"I study the physics of ‘soft condensed matter’. This term refers to a range of substances from polymer solutions (such as engine oil), emulsions (such as mayonnaise) and foams (such as shaving cream).
A big challenge for scientists is to understand how these materials flow. Many have flow behaviour intermediate between liquids and solids, demonstrating what is known as a ‘yield stress’. This means that a solid-like response is seen at weak forcing (a blob of shaving foam does not collapse into a puddle under its own weight) whilst a liquid-like response is seen when pushed harder (the same blob offers little resistance if rubbed between the fingers).
Although soft materials vary a lot chemically, the physical interactions responsible for formation of a yield stress appear to be quite universal.This allows them to be studied using the tools of physics and in particular, the theoretical methods known as ‘statistical mechanics’. These are methods for dealing with large assemblies of interacting units – whether these are molecules in a gas, entangled polymers in a molten plastic, or the numerous tiny bubbles in a shaving foam.
The goal of my research is to use these tools to predict flow behaviour for quite wide classes of soft materials. The ability to predict flow behaviour of soft matter has impact on processing capability in ceramics, plastics, and the food industries; on design of products to meet consumer expectations in terms of shelf-life, and in-use ‘feel’; on the design of display and other devices that use liquid crystals; and on understanding biological soft matter (such as the lubricants in our joints, or the ever-renewing‘cytoskeleton’ which gives shape to the cells in our body)."
A recent focus of my work has been to extend these theories to systems whose interacting units are not inert molecules but have the means of self-propulsion. Such units arise in biology -- for instance molecular motors within a cell, or in a suspension of swimming bacteria -- but also arise in colloidal suspensions whose micron-sized particles are designed to move around by using up a chemical fuel supply. We are in the process of extending statistical mechanics to address the behaviour of these so-called 'active' systems, whose microscopic laws look different from those of conventional materials.
Read more about Professor Michael Cates' work at the University of Cambridge.