arXiv subjects

Pallab Sinha Mahapatra

Publications and source records attributed to Pallab Sinha Mahapatra.

8 recordsLinked to original sources

arXiv2026

Underwater bubble transport on superhydrophobic cylindrical rod

The transport of bubbles along a curved superhydrophobic surface is not governed by buoyancy alone, but also by the interaction between the geometric confinement, the capillary and contact-line resistance, and the hydrodynamic resistance. We systematically investigated the capillary number ($Ca$) and Bond number ($Bo$) of the transported bubble, as well as the size of the rod relative to the bubble. An analytical force-balance model was developed to predict transport velocity, accounting for buoyancy, hydrodynamic drag, and capillary resistance. The transition from transport to detachment was identified. The experiments were carried out sequentially and supplemented by axisymmetric numerical simulations to investigate wake-induced interactions between bubbles. Bubble transport is strongly influenced by rod inclination and the bubble-to-rod size ratio, demonstrating that curvature-induced confinement alters the balance between driving and resistive forces. The analytical model captured the experimental trends with deviations of less than 10\% in most cases. The bubble detaches from the inclined rod at a critical Bond number that depends on rod diameter, facilitating the development of a transport regime map. The study also found that the motion of successive bubbles is strongly coupled. When a following bubble enters the wake of a preceding bubble, it speeds up and attains a higher capillary number than the leading bubble. Both experiments and numerical simulations consistently reproduce this wake-mediated acceleration, which shows that bubble transport is governed by both the force balance on individual bubbles and the hydrodynamic interactions between adjacent bubbles. These findings form the basis for controlling bubble transport, detachment, and collective motion on superhydrophobic interfaces.

physics.flu-dyn↗

arXiv2026

Rheotaxis of microswimmers in colloid-laden channel flow

Microswimmers are often found in heterogeneous and crowded environments within narrow conduits under external flow conditions, enabling them to perform interesting translational and rotational maneuvers, such as swimming in the upstream direction, following walls, and oscillatory motion. Studying such systems helps us understand the motility behaviors of microswimmers (pushers, pullers, or neutrals) and develop applications such as targeted drug delivery. To study the motion of microswimmers in a channel flow with the presence of hard, monodisperse spherical colloids, we adopted the spherical squirmer model to represent the microswimmers, along with a mesoscale simulation framework, multi-particle collision dynamics (MPCD), to represent the background fluid. In the absence of colloids, a squirmer in a microchannel flow develops an increased probability of moving away from the walls and oscillates between the walls as the flow speed increases compared to the squirmer speed, with a dominant upstream orientation near the walls. However, the presence of the colloids makes the pusher swim towards the center of the channel and upstream direction, and the puller swim away from the center of the channel at low flow speeds. At high flow speeds, the flow carries all the squirmers, resulting in a dominant upstream direction in the channel center. We observe that this leads to a decrease in the local velocity of the squirmer in the flow direction for pusher, neutral, and puller-type squirmers. We also observe that, for a constant colloidal packing fraction, the local velocity magnitude of the puller along the flow direction is less than that of the pusher.

cond-mat.soft↗

arXiv2025

Mixing of a binary passive particle system using smart active particles

Controlled activity of active entities interacting with a passive environment can generate emergent system-level phenomena, positioning such systems as promising platforms for potential downstream applications in targeted drug delivery, adaptive and reconfigurable materials, microfluidic transport and related fields. The present work aims to realise an optimal mixing of two segregated species of passive particles by introducing a small fraction of active particles (2% by composition) with adaptive and intelligent behaviour, directed by a trained Artificial Neural Network-based agent. While conventional run-and-tumble particles can induce mixing in the system, the smart active particles demonstrate superior performance, achieving faster and more efficient mixing. Interestingly, an optimal mixing strategy doesn't involve a uniform dispersion of active particles in the domain, but rather limiting their motion to an eccentrically placed zone of activity, inducing a global rotational motion of the passive particles about the system centre. A transition in the directionality of the passive particles' motion is observed along the radius towards the centre, likening the active particles' motion to an ellipse-shaped void with a defined surface speed. Situated at the intersection of active matter and machine learning, this work highlights the potential of integrating adaptive learning frameworks into traditional active matter models.

cond-mat.soft↗

arXiv2025

Entry and penetration of a superhydrophobic sphere into a deep pool

This study experimentally examines the entry and penetration of a superhydrophobic sphere into a quiescent deep pool, with special emphasis placed on the primary and secondary pinch-off of the air cavity existing in its wake. Two aspects are novel in this study. For one, the experiments are performed for a large range of dimensionless sphere densities, where lighter spheres, with their air cavity, exhibit a terminally ascending trajectory and heavier spheres a terminally descending trajectory. The second novel result is a strong correlation of primary and secondary pinch-off times with the Froude number at impact and the dimensionless density. A semi-empirical correlation for the air cavity volume following the primary pinch-off shows excellent agreement with measurements over all dimensionless densities. A scalar force balance predicts a drastic decrease of buoyancy upon pinch-off, reflected also in the abrupt change of deceleration, measured using two orthogonally placed high-speed cameras to capture the time resolved trajectory of the sphere in the pool. Comparisons are drawn between the trajectories of superhydrophobic spheres and those of hydrophilic spheres, measured in a previous study.

physics.flu-dyn↗

arXiv2025

Motion of a rigid sphere penetrating a deep pool

In this study, we experimentally examine the behavior of a free-falling rigid sphere penetrating a quiescent liquid pool. Observations of the sphere trajectory in time are made using two orthogonally placed high-speed cameras, yielding the velocity and acceleration vector through repeated differentiation of the time-resolved trajectories. The novelty of this study is twofold. On the one hand, a methodology is introduced by which the instantaneous forces acting on the sphere can be derived by tracking the sphere trajectory. To do this, we work in a natural coordinate system aligned with the pathline of the sphere. In particular, the instantaneous lift and drag forces can be separately estimated. On the other hand, the results reveal that when decelerating, the sphere experiences a very high drag force compared with steady flow. This is attributed to an upstream shift of the mean boundary-layer separation. The sphere also experiences significant lift force fluctuations, attributed to unsteady and asymmetric wake fluctuations. The trajectories can be reduced to three stages, common in duration for all initial Reynolds numbers and density ratios when expressed in dimensionless time. In addition, the sphere velocity and deceleration magnitude for different initial parameters exhibit a high degree of uniformity when expressed in dimensionless form. This offers prediction capability of how far a sphere penetrates in time and the forces acting on it.

physics.flu-dyn↗

arXiv2023

A plate-type condenser platform with engineered wettability for space applications

Vapor condensation is extensively used in applications that demand the exchange of a substantial amount of heat energy or the vapor-liquid phase conversion. In conventional condensers, the condensate removal from a subcooled surface is caused by gravity force. This restricts the use of such condensers in space applications or in horizontal orientations. The current study demonstrates proof-of-concept of a novel plate-type condenser platform for passively removing condensate from a horizontally oriented surface to the surrounded wicking reservoir without gravity. The condensing surface is engineered with patterned wettabilities, which enables the continuous migration of condensate from the inner region of the condenser surface to the side edges via surface energy gradient. The surrounding wicking reservoir facilitates the continuous absorption of condensate from the side edges. The condensation dynamics on different substrates with patterned wettabilities are investigated, and their condensation heat transfer performance is compared. The continuous migration of condensate drops from a superhydrophobic to a superhydrophilic area can rejuvenate the nucleation sites in the superhydrophobic area, resulting in increased heat transport. We can use the condenser design with engineered wettability mentioned above for temperature and humidity management applications in space.

physics.flu-dyn↗

arXiv2023

Atmospheric water vapor condensation on engineered interfaces: Busting the myths

Condensing atmospheric water vapor on surfaces is a sustainable approach to potentially address the potable water crisis. However, despite extensive research, a key question remains: what is the physical mechanism governing the condensation from humid air and how significantly does it differ from pure steam condensation? The answer may help define an optimal combination of the mode and mechanism of condensation as well as the surface wettability for best possible water harvesting efficacy. Here we show that this lack of clarity is due to the differences in heat transfer characteristics during condensation from pure vapor and humid air environments. Specifically, during condensation from humid air, the thermal resistance across the condensate is non-dominant and the energy transfer is controlled by vapor diffusion and condensate drainage. This leads to filmwise condensation on superhydrophilic surfaces, offering the highest water collection efficiency. To demonstrate this, we measured condensation rate on different sets of superhydrophilic and superhydrophobic surfaces in a wide degree of subcooling (10 - 26 C) and humidity-ratio differences (5 - 45 g/kg of dry air). The resulting condensation rate is enhanced by 57 - 333 % on the superhydrophilic surfaces as compared to the superhydrophobic ones. The findings of this study challenges the nearly century-old scientific ambiguity about the mechanism of vapor condensation from humid air. Our findings will lead to the design of efficient atmospheric water harvesting systems.

physics.flu-dyn↗

arXiv2019

On the brachistochrone of a fluid-filled cylinder

We discuss a fluid dynamic variant of the classical Bernoulli's brachistochrone problem. The classical brachistochrone for a non-dissipative particle is governed by maximization of the particle's kinetic energy resulting in a cycloid. We consider a variant where the particle is replaced by a bottle filled with a viscous fluid and attempt to identify the shape of a curve connecting two points along which the bottle would move in the shortest time. We derive the system of integro-differential equations governing system dynamics for a given shape of the curve. Using these equations, we pose the brachistochrone problem invoking optimal control formalism and show that (in general) the curve deviates from a cycloid. This is due to the fact that increasing the rate of change of bottle kinetic energy is accompanied by increased viscous dissipation. We show that the bottle motion is governed by a balance between the desire to minimize travel time and the need to reach the end point in the face of increased dissipation. The trade-off between these two physical forces plays a vital role in determining the brachistochrone of a fluid-filled cylinder. We show that in the two limits of either vanishing or high viscosity, the brachistochrone for this problem reduces to a cycloid. An intermediate viscosity range is identified where the fluid brachistochrone is non-cycloidal. Finally, we show relevance of these results to the dynamics of a rolling liquid marble.

physics.flu-dyn↗