Research
Our research brings together theoretical modelling and computational mathematics to study problems arising in complex fluids, biological systems, soft matter, and related areas of applied science. Our work sits at the interface of fluid mechanics and soft matter physics. Motivated by experiments and physical observations, we aim to uncover the mechanisms that govern motion, transport, and interactions across scales.
Complex Fluids and Electrohydrodynamics
Complex fluids are materials whose behaviour reflects an interplay between microstructure and flow, rather than a single constant viscosity. Examples include emulsions, suspensions, and biological media, where interfaces, particles, or internal structure influence macroscopic dynamics. A central goal is to understand how microscale physics feeds back into continuum-scale motion and transport.
A major part of our research concerns the dynamics of dielectric particles and liquid drops suspended in another liquid medium and subjected to electric fields. These problems fall within electrohydrodynamics and are often described by the Melcher–Taylor leaky dielectric model. We are particularly interested in weakly conducting particles and drops in strong electric fields, where symmetry-breaking instabilities can lead to Quincke electrorotation. Using a combination of analysis and computation, we study how electric forcing, interfacial mechanics, and hydrodynamic interactions combine to produce rich nonlinear dynamics. This work has helped explain experimentally observed behaviour of particles and drops and reproduce it in simulations. It also connects naturally to active matter, since Quincke-driven particles provide a simple model system for self-propulsion and collective motion. Some representative simulations are shown below.
A three-dimensional small-deformation theory for electrohydrodynamics of dielectric drops
This paper develops a three-dimensional small-deformation theory for dielectric drops in electric fields within the full Melcher–Taylor leaky-dielectric framework. The theory captures both steady deformation in weak fields and the transition to Quincke rotation in stronger fields, where the drop spontaneously tilts and rotates. By retaining both the straining and rotational components of the flow in the surface charge dynamics, the model yields a criterion for the onset of Quincke rotation. The predictions agree well with experiments in the small-deformation regime.
D. Das and D. Saintillan, Journal of Fluid Mechanics 914, A22 (2021).
Electrohydrodynamics of viscous drops in strong electric fields: numerical simulations
This paper develops a three-dimensional boundary element method for the full leaky-dielectric model to study the deformation and dynamics of viscous drops in strong electric fields. Unlike many earlier simulations, it includes interfacial charge convection and therefore captures non-axisymmetric behaviours in the Quincke regime, where drops undergo symmetry breaking and steady electrorotation. The simulations recover a wide range of dynamical responses and show excellent agreement with both experiments and small-deformation theory. The work provides a detailed numerical framework for understanding drop electrohydrodynamics beyond the weak-field and axisymmetric limits.
D. Das and D. Saintillan, Journal of Fluid Mechanics 829, 127–152 (2017).
Active particles powered by Quincke rotation in a bulk fluid
This paper shows that spontaneous Quincke rotation can be converted into translational motion in a bulk fluid without relying on nearby surfaces. Using theory and numerical simulations, it demonstrates that geometric asymmetry alone is sufficient to break symmetry and produce self-propulsion in a plane perpendicular to the applied electric field. The work introduces a new route to electrically driven active particles in unbounded fluids. It also provides a simple model system for exploring active matter powered by Quincke rotation.
D. Das and E. Lauga, Physical Review Letters 122, 194503 (2019).
Biological Fluids, Cilia, and Intracellular Flows
Biological flows arise when fluid motion is driven by living systems, such as cilia, flagella, or molecular motors. These systems typically operate at low Reynolds number, where viscous forces dominate inertia and fluid transport is shaped by geometry, elasticity, and coordinated forcing. Understanding them requires linking local actuation at the microscale to flow organisation and transport at larger scales.
In this area, we study microorganism locomotion, cilia-driven transport, and intracellular flows using mathematical models based on resistive-force theory, slender-body theory, and boundary element methods. These problems are motivated by questions in physiology and cell biology, but they also raise fundamental issues in microscale fluid mechanics. A recurring theme is to understand how activity at the level of filaments, cilia, or molecular motors gives rise to pumping, transport, and collective behaviour that can be observed at the cellular and tissue scales.
Cilia dynamics create a dynamic barrier to penetration of the periciliary layer in human airway epithelia
The study develops a fluorescence-based method to measure fluid flow inside the periciliary layer of the human airway, where direct observation is usually very difficult. The experiments show that transport within this layer is vertically nonuniform and that cilia actively expel fluid near their tips, which may help protect the epithelium from pathogens. Numerical simulations support these observations and highlight the importance of cilia geometry and coordination in shaping respiratory fluid transport.
E. Causa, D. Das, L. Feriani, J. Kotar, and P. Cicuta, Proceedings of the National Academy of Sciences 122 (28), e2419032122 (2025).
Bacterial Hydrodynamics and Microorganism Locomotion
We are also interested in the mechanics of swimming microorganisms, especially bacteria interacting with nearby boundaries. At low Reynolds number, hydrodynamic interactions with walls can strongly influence orientation, stability, and transport. Our work studies how body shape, flagellar forcing, and near-surface flows determine bacterial trajectories and long-time behaviour. These questions are relevant both to the basic mechanics of locomotion and to processes such as confinement, surface accumulation, and the early stages of biofilm formation. More broadly, this area connects microbial behaviour with the general fluid mechanics of active suspensions.
Hydrodynamic hovering of swimming bacteria above surfaces
Flagellated bacteria are hydrodynamically attracted to rigid walls, yet past work shows a hovering state where they swim stably at a finite height above surfaces. We use numerics and theory to reveal the physical origin of hovering. Simulations first show that hovering requires an elongated cell body and results from a tilt away from the wall. Theoretical models then identify two essential asymmetries: the response of width-asymmetric cells to active flows created by length-asymmetric cells. A minimal model reconciles near- and far-field hydrodynamics, capturing all key features of hovering.
P. H. Htet, D. Das, and E. Lauga, Physical Review Research 6 (3), L032070 (2024).
Additional simulations from the same paper, showing representative hovering states and near-surface bacterial trajectories. Spherical-shaped bacterial cell bodies tend to get attracted all the way to the surface.