What is Soft Matter?

In school we generally learn that matter exists in three phases: solid, liquid and gas. If you think about it, you will however encounter a great many examples of materials that seem to fall between the categories solid and liquid. Start with your own skin, for instance. It certainly is not a liquid, but it is not really a solid either. It is soft, just like most tissue in our body. Biomaterials constitute one prominent example of what today is referred to as Soft Matter, a materials class that is today enjoying a strongly increasing attention in the international scientific research community, both among physicists and chemists.

There are many more examples of soft matter playing important parts in your life, perhaps not quite as obvious. You are very likely reading this text from a liquid crystal display, its function relying critically on the unique combination of anisotropic physical properties and molecular mobility that is the key characteristic of liquid crystals. It is their intermediate status between liquid and crystalline that gives them their remarkable behavior and also makes them key representatives of soft matter (liquid crystals are in fact the only examples of thermodynamically stable soft matter phases, metastable states being ubiquitous among other soft matter classes). The same type of liquid crystal will be in displays all over your home, in your phone, laptop, TV and many small gadgets. But even more important for your life is in fact another type of liquid crystal, that you have inside you throughout your whole body: your cell membranes are in a liquid crystalline state in order to function properly. We would thus not exist without liquid crystals.

Another important class of soft matter is colloids, encountered in your everyday life mainly through the food you eat. Butter, milk, mayonnaise, bread, yoghurt and many more foods are colloidal dispersions, sometimes appearing to be liquid (like milk) or close to solid but all with physical properties that ordinary non-colloidal materials cannot have. But colloids are of great importance also well beyond the realm of foods. Blood is a colloid we all carry inside us and in the rooms where we live, we may very well have painted the walls with water-based colloidal paint. The defining characteristic of this broad materials class is that colloids are dispersions of some kind of 'particles' (which here can be solid, liquid or gaseous) in a continuous phase, the size of the former being substantially larger than the molecules constituting the latter, yet small enough that the dispersion is stable on practical time scales. In colloids we encounter matter which we normally know as macroscopic bulk materials (for instance coal, sand or oil) but divided into particles of such a small scale that the total surface area has become immense, with a consequent great importance of interfaces and interparticle interactions. This gives colloids some quite exceptional properties and a behavior that we do not find in ordinary liquids or solids.

The term Soft Matter (which is used for many other materials than the few examples listed above) was coined around 1970 in the Paris school of physicists around Pierre-Gilles de Gennes [1], 1991 Nobel laureate in physics and by many regarded as the father of the research field. It was initially introduced as a joke, but it stuck on and has, for many years since, been the generally used term for describing all these fascinating and immensely important materials that are neither simply solid or liquid. Sometimes the alternative term Complex Fluids is used.

It is no coincidence that biological material generally is soft. Life simply could not be without the unique properties of soft matter, combining the flexibility and suppleness of fluids with the stronger interactions and sometimes also long-range order otherwise seen only in solids. This situation was beautifully expressed by the biophysicist Dikran Dervichian [2], who considered only the liquid crystalline examples of soft matter when he said:

Liquid crystals stand between the isotropic liquid phase and the strongly organized solid state. Life stands between complete disorder, which is death and complete rigidity, which is death again.

One of the most attractive properties of soft matter is the diverse mechanisms of self-organization behind the formation of these states and their inherent dynamics. With the right components at the right temperature and in the right proportions, very complex soft matter structures with peculiar properties can arise all by themselves, sometimes requiring a bit of help in terms of dispersing the components sufficiently. Moreover, a subtle change in the conditions, for instance a change in temperature, application of a weak electric or magnetic field, adhesion of chemically different species, or gentle flow, can dramatically change the appearance and/or properties of the system on a macroscopic scale. This ‘large response function’ is another key characteristic of soft matter and it is at the core of many ways of exploiting soft matter, whether industrially as in a liquid crystal display or by nature, as in our bodies.

References:

[1] P.-G. de Gennes, Soft Matter, 1, 1, pp. 16-16 (2005) “Soft matter: More than words”
[2] D. Dervichian, Mol. Cryst. Liq. Cryst., 40, pp. 19-31 (1977) “The Control of lyotropic liquid-crystals, biological and medical implications”