New Papers in Fluid Mechanics

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

Author(s): Jie Yao, Edgardo García, and Fazle Hussain

Opposition control (OC) mitigates near-wall turbulence by local blowing and suction and is effective in reducing drag at low Reynolds numbers. Using direct numerical simulations up to Reτ=2000, we show that drag reduction progressively deteriorates with increasing Reτ due to the enhanced influence of outer large- scale motions that OC fails to suppress. Our results quantify the Reynolds-number dependence of optimal control parameters and predict that about 15% drag reduction remains achievable at very high Reτ. These findings highlight the need for hybrid strategies targeting both inner- and outer-layer turbulence to sustain drag reduction in realistic high-Reτ flows.

[Phys. Rev. Fluids 10, 094604] Published Fri Sep 26, 2025

Author(s): Tomoaki Itano and Isshin Arai

Flow visualization using reflective flakes presents a puzzle: Why do experimental patterns appear steady and reproducible when orientations of individual flakes follow chaotic paths on the unit sphere, even in steady flows? We introduce a probabilistic framework that treats flake orientation as a probability density field rather than tracking individual particle trajectories. The key insight reveals that small random motions, typically ignored in deterministic approaches, are requisite for eliminating initial condition dependence and achieving the steady patterns observed in experiments. The theory successfully predicted asymmetric flow patterns that deterministic approaches couldn’t explain.

[Phys. Rev. Fluids 10, L092901] Published Fri Sep 26, 2025

Author(s): Berin Šeta, Jon Spangenberg, Mounir M. Bou-Ali, and Valentina Shevtsova

Ternary systems driven by concentration-dependent diffusion coefficients were studied aboard the International Space Station. A range of new patterning possibilities and the coexistence of gravitational instabilities, previously thought impossible in nonreactive systems, were uncovered.

#ClearMotivation #ElegantVisuals

[Phys. Rev. E 112, L033101] Published Thu Sep 25, 2025

Author(s): Xukang Lu, Qiangqiang Li, Guang Yang, Yunfan Huang, Wenhai Lei, and Moran Wang

Migration and retention of particles in flow systems is a long-standing and ever-relevant topic. We identify a new mode of particle accumulation driven by geometry variations in confined spaces and nonhydrodynamic viscosity effects of complex particle systems, which is triggered in the absence of clogging effects and distinct from shear-induced migration in dense suspensions. A new dimensionless number is derived and validated by numerical simulations in typical pore-throat geometries. Investigations in heterogeneous porous structures reveal variable yet predictable accumulation patterns, offering new opportunities for preferential flow control in porous media.

[Phys. Rev. Fluids 10, 093304] Published Thu Sep 25, 2025

Author(s): Juan Li, Baixue Li, Alexander Oron, and Youhua Jiang

Conventional wisdom holds that suppressing droplet penetration through meshes needs to decrease pore sizes. Alternatively, this work demonstrates that a macro-ridge on a superhydrophobic mesh can increase the dynamic pressure threshold for liquid penetration. Using high-speed visualization and force measurements, we show that the ridge breaks the symmetry of droplet retraction, inducing multiple flow-focusing events at different sites in place of one flow-focusing event at the droplet center on a flat mesh. This work provides a new strategy for designing water-repellent surfaces without compromising the breathability.

[Phys. Rev. Fluids 10, 093604] Published Thu Sep 25, 2025

Author(s): Yueke Niu and Yanbo Xie

Liquid films govern processes from bubble lubrication to emerging soft nanofluidic memristors, yet their response to electric fields has remained unclear. We develop an analytical framework considering Maxwell stress, disjoining pressure, and Laplace pressure to capture both equilibrium and dynamic responses of bubble liquid films, which explains the conductance rectification and hysteresis observed in experiments. This work positions liquid films as a concept of electrohydrodynamic soft memristors, which aims for future energy-efficient and neuromorphic computing.

[Phys. Rev. Fluids 10, 093702] Published Thu Sep 25, 2025

Author(s): Jingyi Liu, Yi Man, and Eva Kanso

Sessile ciliates feed by generating ciliary currents rather than by locomotion. While the optimal ciliary stroke in uniform nutrient fields is known, natural habitats are patchy and gradient-rich. We show that, for a stationary cell, a linear nutrient gradient leaves the purely diffusive uptake unchanged; however, when cells generate ciliary flows that align with the gradient, intake is enhanced, following a square-root scaling law with time and Péclet number. Thus, cells using simple flow-generation strategies can exploit environmental asymmetry to optimize resource acquisition.

[Phys. Rev. Fluids 10, 093104] Published Wed Sep 24, 2025

Author(s): Shibo Lee, Chenglin Zhou, Yang Zhang, Yunlong Zhao, Jiaqi Luo, and Yao Zheng

Direct numerical simulation is utilized to resolve the drag and turbulent characteristics of supersonic fully developed channel turbulence in this paper. Distinct from spatially developing turbulence, a relaxation of the skin friction coefficient is discovered after blowing in fully developed turbul…

[Phys. Rev. E 112, 035104] Published Tue Sep 23, 2025

Author(s): Jason R. Picardo and Dario Vincenzi

Most simulations of polymeric turbulence treat the solution as a suspension of elastic dumbbells. Though the feedback-force exerted by the beads of the dumbbell are taken into account while calculating the large-scale flow, the local disturbance flow at the scale of the dumbbell is typically neglected, i.e., inter-bead hydrodynamic interactions (HI) are disregarded. Here, we quantify the effect of these interactions on the dumbbell’s extension. We derive exact analytical results for a Batchelor-Kraichnan random flow, while also performing simulations in homogeneous isotropic turbulence. We find that the effects of HI are mild but extend beyond simply increasing the elastic relaxation time.

[Phys. Rev. Fluids 10, 093303] Published Tue Sep 23, 2025

Author(s): Simon M. Finney, Matthew G. Hennessy, Andreas Münch, and Sarah L. Waters

Analytical solutions for the deformation, translational velocity, and rotational velocity of a linear poroelastic particle in an unbounded Stokes flow are provided. The solutions are specialized to the cases of shear and Poiseuille flows. Surprisingly, the rotation of the particle is not influenced by its poroelastic nature.

[Phys. Rev. Fluids 10, 093603] Published Tue Sep 23, 2025

Author(s): Xiao Shao, Marian Albers, Matthias Meinke, and Wolfgang Schröder

We examine the effect of compressibility on drag reduction in turbulent boundary layers actuated by spanwise traveling transversal surface waves. Wall-resolved simulations are performed at Mach numbers M1=0.2 and M2=0.7 over the same actuation space. The results show that drag reduction is greater at higher Mach number, but at M2=0.7 the increased wave speed induces spanwise shock waves. These shocks disrupt turbulence, redistribute turbulent kinetic energy, and alter skin-friction contributions. The findings highlight the trade-off between enhanced drag reduction and reduced actuation efficiency in compressible flows.

[Phys. Rev. Fluids 10, 094603] Published Tue Sep 23, 2025

Author(s): Rupayan Jana and Shubhadeep Mandal

We investigate the low-Reynolds-number hydrodynamics of rigid circular and spherical particles near a plane wall in a heterogeneous viscous environment comprising both ambient and disturbance viscosity fields. Focusing on the resistance problem, we theoretically and numerically compute the forces and torques acting on the particle. Our results demonstrate that viscosity gradients induce novel hydrodynamic cross-couplings in near-wall particle motion, which are also directly reflected in the resulting trajectories. These findings have potential implications for controlled transport, sorting, and separation in microfluidic applications.

[Phys. Rev. Fluids 10, 094204] Published Mon Sep 22, 2025

Author(s): Tristan P. W. Bunnage, Douglas R. Brumley, and Edward M. Hinton

The paper examines rectilinear two-dimensional miscible viscous fingering in response to both injection and withdrawal of low viscosity fluid. Whether or not substantial fingering occurred during the injection controls the dynamics in the withdrawal period. At the end of injection, the fingers generally have a tip with a localized peak in concentration of the low viscosity fluid. This region then changes direction upon withdrawal. The low viscosity tip retraces the original finger because flow there is preferential as the viscosity is lower relative to the ambient fluid.

[Phys. Rev. Fluids 10, 094501] Published Mon Sep 22, 2025

Author(s): Tao Shu, Yuan Xue, Abhishek Saha, Jialong Huo, Hua Zhou, Zhuyin Ren, and Chung K. Law

Characterization of the turbulent flow field in a constant-pressure, dual-chamber, expanding flame apparatus is presented, based on high-resolution particle imaging velocimetry. Turbulent flame speeds were measured for C2H4/air and NH3/CH4/air mixtures across turbulent Reynolds number ranges of 29–2706 and 69–2560, respectively. An extended scaling correlation for turbulent flame speed is proposed, applicable to both corrugated flamelets and thin reaction zones regimes. The proposed model demonstrates improved accuracy, with the mean absolute percentage error between experimental and predicted values of 7.8% for C2H4/air and 8.5% for NH3/CH4/air flames, demonstrating a comparative advantage over existing scaling models.

[Phys. Rev. Fluids 10, 094602] Published Mon Sep 22, 2025

Author(s): Minki Kim, Shahaboddin Alahyari Beig, and Eric Johnsen

The inertial collapse of a cavitation bubble concentrates potential energy within the bubble. This process redistributes energy between the bubble and the surrounding liquid, while also emitting a shock wave. In the incompressible limit, bubble dynamics are governed by the driving pressure, but at high collapse speeds, compressibility effects become critical. We present a theoretical framework that corrects radiated energy estimates, derives closed-form expressions for bubble energy and volume at collapse, and relates shock pressure directly to governing parameters. Our results provide a foundation for describing more complex systems, such as bubble clouds and bubbles near boundaries.

[Phys. Rev. Fluids 10, 093602] Published Fri Sep 19, 2025

Author(s): Charles Paul Moore, Hiba Belkadi, Brouna Safi, Gabriel Amselem, and Charles N. Baroud

Cells and other soft particles are often forced to flow in confined geometries in both laboratory and natural environments, where the elastic deformation induces an additional drag and pressure drop across the particle. We start by measuring the pressure drop across a single spherical hydrogel particle as it flows in a microfluidic comparator. This pressure is found to depend on the amount of confinement, elastic modulus, fluid viscosity and velocity. A model for the force balance on the particle is then proposed, by incorporating the above ingredients and relying on simulations of bead geometry and lubrication flow considerations.

[Phys. Rev. Fluids 10, L092201] Published Fri Sep 19, 2025

Author(s): Zhiqian Xin, Jiadong Wang, Xingyuan Mao, Bowen Jin, and Jian Deng

This study investigates the influence of varying degrees of freedom (DOFs) on the swimming performance of self-propelled undulatory swimmers navigating a straight path in three flow configurations: an unbounded fluid, near a solid wall, and in a side-by-side arrangement. Vertical and rotational DOFs…

[Phys. Rev. E 112, 035103] Published Fri Sep 19, 2025