Antarip Poddar

The dynamics of artificial microswimmers in confined flows remain poorly understood despite their relevance to microfluidics, drug delivery, and the navigation of active particles through physiological pathways. Here, we investigate the self-diffusiophoretic motion of spherical Janus particles in pressure-driven Poiseuille flow within microchannels, systematically spanning the weak to strong confinement regime. Using bispherical eigenfunction expansions for the solute and flow fields, we analyze swimmer-boundary interactions in weak to moderate confinement, while complementary finite-element simulations are employed to examine the strongly confined regime without invoking artificial steric potentials. Our analysis reveals that confinement fundamentally reshapes swimmer trajectories: near-wall equilibria characteristic of weak confinement is destroyed, giving rise to centerline attractors, oscillatory states with confinement-dependent wavelengths. Furthermore, the non-trivial coupling between diffusiophoretic slip, hydrodynamic interactions with opposing walls, and the curvature of Poiseuille flow yields striking modifications of trajectories, including confinement-induced transitions from centerline to wall-bound states. These results demonstrate that strongly confined environments cannot be captured by far-field or weak-confinement theories, and they emphasize the essential role of geometric confinement in dictating active particle transport. Beyond providing physical insight into the interplay between chemical activity, hydrodynamics, and flow curvature, our findings further establish design principles for the controlled deployment of microswimmers in narrow fluidic passages of technological and biomedical significance.

Understanding how viscoelastic fluids influence the propulsion of catalytic Janus microswimmers is crucial for their application in targeted drug delivery and navigation through complex biofluids. We present a theoretical model of electrocatalytic self-propulsion of a bimetallic Janus microswimmer in a viscoelastic Oldroyd-B medium. The present model couples the charge conservation, ionic transport, the Michaelis–Menten surface reaction kinetics for hydrogen peroxide decomposition, and creeping-flow hydrodynamics, whereas the non-Newtonian viscoelastic rheology is captured using the Oldroyd-B model. The nonlinear governing equations are solved numerically using the finite element method. Results reveal swimming speed exhibits a non-monotonic dependence on fluid elasticity (quantified with the Deborah number De), with suppression at low values followed by recovery and saturation near De≈0.5. Increasing fuel concentration enhances ionic production, strengthens the self-generated electric field, and monotonically raises propulsion speed. A higher polymer concentration deepens the low-De velocity dip but increases the saturation speed at finite De. Reaction kinetics significantly modulate propulsion by altering surface charge asymmetry: stronger reduction kinetics enhance swimming, whereas increases in certain rate coefficients diminish it. We also report that viscoelasticity enhances the conversion efficiency, which quantifies the mechanical power extractable from a given chemical input. Viscoelasticity can improve electrocatalytic performance by up to threefold relative to Newtonian media. These findings underscore the critical interplay between surface reaction kinetics and fluid viscoelasticity in controlling micromotor propulsion, indicating that optimizing electrical parameters and fluid properties can significantly improve performance.

Understanding the effect of intricate surface wettability conditions on microswimmers is crucial for precisely navigating them across narrow microcirculatory networks. Here, we adopt the spherical squirmer model and Navier slip condition to delineate the microswimmer locomotion under a Poiseuille flow in a slit microchannel. Through a combined analytical–numerical approach utilizing bispherical coordinates and the superposition technique, we resolve the slip-modulated simultaneous hydrodynamic interaction with substrate boundaries. Phase portraits reveal that slip significantly alters propulsion mechanisms, destabilizing centreline stable oscillations of pullers beyond a threshold slip length. Superhydrophobic surfaces suppress near-wall rheotaxis states but preserve centreline focusing, facilitating slip-assisted directed transport without surface accumulation. Under strong background flows, subcritical Hopf bifurcation emerges for pullers at a critical slip length, transitioning dynamics from coexisting stable and unstable states to purely unstable behaviour. Contrastingly, for pushers, slip causes a transition from unstable to either stable or fixed-amplitude oscillations. Increased slip length reduces hydrodynamic repulsion on pullers from the walls by enhancing rotational velocity near the walls, whereas it counteracts the torque that causes unstable oscillations of pushers. Three-dimensional analysis of the trajectories reveals the significant role of the out-of-plane orientation of the microswimmer in its transitions between different swimming states. The presented regime maps offer parametric combinations for specific motion behaviours, guiding the development of smart microfluidic drug delivery systems and preventing biofilm deposition in biomedical devices.