arXiv subjects

Atul Srivastava

Publications and source records attributed to Atul Srivastava.

3 recordsLinked to original sources

arXiv2020

A comparative study of the behaviour of forsterite melts under atmospheric and sub-atmospheric conditions

The study is focussed towards understanding the difference in behaviour of forsterite droplets subjected to sub-atmospheric conditions. Melt droplets, 1.5-2.5 mm in diameter, are made to crystallize under levitated conditions. The spherule is initially superheated by about 250oC above its liquids temperature, for both atmospheric and sub-atmospheric conditions, and then, subsequently cooled at different cooling rates. Crystallization of molten droplets was observed from their hypercooled states in all the cases. It was found that the level of undercooling was more (~150K-200K) for sub-atmospheric cases. In addition, the degree of recalescence was also found to be higher. However, the varying cooling rates did not produce any considerable effect on the level of undercooling or, even, the degree of recalescence. For better insight into the dynamics of crystallization, in situ visualization of the recalescence process was made possible by use of high-speed camera. A clear difference in the growth mechanisms was observed. The role of cooling rate in affecting crystallization was seen as the difference in the growth of the crystal-front; from a clear rim front, in high cooling rates, to a faded irregularly shaped crystal front, in low cooling rates. Two proper crystal fronts were observed in all the cases, which correspond to surface and volumetric crystallizations. It was observed that the molten droplets, under sub-atmospheric conditions, undergo high rate of volumetric crystallization, which is marked by the sightings of surface textures during the recalescence process. High-speed camera images of recalescence provided direct observational proof of the delay in volumetric crystallization, compared to surface crystallization.

physics.app-ph↗

arXiv2019

On the mechanism responsible for unconventional thermal behaviour during freezing

In this study, identical experiments of bottom-cooled solidification fluidic mixtures that exhibit faceted and dendritic microstructures were performed. The strength of compositional convection was correlated with the solidifying microstructure morphology, with the help of separate Rayleigh numbers in the mushy and bulk-fluid zones. While the dendritic solidification experienced a monotonic decrease in the bulk fluid temperature, solidification of the faceted case revealed an unconventional, anomalous temperature rise in the bulk liquid, at the initiation of the eutectic phase. Based on the bulk-liquid temperature profile, three distinct regimes of heat transfer were observed in the liquid over the course of solidification, namely - convection-dominated, transition, and conduction-dominated. The observations were analyzed and verified with the help of different initial compositions, as well as other mixtures that form faceted morphology upon freezing. The observed temperature rise was further ascertained by performing an energy balance in an indicative control volume ahead of the solid-liquid interface. The plausible mechanism behind the gain in temperature of the liquid during freezing was further generalized with the help of a simplified one-dimensional numerical model, and was extended to metals which are low Prandtl number mixtures. The study sheds new insights into the role of microstrostructural morphology in governing the transport phenomena in the bulk liquid.

cond-mat.soft↗

arXiv2018

Controlling the size distribution of nanoparticles through the use of physical boundaries during laser ablation in liquids

A simple, yet effective method of controlling the size and size distributions of nanoparticles produced as a result of laser ablation of target material is presented. The method employs the presence of physical boundaries on either sides of the ablation site. In order to demonstrate the potential of the method, experiments have been conducted with copper and titanium as the target materials that are placed in two different liquid media (water and isopropyl alcohol). The ablation of the target material immersed in the liquid medium has been carried out using an Nd:YAG laser. Significant differences in the size and size distributions are observed in the cases of nanoparticles produced with and without confining boundaries. It is seen that for any given liquid medium and the target material, the mean size of the nanoparticles obtained with the boundary-fitted target surface is consistently higher than that achieved in the case of open (flat) targets. The observed trend has been attributed to the plausible role(s) of the confining boundaries in prolonging the thermalisation time of the plasma plume. In order to ascertain that the observed differences in sizes of the nanoparticles produced with and without the presence of the physical barriers are predominantly because of the prolonged thermalisation of the plasma plume and not due to the possible formation of oxide layer, select experiments with gold as the target material in water have also been performed. The experiments also show that, irrespective of the liquid medium, the increase in the mean size of the copper-based nanoparticles due to the presence of physical boundaries is relatively higher than that observed in the case of titanium target material under similar experimental conditions.

physics.plasm-ph↗