured by adding a few drops of food colouring or some fluorescein to the solutions. Tests done on the slime can be qualitative, where pupils simply explore a variety of rheological properties of slime, or quantitative, where pupils accurately measure values and consider relationships between a number of variables. The work can be extended to a whole Sc1 investigation for pupils at KS4, and high levels of attainment can be achieved for the most able pupils. Qualitative tests There are a large number of tests that pupils can enjoy performing on slime such as: • What happens if you just leave slime on a surface? • What happens when you pull it – does it flow or does it snap (or fracture)? • Can it be flattened, extruded or squeezed? • Is it elastic? Does it bounce? • If you stretch it can you detect any change in temperature? • What happens if you add acid or alkali to the slime? Here, pupils can treat small samples of slime with dilute acid (e.g. 0.4M HCl) initially and then see if by adding alkali (e.g. 0.4M NaOH) (irritant), any changes can be reversed. Quantitative tests Investigations of a quantitative nature can involve measuring some property of the polymer and finding the relationship between that and some other factor such as composition. The slump test is a standard test and is easy to do. It involves making a disc of the slime and observing how quickly it spreads out. A simple way of making the discs of slime is to use plastic downpipe cut into rings of width 1 cm and greased inside with Vaseline. Pupils measure out 50 cm3 of slime, put it into the greased ring and allow the slime to settle. They then simply remove the ring and measure how much the slime flows in a certain time, say 5 minutes. They can then repeat for equal volumes of slime of varying compositions. Increase in area can be plotted against concentration of borax. All other variables should be constant (including temperature).

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Pupils can be challenged by asking: • What happens as the borax becomes very dilute? • Can the slime become a solid? Note that since the solubility of borax is only about 6 g / 100 g water at 298 K, this limits the degree of crosslinking that can be achieved between the PVA molecules. Pupils could also try out other quantitative tests. For example, they could investigate how a steel ball or marble bounces on a slime surface or how quickly a ball bearing sinks in slimes of different compositions. Sc1 whole investigation One interesting idea for an investigation is to ask pupils to consider whether slime could be used as a drilling lubricant by the oil industry (this was suggested on the website Terrific Science – see the section References and further reading on page 26). Pupils are asked to imagine that they run a manufacturing company that is trying to expand its portion of the market. The company needs to investigate a variety of slime formulations to see if they might make better lubricants than those currently available. Pupils soon get the idea that they could make up different slime formulations by changing the amounts of borax in their mixtures. They can then be presented with the problem of testing the slime so that they can relate their observations and measurements to a property that would give an indication on the usefulness of their slime formulation as a lubricant. There are plenty of opportunities here to make predictions, and some of the more imprecise aspects of the investigation allow significant evaluation of results, an area that causes the biggest difficulties in Sc1 investigations. Pupils could also be encouraged to consider the importance to the laboratory chemist in any field of chemistry (not just polymer chemistry) of learning the correct terminology, observing variations in the properties of materials and performing simple tests. SAFETY, HANDLING, AND DISPOSAL Although this activity is not unduly hazardous, proper care should be taken when handling any chemical. Chemical resistant eye protection should be worn at all times. Sodium borate (borax) is not classed as hazardous, but take care that this material is not placed in the mouth and wash hands after handling. Those with skin problems would be best advised to wear gloves. Risk assessments should be consulted for the use of acids and alkalis for testing slime. It is unwise to dispose of unwanted slime down the laboratory sink as this can lead to blockages; slime can be destroyed by adding 1M sulphuric acid and then pouring down a foul water drain or toilet.

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CURRICULUM LINKS – WHY USE SLIME? The advantages of using ‘slime chemistry’ to teach aspects of the science curriculum are clear: pupils simply love making and testing slime. The material is innocuous and if lessons are planned carefully, it is not very messy! Pupils can study at any level. and measurements can be approximate or accurate and precise depending on the requirements of the lesson. Relevant topics include: • structures of solids and liquids • molecular structure and polymers • intermolecular attraction, crosslinking • physical and chemical properties • acid-base theory • Sc1 investigations. A number of opportunities within these topics allow pupils to use extended prose to: • describe how the properties of slime vary with its composition • explain why the properties of slime depend on the extent of crosslinking • find out and write about other common polymers • apply the concept of hydrogen bonding to explain crosslinking • use acid-base theory to explain the stability of slime • find out and write about rheology and the behaviour of other unusual materials. BACKGROUND SCIENCE PVA or polyvinyl alcohol, poly(1-hydroxyethene) This is a polymer consisting of chains of carbon atoms with -OH groups positioned on every other carbon atom.

Borax This is the common name for sodium tetraborate decahydrate, Na2B4O7.10H2O. Tetraborate ions can react with mineral acid forming boric acid. Boric acid further reacts with water to produce the [B(OH)4]- ion (functioning as a Lewis Acid).

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Crosslinking The [B(OH)4]- is believed to form crosslinks between polymer chains using hydrogen bonds as follows:

Some sources consider that the crosslinks are formed as a result of condensation reactions producing B-O-C covalent bonds and the structure of the ‘slime molecule’ is as shown below. The actual structure is very complex and a number of condensation reactions and hydrogen bonds crosslink the polymer molecules. A good analogy is to think of slime as being like a building made of steel girders joined together. It’s the way the girders are joined together that gives the building its shape, and it’s the way the PVA molecules are crosslinked that gives slime its structure. In addition, the steel girders of a building provide the framework – inside this framework there is a lot of space. It is the same with the slime. In between the crosslinked PVA molecules there are lots of water molecules and about 95% of commercial slime is water. It is the crosslinking of the PVA molecules that makes slime behave a bit like a solid, and it is the water within its structure that gives it its flexibility. Note that some of the molecular structures shown have been drawn using ChemSketch. This is a free computer program, which is easy to use and would find many uses in a school science department. Its extensive range of features are described in a good article in School Science Review ‘The ChemSketch windfall’, and the program can be downloaded from the Advanced Chemistry Development website. (For details, see the section References and further reading on page 26.)

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POLYMORPH: A MOULDABLE POLYMER Polymorph (polycaprolactone) is a biodegradable polymer with a low melting point, and is available with different ranges of molecular masses. Middlesex University Teaching Resources (see back cover for contact details) supply two caprolactone polymers of differing molecular masses. The most useful is solid Polymorph (Mr ~80,000); also available is liquid Polymorph (Mr ~400). Solid Polymorph When softened, the solid polymer can be easily shaped or moulded, and provides very interesting opportunities for practical work. The material becomes pliable at Liquid Polymorph and 62oC, and this can easily be achieved by solid Polymorph granules immersion in hot water or by using a hairdryer. Granules of the polymer covered with hot water change from opaque to clear when it has softened sufficiently. The material can then be removed with tongs and shaped easily by hand. It remains pliable for some time, and when cooled completely, the plastic takes on the appearance of a lump of polythene. It is hard and very robust. Interestingly, the solid has a density of 1.05 g cm-3 and so will sink in tap water but it Softening Polymorph granules in hot will float quite well in salt water. This suggests water an interesting possibility. A moulded shape can be placed into salt water of a specific concentration such that it will neither sink nor float. If the water is heated or cooled the shape will then rise or drop (cf. a Galilean thermometer). Liquid Polymorph provides opportunities to investigate phase change since the polymer melts at about 2oC. The transition liquid-solid can be investigated by putting the polymer into an ice-salt bath. PRACTICAL ACTIVITIES IN THE CLASSROOM Streamlining investigation Many pupils at KS3 enjoy a popular investigation based on streamlining. The usual method is to make plasticine shapes and time how long they take to fall through a column of wallpaper paste. Polymorph invites a more convenient alternative, since with careful choice of salt concentration, convenient flow times for the experiment can be ensured. It is not messy and avoids the need to store the wallpaper paste, which soon deteriorates and grows mould. Pupils enjoy making polymer shapes since the material is very tactile, and the shapes can be re-moulded for other pupils. Pupils can be quizzed on their choices of shape. Short ‘tails’ made from fishing line can be attached to the shapes to make them flow more efficiently. Fair testing principles can be explored since some pupils may casually drop their shapes into the salt solution whilst others may take great care about introduction into the liquid. They should be asked if the mass of the shape or the temperature of the solution affects the results. A Polymorph sphere sinks

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Accuracy of measurement should be also examined. In trials, pupils measured the time it took the shape to fall between two thin lines drawn on a clear plastic tube. Pupils should consider the reliability of their results, how many repeated measurements to make and when to ignore outlying values. At the end, pupils can be asked to draw shapes of fish or racing cars based on their experimental results. Floating and sinking Candle wax or softwood will float in water; metals and some hardwood will sink. Cubes of these materials purchased from school laboratory suppliers can be used to demonstrate the concept of density. Pupils normally predict whether a substance will float or sink based on its mass for a given volume. Polymorph allows an extension to this work since an identically-sized cube will sink in water but float in ‘sea water’. Pupils can be challenged to make a salt solution in which a cube of Polymorph neither floats nor sinks. This activity involves pupils in learning and practising a number of different skills including the measurement of volume and mass, and handling the calculations relating to concentrations of solutions and their dilution. An extension to the work would be to try making a Galilean thermometer. Pupils should fill a test tube with a salt solution of critical concentration to support a small Polymorph shape. If the test tube is then immersed in beakers of hot water and then cold water the position of the shape should change. The phenomenon arises because of the difference in the thermal expansivity of Polymorph and water, and the results allow pupils to study how materials expand and contract with change of temperature.

A Galilean thermometer

Change of state The previous activities have all used the Polymorph with the higher range of molecular masses, and which is a solid at room temperature. ‘Liquid Polymorph’ has a lower range of molecular masses, and is a liquid at room temperature. It can be used to investigate change of state. The softening temperature of the polymer is 2oC. The liquid can be placed in a test tube immersed in and ice-salt bath and the temperature recorded every 30 seconds or so (using a standard thermometer or a temperature probe). The usual graph of temperature against time can be plotted to elucidate what happens during the phase change. Alternatively, a beaker of liquid Polymorph at room temperature can be placed in a freezer containing an SEP iButton (available from Middlesex University Teaching Resources – see back cover for contact details). The iButton is a portable temperature datalogger about the size of a £1 coin that can be connected to a computer to programme it, then used in a remote location to collect data, and then connected to the computer again to download the results.

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Another method is to allow a test tube or beaker of Polymorph to warm up in a room or in a warm water bath. The rising temperature measured by a thermometer, temperature probe or iButton datalogger can be plotted in the usual way (as on the graph shown). Pupils should be asked to compare the graph for liquid Polymorph with that for the melting or freezing of water, and to relate the changes in temperature with energy taken in or released during phase change. Water shows a longer ‘straight portion’ because it is a single substance. Polymorph contains a range of molecules with differing molecular masses, and so it does not have a sharp melting point. SAFETY, HANDLING, AND DISPOSAL The polymers are fully biodegradable thermoplastics. The polymers are non-toxic; ingestion should be avoided. They do not present any real hazards but the liquid forms could stick to clothes and other material such as hair. Moulding the polymer can be comfortably accomplished without protective gloves when its temperature is approximately 60 - 65oC. If the solid polymer is melted at temperatures significantly above this then skin contact with the material could cause minor scalding or burning. Under no circumstances should the material be heated in a microwave oven. The temperature will rise very quickly and it will boil and splash. The polymers would become flammable at elevated (boiling) temperatures just as paraffin wax might. If the solid form is melted by immersion in hot water (at temperatures between 65 - 90oC) then spillage should be avoided and removal of the molten polymer must be attempted with tongs. CURRICULUM LINKS – WHY USE POLYMORPH? Activities using Polymorph are expected to support the curriculum at KS3 but pupils with learning difficulties and low achievers at KS4 may find some of the possible activities quite stimulating. (For example, Polymorph proved to be an ideal material in an experiment trialled with some Y9 SEN pupils to investigate ‘Does Volume Change with Shape?’) The list below is by no means exhaustive but simply underlines a number of basic areas of knowledge, skills and understanding required within the science curriculum. Relevant topics at Key Stage 3 include: • structures of solids and liquids • changes of state • density • floating and sinking • frictional forces and streamlining • measurement (as a topic in itself) • Sc1 investigations.

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A number of opportunities within these topics allow pupils to use extended prose to: • describe their activities • explain the idea of streamlining and the concept of friction • explain the difference between solids and liquids, and changes of state • explain fair testing procedures • explain the importance of accuracy, precision and reliability in their activities • describe some ICT activities and compare them with the use of standard measuring instruments • relate what they have discovered and evaluate their method. BACKGROUND SCIENCE Polymorph is an interesting and useful material for science lessons, but caprolactone polymers also have a wide variety of commercial uses. They are used as additives for other resins, being compatible with many thermosetting and thermoplastic materials, and are used to improve their processing characteristics and physical properties. For example, the polymer aids mould release of thermosetting resins, and is used as a plasticiser with PVC in order to increase Degradation time for its flexibility. polycaprolactone (PCL) is very Polycaprolactones are biodegradable, and the low melting point of Polymorph makes the material suitable for composting as a means of disposal since the temperature obtained during composting routinely exceeds 60°C. The softening of the material above this temperature aids its decomposition.

short. In Sweden, there has been an attempt to produce PCL bags, but they degraded before reaching the customers.

Manufacture of caprolactone monomer This is manufactured by using an aqueous peracid oxidising agent to oxidise cyclohexanone (Baeyer-Villiger reaction). This process yields a high-purity, highly reactive caprolactone monomer which can then undergo a special type of polymerisation reaction (see page 12) to form the polymer. The monomer can also be co-polymerised with a number of other compounds (normally substituted diols) to make wide variety of polyesters. Many of these are biodegradable thermoplastics with low melting temperatures.

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Manufacture of polycaprolactone The polymers are all produced by a ring-opening addition polymerisation reaction. This is in contrast to the condensation polymerisation reaction that is normally used for polyester production. This simple but highly versatile chemistry enables a wide range of polymers and copolymers to be produced. The first step in the manufacturing process involves the use of an electron-deficient metal halide, MX2 as a catalyst; in the initiation step, a bond is formed between the metal and the carbonyl oxygen, allowing attack by the -OH group of an alcohol. This leads to ring opening, and the -OH group on the new molecule produced can take part in the next propagation step. Thus, each new, growing molecule becomes the attacking species for the next step, hence the term ‘addition polymerisation’.

Coloured Polymorph In industry coloured Polymorph is manufactured by using a cryogenic process. The polymer is cooled to a very low temperature when it becomes quite brittle. Under these conditions it can be ground into a very fine powder. The powder is then mixed with finely divided colouring agents, similar to those which are added to paints and then re-melted, extruded and turned into small pellets. It is possible to add colour to Polymorph yourself by mixing a very small amount of acrylic paint dye (available in DIY stores) with the molten polymer. The mixture needs to be kneaded for some time wearing gloves to avoid staining. The colour is rather uneven and the polymer is more difficult to mould, but when solid, the polymer retains its physical properties and is colour-fast.

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FERROFLUID: A MAGNETIC MATERIAL THAT FLOWS Ferrofluid was originally developed by scientists at NASA engaged in research for the space programme, who were looking for a way to control the flow of fuel under weightless conditions in space. One idea was to magnetise the fuel by dispersing finely-ground iron oxide particles within it, and then using a magnetic field to feed the fuel into the engine. Though this was never used, the research led to the development of Ferrofuid. The liquid’s flow characteristics are affected by the strength of a magnetic field around it, and this property leads to important uses in a wide variety of appliances and industrial applications. Ferrofluid is a colloidal mixture comprising extremely small magnetic particles suspended in a synthetic oil. The particles are coated with a surfactant (cf. a soap or detergent) to prevent them from clumping together. It responds uniquely to magnetic fields, and when a magnet is held close the fluid, it produces a characteristic spike pattern as seen in this photograph. The fluid is not magnetic itself, but when subjected to a magnetic field the particles align themselves along the lines of force and produce an attractive ‘hedgehog’ pattern. This allows 3-dimensional magnetic fields to be visualised. When the magnetic field is removed, the particles randomise and the pattern disappears. The fluid is a commercially available product that can be obtained from a number of sources including Middlesex University Teaching Resources (see back cover for contact details).

A magnet is held near a bottle containing some Ferrofluid

PRACTICAL ACTIVITIES IN THE CLASSROOM Exploring the behaviour of Ferrofluid Pupils can investigate the liquid qualitatively and can supplement their observations by viewing the many experiments and images that can be found on the internet. This may be a more convenient activity than using Ferrofluid in an open dish or beaker since, while this is extremely engaging, spillage of the material causes real problems with staining and cleaning. Teachers may choose to demonstrate the properties of the fluid showing for example that a 1p or 10p coin in a dish of Ferrofluid suddenly floats as soon as a magnet is placed under the dish. The strong attraction between the fluid and the magnet pushes the coin upwards. A far more convenient method for handling Ferrofluid is to suspend a few cm3 of the liquid in a sealed tube containing a 50% solution of propan-2-ol (highly flammable and harmful) and de-ionised water. It is important that the tube is exceptionally clean. Pupils can then examine the liquid closely and view the changes by applying a magnet to the surface of the tube. A hand lens allows pupils to see the dramatic ‘hedgehog’ pattern. Quantitative investigations It is possible to investigate the material quantitatively. The rate of flow of the fluid through a small tap varies according to the strength of the magnetic field around it. So the position and strength of a magnet or an electromagnet can be adjusted to control the flow rate. This property is exemplified by the use of Ferrofluid in industry (see the section Background science on pages 15-16).

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The density of aluminium is greater than that of Ferrofluid, so an aluminium rod will sink into the fluid. However, the viscosity of the fluid increases in the presence of a magnetic field, and so the rate of descent of the rod depends on the magnetic field strength around it. Pupils could perform this experiment by varying the distance of their magnet from the Ferrofluid using strips of plastic as spacers. They could then plot a graph of distance (number of strips or thickness) against speed of descent. Great care needs to be taken to avoid spillage (see Safety and handling below) as for any experiment involving measurement of ‘drainage rates’. If the magnet is brought very close to the fluid, then instead of sinking, the rod may float, just like the coin in the dish. It has been reported that a ‘magnetic egg timer’ has been made using Ferrofluid so that the fluid flows upwards towards a magnet, quite the opposite of a traditional sand timer. SAFETY AND HANDLING The liquid although innocuous does create problems for the handler and, if spilled, will stain clothes. Pupils should wear protective gloves and eye protection. Small spillages can be cleaned up with heptane (highly flammable). Ferrofluid examined in an open beaker or Petri dish should be well-supported and placed over paper towels so that spillages can be mopped up. Cat litter is an excellent material for mopping up big spills of many liquids in a school laboratory and can be stored in small buckets. This information below is a summary of the Manufacturer’s Safety Data Sheet (see www.sep.org.uk/practical for further information).

Ferrofluid is potentially very messy and will stain clothing etc. Do not attempt to dispense it through a small aperture (e.g. syringe nozzle), as it will tend to clog. It is advisable to wear eye protection and protective clothing when handling Ferrofluid. • • • • • •

Use general precautions when handling chemicals. Wear protection to avoid skin/eye contact and contact with cloth (it stains fabrics). No special ventilation required (unless smoke or mist is produced). Prolonged or repeated contact with skin or eye contact may cause irritation. Inhalation of mist or vapour at high temperatures may irritate respiratory passages. Extinguishing media: CO2 foam, dry chemical, water spray.

First aid Skin contact: wash with soap and water. Eye contact: Flush with water and seek medical attention if necessary. If spilt, remove with powder absorbent (e.g. sand, sawdust, cat litter). Wipe up residue with soapy water to prevent slipping on surface.

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CURRICULUM LINKS – WHY USE FERROFLUID? Ferrofluid is a very novel and exciting material for pupils to see and to explore. There are perhaps fewer links to the science curriculum than for the other materials described in this booklet, though there are certainly possibilities for using Ferrofluid effectively in some areas. Relevant topics at KS3 and KS4 include: • magnets and magnetic fields • electromagnets and electromagnetic effects • Sc1 investigations. A number of opportunities within these topics allow pupils to use extended prose to: •

describe some interesting applications or experiments using Ferrofluid after researching the internet

•

explain ‘lines of force’ and compare experiments with Ferrofluid and the typical practical done with iron filings

•

plan an experiment with Ferrofluid to illustrate its flow properties under the influence of a magnetic field and make a prediction of the outcome.

BACKGROUND SCIENCE Composition Ferrofluid consists of colloidal particles suspended in a synthetic oil. These particles are composed of magnetite, Fe3O4 (a ferrimagnetic substance), the average size of these magnetite particles being about 10 nm. They are prevented from aggregating by adding a soap like material called a surfactant, consisting of polar, long chain fatty acid molecules. The magnetite particles are attracted to the polar end of these molecules by ion-dipole forces. The long, non-polar tails of the particles are attracted by weak, van der Waals forces to the molecules of the synthetic oil that acts as the liquid medium. These structures resemble the micelles formed by soaps and detergents, which are also surfactants. In the case of soaps, the non-polar tails (e.g. of C17H35COO-) are directed toward the interior, where they surround waterinsoluble oils, while the polar ends, -COO- or -SO3- groups are strongly attracted to water molecules by ion-dipole forces. Ferrofluid may typically contain by volume 5% magnetic solid, 10% surfactant and 85% synthetic oil.

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Uses of Ferrofluid Ferrofluids can be precisely positioned and controlled by external magnetic fields. The forces holding the magnetic fluid in place can be adjusted by changing either the composition of the fluid or the magnetic field in the region in which it is employed. This property allows it to be used as a component that improves the performance of a number of useful devices. Examples of commercial applications include: •

mechanical devices, e.g. seals and bearings

•

electromechanical devices, e.g. loudspeakers and sensors

•

non-destructive testing of other items, e.g. magnetic tapes and stainless steels.

Though the research that led to Ferrofluid was initially undertaken as part of NASA’s space programme, it is now finding applications much closer to home. One such example of the use of Ferrofluid is in loudspeakers, where it is used to fill the gap between the moving coil and the permanent magnet. This serves to dampen the moving coil. There is a wide range of Ferrofluids available with Ferrofluid is now being used different viscosities, so a fluid can be chosen that will give the amount of in loudspeakers damping required for a particular loudspeaker. Because the fluid is attracted by the permanent magnet, it does not need to be enclosed in a container, but is held in the gap by the magnetic field. In addition, the thermal conductivity of Ferrofluid is about five times that of the air that would otherwise fill the magnet gap. This means that energy is dispersed more rapidly from the moving coil to the magnet structure and thus dissipated. One very important example is the use of Ferrofluid in vacuum seals for bearings and shafts. Conventionally these use greased O-rings, but the disadvantage of these are that they have to be checked and maintained regularly, and, because they tend to become brittle with use, they may fracture and leak. Ferrofluid offers a more efficient and permanent solution.

The fluid can substitute for the O-rings and be held in place with a magnetic field. This has obvious mechanical advantages because the Ferrofluid is not prone to fracture and provides a seal that will not leak.

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SILICONE POLYMERS AND SILLY PUTTY Silicones are synthetic polymers consisting of a backbone of repeating silicon-oxygen atoms, with organic groups attached to the silicon atoms. A huge number of silicone polymers are manufactured by Dow Corning. They are used in a multitude of applications such as cosmetics and shampoos; sun creams and health care products; food processing materials; polishes and cleaning materials; glazing materials and sealants; and plastics, textiles and rubber. Two very interesting silicone formulations with fascinating rheological properties are worth exploring. One of these is officially known as ‘3179 Dilatant Compound’ but it is much better know as ‘Silly Putty’. The other is ‘silicone gum’. These materials are safe, non-toxic and provide opportunities for fun activities supported by genuine scientific principles. Silly Putty is plastic material that looks like glazier’s putty and is marketed as a toy for children by Binney & Smith, Inc. Silly Putty is a classic example of serendipity - one of those scientific accidents that sometimes happens when trying to solve a different problem. During World War II, the USA was looking for a synthetic rubber compound because of the difficulties in obtaining natural rubber from the Far East. In researching this problem, James Wright of General Electric reacted boric acid with silicone oil and produced a gooey material – though it bounced Exploring the rheological properties of Silly Putty it was certainly not a rubber substitute. No uses for it were found until the 1950s when its potential as a toy was realised. Ironically, it was only after its success as a toy that more serious uses were found. It has found applications in medical and scientific simulations, and has also been used in stress-reduction and physical therapy. In the home it can be used to pick up dirt, lint and pet hair, and it was even used by Apollo astronauts to secure tools in zerogravity. Silicone gum has a different formulation to Silly Putty and has slightly different rheological properties, having a greater tendency to flow. While Silly Putty has a density greater than that of water and sinks, silicone gum floats. Both of the materials described here can be obtained from Middlesex University Teaching Resources marketed under the names ‘Smart Putty’ and ‘Smart Gum’ (see back cover for contact details). PRACTICAL ACTIVITIES IN THE CLASSROOM Apart from the obvious tactile properties of Silly Putty and silicone gum, pupils can perform a range of qualitative and even some quantitative experiments. Useful applications of the materials Younger pupils enjoy making ‘fossils’ using plasticine, seashells and Plaster of Paris. The normal routine is to make a mould by covering the seashell with plasticine; then, after careful removal, the mould is filled with Plaster of Paris and set aside to harden. The plasticine is often difficult to remove without spoiling the shape and it tends to adhere to the shell. Silly Putty is better than plasticine for this, being easier to remove and it retains its shape for a long time. Silly Putty is an excellent material for modelling mini-beasts. Legs and body segments ‘spread’ into each other and the completed animal is quite robust.

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Both Silly Putty and silicone gum lend themselves to floating and sinking experiments. Again, these materials could be used to answer the question ‘does volume change with shape?’. Both materials are electrical insulators and could be used in experiments with electrical circuits where wires, etc. must be kept apart. Exploring the behaviour of the materials The non-Newtonianess of these materials is exemplified dramatically by their elasticity. Silly Putty rebounds to approximately 80% of its drop height. Using small spheres of the material, pupils could explore the temperature dependence of this effect (its ‘bounciness’ decreases if it is cooled). In addition, they could try out different surfaces to find out which type of material absorbs energy the most from a bouncing ball. The flow properties of silicon gum can be explored using the “slump test” (see section on Slime on page 4). Its viscosity is temperature dependent (it flows faster when it is warmed). Equal sized balls of gum can be placed in a freezer, refrigerator, oven and a normal room so that the effect of temperature on its flow characteristics can be measured.

Silicone gum flows quite quickly whilst Silly Putty retains its shape longer

Silly Putty and silicon gum resist the sudden application of force, and so will not change their shape when stuck sharply with a rubber mallet. In fact, if Silly Putty is hit hard and very suddenly with a steel hammer on a hard surface, e.g. concrete, it shatters into small pieces. Homemade bouncing putty Various published formulations offer pupils the opportunity to make their own ‘bouncing putty’. The methods are similar to that for making slime but the polyvinyl alcohol solution is replaced by a dilute solution of PVA wood glue. How successful this is will depend on the formulation of the wood glue. Some manufacturers use a polyvinyl alcohol / polyvinyl acetate mixture which may work. If the glue only contains polyvinyl acetate, the crosslinking will not occur because there are no – OH groups. Often the wood glue is mixed with an equal volume of water and then stirred with borax solution (approximate concentration 4% w/v). Colour can be added with small amounts of food colouring. Different combinations of PVA/borax should be tried to find the best product. Some suitable recipes are shown in the following tables. Bouncing Custard Balls (use eye protection) You will need: •

15 cm3 PVA glue

•

borax

•

custard powder

1. Make a solution of Borax (about one spatula measure to 10 cm3). 2. Measure 15 cm3 PVA glue into a beaker. 3. Stir in 2 spatula measures of custard powder and 1 of solid, powdered borax. 4. Add 0.5 cm3 0f borax solution and stir vigorously until the mixture is smooth. 5. Remove the mixture and rinse well with water before handling too much (borax is moderately alkaline). 6. Shape into a ball and knead it for 2 minutes.

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Bouncing Talc Balls (use eye protection) You will need:

1. Add the water to dilute the PVA glue.

•

15 cm3 PVA glue

2. Mix the talcum powder thoroughly with glue solution.

•

5 cm3 borax

•

5 cm3 talcum powder

3. Add food colouring followed by borax solution and stir until mixture thickens.

•

25 cm3 water

•

food colouring

4. Remove the mixture and rinse well with water before handling too much (borax is moderately alkaline). 5. Shape into a ball and knead it for 2 minutes.

SAFETY, HANDLING AND DISPOSAL Both Silly Putty and silicone gum are virtually non-toxic and non-irritating to the skin and eyes. Silicone gum contains a small amount of octamethylcyclotetrasiloxane (harmful). (N.B. Octamethylcyclotetrasiloxane is commonly used in school science under the trade name Volasil, as a solvent partly to replace some chlorinated hydrocarbons.) Small quantities of both materials can be handled without the need for gloves or eye protection. Hands should be washed after handling the materials. If the materials are trodden into carpet or worked into fabric, most of it should be scraped up before cleaning gently with propan-2-ol (highly flammable and irritant). Silicone gum causes some staining of material, and should be handled with greater care. The materials should not be heated strongly or ignited since hazardous vapours are evolved. Silly Putty materials are not significantly harmful to the environment, but careless disposal should be avoided, and should not be put in sinks and drains which may become blocked. They may be wrapped in newspaper placed in a wastebin. CURRICULUM LINKS – WHY USE SILLY PUTTY? Activities using silicone polymer formulations are expected to support the curriculum at KS3 but pupils with learning difficulties and some low achievers at KS4 could also find some of the activities available quite stimulating. Relevant topics at KS3 include: •

structures of solids and liquids

•

density

•

floating and sinking

•

frictional forces and streamlining

•

measurement (as a topic in itself)

•

electricity, insulation

•

biology, classification

•

simple Sc1 investigations.

A number of opportunities within these topics allow pupils to use extended prose to: •

describe their activities

•

explain fair testing procedures

•

comment on the difficulty in obtaining accuracy, precision and reliability in activities using these unusual materials

•

relate what they have discovered and evaluate their method.

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BACKGROUND SCIENCE Silly Putty and silicone gum are complex formulations containing a number of constituent materials. The important compounds are siloxanes (substances containing Si-O-Si bonds, with organic groups attached to the silicon atoms), which are manufactured from silicon and chloromethane. The basic raw material for the silicone industry is silica (SiO2) in the form of sand. It is reduced to elemental silicon in a smelting process.

After grinding the silicon to a fine powder, it is reacted with chloromethane to yield a complex mixture of chlorosilanes (similar to chloroalkanes but with an Si atom instead of a C atom). The major constituent (70 - 90%) of this is dimethyldichlorosilane, (CH3)2SiCl2 .

After purification, the primary product (i.e. dimethyldichlorosilane) is reacted with water to give short chains of poly(dimethylsiloxane) [(CH3)2SiO]n . These molecules are intermediate in size between a monomer and a polymer and is called an oligomer.

The hydrogen chloride produced by this reaction is recycled and used to make more choromethane from methanol. In addition to poly(dimethylsiloxane) oligomer, a number of other linear and cyclic siloxane oligomers are formed simultaneously in the hydrolysis reaction. These siloxanes are the basic raw materials (monomers) from which large numbers of silicone products are made by polymerisation. 'End-blockers' are used to control the molecular masses of the products by terminating the chains, and the resulting products may be processed further to create crosslinks between the molecules and materials with a variety of properties. Silicon fluids consist usually of only straight chains, while introducing an increasing degree of crosslinking leads to silicon gels, elastomers and resins.

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FURTHER INFORMATION ON RHEOLOGY The science of rheology began in the late 1920's. Two scientists, Eugene Bingham and Markus Reiner met and found that they both had the same need to describe fluid flow properties. The term rheology is derived from the Greek word rheos which means 'to flow'. and was coined by Bingham after a suggestion by Reiner. He took the idea from the famous expression of the Greek philosopher Heraclitus 'panta rei' or 'everything flows'. Heraclitus' view was that the only thing permanent about the universe was change - no person ever steps into the same river twice - and the idea that 'everything flows if you leave it long enough' is a good starting point for understanding what rheology is about. Rheology is used to describe the properties of a wide variety of materials such as oils, foods, inks, polymers, clays, concrete, asphalt, and so on. These all exhibit some sort of flow and cannot be treated as solids. Materials can be divided into groups according to their rheological characteristics. To describe the behaviour of different materials in rheological terms, two key properties are used - viscosity (or 'thickness') and elasticity (or 'stretchiness'). If a force is applied to a viscous material (e.g. syrup), it flows and all the energy added is dissipated thermally; if a force is applied to an elastic material (e.g. rubber), it deforms and all the energy added is stored mechanically in the material. Some materials are viscoelastic (e.g. slime) and exhibit both viscous and elastic behaviour. FLOW PROPERTIES OF FLUIDS The flow behaviour of some fluids, for example, water and syrup, can be described using simple equations. Such fluids are called Newtonian fluids. If a force is applied to a Newtonian fluid contained in a syringe, then the rate at which it flows is proportional to the applied pressure (the shear stress). If shear stress is plotted against shear rate then the slope of the line is a measure of the fluid's resistance to flow. This is called its viscosity. For a Newtonian liquid, the plot is a straight line, meaning that the viscosity remains constant. This is not the case for fluids, such as toothpaste and quicksand - called non-Newtonian fluids, and it is this difference that makes them so interesting. Syrup is a highly viscous Newtonian fluid

Salad cream is a thixotropic fluid

There are two main types of non-Newtonian fluids, thixotropic and rheopectic (or anti-thixotropic) fluids. Thixotropic fluids become less viscous under higher shear stress (shear-thinning behaviour). Paints are usually thixotropic fluids: in the tin they are thick, but when stirred or brushed onto a surface they flow easily. Silly Putty is a rheopectic fluid, which means that it shows an increase in viscosity under an applied stress (shear-thickening behaviour). If you push your fingers into it slowly it offers less resistance than it does if you hit it hard with your fist. This is why Silly Putty would shatter if it was hit by a bullet fired from a gun, and why you can run across wet sand on a beach, but if you stand still you slowly sink into it.

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EXAMPLES OF NEWTONIAN AND NON-NEWTONIAN FLUIDS Newtonian fluids The viscosity of a Newtonian fluid does not depend on the force applied to it, but is dependent only on temperature. Examples of Newtonian fluids include water, milk, sugar solution, motor oil, washing-up liquid, glycerol and liquid honey. The viscosity of these materials varies a great deal as is shown in the table. For example, it can be seen that honey is 10 000 times more viscous than water. Non-Newtonian fluids: thixotropic These materials exhibit shearthinning behaviour, that is, their viscosity decreases with increased shear rate. They are the most common type of non-Newtonian fluid, and examples include paint, salad cream, ketchup, shampoo, grease, slurries and fruit juice concentrates.

relative viscosities of some common substances air

2 x 10-5

petrol

3 x 10-4

water

10-3

motor oil

10-1

washing-up liquid

3 x 10-1

glycerol

1

liquid honey

10

corn syrup

103

bitumen

109

Non-Newtonian fluids: rheopectic These are also known as antithixotropic fluids, and they exhibit shear-thickening behaviour, that is, their viscosity increases with increased shear rate. They are rarer than thixotropic fluids. Examples include slime, Silly Putty, wet sand and concentrated starch suspensions. APPLICATIONS OF RHEOLOGY Scientists responsible for the measurement of flow in a variety of materials work in a wide range of industries. The table below shows the typical areas of interest and types of liquids investigated.

RHEOLOGY OF FOOD PRODUCTS Of particular interest is food technology. Pupils learn how to prepare and cook food and how to produce a balanced diet, but to what extent are they aware of the importance of rheology as it applies to creams, oils and sauces for example? Apart from the smell, taste and colour of foodstuffs, their flow properties are important for a consumer. The ways in which food behaves in the mouth and the throat present a rheologist with a complex series of problems, and involves developing a rheological description of foodstuffs in terms of their properties during eating, drinking and swallowing. Flow properties are also important in food preparation as well as consumption. Food ingredients include sugars, polysaccharides, lipids, proteins, colours, flavours, vitamins, preservatives, minerals and water. Food rheologists are concerned with how these ingredients can be converted into a palatable form with the correct microstructure and acceptable rheological properties.

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Liquid-like food is usually made up of a complex mixture of dissolved natural polymers and dispersed insoluble biological material, usually in an aqueous phase containing various electrolytes. Such foodstuffs cover a range from thin liquids to soft solids as shown in the table. Thick or thin, runny or set? The following qualitative 'map' gives some indication of the variation of viscosity with the degree of 'non-Newtonianess' for a range of foodstuffs.

For example, from the diagram it can be seen that water has a low viscosity and is a Newtonian fluid.

Honey is also a Newtonian fluid, but with a much greater viscosity. Tomato ketchup is less viscous that honey, but has a high degree of 'non-Newtonianess' - in fact it is a thixotropic fluid, meaning that its viscosity decreases with applied pressure, which is why it needs a good 'bash' to get it out of the bottle.

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GLOSSARY Addition polymerisation The formation of a polymer from monomers with no other products being formed. The monomer usually has a double bond and the polymer has the same empirical formula as monomer. Anti-thixotropic A term used to describe a fluid whose viscosity increases with shear stress. It means the same as rheopectic. Baeyer-Villiger reaction Conversion of ketones into esters, and cyclic ketones into lactones, by treatment with a peracid. Borax Common name for sodium tetraborate decahydrate, Na2B4O7.10H2O. Caprolactone Monomer used to manufacture polycaprolactone (Polymorph). Condensation polymerisation The formation of a polymer from monomers by the removal of a smaller molecule, often water. Colloidal A term used to describe a material in which very small particles (between 1 and 1000 nm) are suspended in a medium. Copolymerisation The formation of a polymer from two different monomers. Crosslink A 'bridge' between one polymer chain and another. Ferrofluid A fluid consisting of magnetic particles suspended in an oil. Fluid A material that flows. A fluid does not have a fixed shape. Homopolymerisation The formation of a polymer from only one kind of monomer. Lewis acid A substance that forms a covalent bond by accepting a lone pair of electrons from a base. Monomer This consists of simple molecules that can combine with each other to form larger polymers. Newtonian fluid The viscosity of a Newtonian fluid is independent of shear stress and is affected only by temperature. Its flow can therefore be described by simple equations. Non-Newtonian fluid The viscosity of a non-Newtonian fluid is affected by the shear stress. If the viscosity decreases with shear stress, they are called thixotropic; if the viscosity increases they are called rheopectic. Oligomer A molecule formed from just a few monomer molecules, and thus smaller than a polymer. Peracid A compound containing the functional group -COOOH. Plasticiser A material added to a polymer that would normally be rigid so that it becomes plastic. Polycaprolactone Polymer manufactured from caprolactone monomer and marketed under the name Polymorph.

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Polymer A large molecule formed from repeating units called monomers joining together. There are usually more than 1500 units and polymers tend to have a relative molecular mass of over 30 000. Polymerisation The joining of monomers to produce a polymer. The two main types of polymerisation are addition polymerisation or condensation polymerisation. Polymorph A polymer (polycaprolactone) with a low melting point that can be easily moulded. PVA Abbreviation for polyvinyl alcohol. Its systematic name is poly(1-hydroxyethene). Rheology The study of the deformation and flow of matter. Rheopectic A term used to describe a fluid whose viscosity increases with shear stress. It means the same as anti-thixotropic. Silicon Non-metallic element, symbol Si. Silicone A polymer with a backbone of repeating silicon and oxygen atoms, with each silicon atom being joined to two other functional groups, e.g. one common silicone has the repeat group -(Si(CH3)2-O)-. Silly Putty A silicone polymer with interesting rheological properties, being able to both bounce and flow. Siloxane A substance containing Si-O-Si bonds, with organic groups attached to the silicon atoms. Shear rate The velocity gradient (dv/dx) in a flowing fluid - it is a measure of the change in velocity from one layer of the fluid to the next, and has units of s-1. Shear stress The force per unit area that results in the deformation or flow of a fluid. It has the same units as pressure, Pa. Shear-thinning The behaviour of a fluid whose viscosity decreases with shear stress. Shear-thickening The behaviour of a fluid whose viscosity increases with shear stress. Slime A gel formed by reacting PVA (polyvinyl alcohol) solution with borax solution. Surfactant A material added to a liquid to change its properties related to surface tension (abbreviation of 'surface active agent'). Thermosetting A term used to describe a plastic material that does not soften when it is heated. Thermoplastic A term used to describe a plastic material that can be heated repeatedly to melt or soften it. Thixotropic A term used to describe a fluid whose viscosity decreases with shear stress. Viscosity The resistance of a fluid to flow due to friction due to friction within itself. It is given by the ratio of shear stress to shear rate (and has units of Pa s).

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REFERENCES AND FURTHER READING For further information and links to useful websites, visit the 'Practical science' section on the SEP website (www.sep.org.uk/practical). Some particularly useful references are listed below. BOOKS AND ARTICLES A Handbook of Elementary Rheology Barnes, H A (2000) Publisher: Institute of Non-Newtonian Fluid Mechanics, University of Wales (ISBN 0 9538032 0 1) Assumes no background knowledge of rheology, and the emphasis is on explanation rather than mathematics. Classic Chemistry Experiments Hutchings, K (2000) Publisher: The Royal Society of Chemistry (ISBN 0 85404 919 3) Offers 100 tried and tested experiments including a description of the slime experiment. Soft matter: food for thought Ogborn, J (2004), Physics Education, 39(1), pp45-51 Very interesting article on the rheological properties of various foodstuffs (and recipes too). The ChemSketch windfall Mumford, C (2003), School Science Review, 84(309), pp 19-23 Decsribes the extensive facilities of a free computer programme for drawing chemical structures. Can be downloaded from the Advanced Chemistry Development website (see below).

WEBSITES The Royal Society of Chemistry (www.rsc.org, or better, (www.chemsoc.org/learnnet) A mine of help and information for chemistry teachers. Links also to the ChemIT site, which supports ICT in chemistry teaching (www.chemIT.co.uk). Advanced Chemistry Development, Inc. (www.acdlabs.com) Chemistry software company offering downloadable ChemSketch freeware. This is an excellent resource, particularly for drawing molecular structures and chemical apparatus. How Stuff Works (www.howstuffworks.com) A really useful site with lots of information on a wide range of materials and devices, including slime and slime experiments. Ferrotec (www.ferrotec.com) Manufacturers of Ferrofluid, with background information on Ferrofluid and its applications Dow Corning (www.dowcorning.com) Manufacturers of silicone products. This site offers details on the manufacture of silicone polymers and gives information on applications and a wide range of products. Cambridge Polymer Group (www.campoly.com) Details on latest polymer research and provides useful account on the science of Silly Putty. Terrific Science (www.terrificscience.org) A large online resource for chemistry produced by Miami University's Center for Chemistry Education. It contains quite a number of pdf files including a protocol for practical work on elasticity of Silly Putty and investigations on slime. The Home of Higher Silly Putty Learning (www.sillyputty.com) For youngsters, and for teachers who like to be utterly childish. The site contains, however, some useful science experiments on Silly Putty as well. The Salters' Chemistry Club Handbook (www.schoolscience.co.uk/teachers/chemclub) This site offers a mine of activities for 11 - 14 year olds, including a pupil worksheet for 'bouncing custard balls'.

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You can download the written materials in this booklet, and find further information from: Science Enhancement Programme (SEP) www.sep.org.uk The Science Enhancement Programme (SEP) is part of Gatsby Technical Education Projects. It undertakes a range of activities concerned with the development of curriculum resources and with teacher education.


Science Enhancement Programme Department of Education and Professional Studies King's College London Franklin-Wilkins Building (Waterloo Bridge Wing) Waterloo Road London SE1 9NN Tel: 020 7410 7129 Email: [email protected] Web: www.sep.org.uk

You can order slime, polymorph, ferrofluid and other resources from: Middlesex University Teaching Resources (MUTR) www.mutr.co.uk Teaching Resources Middlesex University Unit 10 The IO Centre Lea Road Waltham Cross Hertfordshire EN9 1AS Tel: 01992 716052 Fax: 01992 719474 Email: [email protected] Web: www.mutr.co.uk

www.sep.org.uk The mission of SEP is to enhance and enrich science education and training Cover design and production: Aukett Media