Arvind Kumar Gautam's Talk

Aug 14, 2015 - 16:00

Aug 14, 2015 - 17:00

Room 118 Creativity Hall Chemical Engineering

Seminars

Arvind Kumar Gautam Chemical Engg. Dept. I.I.T Kanpur 208016

Abstract: The liquid phase in a supercooled state (i.e. at a temperature below the melting temperature) eventually relaxes towards the stable crystal phase. The estimation of the properties of the supercooled liquid close to the limit of metastability could be challenging because of the fast relaxation process. The properties of the supercooled tetrahedral liquids (such as silicon and water) show unusual behaviour. One such well known anomaly is the increase in heat capacity as the temperature is reduced. To understand the origin of such behavior it is important to study in detail the equilibrium (i.e. average) properties and the statistical distributions of these properties. In this work we investigate the supercooled tetrahedral liquids of silicon and water (modelled using widely used empirical potentials) close to the limit of metastability. In case of supercooled silicon we identify the equilibrium properties based on precise computation of Gibbs free energy (relative to the stable crystalline phase) by means of thermodynamic integration. Further we study the equilibrium distribution of potential energy of the supercooled liquid. We find that as the relaxation process initiates the distribution of potential energy develops a straight line region (SLR) with a configurational temperature close to 1060 K which was identified as the freezing temperature of 4 coordinated particle network in an earlier study. We find that the equilibrium distribution is tangential to the SLR region. We attribute the increase in heat capacity (with supercooling) to the appearance of this straight line region in the potential energy distributions. In the second part of our work we study the relaxation process in detail using supercooled water. The relaxation of supercooled liquid at a macroscopic level is generally described in the framework of classical nucleation theory. However in a deeply supercooled state near the limit of metastability inherent structures that influence the liquid properties may play an important role. We find that the relaxaion of the supercooled liquid is accompanied by a sharp change in the potential energy and density of the system as well as a rapid increase in size of the 4 -coordinated particle network. During this process the perparticle 2-body energy of the 4-coordinated network approaches the crystalline value very closely (with less than 0.35 % deviation) while the per particle 3-body energy of the network approaches a value that shows a large (about 60--90 %) deviation with respect to the crystalline value. During relaxation the fluctuations in the 2-body energy of the network decrease continuously resulting in the tightening of the bonds which is expected to reduce the mobility of the 4-coordinated particles. We describe this process as the freezing of the 4- coordinated particle network. Our work shows that close to the limit of metastability the relaxation process may be better described at a macroscopic level in terms of average mechanical changes in the inherent structures (i.e. the 4-coordinated particle network in our case) that develop naturally in the liquid as a result of supercooling.Bio-Data: Arvind Kumar Gautam is a research scholar in chemical engineering department of I.I.T Kanpur.He has recently submitted his Ph.D. thesis. In his Ph.D. he has worked in the field of modeling and simulation with the specialization of thermodynamics.