MultiScale Materials Science for Energy and Environment
13th, May 2019, Friday, 12:00‐1:00 pm, ChE Seminar hall
Abstract Viscoelastic materials are ubiquitous in our everyday life. From rubber to butter to bitumen, we are surrounded by materials that display both a viscous and an elastic response under deformation. In the limit of linear deformation, the viscoelastic materials generally follow principles of linear viscoelasticity. However, aging soft glassy materials demonstrate temporal evolution of physical properties such as viscosity and modulus, invalidating time – translational invariance. Consequently, the fundamental principle of linear viscoelasticity - the Boltzmann superposition principle (BSP) is not applicable to this class of materials. Recently it has been proposed that the BSP can be applied to aging soft materials by transforming the material from the real time to the effective time domain. The effective time is defined by normalizing the real time by time dependent mean relaxation time so that the time dependency ceases to exist in the effective time domain. In this work we test applicability of BSP for aging colloidal suspensions of Laponite subjected to different stress fields. We observe that influence of deformation field on the shape of evolving spectrum of relaxation times is crucial to the validation of BSP. Beyond the linear deformation regime, such materials often exhibit complex flow profiles, from spatially heterogeneous ductile flow to crack and fractures. In this presentation, we also focus on the latter category of materials and show that a scratch technique, which has been extensively applied for determining the resistance to fracture of solids, can be successfully applied to viscoelastic materials. Using a Rockwell diamond probe of conical shape mounted on a micro scratch tester, we perform direct measurements of the fracture toughness of various type of bitumen samples at ambient temperature. We show that the fracture toughness of bitumen increases as a power-law of the horizontal scratch speed, independently of the vertical loading rate. The power-law exponent is characteristic of the sample and increases for decreasing penetration grade. Finally, we use the scratch test to quantify the impact of various additives (polymer, epoxyilane) on the fracture toughness of bitumen samples.
Biosketch Asheesh is currently working as a Post-Doctoral Associate at the joint CNRS-MIT lab, MultiScale Materials Science for Energy and Environment [MSE] 2 , at the Massachusetts Institute of Technology, Cambridge. At MIT, he is working towards understanding, quantifying and improving performance of bitumen as a road building material. He is trying to measure fracture toughness of bitumen and relate that with its microstructure. He obtained his B.Tech- M.Tech (dual degree) in Chemical Engineering from IIT Kanpur, in 2008. Thereafter, he worked for 3 years at the Jamnagar refinery of Reliance Industries Limited. He later rejoined IIT Kanpur to pursue PhD degree under the supervision of Prof. Yogesh M. Joshi, at the Department of Chemical Engineering. During Ph.D. he worked on understanding the yielding of soft surfactant pastes and applying the Boltzmann superposition principle to the aging soft glassy materials.