February 14 , 2007

Soft Materials: Controlling and Probing Structure Formation at the Meso Scale

Hans Wyss

Physics and DEAS, Harvard University

Soft materials such as polymers, colloids or surfactant systems exhibit a wide range of fascinating structural, dynamic and mechanical behaviors. They contain structural elements at the meso scale, larger than molecules, but smaller than macroscopic objects. Examples of soft materials are ubiquitous; they include for instance paints, food systems, and many other industrial products.  My interest is focussed on understanding the physical mechanisms that govern the behavior of such systems.

Studying the macroscopic mechanical properties of a variety of soft materials, we find remarkable similarities in their rheological behavior both in linear and nonlinear viscelastic measurements. Our experiments show that these properties can be unified in a single physical picture by considering the effect of the strain-rate amplitude on the structural relaxation of the material. We present a new form of oscillatory rheology, Strain-Rate Frequency Superposition (SRFS), where the strain-rate amplitude is fixed as the frequency is varied. We show that SRFS can isolate the response due to structural relaxation, even when it occurs at frequencies too low to be accessible with standard techniques.

I will further show that glass-like behavior in soft materials is not limited to systems that are very densely packed, but can be induced even at relatively low volume fractions, implying that SRFS will be applicable to a very broad range of soft materials.

Based on our experimental data on a colloidal model system we develop a simple physical picture that bridges the gap between colloidal gel networks and colloidal glasses, two distinctly different out-of-equilibrium states of soft matter.