Rheology of soft matter

Understanding the complexity of soft matter is a challenge for physics in the 21st century. One of the first steps must be a deeper understanding of the properties of soft matter which is far from equilibrium.

Equilibrium and determinism play a central role in physics. On the contrary, living processes and complex fluids are not at equilibrium. Here motion often appears chaotic, or has periods of apparent stability punctuated by periods of rapid change. The problem in understanding these systems arises because of their complexity.

Living cells are a good example of complex soft matter. They are made up of molecular assemblies with mesophase (between a solid and a liquid) structures that have multiple length scales whose dynamics exhibit multiple time scales.

Simple complex systems

Synthetic models coined 'simple complex systems', include high molecular mass polymers and elastomers, micellar structures, liquid crystals, foams, emulsions, micro-emulsions and bicontinuous phases. These systems are also important in advanced materials synthesis, food technology, oil recovery, and biotechnologies such as drug delivery and therefore have important practical applications.

The 'profound' motivation is thereby augmented by some mundanely practical goals, goals that call on our understanding of the mesophase origins of mechanical complexity in which out-of-equilibrium conditions are imposed through flow. Here competition arises between the molecular organisational dynamics and the rate of strain with outcomes including conformational distortion, re-organisation of mesophase structure, double-valuedness in the constitutive properties, banded flow, the driving of the material through nearby phase transitions, and soft glassy dynamics-the slow aging of a system as the structure reorganises. In a sense, the precise molecular system studied is less important than its potential to act as a platform for the essential physics, a physics that will only be revealed by the most imaginative experimental tools.

Rheo-NMR

To meet the challenges of the soft matter research we seek to extend the power of Rheo-NMR, examining some of the central questions around molecular organisation and fluctuations under deformational flow. We also want to build links with established methods to increase their acceptance.

We have already shown that Rheo-NMR measurements of shear-induced orientational order agree perfectly with neutron scattering, while disagreeing with birefringence studies. Further objectives with respect to the Rheo-NMR methodology includes the development of rapid-scan, movie-frame-rate Rheo-NMR microscopy, the incorporation of devices for extensional flow or the further exploitation of deuterium NMR spectroscopy and imaging.