Physical Review Fluids

Author(s): Shunta Kikuchi and Hiroshi Watanabe

Molecular dynamics simulations show how the Rayleigh-Plateau instability of a nanoscale liquid filament evolves with and without imposed interfacial perturbations between two immiscible liquids of equal viscosity. For a single-mode perturbation, the growth rate deviates from classical theory at small radii but converges to macroscopic predictions for thicker filaments. Without imposed perturbations, thermally induced breakup has a power-law dependence of breakup time on minimum filament radius. These results demonstrate that continuum theory remains valid down to scales of about 15 molecular diameters, while thermal fluctuations are increasingly important in thinner liquid filaments.

[Phys. Rev. Fluids 11, 023901] Published Wed Feb 04, 2026

Author(s): Xiaohui Meng and Ewe-Wei Saw

Predicting particle collisions in turbulence is critical for applications from cloud formation to industry, yet a model for mean radial relative velocity (MRV) remains elusive. Using direct numerical simulations, we investigate the motion of colliding particles and establish the relationship between MRV at collisional distances, and the particle Stokes and flow Reynolds numbers. A resonant length scale, the spatial scale where inertial particles capture momentum from turbulent flow, is introduced. By finding the relationship between this scale and particle and flow parameters, we provide inputs for a robust framework to predict particle MRV at collisional scale across a range of conditions.

[Phys. Rev. Fluids 11, 024301] Published Wed Feb 04, 2026

Author(s): Adam Jiankang Yang, Mary-Louise Timmermans, and Mona Rahmani

Kelvin–Helmholtz (KH) instability can dramatically reshape how particles settle and spread in stratified shear flows. Using direct numerical simulations with Lagrangian tracking, we show that KH billows can either slow, trap, or strongly accelerate particle settling depending on particle size, with small particles settling up to seven times faster than their Stokes velocity. The results reveal how preferential sampling of coherent flow structures and limited encounter times with turbulence fundamentally alter particle dispersion and sediment transport in mixed layers.

[Phys. Rev. Fluids 11, 024302] Published Wed Feb 04, 2026

Author(s): Serge Mora, Martine Le Berre, and Yves Pomeau

The free fall of a sphere is a seminal problem in fluid mechanics that becomes remarkably complex as its velocity approaches the “drag crisis” regime. This study demonstrates that intermittency in the drag coefficient at this critical stage renders the temporal evolution of the velocity intrinsically unpredictable. By combining numerical simulations with a stochastic two-state model, we reveal huge velocity dispersions. These findings highlight the necessity of statistical descriptions for bodies falling at high Reynolds numbers, with direct implications for ballistics and sports aerodynamics.

[Phys. Rev. Fluids 11, 024401] Published Wed Feb 04, 2026

Author(s): Outi Supponen

Experimental high-speed visualization techniques are evolving rapidly and provide valuable tools for learning about ultrafast bubble dynamics that cause unwanted but also desirable damage, such as cavitation and acoustically driven bubbles and droplets relevant for biomedical applications. This article offers my personal perspective on the quest to illuminate the hidden physics of externally stimulated bubbles that have a remarkable ability to focus energy. The focus is given to advanced experimental techniques including ultrafast videomicroscopy and synchrotron x-ray imaging to characterize bubble jetting, vapor bubble nucleation and shape deformations of periodically driven bubbles.

[Phys. Rev. Fluids 11, 023601] Published Tue Feb 03, 2026

Author(s): J. Tauber, J. Asnacios, and L. Mahadevan

With experiment and theory we consider the branching morphodynamics of injecting a shear-thinning liquid from a point source to a point sink in a Hele-Shaw cell filled with a yield-stress fluid which has a sudden transition in its response as the local stress crosses a threshold. As the injection rate is increased an abrupt transition occurs, from a direct path connecting source to sink at low flow rates, to a rapid branching morphology at high flow rates, eventually converging to the sink. We show that global constraints imposed by boundary conditions, including source-sink separation, injection rate, and plate properties, shape branching morphodynamics and determine the transition point.

[Phys. Rev. Fluids 11, 023301] Published Mon Feb 02, 2026

Author(s): Zhanwen Wang, Michael J. Miksis, and Petia M. Vlahovska

The dynamics of a spherical particle undergoing Quincke electro-rotation in the vicinity of a planar electrode are investigated. Increasing the electric field induces a transition from steady rolling to periodic and then chaotic oscillations, with the onset threshold depending on the particle–surface gap and particle inertia. When allowing for normal motion of the particle the electrostatic attraction reduces the gap, which in turn suppresses chaotic behavior and reestablishes a steady rolling state.

[Phys. Rev. Fluids 11, 023701] Published Mon Feb 02, 2026

Author(s): M. Rubio, S. Rodríguez-Aparicio, M. G. Cabezas, J. M. Montanero, and M. A. Herrada

Tip streaming in a coflowing device is probably the simplest way to generate quasi‑monodisperse droplets that are much smaller than the device’s fluid passages. We show that flow stability cannot be determined from the linear stability analysis of the steady microjetting mode but from the linear superposition of decaying eigenmodes triggered by an initial perturbation. The red line in the image corresponds to a direct numerical simulation of an asymptotically stable microjetting. The experiment and simulation show the perturbation growth leading to jet breakup. These results call into question the validity of linear stability analysis applied to coflowing and similar configurations.

[Phys. Rev. Fluids 11, 024001] Published Mon Feb 02, 2026

Author(s): Yunpeng Wang, Huiyu Yang, Zelong Yuan, Zhijie Li, Wenhui Peng, and Jianchun Wang

A machine-learning-based surrogate model is proposed for the large-eddy simulation of three-dimensional turbulent flows over curved boundaries with strong flow separation. The model, termed as hybrid U-Net and Fourier neural operator (HUFNO), is based on an integrated framework of convolutional neural networks and Fourier neural operators, tailored for problems involving mixed periodic and non-periodic boundary conditions. The HUFNO model is validated in the fast prediction of turbulent dynamics of periodic-hill flow, with transferable accuracy to unseen initial conditions, Reynolds numbers, and hill shapes.

[Phys. Rev. Fluids 11, 024601] Published Mon Feb 02, 2026

Author(s): Yi Zhang, Apurav Tambe, and Zhao Pan

The maximum droplet volume that a fiber can retain is a classic problem in the physics of droplet-fiber interactions, with established results for horizontal and Λ-shaped bent fibers. However, this question has remained less explored for V-shaped bent fibers, despite their relevance to fog harvesting and condensation technologies. Here, we develop a free-energy- based analytical model to predict the maximum droplet volume on V-shaped fibers and validate it experimentally using multiple liquid-fiber pairs. We reveal a non-monotonic dependence of the maximum droplet volume on the fiber opening angle, identifying a transition regime that facilitates droplet detachment.

[Phys. Rev. Fluids 11, 013604] Published Thu Jan 29, 2026

Author(s): Santiago Caro, Riccardo Artoni, Patrick Richard, Michele Larcher, and James T. Jenkins

Sheared polydisperse granular materials exhibit a subtle balance between size segregation and diffusion that governs their transverse dynamics. Combining annular shear cell experiments with discrete numerical simulations, we investigate how confinement, shear localization, granular temperature, and mixture composition control segregation in nonuniform flows. Beyond the classical gravity-driven mechanism, we identify inverse and horizontal segregation modes that emerge from flow kinematics and geometry. These mechanisms hinder complete segregation, explaining the persistence of mixing in steady-state granular systems.

[Phys. Rev. Fluids 11, 014305] Published Thu Jan 29, 2026

Author(s): Beverley McKeon and Eric Lauga

[Phys. Rev. Fluids 11, 010001] Published Wed Jan 28, 2026

Author(s): Qian Mao, Umberto D'Ortona, and Julien Favier

Micro-scale cilia play a vital role in mucociliary clearance (MCC) in the human respiratory airways. We develop a three-dimensional model for predicting MCC with two-way coupling between the cilia and the two-phase airway surface liquid, comprising the periciliary layer (PCL) and the mucus layer (ML). Focusing on a single cilium, we systematically examine the effects of PCL thickness and the viscosity ratio between the PCL and ML, which can vary markedly under pathological conditions. The fluid transport mechanisms are clarified by identifying two competing effects, namely the balance between drag and elastic forces and the viscous diffusion of momentum, and by establishing quantitative relationships between the flow rate and the beating pattern.

[Phys. Rev. Fluids 11, 013102] Published Wed Jan 28, 2026

Author(s): Leo R. M. Maas and Eyal Heifetz

The dynamics of a stratified, rotating fluid, contained in a box and subject to differential heating in the horizontal direction, is approximated by a low-order set of five nonlinear ordinary differential equations (ODEs). Its forced and damped versions reduce to well-known ODEs for convection or long-wave dynamics. In the ideal fluid limit, one integral of motion represents initial stratification and motion. The remaining equations, capturing the essence of ‘rotating horizontal convection’, are integrable in the absence of rotation or differential heating. In general, they represent a forced, complex Duffing equation that appears to be a generalized nonintegrable 2 DOF Hamiltonian system.

[Phys. Rev. Fluids 11, 013506] Published Wed Jan 28, 2026

Author(s): Owen Thompson, Kyriakos Tapinou, Daryl Bond, and Vincent Wheatley

Shock-driven Richtmyer–Meshkov instability is central to astrophysical and inertial-confinement-fusion plasmas, where kinetic-scale effects invalidate single-fluid MHD descriptions. Using an ideal two-fluid plasma model, we show that a magnetic field parallel to the interface suppresses instability growth by transporting and phase-mixing interfacial vorticity on plasma wave packets, with increasing efficacy at smaller plasma length scales. In contrast, an out-of-plane magnetic field fails to suppress the instability and instead promotes Kelvin–Helmholtz–like roll-up through charge-separation-driven vorticity generation.

[Phys. Rev. Fluids 11, 013702] Published Wed Jan 28, 2026

Author(s): P. Beaumard, J. P. Laval, and J. C. Vassilicos

Existing large eddy simulation models suffer from a lack of physical justification. In this paper, the links between two-point equations derived from the Navier-Stokes equation and Large Eddy Simulation (LES) are examined and an approximation of an exact equation is used to design a new subgrid-scale model. This new model is tested both a priori and a posteriori and is found to capture the correct physical energy transfer between filtered scales and residual subfilter scales. This is a proof of concept that two-point equations can be used to develop new LES models and this strategy may be the right one for developing more efficient models.

[Phys. Rev. Fluids 11, 014607] Published Wed Jan 28, 2026

Author(s): David Martín, Joan Grau, and Lluís Jofre

Turbulence in supercritical fluids differs from its low-pressure counterpart due to strong thermodynamic coupling and pseudoboiling effects, yet their influence on velocity and thermodynamic fluctuations remains unclear. Using direct numerical simulations of isotropic turbulence in supercritical fluids, small-scale statistics are examined. Temperature-related quantities are particularly sensitive, exhibiting increased intermittency of temperature variance dissipation rate and reduced production of mean-square temperature gradients. Topological analysis also shows that regions of intense pseudoboiling activity suppress strain-dominated structures while enhancing vortical motions.

[Phys. Rev. Fluids 11, 014609] Published Wed Jan 28, 2026

Author(s): Ran Qiao, Kai Mu, Chengxi Zhao, and Ting Si

Although thermal fields are known to influence liquid jet instability, prior studies have focused primarily on axisymmetric disturbances. This work reveals that a temperature field can excite a dominant non-axisymmetric helical mode, driven by azimuthal Marangoni stresses. We show that enhancing the Marangoni effect or suppressing thermal diffusivity promotes this helical instability, triggering a fundamental transition from Rayleigh-Plateau to azimuthal Marangoni-driven destabilization. Phase diagrams provide criteria for predicting this mode transition, offering new insights into controlling jet stability in applications such as ink printing and fiber production.

[Phys. Rev. Fluids 11, 014006] Published Tue Jan 27, 2026

Author(s): Mano Grunwald and Claudia E. Brunner

We experimentally investigate the impact of different inflow conditions on the breakdown of wind turbine tip vortices in a high Reynolds number wind tunnel. The data in this paper is obtained through hot wire spectral analysis. While downstream evolution of the spectra exhibits a complex scale dependent behavior, here we focus on the decay of the signature of the tip vortices for which we identify three distinct regimes. These regimes are linked to an initial advection phase, vortex breakdown, and turbulence decay. Variations in the tip speed ratio have a significant impact on the breakdown rate in the second regime, while effects of mean shear and turbulence intensity are less pronounced.

[Phys. Rev. Fluids 11, 014608] Published Tue Jan 27, 2026

Author(s): Gonzalo G. de Diego and Georg Stadler

Polar sea ice is a crucial component of Earth’s climate system which is generally modeled as a non-Newtonian fluid in climate simulations. To overcome the accuracy limitations of existing non-Newtonian models for sea ice, we present a framework for learning an effective shear viscosity function for sea ice from velocity data. We apply our approach to data generated from a complex sea ice discrete element method (DEM). The learned rheology is capable of reproducing the DEM velocity data accurately.

[Phys. Rev. Fluids 11, 013301] Published Mon Jan 26, 2026