Research Interests
"If we knew what it was we were doing, it would not be called research, would it?" - Albert Einstein
"No problem is too small or too trivial if we can really do something about it." - Richard Feynman
Microfluidics and Complex Fluids
Complex Fluids (aka Soft Matter) comprise a large class of deformable materials whose behaviour is significantly different from conventional solids or Newtonian fluids due to the presence of mesoscale structures. Such materials include colloidal suspensions, emulsions, surfactants, ferrofluids, polymeric liquids, liquid crystals, etc. A comprehensive understanding of the hydrodynamics of complex fluids is essential in various industrial applications, ranging from chemical, food and pharmaceutical industries to modern-day lab-on-a-chip technologies used in biomedical and healthcare applications. In this group, we are interested in understanding the motion of passive and active particles suspended in non-Newtonian fluids. We have developed a mesoscale simulation tool (nematic MPCD) for a complex fluid called nematic liquid crystals.
Nematic Liquid Crystals: The dynamics of liquid crystals in the presence of spatially and temporally varying active or passive forces is an important topic in the field of nonequilibrium soft matter. Nematic liquid crystals possess long-range molecular orientational order due to their rodlike molecules. Liquid crystals establish a nearly unique combination of thermodynamic, hydrodynamic, and topological behaviour. This poses a challenge to their theoretical understanding and modelling. The arena where these effects come together is the mesoscopic (micron) scale.
Mesoscopic Modelling of Nematic Liquid Crystals: Liquid crystals establish a nearly unique combination of thermodynamic, hydrodynamic, and topological behaviour. This poses a challenge to their theoretical understanding and modelling. The arena where these effects come together is the mesoscopic (micron) scale. It is then important to develop models to capture this variety of dynamics. We have generalized the particle-based multiparticle collision dynamics (MPCD) method to model the dynamics of nematic liquid crystals.
Microswimmers and Biofluid Dynamics
Biological Fluids are a special class of complex fluids made of biological matter. Such materials include blood, mucus, saliva, synovial fluid, sperm suspension, bacterial suspension, etc. Research and innovation in this field may revolutionize the delivery of cargo and drugs, minimally invasive surgery, in vitro fertilization, and several lab-on-a-chip technologies. In this group, we are interested in understanding the motion of microorganisms in biological environments.
Dynamics of a microswimmer in non-Newtonian fluid
Microswimmers are self-driven micron-sized bodies (particles, droplets or organisms) capable of utilizing internal or external energy in a systematic motion. Recently, there has been tremendous attention in Biological Fluid Dynamics and Active Matter research towards understanding the motion of biological and artificial microswimmers. Research and innovation in this field may revolutionize the delivery of cargo and drugs, minimally invasive surgery, in vitro fertilization, and several lab-on-a-chip technologies. Towards achieving these technological goals, the major challenge is the controlled manipulation of the trajectory and orientation of the microswimmers in complex environments.
Force dipole in nematic liquid crystals: We have investigated the flow field around a force dipole in a nematic liquid crystal, representing the leading-order flow field around a force-free microswimmer. The anisotropy of the medium not only affects the magnitude of the velocity field around the force dipole but can also induce hydrodynamic torques depending on the orientation of the dipole axis relative to the director field. A force dipole experiences a hydrodynamic torque when the dipole axis is tilted with respect to the far-field director. The direction of hydrodynamic torque is such that the pusher- (or puller-) type force dipole tends to orient along (or perpendicular to) the director field.
Squirmer in nematic liquid crystals: Recently, we have studied the motion of a model microswimmer, squirmer, in a nematic liquid crystal media. A recently developed nematic MPCD method [Phys. Rev. E 99, 063319 (2019)] which employs a tensor order parameter to describe the spatial and temporal variations of the nematic order is used to simulate the suspending anisotropic fluid. Considering both nematodynamic effects (anisotropic viscosity and elasticity) and thermal fluctuations, in the present study, we have coupled the nematic MPCD algorithm with a molecular dynamics (MD) scheme for the squirmer. A unique feature of the proposed method is that the nematic order, the fluid, and the squirmer are all represented in a particle-based framework. To test the applicability of this nematic MPCD-MD method, we have simulated the dynamics of a spherical squirmer with homeotropic surface anchoring conditions in a bulk domain. The importance of anisotropic viscosity and elasticity on the squirmer’s speed and orientation is studied for different values of self-propulsion strength and squirmer type (pusher, puller or neutral). In sharp contrast to Newtonian fluids, the speed of the squirmer in a nematic fluid depends on the squirmer type. Interestingly, the speed of a strong pusher is smaller in the nematic fluid than for the Newtonian case. The orientational dynamics of the squirmer in the nematic fluid also shows a non-trivial dependence on the squirmer type. The full particle-based framework could be easily extended to model the dynamics of multiple squirmers in anisotropic fluids.
Droplet Microswimmers
Self-propelled Droplet or Swimming Droplet: Self-propelled droplet or droplet swimmer is a relatively new class of active matter system consisting of liquid droplets that can move autonomously without the intervention of externally imposed flows/fields. The self-propulsion is achieved by the generation of Marangoni stress at the droplet interface due to gradient in surfactant concentration. Though the typical driving mechanism is same for all self-propelled droplets, the way gradient in surfactant concentration is created and continuously sustained depends on the physical system (i.e. properties of two interacting liquids and surfactant molecules). The following two physical systems have been studied extensively: propulsion by chemical reaction, and propulsion by solubilization.
We are interested to understand the dynamics of self-propelled droplets in a confined environment using both experiment and numerical simulation.
Droplet Microfluidics and Marangoni Flows
Thermocapillary Migration of Droplets
Thermocapillary Migration of Droplet in Poiseuille Flow: Intricate manipulation of droplets in fluidic confinements may turn out to be critically important for achieving their controlled transverse distributions. Here, we study the migration characteristics of a suspended deformable droplet in a parallel plate channel under the combined influence of a constant temperature gradient in the transverse direction and an imposed pressure driven flow. For the analytical solution, an asymptotic approach is used, where we neglect any effect of inertia or thermal convection of the fluid in either of the phases. To obtain a numerical solution, we use the conservative level set method. We perform numerical simulations over a wide range of governing parameters and obtain the dependence of the transverse steady position of the droplet on different parameters. In order to address practical microfluidic set-ups, the influence of a bounding wall as well as the effect of thermal convection and finite shape deformation on the cross-stream migration of the droplet is investigated through numerical simulations.
Surfactant-laden Droplets
Surface tension at liquid-liquid interface depends on the temperature and surfactant concentration. The presence of surfactants not only reduces the interfacial tension but also creates a local gradient in interfacial tension (i.e., Marangoni stress) which has the ability to affect the motion dynamics of the droplets dramatically.
Surfactant-laden Droplet in Poiseuille Flow: The motion of a viscous deformable droplet suspended in an unbounded Poiseuille flow in the presence of bulk-insoluble surfactants is studied analytically. Assuming the convective transport of fluid to be negligible, we perform a small-deformation perturbation analysis to obtain the droplet migration velocity. The droplet dynamics strongly depends on the distribution of surfactants along the droplet interface, which is governed by the relative strength of convective transport of surfactants as compared with the diffusive transport of surfactants. The present study is focused on the following two limits: (i) when the surfactant transport is dominated by surface diffusion and (ii) when the surfactant transport is dominated by surface convection.
Droplet Microfluidics and Electrohydrodynamics
Electrohydrodynamics of Droplets and Emulsions
Not only in conventional industrial applications such as chemical, food and pharmaceutical industries, droplets also play a central role in interdisciplinary microfluidic and nanofluidic research. Droplets are an integral part of multiphase microfluidic devices. Droplets are used as micro-mixers and micro-reactors in several chemical processes. Droplets are used in encapsulation of macromolecules and cells. Note that cell encapsulation plays an important role in single-cell analysis used for efficient detection techniques. Thus, active control over the generation, shape deformation and transport of droplets through these small-scale devices are essential for optimal functionalities of the concerned applications.
Motivated by these applications, we are interested in understanding droplet motion and emulsion rheology in presence of external flows and fields. The aim is to study the electrohydrodynamic manipulation of suspended droplets and emulsions in the combined presence of imposed electric field and imposed flow field. The following two important aspects are studied: modelling of the motion and shape deformation of an isolated droplet; modelling of the emulsion stress of a dilute suspension of droplets. We have solved a couple of important problems. First, we have studied the gravitational settling of droplet under the action of uniform electric field. This setup is relevant for separation of water in water-in-oil emulsions in oil industries. Second, we have studied the combined influence of pressure driven flow and electric field. This setup is relevant for microscale droplet manipulation. Third, we have studied droplet dynamics in the combined presence of shear flow and electric field. This setup is relevant for emulsion electro-rheology.
Sedimentation of Droplet in Uniform Electric Field: We have obtained analytical expressions for the velocity of a sedimenting droplet in the presence of a uniform electric field acting in an arbitrary direction, considering small shape deformation and small charge convection effects. We have shown that tilt angle, which quantifies the angle of inclination of the applied electric field with respect to the direction of gravity, has a significant effect on the magnitude and direction of the drop velocity. Droplet deformation and charge convection independently contribute to the alteration in drop velocity in both the longitudinal as well as lateral directions in presence of tilted electric field. Experimental measurements also confirm the lateral migration of the drop in the presence of tilted electric field.
