Active Matter

"Active matter is composed of large numbers of active "agents", each of which consumes energy in order to move or to exert mechanical forces. Due to the energy consumption, these systems are intrinsically out of thermal equilibrium."

Examples of active matter are schools of fish, flocks of birds, bacteria, artificial self-propelled particles, and self-organising bio-polymers such as microtubules and actin, both of which are part of the cytoskeleton of living cells. Most examples of active matter are biological in origin; however, a great deal of current experimental work is devoted to synthetic systems. Our group has worked to develop and apply theoretical and computational descriptions of several model active systems.

Recent research highlights...

Local Polar and Long-Range Isotropic Activity Assisted Swelling and Collapse Dynamics of an Active Ring Polymer

Active polymer is an important class of active matter system that has immense application in the modelling and understanding of bio-molecules. Intense research has been carried out on bio-polymers modeled as linear or open chains, as well as on cyclic chains or rings under various kinds of activity. This current research includes the class of ring polymer which is gaining academic interest owing to their cyclic topology that leads to different physical properties. For example, tangentially active ring polymers exhibit a non-monotonic dependence of their size on active force, small rings swell whereas large rings collapse to a globule-like structure at high activity. Additionally, when the active and passive units of such polymers are allowed to interact via a long-range isotropic attraction, the non-monotonicity slowly fades away. The closest possible resemblance of such model in biological system are the long chromatin chains that are folded or looped into different level of compaction which allows contact between spatially distant segments of chromosomes. 

Particle-based mesoscopic model for phase separation in a binary fluid mixture

Liquid-liquid phase separation is an ubiquitous phenomena ranging from living cells to our daily life for example bio molecular condensates like P granules, stress granules etc. to oil water mixture. To understand such complex liquids many simulation models have been proposed which are based on molecular dynamics. This work describes the modelling of such complex liquids using the coarse-grained simulation technique named as multi particle collision dynamics (MPCD) which does not involve any direct inter-particle interaction and can be implemented to long length and time scales. 

Activity induced non-monotonic aggregation in a mixture of chemically active and passive colloids

Spontaneous symmetry breaking has been shown to be the genesis of self-assembly in a mixture of spherically symmetric chemically active and passive colloids, forming dense clusters. We study the kinetics of such self-assembly, driven by the phoretic motion of passive colloids following the chemical gradient generated by the active seeds. A non-monotonic effect of activity on aggregation is the key observation in this work. We rationalize such non-monotonicity in the clustering by the hybrid coarse-grained simulations. 

Cargo transportation using an active polymer

We explore the efficiency of the directional transport of the colloidal cargo by attaching it either at the front (pushing) or at the back (pulling) of the filament. The filament is chemo-mechanically active and acquires the activity by attaching chemically active beads that provide local tangential force along the chain. The effect of the size and location of the load, activity, and bending rigidity is comprehensively explored. With the help of dynamical properties, we show that the modes of propulsion and their efficiency is different for pushing and pulling, which depend on the load size.

Collapse Dynamics of Chemically Active Flexible Polymer

In the quest for synthetic active motors, it is observed that many biological active systems such as actin filaments, microtubules, and RNA strands exist in polymeric structures that can undergo easy deformation. Motivated by these naturally occurring active polymers our work models a chemically active polymer that experiences an effective tangential active force along its backbone by the chemical reaction on some of the monomers distributed along its length. The tangential local forces responsible for activity are provided by these monomers. We present the conformational properties of the flexible active polymer in an explicit solvent bath by performing a large-scale computer simulation to study the collapse dynamics of such a heteropolymer. 

Active particles in explicit solvent: Dynamics of clustering for alignment interaction

We use a combination of molecular and multiparticle collision dynamics methods for studying clustering in systems of Vicsek-like active particles in a hydrodynamic environment. Results are obtained for low overall density of active particles, for which the state point is close to the vapor branch of the coexistence curve, and thus the morphology consists of disconnected clusters. In such a situation, the mechanism of growth switches among particle diffusion, diffusive coalescence, and ballistic aggregation, depending upon the presence or absence of active and hydrodynamic interactions providing different kinds of mobilities to the clusters. Corresponding growth laws have been quantified and discussed in the context of appropriate theoretical pictures.

Autonomous movement of chemically powered vesicle

We investigate the diffusio-phoretic motion of a deformable vesicle. A vesicle is built from the linked catalytic and noncatalytic vertices that consumes fuel in the environment and utilize the resulting self-generated concentration gradient to exhibit propulsive motion. Under nonequilibrium conditions it is found that the self-propulsion velocity of the vesicle depends on its shape, which in turn is controlled by the bending rigidity of the membrane and solvent density around it. The self-propulsion velocity of the vesicle for different shapes has been calculated and the factors which affect the velocity are identified.