We study how particles — cloud droplets, ice crystals, sediments, microplastics, microswimmers — interact with the flows that carry them. Using a combination of theoretical analysis, large-scale computation, and asymptotic methods, our research reveals how microscale particle physics profoundly shapes macroscale geophysical phenomena: from rain formation in warm clouds to cyclone intensification over tropical oceans.
Droplet collisions, ice crystal dynamics, charge transfer and precipitation initiation in warm and mixed-phase clouds.
Inertial particles in vortices, suspensions in falling films, pulsatile flows, anisotropic particles and microswimmers.
Internal gravity waves, surface gravity waves, stratified shear instabilities and viscoelastic flow transitions.
How does a cloud, born from vapour, produce rain within minutes? Classical condensation theory predicts timescales of hours for droplets to grow large enough to precipitate — yet real clouds routinely accomplish this in 15–20 minutes. The answer lies in the collision-coalescence process, where turbulence, gravity, electrostatics, and short-range hydrodynamic forces conspire to accelerate droplet growth across the so-called condensation-coalescence bottleneck. Our group develops the theoretical and computational machinery to understand and quantify this process.
Using direct numerical simulations (DNS), stochastic collision models, and pairwise hydrodynamic interaction theory, we have quantified how turbulence, inertia, gravity-induced differential sedimentation, and short-range forces (van der Waals, non-continuum lubrication) influence collisional growth of cloud droplets. Our collision kernels incorporate both hydrodynamic and environmental factors, providing more accurate inputs for models that couple microphysical processes with cloud-scale dynamics.
Cloud droplets carry electric charge, and a naïve point-charge argument suggests that like-charged droplets should always repel each other. Surprisingly, our work has demonstrated that like-charged dielectric spheres can attract each other at close separations: induced-dipole interactions arising from finite droplet size overpower the point-charge repulsion, transforming the dynamics into a near-field attractive force. This finding is pivotal for understanding collision processes during lightning discharge and in electrified cloud environments. In scenarios with high charge ratios, the collision efficiency between like-charged droplets significantly exceeds that of uncharged pairs.
A parallel thread of our research focuses on ice microphysics in mixed-phase and convective clouds. Ice particle growth, aggregation, and charging play critical roles in hail formation, lightning initiation, and storm intensity. Unlike fluid droplets, ice particles introduce additional complexities: diverse crystal habits, anisotropic mechanical behaviour, and quasi-liquid layers. We develop models for ice–ice collision outcomes, rebound dynamics, charge transfer, and fluid-mediated reorientation during settling and turbulent motion.
Our earlier work has shown that sedimenting asymmetric rods can achieve nontrivial alignment due to the competition between gravitational and hydrodynamic torques — a transition identified as a supercritical pitchfork bifurcation, validated experimentally. This alignment of ice crystals by inertial torques produces the atmospheric optical phenomenon known as sundogs, and the competition between gravitational alignment and turbulent randomisation is central to understanding electromagnetic scattering from icy clouds — one of the largest sources of errors in global climate models.
We develop multiscale simulation frameworks that combine particle-resolved microscale simulations with mesoscale cloud-resolving or large eddy simulations (LES). This includes developing super-droplet models (SDM) that represent billions of droplets via reduced statistical particles, demonstrating how accurate collision kernels from DNS can be embedded within SDM frameworks to improve rain initiation modelling in LES. This effort is strengthened by ongoing collaboration with the Mesoscale and Microscale Meteorology (MMM) Lab at NCAR, where insights from our group help shape next-generation cloud-resolving models, and with researchers at Microsoft toward physics-informed surrogate models for scalable, cloud-based forecasting systems.
To connect in situ cloud processes to remotely sensed observations, we have developed numerical tools for evaluating electromagnetic scattering by cloud particles. Using a Lattice Boltzmann Method-based electromagnetic solver capable of handling complex particle geometries and irregular orientations, we link microphysics outputs to radar reflectivity and optical signatures used in observation systems for conducting, dielectric and bi-dielectric (Janus) particles.
Flows laden with particles are ubiquitous — from dust devils and atmospheric cyclones to coating processes and the respiratory airways. When particles have inertia, they don't passively follow the fluid; they centrifuge out of vortices, cluster in straining regions, alter the carrier-phase rheology, and can destabilise flows that would otherwise remain stable. Our group investigates these phenomena across scales, from granular media and fibre suspensions to microswimmers, combining theoretical analysis with computational modelling.
Gravity-driven interactions among charged dielectric spheres in a gaseous medium are examined, revealing that non-continuum lubrication interactions drive contact within finite time. Attraction is observed between closely approaching dielectric spheres with similar charges across various size and charge ratios. We have derived the asymptotic interparticle force under lubrication limits for arbitrary size ratios, showing how the attractive electric force diverges as separation approaches zero, overcoming continuum lubrication resistance.
Vortical flows suspended with heavy inertial particles occur widely — from dust devils and atmospheric cyclones to Jupiter's Great Red Spot. Heavy particles centrifuge away from vortical regions and accumulate in straining regions. We model the dynamics of inertial particles in flows generated by isolated elliptic vortex patches and study their clustering behaviour. Particles sample the flow field according to stable and unstable manifolds, and the inclusion of external shear induces chaos in particle trajectories.
Active suspensions — assemblies of self-propelling micro-organisms — exhibit remarkable collective behaviour driven by the interplay between self-generated flows and confinement. We explore how geometric confinement creates flow instabilities that give rise to collective motion. Modelling micro-swimmers as force dipoles, we have uncovered instabilities driven by the interplay between active stress and perturbations in swimmer density. An attractant gradient introduces anisotropy to the orientation field, revealing a novel instability mode driven by interface deformation due to active stress — even in suspensions of chemotactic pullers previously considered stable.
In surface gravity waves, an anisotropic particle exhibits orbital motion whose character depends on its orientation. A negatively buoyant particle settling into a deep flow field couples translational and rotational dynamics: as the particle settles, its orientation evolves in time and reaches a steady state, with different steady orientations leading to qualitatively different trajectories.
The dynamics of inertial particles in pulsatile (Womersley) flows are crucial for understanding biological processes and designing microfluidic devices. In collaboration with Prof. Mahesh Panchagnula (IIT Madras), we study the transport of particulate matter in respiratory airways using Maxey-Riley and Langevin models, and investigate dispersion in axially diverging oscillatory conduits with focus on the interplay of particle inertia, temporal oscillation, and geometric divergence.
Falling liquid films laden with particles appear in industrial coating processes, the tear film protecting our eyes, and many natural settings. We studied the stability of particle-laden gravity-driven falling films and discovered that the viscosity stratification created by suspended particles can solely destabilise the flow — a finding highlighted by the J. Fluid Mech. Focus on Fluids. In parallel, we characterised the instability of a two-dimensional Rankine vortex laden with inertial particles using linear stability analysis, revealing mechanisms driving instability in dusty semi-dilute flows.
Active suspensions display medium-like behaviour arising from the self-propulsion of their constituent organisms. We explore the role of geometric confinement on the creation of flow instabilities that eventually give rise to collective motion. Extending from micron-sized swimmers to virus particles at the nanometre scale, we also investigate the role of convective fluid motion in the aerosol transmission of diseases.
Oceans and atmospheres host a rich spectrum of wave phenomena across length and time scales, from internal gravity waves deep in the stratified ocean to wind-generated surface waves at the air-sea interface. These waves sustain large-scale circulations, mediate energy transfer, and undergo nonlinear interactions and instabilities that profoundly affect climate. Our group combines linear stability analysis, non-modal theory, and direct numerical simulation to understand these processes.
In a density-stratified ocean or atmosphere, displaced fluid parcels oscillate and radiate internal waves that contribute to deep-ocean mixing and sustain large-scale circulations. These highly nonlinear waves interact through triadic resonances, exchanging energy and amplifying. We study the underlying nonlinear interactions and resonances amongst internal wave modes, including the effects of background shear.
Surface gravity waves, generated by wind over a disturbed interface, are parameterised into global climate models since they regulate fluxes of heat and momentum between ocean and atmosphere. We perform linear stability calculations to identify parameter regimes of instability, determine growth rates, and validate predictions with DNS. Our work addresses the Miles mechanism (energy transfer from a critical layer in air to the interface), the rippling instability from the critical layer in water, and their viscous counterparts.
Stratified shear flow instability plays a key role in geophysical turbulence and mixing. Our group has studied the linear stability and nonlinear evolution of various stratified instabilities — Kelvin-Helmholtz, Holmboe, and Taylor instabilities. Not all stratified shear flows are linearly unstable, yet they still transition to turbulence. We have identified an algebraic instability in stably stratified shear flows where the eigenspectrum is purely continuous, followed by optimal perturbation analysis to find initial conditions that maximise energy amplification.
Vortex column stability has been studied since Lord Kelvin's classical work in 1880. We have identified the singular eigenfunctions of a vortex column and shown how their inclusion completes the spectrum, enabling description of how a vortex interacts with external turbulence — a feature previously understood only through the initial value problem. In parallel, we discovered a novel inertio-elastic instability of a vortex in a dilute polymer solution, arising from the resonance of elastic shear waves aided by background shear. This connects to the broader question of turbulent drag reduction: why adding a few parts per million of polymer to a turbulent flow can reduce drag by up to 80%.
Our research sits at the intersection of suspension dynamics and geophysical fluid dynamics — two communities that traditionally don't converse but have much to contribute as a team. This interdisciplinary vantage point drives the following research frontiers we are actively pursuing.
Developing physically-based, scale-aware models for droplet collision, coalescence and breakup that operate across different turbulence regimes and size distributions. This integrates high-fidelity DNS, stochastic collision frameworks, super-droplet cloud-resolving LES, and machine-learning surrogate models. Collaboration with NCAR-MMM ensures seamless interfacing with next-generation weather models.
Developing physically consistent collision and rebound models for realistic ice crystal habits, quantifying charge transfer during ice-ice interactions under turbulent flow, and embedding microphysical charge models into cloud-resolving simulations to predict charge stratification and lightning precursors.
Building the physical bridge between cloud particle microphysics and radar/lidar observations by extending Lattice Boltzmann electromagnetic solvers to wet and irregular particle shapes, generating scattering lookup datasets, and coupling these with evolving particle states from LES to support cloud profiling radars and Earth observation missions.
Understanding how sea spray — the droplet-laden boundary layer between ocean and atmosphere — modifies surface drag and heat transfer coefficients, acting as a lubrication layer for the wind and accelerating cyclones. We develop modified Kolmogorov-Prandtl models for particle-laden turbulent shear flow and two-way coupled DNS incorporating particle inertia, polydispersity, mass loading and thermodynamics. (SERB-SUPRA project, collaboration with IIT Bombay and IIT Ropar)
Microplastics are among the most pervasive ocean pollutants. Classical Stokes drift theory applies only to tracers, but microplastics have significant inertia and anisotropy. We investigate particle transport in flows that combine waves and turbulence — studying the role of inertia and shape on Stokes drift, and performing numerical simulations of Langmuir turbulence via the Craik-Leibovich equations to understand the vertical transport of finite-sized particles.
Erosion, deposition and transport in coastal regions involve a complex multiphase problem. We have shown that the particulate microstructure can destabilise otherwise stable flows. Using complementary Euler-Lagrange and Euler-Euler simulations, we model particle dynamics, mass transport by complex wave fields, and wave-field modulation by the dispersed phase in turbulent sediment-laden flows.