Levich Institute Seminar Announcement, 10/19/2010
Steinman Hall, Room #312
(Chemical Engineering Conference Room)
Professor Chinedum Osuji
Department of Chemical Engineering
"Evolution of Dynamic Viscoelastic Properties During Structural Arrest and Aging of Colloidal Glasses"
Aging at rest in sufficiently dense colloidal suspensions following the cessation of shear flow leads to the formation of a solid-like material (G'>G") from an initially fluid state. Evolution of the energy landscape during such physical aging of glassy materials can be understood from the frequency and strain dependence of the shear modulus but the non-stationary nature of these systems frustrates investigation of their instantaneous underlying properties. We show that parallel superposition rheology can be used effectively to study the dynamic rheological properties of glassy colloidal systems during this structural arrest transition, as a function of the stress imposed on the system. Using a series of time dependent measurements we systematically reconstruct the frequency and strain dependence as a function of age for a system undergoing structural arrest and subsequent aging in the presence of zero and small finite stresses. In this manner, we are able to unambiguously observe the structural relaxation time, which increases exponentially with sample age at short times. The yield stress varies logarithmically with time in the arrested state, consistent with recent simulation results, whereas the yield strain is nearly constant in this regime. Enhanced viscous dissipation during yielding is featured only in the arrested state, with the magnitude of dissipation increasing linearly with sample age. Strikingly, the frequency dependence at fixed times can be rescaled onto a master curve, implying a simple connection between the aging of the system and the change in the frequency dependent modulus.
BRIEF ACADEMIC/EMPLOYMENT HISTORY
RECENT RESEARCH INTERESTS
Structure and dynamics of soft matter and complex fluids; rheology of colloidal gels and dispersions; microfluidic studies of multiphase flows; directed self-assembly of soft mesophases and templated assembly of nanomaterials