Soft Matter: The Science of Squishy Materials

Be it in their appearances or applications, soft matter or materials such as toothpastes, shampoos, lotions, sand, liquid detergents, milk, ice cream, and mayonnaise are different.  Despite their apparent differences, they are all very similar in one regard – they can behave as rigid solids or flowing liquids depending on the external forces acting upon them. However, sand on a beach behaves like a solid and can support our weight when we walk on it. Take some of that sand in your palm and separate your fingers. The sand will flow right through under gravity, just like a liquid. Toothpaste stays in its tube till the time you squeeze it onto your toothbrush. Further, that little squeeze you applied was sufficient to convert the solid-like paste into a more liquid-like consistency. 

Water and other small-molecule liquids obey Newton’s law of viscosity, while metals follow Hooke’s law of elasticity. Toothpastes, lotions, polymers, sand mixtures, and most foodstuffs that can be squished easily cannot be described by Hooke’s or Newton’s law alone. We can only describe them by combining both laws, and we call them viscoelastic materials. 

What is Soft Matter?

Characteristics and Properties

Soft matter includes materials that are neither fully solid nor liquid. They exist in a unique state that allows them to deform easily under stress. This makes them incredibly versatile. You can think of materials like gels, foams, and polymers. They all share some fascinating characteristics.

Characteristic	Description
Low Elastic Moduli	These materials deform easily because of their low stiffness.
Dissipative Nature	They absorb energy during deformation, making them great for damping vibrations.
Disordered Matter	Their internal structure is often irregular, which affects how they behave under force.
Non-linear Behavior	They can change drastically under small forces, unlike rigid materials.
Far-from-equilibrium States	They often require constant energy input, leading to dynamic changes.
Thermal and Entropic Factors	Heat and entropy play a big role in shaping their structure and behavior.

These properties make soft matter unique. They allow these materials to adapt and respond to their environment in ways that rigid materials cannot.

Macromolecules Determine the Properties of Soft Materials

An incredibly large number of atoms and molecules compose solids and liquids. The constituent atoms or molecules and the forces between them govern their properties. In contrast, complex entities called macromolecules, made up of many small molecules or monomers held together by very weak forces, form the building blocks of squishy or soft materials. These ‘inter-macromolecular’ forces are weak enough that soft materials can be ripped apart by thermal energy (4×10-21J at room temperature), or by shaking and stirring with a spoon. Soft matter or materials are therefore sometimes called sensitive matter.

Macromolecules are giant molecules that can easily be seen under a microscope. Soft materials are therefore often studied to model the complex structures and dynamics. Also, phase transitions and flows of a very broad range of materials composed of atoms and molecules, such as supercooled water and window glass. Knowledge about their structure, dynamics, and stability is also important to fabricate better products for the kitchen (foods) industry. For example, plastics are very long polymers, and therapeutics (drug and gene delivery capsules using self-assembled soft structures called micelles). 

How Do We Study Soft Materials?

Understanding squishy materials, including living matter like cells and tissues, requires knowledge of chemistry, biology, and physics, making soft matter an extremely interdisciplinary field of study. 

X-rays are often used to understand how atoms and molecules are arranged in materials. As macromolecules are 1000s of times larger than atoms and molecules, they can scatter visible light. You can study the structure, dynamics, and phases of soft materials by using microscopes or even your mobile phone camera. 

Studies of how soft materials respond to stresses, strains, pushes, and pulls form the basis of the science of rheology, a term coined by the famous scientist Eugene Bingham from the Greek words rheo, meaning to flow, and logia, meaning to study. In the laboratory, rheometers are used to apply strains or stresses to study the deformation and flow of soft materials.  

Colloidal Suspensions: An Avatar of Soft Matter

Colloidal suspensions are a category of soft material. For example, substances like blood, ink, fog, smog, and foods like milk, ice cream, mayonnaise, and batter. Each colloidal material consists of two distinct phases. Firstly, a dispersed phase of macromolecules that remain suspended, and secondly, a continuous dispersing phase like water or oil. For example, blood contains blood corpuscles and protein macromolecules (the dispersed phase) that float in plasma (the dispersing medium). Moreover, the term ‘colloid’ comes from the Greek words kolla (meaning glue) and aoeides (meaning like). In 1926, Jean-Baptiste Perrin earned the Nobel Prize in Physics. He demonstrated that Brownian motion causes glue macromolecules to diffuse in water. Thus, perfectly balancing their gravitational tendency to settle. The colloidal glue particles, therefore, stay suspended in water. 

Clay-Water Mixtures: A Fascinating  Colloidal Suspension

Clay is a major ingredient of soil. It forms a colloidal suspension when mixed with water and flows in really unique ways. Water and honey are Newtonian liquids. If you apply a force to Newtonian liquids, their resistance to flow, or their viscosities, does not change. When you apply a force to a clay suspension, in contrast, its viscosity decreases very rapidly, and it starts flowing very easily. This decreased resistance to flow under the application of external forces occurs due to the rearrangements of the disk-shaped clay particles.  This property, called shear thinning, is also seen in other soft materials such as foams and polymers. 

Clay particles in suspension are arranged randomly in disordered configurations, just as in a liquid. Even though the structure of clay suspensions resembles that of a liquid, they are hard to the touch, just like solids. Glasses like window glass typically combine a liquid-like structure with a solid-like feel. Because the molecules in window glass are very small and hard to study, researchers often use dense clay suspensions to model glass formation. This lets them observe the behavior of individual glass molecules.  

Soils made in laboratories from clay colloids, salt, and water have been used to create artificial mini river deltas. Additionally, adding salt to clay suspensions encourages rapid sedimentation of the clay particles. Therefore, these studies show how river deltas form where a river meets the sea. The river carries mud and clay, and the sea has very salty water. 

Soft Matter is Fun!

Physics should be made simple enough to be amusing, but not so trivial as to spoil the fun.

Michel Mitov’s delightful book offers fascinating insights into the incredible properties of polymers, colloids, foams, and gels. Moreover, exploring their diverse uses in fields ranging from molecular gastronomy to the fabrication of liquid crystal screens. 

Simple experiments with soft materials

‘Blue milk’ effect

Materials needed: 300 ml milk in a glass, a flashlight. 

Experiment:  Dim the lights in the room. Shine the flashlight through the glass of milk. 

Observation: The glass of milk glows with a blue hue.  

Explanation: The stronger scattering of blue light by the colloidal fat and protein particles suspended in milk gives it a bluish appearance. This is called the Tyndall effect and explains why the sky is blue. 

Rod-climbing food batters

Materials needed: 300 ml pancake or idli batter, 300 ml water, 2 separate glasses for the batter and water, 2 stirring rods.

Experiment:  Take the pancake batter in one of the glasses. Pour water into the second glass. Stir the batter and the water vigorously using the two stirring rods. 

Observation: The curved meniscus of water is highest at the glass wall and lowest at the centre. In contrast, for the vigorously stirred batter, the meniscus has the opposite curvature and the batter eventually even climbs up the stirring rod. This is called the Weissenberg or rod-climbing effect.

Explanation: Unlike water, which is a Newtonian liquid, soft materials possess elasticity. The circular lines of flow generated by stirring the batter are like stretched elastic bands. These lines of flow store elastic energy and try to contract inwards. This drives the batter up the rod. 

Segregation of granular mixtures

Materials needed: A single walnut in a shell, 200-300 peanuts, and a small transparent jar with a lid. 

Experiment: Fill the jar with peanuts to form 2-3 layers. Place the walnuts on top of these peanuts, and then fill up the jar with the remaining peanuts. Close the lid of the jar and thump the jar from the bottom. 

Observation: After a few thumps, the walnut will emerge at the top of all the peanuts. This phenomenon is called the Brazil nut effect.

Explanation: Thumping the jar from below primarily drives the walnut’s vertical motion through void filling and granular convection of the peanuts. 

A cornflour-water mixture behaves like a solid or a liquid based on how you stir it:

Materials needed: A box of cornflour or cornstarch, a beaker, clean water, and a stirring rod.

Experiment: Mix the cornflour and water in the beaker till you get a homogeneous opaque white mixture. Further, stir the mixture gently first and then very vigorously. 

Observation: It is easy to stir the mixture gently but it is very difficult to stir it vigorously.

Explanation: Thin disks around 15 micrometres in diameter make up cornflour powder. When stirred gently, the cornflour particles in the cornflour-water mixture align along the direction of flow. Thus, this makes stirring easy. When stirred rapidly, the particles crash or jam into each other, and the mixture cannot flow anymore. 

Experiments That Make You Think And Laugh

Every year, Ig Nobel awards are awarded to unusual research studies “that first make people laugh and then make them think”. Notable among the soft matter research to have earned the Ig Nobel recognition is the pitch drop experiment. It ran non-stop for 90 years. Thus, this experiment reveals that pitch (used to make roads and is a petroleum derivative) is liquid-like. It has a viscosity that is 10¹¹ times higher than that of water. Then there is the demonstration of how to unboil a boiled egg and the detailed work on why cats are viscoelastic.

Where Do We Go From Here?

Researchers extensively use soft materials to study fundamental physical phenomena. Soft materials are very important in our daily lives, and their diverse applications drive significant scientific interest. Therefore, synthesizing and studying new and improved versions with novel properties is cutting-edge research. Several experts have laid out an extensive roadmap for the reader to explore what lies ahead.

References

1. Mezzenga, R. (2021). Grand challenges in soft matter. Frontiers in Soft Matter, 1. https://doi.org/10.3389/frsfm.2021.811842
2. Van Der Gucht, J. (2018). Grand challenges in soft matter Physics. Frontiers in Physics, 6. https://doi.org/10.3389/fphy.2018.00087
3. Chopard, B., Ansumali, S., Patil, D. V., Karlin, I., & Venkatesan, D. S. (2020). Fluid dynamics, soft matter and complex systems: recent results and new methods. Philosophical Transactions of the Royal Society a Mathematical Physical and Engineering Sciences, 378(2175), 20190395. https://doi.org/10.1098/rsta.2019.0395

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