arXiv subjects

Akash Choudhary

Publications and source records attributed to Akash Choudhary.

15 recordsLinked to original sources

arXiv2026

Colloid recovery from porous structures under ambient flow: enhanced extraction via phoretic and osmotic mechanisms

Chemical gradients are widely employed to enhance particle transport in porous media, such as laundry detergency and enhanced oil recovery. Diffusiophoresis and diffusioosmosis refer to the movement of colloid and movement of near-surface fluid in response to electrolyte gradients, respectively. These mechanisms play a crucial role in colloid and drug transport in constricted regions where bulk transport is infeasible. Our earlier work [Tiwari et al., Langmuir 41, 18583 (2025)] has shown that phoretic and osmotic transport in dead-end micro-pores can be controlled by orienting salt gradients into or out of the pores; however, the extent to which this orientation influences large-scale spatiotemporal patterns and colloid extraction is not thoroughly explored. In this work, we study the phoretic and osmotic colloidal extraction from porous structure exposed to an ambient flow. We characterize the impact of solute gradient orientation, such as solute-out (i.e., solute-emitting porous media) and solute-in (i.e., solute-consuming media) modes. The two-dimensional porous structure is made of a number of pillars/fibers arranged in a lattice ordered hexagonal packing with equal spacing. The results from finite-element simulations show that phoretic colloidal extraction exhibits a qualitatively distinct behavior in the two modes: in the solute-out mode, colloids are extracted from the peripheral region of the porous structure, whereas in the solute-in mode, extraction predominantly occurs from the stagnant core. Diffusioosmotic slip on the internal surface of pillars/fibres further amplifies extraction in both modes, with a relatively larger enhancement in the solute-in mode due to internal spatiotemporal flow patterns. Beyond demonstrating the sensitivity of osmotic transport in porous media, these insights can guide enhanced membrane filtration, laundry detergency, and enhanced oil recovery.

cond-mat.soft↗

arXiv2026

Propulsion and far-field hydrodynamics of linked-sphere microswimmers with viscoelastic deformability

Viscoelasticity governs the locomotion strategies of deformable microorganisms, rendering it a fundamental mechanical property of microbial motility and an integral component in the design of envisioned microbots. Recent studies have shown that it can enable effective propulsion through non-reciprocal body deformations, even under time-reversible actuation. In this work, we investigate the dynamics of model microswimmers driven by reciprocal actuation, wherein the passive body exhibits viscoelastic deformability. We consider two linked-sphere designs, distinguished by the location of actuation: applied at one end (3-sphere design) or at the midpoint of the swimmer body (4-sphere design). Adopting Kelvin-Voigt deformability, we characterize the kinematic performance of both designs: the three-sphere swimmer possesses an optimal actuation frequency, while the four-sphere swimmer exhibits a critical frequency at which the locomotion direction reverses. We examine the swimmer's far-field hydrodynamic signature and find that resulting flow field is characterized by dominant dipolar and quadrupolar contributions, whose magnitudes are sensitive to the relative length of the actuator segment.

cond-mat.soft↗

arXiv2026

Tuning Cross-stream Lift in Viscoelastic Shear: Distinct Hydrodynamic Signatures of Force-bearing and Force-free Mechanisms

We investigate the lift and drag corrections acting on a particle suspended in a planar viscoelastic shear flow when the particle is tuned to translate relative to the flow by an external mechanism. A cross-stream lift force arises when particle is driven in streamwise direction; we find that the nature of the driving mechanism dictates the lift direction: force-bearing mechanisms (such as gravity acting on non-neutrally buoyant particles) and force-free mechanisms (such as electrophoresis) generate lift forces of opposite sign. By explicitly deriving the first-order fields and stresses, we demonstrate that this reversal originates from distinct hydrodynamic disturbances induced by each mechanism, which produce qualitatively different polymeric stress distributions. This analytical result is further verified through an independent derivation using the reciprocal theorem. Further, we find that driving the particle in the gradient direction gives rise to a streamwise drag correction that is of the same sign for both mechanisms. Beyond microfluidic particle manipulation, these results have broader implications for understanding the locomotion of microswimmers in viscoelastic shear flows, where distinct force-free propulsion mechanisms are expected to generate unique force and torque modifications.

physics.flu-dyn↗

arXiv2026

Clustering Dynamics of SiO2-Pt Active Janus Colloids

Active colloid clustering is central to understanding non-equilibrium self-organization, with implications for programmable active materials and synthetic or biological assemblies. While most prior studies have focused on dimers or small aggregates, the dynamics of larger clusters remain relatively unexplored. Here, we experimentally investigate chemically active, monodisperse SiO2-Pt Janus colloid (JC) clusters as large as n=9 in a dynamic clustering regime, where clusters continuously form, dissolve, and merge as swimmer density increases. We show that clusters move in circular trajectories, and that both their translational and rotational dynamics can be predicted directly from the orientations of constituent JCs. Furthermore, we identify that their formation undergoes a mechanistic transition: while small clusters are mediated by chemical interactions, larger clusters are predominantly formed by steric effects. This transition arises from a mismatch of motilities between incoming JCs and clusters, combined with increased Pt-surface exposure. Our results extend prior dimer-focused studies to larger aggregates and establish a predictive description that bridges individual swimmer behavior with collective dynamics.

cond-mat.soft↗

arXiv2025

On the influence of electrolytic gradient orientation on phoretic transport in dead-end pores

Electrolytic diffusiophoresis refers to directional migration of colloids due to interfacial forces that develop in response to local electrolytic concentration ($c$) gradients. This physicochemical transport provides an efficient alternative in numerous microscale applications where advection-induced transport is infeasible. Phoretic withdrawal and injection in dead-end pores can be controlled by orienting salt gradients into or out of the pore; however, the extent to which this orientation influences spatiotemporal transport patterns is not thoroughly explored. In this study, we find that it has a significant influence: colloidal withdrawal in solute-out mode ($β=c_\infty/c_{\text{pore}}<1$) is faster and shallower, whereas the solute-in mode enables deeper withdrawal. Similarly, solute-out injection features rapidly propagating wavefronts, whereas the solute-in mode ($β>1$) promotes uniform and gradual injection. Each mode's transport is found to evolve and persist over different time scales. We characterize the performance of these modes and find that while persistence of the solute-out mode strengthens with a growing electrolytic gradient [$\sim \ln(β^{-0.4})$], solute-in mode diminishes and eventually its persistence is insensitive to $β$. We also incorporate the variable mobility model to examine the impact of large zeta potentials, which intensifies the transport of solute-out mode further and weakens the solute-in mode. Additionally, we investigate how osmotic flows of the two modes affect injection and withdrawal patterns. We find that osmosis-induced mixing can counterintuitively inhibit injection effectiveness in solute-out mode. These insights bring attention to the distinctions between different phoretic transport modes and contribute to the rational design and setup of electrolytic gradients in numerous microscale applications.

cond-mat.soft↗

arXiv2024

A bi-directional low-Reynolds-number swimmer with passive elastic arms

It has been recently shown that it is possible to design simple artificial swimmers at low Reynoldsnumber that possess only one degree of freedom and, nevertheless, can overcome Purcell's celebratedscallop theorem. One of the few examples is given by Montino and DeSimone, Eur. Phys. J. E, vol.38, 2015, who consider the three-sphere Swimmer of Najafi and Golestanian, replacing one active armwith a passive elastic spring. We further generalize this idea by increasing the number of springs andshow that it is possible to invert the swimming direction using the frequency of the single actuatedarm.

cond-mat.soft↗

arXiv2023

Pair-Interactions of Self-Propelled SiO2-Pt Janus Colloids: Chemically Mediated Interactions

Driven by the necessity to achieve a thorough comprehension of the bottom-up fabrication process of functional materials, this experimental study investigates the pair-wise interactions or collisions between chemically active SiO2-Pt Janus Colloids. These collisions are categorized based on the Janus colloids orientations before and after they make physical contact. In addition to the hydrodynamic interactions, the Janus colloids are also known to affect each others chemical field, resulting in chemophoretic interactions, which depend on the degree of surface anisotropy in reactivity and solute-surface interaction. These interactions lead to a noticeable decrease in particle speed and changes in orientation that correlate with the contact duration and yield different collision types. Our findings reveal distinct configurations of contact during collisions, whose mechanisms and likelihood is found to be dependent primarily on the chemical interactions. Such estimates of collision and their characterization in dilute suspensions shall have key impact in determining the arrangement and time scales of dynamical structures and assemblies of denser suspensions, and potentially the functional materials of the future.

cond-mat.soft↗

arXiv2023

Orientational dynamics and rheology of active suspensions in weakly viscoelastic flows

Microswimmer suspensions in Newtonian fluids exhibit unusual macroscale properties, such as a superfluidic behavior, which can be harnessed to perform work at microscopic scales. Since most biological fluids are non-Newtonian, here we study the rheology of a microswimmer suspension in a weakly viscoelastic shear flow. At the individual level, we find that the viscoelastic stresses generated by activity substantially modify the Jeffery orbits well-known from Newtonian fluids. The orientational dynamics depends on the swimmer type; especially pushers can resist flow-induced rotation and align at an angle with the flow. To analyze its impact on bulk rheology, we study a dilute microswimmer suspension in the presence of random tumbling and rotational diffusion. Strikingly, swimmer activity and its elastic response in polymeric fluids alter the orientational distribution and substantially amplify the swimmer-induced viscosity. This suggests that pusher suspensions reach the superfluidic regime at lower volume fractions compared to a Newtonian fluid with identical viscosity.

cond-mat.soft↗

arXiv2021

On the cross-streamline lift of microswimmers in viscoelastic flows

The current work studies the dynamics of a microswimmer in pressure-driven flow of a weakly viscoelastic fluid. Employing the second-order fluid model, we show that the self-propelling swimmer experiences a viscoelastic swimming lift in addition to the well-known passive lift that arises from its resistance to shear flow. Using the reciprocal theorem, we evaluate analytical expressions for the swimming lift experienced by neutral and pusher/puller-type swimmers and show that they depend on the hydrodynamic signature associated with the swimming mechanism. We find that for neutral swimmers focusing towards the centerline is accelerated by two orders of magnitude, while for force-dipole swimmers no net modification in cross-streamline migration occurs.

physics.flu-dyn↗

arXiv2021

How inertial lift affects the dynamics of a microswimmer in Poiseuille flow

We analyze the dynamics of a microswimmer in pressure-driven Poiseuille flow, where fluid inertia is small but non-negligible. Using perturbation theory and the reciprocal theorem, we show that in addition to the classical inertial lift of passive particles, the active nature generates a `swimming lift', which we evaluate for neutral and pusher/puller-type swimmers. Accounting for fluid inertia engenders a rich spectrum of novel complex dynamics including bistable states, where tumbling coexists with stable centerline swimming or swinging. The dynamics is sensitive to the swimmer's hydrodynamic signature and goes well beyond the findings at vanishing fluid inertia. Our work will have non-trivial implications on the transport and dispersion of active suspensions in microchannels.

cond-mat.soft↗

arXiv2021

Self-propulsion in 2D Confinement: Phoretic and Hydrodynamic Interactions

Chemically active Janus particles generate tangential concentration gradients along their surface for self-propulsion. Although this is well studied in unbounded domains, the analysis in biologically relevant environments such as confinements is scarce. In this work, we study the motion of a Janus sphere in weak confinement. The particle is placed at an arbitrary location, with an arbitrary orientation between the two walls. Using the method of reflections, we study the effect of confining planar boundaries on the phoretic and hydrodynamic interactions, and their consequence on the Janus particle dynamics. The dynamical trajectories are analyzed using phase diagrams for different surface coverage of activity and solute-particle interactions. In addition to near wall states such as `sliding' and `hovering', we demonstrate that accounting for two planar boundaries reveals two new states: channel-spanning oscillations and damped oscillations around the centerline, which were characterized as `scattering' or `reflection' by earlier analyses on single-wall interactions. Using phase diagrams, we highlight the differences in inert-facing and active-facing Janus particles. We also compare the dynamics of Janus particles with squirmers for contrasting the chemical interactions with hydrodynamic effects. Insights from the current work suggest that biological and artificial swimmers sense their surroundings through long-ranged interactions, that can be modified by altering the surface properties.

cond-mat.soft↗

arXiv2021

Comment on "Migration of an electrophoretic particle in a weakly inertial or viscoelastic shear flow"

A recent article (Khair and Kabarowski; Phys. Rev. Fluids 5, 033702) has studied the cross-streamline migration of electrophoretic particles in unbounded shear flows with weak inertia or viscoelasticity. That work compares their results with those reported in two of our previous studies (Choudhary et al.: J. Fluid Mech. 874; J. Fluid Mech. 898) and reports a disagreement in the derived analytical expressions. In this comment, we resolve this discrepancy. For viscoelastic flows, we show that Khair and Kabarowski have not accounted for a leading order surface integral of polymeric stress in their calculation of first-order viscoelastic lift. When this integral is included, the resulting migration velocity matches exactly with that reported in our work (J. Fluid Mech. 898). This qualitatively changes the migration direction that is reported by Khair and Kabarowski for viscoelastic flows. For inertial flows, we clarify that Khair and Kabarowski find the coefficient of lift to be 1.75$π$, compared to 2.35$ π$ in our previous work (J. Fluid Mech. 874). We show that this difference occurs because Khair and Kabarowski accurately include the effect of rapidly decaying $ \sim O(1/r^{4}) $ velocity field (a correction to the stresslet field $ \sim 1/r^{2} $), which was neglected in our previous work (J. Fluid Mech. 874).

physics.flu-dyn↗

arXiv2021

Motion of an active particle in a linear concentration gradient

Janus particles self-propel by generating local tangential concentration gradients along their surface. These gradients are present in a thin layer whose thickness is small compared to the particle size. Chemical asymmetry along the surface is a prerequisite to generate tangential chemical gradient, which gives rise to diffusioosmotic flows in a thin region around the particle. This results in an effective slip on the particle surface. This slip results in the observed "swimming" motion of a freely suspended particle even in the absence of externally imposed concentration gradients.Motivated by the chemotactic behaviour of their biological counterparts(such as sperm cells, neutrophils, macrophages, bacteria etc.), which sense and respond to external chemical gradients, the current work aims at developing a theoretical framework to study the motion of a Janus particle in an externally imposed linear concentration gradient. The external gradient along with the self-generated concentration gradient determines the swimming velocity and orientation of the particle.The dominance of each of these effects is characterised by a non-dimensional activity number A. The surface of Janus particle is modelled as having a different activity and mobility coefficient on the two halves.Using Lorentz Reciprocal theorem, an analytical expression for the rotational and translational velocity is obtained. The analytical framework helps us divide the parameter space of surface activity and mobility into four regions where the particle exhibits different trajectories.

cond-mat.soft↗

arXiv2020

Electrokinetically enhanced cross-stream particle migration in viscoelastic flows

Advancements in understanding the lateral migration of particles have helped in enhanced separation in microfluidic devices. In this work, we investigate the effects of electrokinetics on the particle migration in a viscoelastic flow, where the electric field is applied parallel to the flow. Through experiments and theoretical analysis, we show that the interaction of electrokinetic and rheological effects can result in an enhancement in the migration by an order of magnitude. The theoretical analysis, in agreement with experiments, demonstrates that the particles can be focused at different equilibrium positions based on their intrinsic electrical properties.

cond-mat.soft↗

arXiv2020

Non-Newtonian effects on the slip and mobility of a self-propelling active particle

Janus particles propel themselves by generating concentration gradients along their active surface. This induces a flow near the surface, known as the diffusio-osmotic slip, which propels the particle even in the absence of externally applied concentration gradients. In this work, we study the influence of viscoelasticity and shear-thinning (described by the second-order-fluid and Carreau model, respectively) on the diffusio-osmotic slip on an active surface. Using matched asymptotic expansions, we provide an analytical expression for the modification of slip induced by the non-Newtonian behavior. The results reveal that the modification in slip velocity arising from polymer elasticity is proportional to the second tangential derivative of the concentration field. Using the reciprocal theorem, we estimate the influence of this modification on the mobility of a Janus sphere. The current study also has direct implications on the understanding of the transport of complex fluids in diffusio-osmotic pumps.

physics.flu-dyn↗