New Papers in Fluid Mechanics

Author(s): Joseph J. Webber and Thomas D. Montenegro-Johnson

Self-oscillating gels that swell and deswell due to an oscillating chemical reaction can be used to pump fluid in pulses. This allows us to design a two-sphere microswimmer that can locomote in the Stokes regime from responsive hydrogels, with an external driving force arising from the chemical field. Using a model for the swelling and deswelling of such gels, and the flows that they generate, we compute analytical expressions for the swimming velocity and how it depends on the asymmetry of the gel spheres, and further show that swimmers can ‘surf’ rapidly along chemical waves in a reaction-diffusion system.

[Phys. Rev. Fluids 10, 100501] Published Fri Oct 03, 2025

Author(s): Blaise Delmotte and Florencio Balboa Usabiaga

This Perspective highlights the rigid multiblob framework, a numerical method for modeling suspensions of complex-shaped particles influenced by hydrodynamics, thermal fluctuations, activity, and other interactions. This review illustrates the effectiveness and versatility of the method in tackling a wide range of physical problems in fluid mechanics, soft and active matter, biophysics, and colloidal science.

[Phys. Rev. Fluids 10, 100701] Published Fri Oct 03, 2025

Author(s): Rupayan Jana and Shubhadeep Mandal

In this work, we semianalytically investigate the influence of an imposed spatially linearly varying viscosity field on near wall motion of a model microswimmer (squirmer). We explore its associated phase portraits and swimming trajectories for different swimming gaits and compare them with their constant viscosity analogs. The results indicate that even simplistic ambient viscosity gradient has substantial role on near-wall squirmer motility, which provides valuable insights for understanding and controlling microswimmer motion in relatively complex biological and microfluidic systems.

[Phys. Rev. Fluids 10, 104201] Published Fri Oct 03, 2025

Author(s): Cong Han and Arindam Banerjee

Tidal turbines deployed at tidal energy sites suffer high-turbulence flows, posing challenges for the estimation of their survivability and energy production. Our work implements an active grid to generate a homogeneous, high-turbulence flow replicating the flow characteristics at those sites in a water tunnel. Important terms in the Reynolds-averaged Navier–Stokes equations are quantified based on the measured wake field data for a comprehensive wake recovery analysis. The results further demonstrate that the tip vortices become extremely unstable under turbulence within one diameter downstream, reshaping the distribution of turbulence kinetic energy production and Reynolds shear stresses.

[Phys. Rev. Fluids 10, 104601] Published Fri Oct 03, 2025

Author(s): Joseph J. Kilbride, F. Fouzia Ouali, and David J. Fairhurst

If you have ever breathed on a window, you will have seen fog condense, which quickly evaporates from its edge to its center. The fog contains millions of individual micron-sized droplets, which when confined evaporate much slower and can be studied under a microscope. Interestingly, despite the fog evaporating overall, in the center of the fog, individual droplets can grow whilst small droplets shrink and Ostwald ripening is observed. In this paper, we track thousands of individual droplets and compare to classic Ostwald ripening theory, finding good agreement. We then show that a mean field model can predict the dynamics of the hundreds of individual droplets imaged.

[Phys. Rev. Fluids 10, L101601] Published Fri Oct 03, 2025

Author(s): Rotem Soffer, Eliezer Kit, and Yaron Toledo

The potential approach has been considered a reliable standard in computing wave directional spectra from instrumental measurements. However, by definition, it neglects ambient shearing currents. We present a proof that this oversight can lead to significant deviations of first-order in wave directional spectra estimates (height and direction), and propose a methodology based on rotational theory. The study demonstrates that shearing currents must not be neglected in wave data processing. A comparison of the rotational and potential approaches using the Acoustic Doppler Current Profiler (ADCP) dataset reveals notable and consistent differences in wave parameter estimation for in situ data.

[Phys. Rev. Fluids 10, 104801] Published Thu Oct 02, 2025

Author(s): Zachary G. Lipel, Yannick A. D. Omar, and Dimitrios Fraggedakis

The finite thickness of cell membranes is an often-overlooked detail in continuum models that can change their physics in fundamental ways. We show that when the bilayer bends, shear flows arise at its two surfaces, producing flow reversal, pressure inversion, and stagnation points that standard two-dimensional models cannot capture. Our findings highlight a missing element in existing continuum theories: resolving coupling at the membrane’s two surfaces requires accounting for thickness. This refinement predicts new nanoscale dissipation mechanisms and suggests experimental signatures in fluctuation spectra, pointing toward a more unified description of membrane dynamics.

[Phys. Rev. Fluids 10, 103101] Published Wed Oct 01, 2025

Author(s): Pijush Patra, Anubhab Roy, and J. S. Wettlaufer

Rain formation in warm clouds begins with the collision and coalescence of tiny water droplets, influenced by the electric charges they carry and the electric fields within clouds. The collision rate determines how quickly larger droplets form, which ultimately influences precipitation. Understanding the effects of electrostatic forces on droplet collisions is vital for improving cloud microphysics parameterizations and thus applications such as weather forecasting. This study shows that strong electric fields generated during cloud electrification can significantly enhance the collision efficiency of droplets subject to a laminar flow.

[Phys. Rev. Fluids 10, 103601] Published Wed Oct 01, 2025

Author(s): Bin Wu, Yunhua Jiang, and Zhihui Zou

A novel method utilizing an air jet to assist the water exit of a model is proposed. The air jet penetrates the water surface and generates a cavity, enabling the model to move within the jet-induced air environment. This ventilated cavity moves synchronously with the model, showing significant potential to mitigate asymmetric impact loads caused by cavity collapse. This study enhances the understanding of artificial cavity formation and is expected to address issues such as structural failure and trajectory instability during vehicle water exit, which arise from asymmetric impact loads caused by the cavity collapse.

[Phys. Rev. Fluids 10, 104001] Published Wed Oct 01, 2025

Author(s): Akira Yoshimori and Shankar P. Das

We obtain the equations of fluctuating hydrodynamics for many-particle systems whose microscopic units have both translational and rotational motion. The orientational dynamics of each element are studied in terms of Langevin equations for the rotational motion of a corresponding fixed-length direct…

[Phys. Rev. E 112, 045101] Published Wed Oct 01, 2025

Author(s): Oleksander Krul and Prosenjit Bagchi

In the microcirculation, highly deformable red blood cells flow through vessels of comparable size. Many of these vessels are compliant, but the impact of their deformation on the cell dynamics remains largely unexplored. Motivated by this, a computational study using a three-dimensional fully coupled fluid-structure interaction model is presented on the flow of deformable capsules in a compliant, inflating tube. The tube’s inflation is found to significantly alter the capsules’ transient deformation and velocity. Additionally, interactions between the capsule and tube create flow rate oscillations that are absent in a rigid tube under similar flow conditions.

[Phys. Rev. Fluids 10, 093105] Published Mon Sep 29, 2025

Author(s): Tianyi Chu, Benjamin Wilfong, Timothy Koehler, Ryan M. McMullen, and Spencer H. Bryngelson

Interfacial Rayleigh–Taylor (RT) and Faraday instabilities are usually studied separately, one driven by pressure gradients, the other by parametric resonance. Their coexistence produces a previously unidentified multi-modal instability. Floquet analysis and numerical simulations reveal a bidirectional competition: the Faraday mechanism, amplified by vibration, suppresses RT modes, while residual RT dynamics nonlinearly attenuate Faraday responses. This interaction advances understanding of interfacial mixing under combined forcing.

[Phys. Rev. Fluids 10, 093904] Published Mon Sep 29, 2025

Author(s): Robert Hicks and H. Reed Ogrosky

Thin-film modeling provides a computationally inexpensive way to quantify viscous film transport arising due to gravity and/or shear flow in tubes. Previous work has largely focused on tubes with fixed mean radius; this study quantifies the impact of a time-dependent radius on film transport. Linear stability analysis of this periodically-forced model shows that tube contractions/expansions enhance instability growth relative to a rigid tube. Simulations of the full nonlinear model equation highlight the role of free-surface waves in enhancing transport. Parameter values used here are motivated by the human lung/airway system.

[Phys. Rev. Fluids 10, 094008] Published Mon Sep 29, 2025

Author(s): Ludovico Fossà and Pierre Ricco

We investigate compressible laminar boundary layers over isotropic porous substrates. A new self-similar solution with nonlinear drag and heat conduction shows that high porosity, large grains, and elevated Mach numbers reduce adiabatic recovery temperature and velocity gradients, while the substrate’s bottom temperature has little effect on shear.

[Phys. Rev. Fluids 10, 094101] Published Mon Sep 29, 2025

Author(s): Kavin Kabilan, Swapnil Sen, and Arun K. Saha

We numerically study the buoyancy-aided mixed convective flow of air past a square cylinder inclined at 45 degrees. Buoyancy is progressively increased through the Richardson number (Ri) in the range (0.0-1.0), keeping the Reynolds number (Re) constant at 100. With increasing Ri, the flow transitions from an inertia-dominated to a natural convection-dominated one, showing dual wake-plume flow. Vortex shedding occurring at low Ri is found to be suppressed beyond a critical Ri, with a simultaneous emergence of far-field plume-like unsteadiness. Competing effects of inertia and natural convection result in a vorticity inversion observed at a certain distance downstream of the cylinder.

[Phys. Rev. Fluids 10, 094102] Published Mon Sep 29, 2025

Author(s): R. Wibawa, M. Alasker, and B. Jha

When one fluid moves through rocks or porous materials and displaces another, it creates branching structures known as “viscous fingering.” This phenomenon affects oil extraction, carbon dioxide storage, and drug mixing in microfluidic experiments. Traditionally, researchers have relied on costly computer simulations to study these patterns, which can become unstable. We present a new AI-based approach that accurately captures both small-scale details, such as the growth and merging of fingers, and large-scale outcomes, such as overall mixing of the two fluids. It operates significantly faster and remains reliable even where traditional methods struggle.

[Phys. Rev. Fluids 10, 094502] Published Mon Sep 29, 2025

Author(s): Tarun K. Jain and Chunendra K. Sahu

Understanding the dynamics of gravity currents is crucial in various engineering and environmental applications, such as CO2 sequestration and underground hydrogen storage. Gravity currents in porous media have often been studied considering a sharp interface between two fluids of different densities in the medium, overlooking the effects of mixing. In our study, we mark the importance of mixing in axisymmetric gravity currents and show that mechanical dispersion significantly alters the behavior of the gravity currents. We present formulations for estimating the rate of mixing and consequent evolution of gravity current length and volume.

[Phys. Rev. Fluids 10, 094503] Published Mon Sep 29, 2025

Author(s): Simon Gsell and Patrice Le Gal

Macroscopic pilot-wave systems, such as bouncing droplets, have long provided analogies to quantum-like wave–particle duality. We introduce a new platform: a particle oscillating in a density-stratified fluid, which couples to its self-generated internal gravity waves. Using theory and simulations, we show that a Doppler force drives spontaneous horizontal motion, while wave reflections from boundaries create a Casimir-like potential that constrains long-term dynamics. These results establish the ludion as a three-dimensional hydrodynamic pilot-wave system, opening new avenues for exploring macroscopic wave–particle phenomena.

[Phys. Rev. Fluids 10, L092801] Published Mon Sep 29, 2025

Author(s): Shiduo Wang, Fei Lyu, Zhenkun Zhang, Xingqian Mao, Haiqiao Wei, and Jiaying Pan

Understanding the high-pressure characteristics of nanobubbles is a significant step forward in unraveling the complexities of bulk nanobubble behavior. In this study, the effect of external pressure on the bulk nanobubbles is systematically investigated. The results demonstrate that as external pressure increases, both the concentration and lifetime of nanobubbles in methanol and water are significantly enhanced, indicating improved stability. Notably, the effect of external pressure on the surface charge is minimal, with the enhanced stability primarily attributed to increased supersaturation, reduced gas molecular diffusion, and the reinforcement of the hydrogen bond network.

[Phys. Rev. Fluids 10, 093605] Published Fri Sep 26, 2025

Author(s): Théophile Gaichies, Bryan Giraud, Anniina Salonen, Arnaud Antkowiak, and Emmanuelle Rio

What happens when a jet plunges into a liquid foam? In this study, we show that it can generate new, tiny bubbles inside the foam and, at higher speeds, stretch the soap films to the point of rupture. To explain these behaviors, we reproduce them in an elementary foam. There, we show that the tiny bubbles arise from an inversion of the Plateau border, while the spreading of the jet entrains films’ surface, triggering an elastic transition that leads to thinning. The resulting scaling laws extend to the two-dimensional foam phase diagram.

[Phys. Rev. Fluids 10, 094007] Published Fri Sep 26, 2025