The term 'soft matter' describes a group of materials that are assembled from components whose size scale is of order microns or nanometers -- much bigger than a typical molecule or atom. Examples include polymers (very long flexible molecules), colloids (small hard spheres), emulsions (droplets of one fluid in another), foams (gas bubbles in a fluid), detergent molecules (with a water-loving head and a water-hating tail -- these clump together into complex shapes), powders (small dry grains), and many analagous systems of biological origin. Familiar examples are respectively engine oil, paint, mayonnaise, shaving cream, shampoo, and talc; the biological analogues include mucus, slime moulds, saliva, and various components of the living cell.
In many cases, the system's behaviour is controlled not by the chemical details of its components, but by their physical interactions, which are generic to each class of material. The softness of these materials, compared to (say) a piece of metal, arises from the fact that these interactions are generically weaker than those between atoms. This makes it easy to bend and shape the materials, and to subject them to extremes of flow (causing disruption to the structure) that cannot easily be achieved with metals or other forms of 'hard' condensed matter. The weakness of the interactions means that there is a lot of random motion (the motion we call heat) even at room temperature; the properties of soft materials are often closer to those found by maximising the entropy (randomness) of the system than to those found by minimizing its energy. Under these conditions, one must use the tools of 'statistical mechanics' to understand how the microscopic interactions, combined with entropy, come to determine the properties of the material.
Prior to award of this grant, our Group had developed experimental and theoretical techniques for understanding how the ingredients of a soft material come to determine its properties -- particularly those properties related to how the material flows (the science of 'rheology'). Our approach focusses on making detailed studies of a small number of model systems, each representative of a larger class: by understanding these in depth, we hope to find general principles that might not be obvious by collating more superficial results for a wider range of samples.
This project continued our integrated programme in experiment and theory, to address new topics in soft condensed matter, increasingly those at the interface with biology. The five main projects were:
1. Rheophysics -- we improved our understanding of the behaviour of colloids and other soft materials under conditions of strong flow. Often, flow can totally alter the internal structure of such materials.
2. Physics of barriers in soft matter and biology -- we improved our understanding of how soft and biological systems undergo 'rare events' taking them from one state of organization to another. These include events that alter the way genes are expressed in a cell, and also the nucleation of one phase of matter within another.
3. New soft materials -- building on our recent discoveries, we used physics to create new and interesting materials with properties potentially relevant to computer displays, drug delivery, catalysis and other fields.
4. Physics of cellular motion -- we made progress in understanding how bacteria (which, if they were dead, would be effectively colloids) behave when swimming, either individually, or collectively. At a smaller scale, within the cell there are various soft matter components which use a constant supply of chemical energy to maintain an 'active' (i.e. living) state. We made progress understanding these too.
5. New statistical mechanics tools -- we developed new and better theories and simulation models that will, over the longer term, help us connect the microscopic components in soft materials to their macroscopic properties.