Additional Seminar

Tuesday, 26th March 2013 at 16h00
(coffee at 15h45)

Campus Limpertsberg
Room BSC-E00-007

Talk by
Dr. Hans Wyss
TU Eindhoven, The Netherlands

Relaxation and flow of soft colloids

Suspensions of soft, deformable and compressible particles exhibit dynamics and flow behavior that is qualitatively different from that observed for hard particles. A wide range of materials including foods, creams and biological systems contain such soft particles. Nevertheless, their behavior remains poorly understood.
As a basis for elucidating this behavior, it is important to first quantify the properties of the particles themselves. I will give a short overview of methods used to experimentally access the elastic properties of single soft particles, focusing on a new method, Capillary Micromechanics, that we have recently introduced. It enables us to access both the elastic shear modulus as well as the elastic compressive modulus of single soft particles in a single experiment.

I will further illustrate the importance of particle softness by discussing the use of soft particle suspensions as model glass formers. Colloidal particles have been widely employed as models for understanding glass formation; their behavior is surprisingly similar to that found for molecular glass formers. However, for different molecular glass-forming liquids, the dynamic behavior upon approach to the glass transition shows broad variations, quantified by the so-called fragility, which describes the sensitivity of their viscosity or structural relaxation time to changes in temperature. In analogy, for colloidal systems, fragility can be defined based on the sensitivity of the structural relaxation time to changes in volume fraction. In this analogy, hard-sphere colloids only exhibit a highly fragile behavior.

By using soft, deformable particles we extend the concept of fragility to colloidal soft materials and capture the entire range of dynamic behaviors merely by varying the softness of the individual mesoscopic particles. Hard particles make « fragile » glasses and soft particles make non-fragile, or « strong », glasses.

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