Exploring the physics of active particles and active matter
Active particles are motile objects, which can be either living or inert. Some examples are bacteria, artificial microswimmers, actin+myosin assembly in living cells. Collectively they form fluids, liquid-crystals, etc. in the conventional sense. However motility of the constituent particles brings in an unusual twist — activity. As a result a variety of striking and surprising properties with novel physics emerge.
Here, at IISER Tirupati, APP-G has begun to unravel the mysteries of active systems in some of the hardest to understand limits.
Current Projects
Fluid-Coupled Physics of Flexible, Head-Tail Asymmetric Active Particles
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In addition to head-tail asymmetric shapes, several particles, in order to generate thrust, have appendages. These have varying degrees of flexibility. Job is uncovering remarkably new states of motion that occur in populations of flexible "microswimmers”. He is fascinated by the novel correlations and collective behaviour that are the consequence of flexibility and the resulting non-axisymmetry, and he is striving for a theory of dense suspensions, where flexibility naturally plays a prominent role.
Most active particles, both living and nonliving, have pronounced head-tail shape asymmetry, giving them a preferred sense of direction. Derek’s work predicted that when such particles are suspended in fluid media, this asymmetry should have the additional effect of crucially altering its hydrodynamic interactions with other suspended particles and/or ambient flows.
Derek has incorporated this hitherto neglected asymmetry into active particle dynamics. His work predicts novel particle transport in nonuniform flows, some of which have been experimentally realized, while the developed framework extends to diverse flow scenarios. He is most interested in analyzing active matter made up of these shape-asymmetric constituents, where he expects new physics to emerge.
Active particles in confined fluids
Hydrodynamics of Biologically-Inspired Microswimmers Near Surfaces
In nearly all biological and technological settings, microswimmers operate not in infinite fluids but in complex, confined spaces. From bacteria in biofilms to artificial swimmers in microfluidic devices, interactions with surfaces are unavoidable and often lead to swimmers accumulating near them. Ambareesh is probing the fundamental question of why this near-surface accumulation occurs. He is developing a detailed theoretical model that account for the intricate hydrodynamic interplay between a swimmer's body, its flagellum, and a nearby boundary. His goal is to build a robust theoretical framework that can accurately capture the physics of this interaction, moving beyond simplified models to better understand this universal behavior.
Hydrodynamics of Rod-Shaped Swimmers in Thin Liquid Films
Confinement can be more complex than a single solid surface. A common and important environment is the thin liquid film, such as those found in bacterial biofilms, which are bounded by a solid surface below and a liquid-air interface above. Ambareesh is investigating how swimmers navigate these unique environments. By modeling a rod-shaped swimmer within a doubly-confined channel, he is exploring how the fundamentally different physics at the solid-liquid and liquid-air boundaries cooperatively shape the swimmer's trajectory and spatial distribution. He aims to uncover the principles governing locomotion in these challenging and biologically relevant geometries. To ground the theoretical predictions, this project is being carried out in synergy with experiments conducted by Dhananjay in Dr. Sivasurender Chandran’s lab at IIT Kanpur.
Collective Dynamics of Quincke Rollers Near a Conducting Boundary
The behavior of artificial microswimmers is often governed by a complex interplay of different physical forces. Quincke rollers, which are propelled by electric fields, are a prime example. When placed near a conducting boundary, the problem becomes even more challenging, as the wall modifies both the electrical and the fluid-mediated forces between particles. Ambareesh is probing how these intertwined fields govern the collective behavior of many rollers. Through multi-particle simulations that couple the electrostatic and hydrodynamic interactions, he seeks to understand the mechanisms that drive the formation of complex, large-scale patterns and structures observed in experiments. This work is supported by experimental data from our colleagues here at IISER Tirupati, with experiments conducted by Piyush Sahu in the lab of Dr. Ravi Kumar Pujala.
The Role of Shape Anisotropy in Quincke Roller Dynamics
While many studies on active particles assume they are simple spheres, real-world particles often have complex shapes. Ambareesh and Rasheed are exploring how shape anisotropy influences the dynamics of Quincke rollers. They are investigating "snowman-shaped" particles, which break the symmetry of a simple sphere. Using a theoretical framework that rigorously models the particle's interaction with a nearby wall, the simulations are designed to reveal how an anisotropic shape alters the particle's motion. The research aims to understand how shape-dependent forces and torques lead to new behaviors, such as reorientation and distinct modes of motion near surfaces, which are not seen with their spherical counterparts.
Internal Complexity and Collective Motion in Synthetic Swimmers
Complexity in active matter can arise not only from external boundaries but also from the internal design of the swimmers themselves. Ambareesh is exploring how the internal architecture of an active particle governs its behavior. He is theoretically investigating how a particle's propulsion mechanism can be designed to produce specific, targeted dynamics. The central question is whether engineering the internal workings of a particle can lead to new and predictable collective phenomena, providing novel design principles for creating functional synthetic active systems.
Control Mechanism of the Motility of E. coli Over Biological Surfaces
While motility of flagellated bacteria is poorly studied in complex media such as colloids and liquid crystals, this is where they are typically found, and the viscoelastic nature of these media appears to contribute to some remarkable phenomena. Experiments often have limitations of resolution and visibility, in verifying plausible theories. Undeterred by the challenges, Vignesh has instead turned to simulations. He is interested in finding the underlying physics influencing the control mechanisms of bacterial motility. His simulations uses a realistic model of E. coli developed by us in collaborations with Prof. Holger Stark's group at TU Berlin, and the project is carried out with a strong collaboration between the two groups.
Life in a living liquid crystal
Living systems are often immersed in environments that are far from equilibrium, exhibiting properties that challenge conventional understanding of matter. Among such systems, active liquid crystals particularly active nematics stand out due to their dynamic behavior driven by internal energy consumption at the microscopic level. Fascinatingly, such systems not only occur naturally, such as within the cytoplasm of living cells, but are also engineered in laboratory conditions, offering an ideal playground to study the interplay between activity, elasticity, and structure formation.
Chinmay and Pranav delved into this area by studying passive, anisotropic colloids suspended within an active nematic liquid crystal. We aim to uncover the fundamental principles governing how an active medium imparts motion and order to passive objects, a crucial step in understanding the self-organization of complex biological structures.
Following the foundational investigations by Chinmay and Pranav, this project is now being advanced by Mahendradev to systematically investigate how the collective behavior of several passive colloids emerges from the intricate interplay between the medium's activity, its elastic properties, and the dynamics of topological defects.
Modeling Mammalian Spermatozoa
Capitalizing on our lab's expertise in microswimmer modeling, we are delving into the hydrodynamic modeling of sperm near a boundary. Building upon our current model for E. coli, we are creating a sophisticated representation of mammalian spermatozoa. This work is being advanced by Rasheed, who is incorporating 3D flagellar kinematics and the detailed morphology of the sperm cell into the model. The research mainly focuses on sperm-substrate interactions and the coupled dynamics of sperm clusters. Through this approach, we seek to understand the relationship between a sperm's shape and its movement, aiming to identify novel and testable control mechanisms to address challenges in reproductive fertility.