My research interest is aimed towards the understanding of the 'dynamics' of soft matter systems, within the "classical statistical mechanics" framework. Particular focus is on the non-equilibrium dynamics of fluids, close to a second-order phase transition. Few topics that I work on: (a) optically heated active colloids, (b) confined near-critical fluids and surface effects, (c) Casimir forces, (d) collective dynamics in fluids, (e) droplet coalescence, (f) phase separation, and aging, etc. For this purpose, I exploit a variety of computational techniques such as GPU-based molecular dynamics simulations, Monte-Carlo, numerical solution of Ginzburg-Landau like continuum dynamical models and Stokes equations, etc. Simulation results are analyzed by using finite-size scaling concepts and simple theoretical arguments.
A brief description is presented below.
1. Optically heated active colloids:
In the last decade, inspired by biological molecular motors, scientists tried to construct artificial devices that can deliver mechanical work or propel themselves in a liquid environment. One approach is to use phoretic transport mechanisms. An interesting candidate for self-propellers is
light-activated colloidal particles that are being used extensively of late. Micron-sized Janus particles, chemically functionalized appropriately in order to give rise to different surface adsorption properties and suspended in a near-critical solvent, undergo active motion when illuminated with a laser of sufficient intensity. Motility of these active particles depends on the non-equilibrium dynamics surrounding the particle which leads to coupled inhomogeneous temperature and concentration fields. This rich mechanism is sensitive to a variety of system parameters, viz., wetting properties of the colloids, illumination intensity, the concentration of the background solvent, hydrodynamic effects, confinement, etc.
2. Confined near-critial fluids:
Physical properties of liquids confined in channels of a few nanometers in diameter can differ significantly from their behavior in bulk. In particular, phase transitions can be suppressed or altered in comparison to the bulk counterpart. Upon approaching the critical point of a continuous phase transition, the order parameter (OP) of fluids near a solid wall deviates in normal direction from its bulk value on the scale of the diverging bulk correlation length. The kind of thermodynamic singularities occuring in this surface layer depends on the boundary conditions for the OP such that each bulk universality class splits up into various surface universality classes. Generically, fluids belong to the so-called normal surface universality class which is characterized by the presence of a symmetry breaking surface field which induces order at the surface even if the bulk is in the disordered phase. Using GPU-based molecular dynamics simulations, we study various dynamical properties of such systems.
3. Collective dynamics in fluids:
With the help of GPU-acclerated molecular dynamics simulations, we study the collective transport properties of binary liquid mixtures near the relevant demixing transition. Associated critical anomalies are quantified with the help of finite-size scaling analysis.
4. Phase separation and aging:
When an initially homogeneous system is quenched inside the binodal, the system phase separates into particle-rich and particle-poor domains. The kinetics of this non-equilibrium process is studied for the elongated percolating morphology as well as for droplet nucleation. In particular, universality of the growth exponent of average domain size is characterized in connection with the underlying growth mechanism. We also study the aging behavior of the fluid which is characterized by the two-time correlation functions of the order parameter.
5. Droplet coalescence:
When two liquid droplets come in contact with each other they form a liquid bridge and the composite structure finally relaxes to a single big drop —a kinetic process known as coalescence. One key aspect of coalescence is the so-called coalescence preference: the product drop (bubble) which emerges from the coalescence of two different-sized parent droplets (bubbles) tends to be placed closer to its larger parent. Spatial and temporal properties of this is studied for droplets and bubbles.
Soft Matter, 16, pages: 8359-8371, Royal Society of Chemistry, August 2020 (article)
A gold-capped Janus particle suspended in a near-critical binary liquid mixture can self-propel under illumination. We have immobilized such a particle in a narrow channel and carried out a combined experimental and theoretical study of the non-equilibrium dynamics of a binary solvent around it – lasting from the very moment of switching illumination on until the steady state is reached. In the theoretical study we use both a purely diffusive and a hydrodynamic model, which we solve numerically. Our results demonstrate a remarkable complexity of the time evolution of the concentration field around the colloid. This evolution is governed by the combined effects of the temperature gradient and the wettability, and crucially depends on whether the colloid is free to move or is trapped. For the trapped colloid, all approaches indicate that the early time dynamics is purely diffusive and characterized by composition layers travelling with constant speed from the surface of the colloid into the bulk of the solvent. Subsequently, hydrodynamic effects set in. Anomalously large nonequilibrium fluctuations, which result from the temperature gradient and the vicinity of the critical point of the binary liquid mixture, give rise to strong concentration fluctuations in the solvent and to permanently changing coarsening patterns not observed for a mobile particle. The early time dynamics around initially still Janus colloids produces a force which is able to set the Janus colloid into motion. The propulsion due to this transient dynamics is in the direction opposite to that observed after the steady state is attained.
Soft Matter, 15(23):4743-4750, Royal Society of Chemistry, Cambridge, UK, May 2019 (article)
We performed molecular dynamics simulations to study relaxation phenomena during vapor–liquid transitions in a single component Lennard-Jones system. Results from two different overall densities are presented: one in the neighborhood of the vapor branch of the coexistence curve and the other being close to the critical density. The nonequilibrium morphologies, growth mechanisms and growth laws in the two cases are vastly different. In the low density case growth occurs via diffusive coalescence of droplets in a disconnected morphology. On the other hand, the elongated structure in the higher density case grows via advective transport of particles inside the tube-like liquid domains. The objective in this work has been to identify how the decay of the order-parameter autocorrelation, an important quantity to understand aging dynamics, differs in the two cases. In the case of the disconnected morphology, we observe a very robust power-law decay, as a function of the ratio of the characteristic lengths at the observation time and at the age of the system, whereas the results for the percolating structure appear rather complex. To quantify the decay in the latter case, unlike the standard method followed in a previous study, here we have performed a finite-size scaling analysis. The outcome of this analysis shows the presence of a strong preasymptotic correction, while revealing that in this case also, albeit in the asymptotic limit, the decay follows a power-law. Even though the corresponding exponents in the two cases differ drastically, this study, combined with a few recent ones, suggests that power-law behavior of this correlation function is rather universal in coarsening dynamics.
Comptes Rendus Physique, 16(3):303-315, April 2015 (article)
We review the understanding of the kinetics of fluid phase separation in various space dimensions. Morphological differences, percolating or disconnected domains, based on overall composition in a binary liquid or on density in a vapor–liquid system, are discussed. Depending upon the morphology, various possible mechanisms for domain growth are pointed out and discussions of corresponding theoretical predictions are provided. On the computational front, useful models and simulation methodologies are presented. Theoretically predicted growth laws have been tested via molecular dynamics simulations of vapor–liquid transitions. In the case of a disconnected structure, the mechanism has been confirmed directly.
Our goal is to understand the principles of Perception, Action and Learning in autonomous systems that successfully interact with complex environments and to use this understanding to design future systems