Chemical Engg. seminar series: Dr. Yogesh Joshi

Evolution of Linear Viscoelasticity across the Sol-Gel Transition

Gels comprise of synthetic polymers to colloidal systems and represent a unique state of soft matter characterized by a space spanning percolated network swollen by liquid solvent. As material transforms from a flowing sol state to a soft solid-like gel state comprising of a space-spanning percolated network, its mechanical response undergoes a remarkable change. At the critical gel state, which is an intriguing unique point between the sol and the gel states, the relaxation of stress becomes scale-free, and characteristic relaxation times diverge. Literature reports that evolution of relaxation dynamics on the either side of the critical gel state, intriguingly, follows a symmetric divergence and several other fascinating features including observation of scaling and hyper-scaling laws. Despite over 50 years of research on this topic, many of these issues remained unexplained. To address this, we develop a rigorous theoretical framework for the evolution of linear viscoelastic properties across the sol-gel transition. More specifically, we derive general admissible expressions for the relaxation modulus and dynamic moduli as the critical gel state is approached from the pre-gel or the post-gel side. We validate these expressions against the experimental data for various polymeric and colloidal systems. We show that the symmetry, scaling, and hyperscaling properties of the sol-gel transition are all consequences of a single unifying physical requirement originating from the continuity of the linear viscoelastic properties at the critical gel point.

Bio :

Dr. Yogesh Joshi earned his B.E. from Pune University in 1996 and his Ph.D. in Chemical Engineering from the National Chemical Laboratory Pune and the Indian Institute of Technology Bombay in 2001. He then joined the Benjamin Levich Institute in New York for post-doctoral studies. In 2004, Dr. Joshi joined the Department of Chemical Engineering at the Indian Institute of Technology Kanpur, where he is currently the Sir J C Bose National Fellow and Mr. and Mrs. Gian Singh Bindra Chair Professor. His research interests include the rheology of complex fluids, soft matter, interfacial science and engineering, clay dispersions, and polymer science and engineering. He is a senior editor of Langmuir and serves on the editorial advisory boards of Physics of Fluids, Journal of Rheology, Transport Phenomena, and Rheologica Acta. Dr. Joshi received Shanti Swarup Bhatnagar award in Engineering Sciences in 2015. He is an elected fellow of the Society of Rheology (US), the Indian National Science Academy, the Indian Academy of Sciences, the Indian National Academy of Engineering, and the National Academy of Sciences, India. He is also closely associated with many industries, helping to improve the flow behavior and stability of soft materials. More information is available on his group website: http://home.iitk.ac.in/~joshi/