Research Overview

Numerous natural and industrial processes involve systems of particles suspended in fluids. Active suspensions, however, are distinguished from these systems as they are driven out of equilibrium through either external forces or self-propulsion, resulting in a deliberate and organized movement. Such systems are represented by motile microorganisms and motor-protein in the cytoskeleton, which are pervasive in living beings and natural habitats. Their artificial counterparts, active colloids, translate in potential or chemical gradients, generated either externally or internally via asymmetry in surface properties. Due to this localized energy-work conversion, these systems exhibit unusual macroscale properties that are in stark contrast to passive suspensions; for instance, enhanced diffusion, reduction of effective viscosity, and generation of normal stresses. Fluids and particles can also be driven via externally applied potential or chemical gradients that have applications at the micro and nanoscale. The suspended matter is thus subjected to evolve over a certain time scale enforced by the gradients in flow or chemical potential, relevant to the application. Competition with the inherent time scales of the activity of suspended matter can yield emergent properties. Our research group aims to model and predict the behaviour of such systems at two length scales:

(i) Microscopic view: How do these additional time scales alter the hydromechanics and navigation strategies of motile pathogens and artificially active colloids? What dominant mechanisms govern the orientational dynamics in sheared flows and how they interact with other swimmers and boundaries? And how does all this alters in biologically relevant non-Newtonian media?

(ii) Macroscopic view: What is the impact of these competing time scales on the rheological response, suspension stability, and spatiotemporal patterns?

Our group utilizes both analytical and computational techniques to gain fundamental insights. The former tools exploit the separation of time/velocity scales in the system and consist of asymptotics & perturbation techniques, and integral representation of experimentally measurable quantities. These provide handy expressions that can be readily used by experimentalists. However, these are mostly valid under limiting conditions. Thus, computational techniques based on finite difference or harmonic expansion of unabridged transport equations and mean-field kinetic models offer a more comprehensive understanding. Furthermore, motivated by the ubiquity of collective motion and the spreading of microbial systems, our group is also developing particle-based simulations governed by overdamped Langevin dynamics.

Awards

• Scholarship for Côte d'Azur University Fall program on Complex Systems, 2021.
• Alexander von Humboldt fellowship for postdoctoral research, 2020.