Yogesh M Joshi

Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016. INDIA

A Constitutive Model for Aging Soft Glassy Materials: Prediction of Yield Stress, Shear Banding, and Residual Stresses and Strains



Glassy soft materials such as concentrated suspensions and emulsions, foams, colloidal gels and variety of different pastes are routinely used in industry as well as in everyday life. In this class of materials either the crowding of constituting entities and/or inter-particle attractive/repulsive interactions kinetically restrict the same from achieving the equilibrium structures. However, microscopic mobility of the constituents arising from the thermal energy induces slow but steady structural evolution to form progressively stable structures. This process of structural recovery is also known as physical aging, wherein relaxation time increases with time. If such material is subjected deformation field, the structure evolved during aging gets altered, which usually causes reversal of physical aging. The corresponding process is termed as rejuvenation.
In this talk we shall discuss a recently proposed constitutive model, which considers aging and rejuvenation in a soft glassy material as respectively a decrease and an increase in free energy. The aging term is weighted by inverse of relaxation time suggesting greater mobility of the constituents induce faster aging in a material. The rejuvenation term, on the other hand, is weighted by timescale of deformation field. A dependence of relaxation time on free energy is proposed by drawing analogy with mode coupling theory, which under quiescent conditions, leads to power law dependence of relaxation time on time as observed experimentally. Interestingly when we use a framework of Maxwell model, the time dependency of physical variables leads to non-monotonic relationship between steady state shear stress and shear rate. We show that such non-monotonicity renders a natural framework to demonstrate existence of true yield stress, thixotropy, shear banding, and presence of residual stresses and strains. Remarkably the model predictions qualitatively match the experimental observations very well.

Date: 18/5/2015
Time:15:00 (coffee & cookies will be served at 14:45)
Place:FORTH Seminar Room 1