# New Papers in Fluid Mechanics

### Induced-charge electrophoresis of a tilted metal nanowire near an insulating wall

Author(s): Jose Eladio Flores-Mena, Pablo García-Sánchez, and Antonio Ramos

Electric fields are commonly used to control the orientation and motion of microscopic metal particles in aqueous suspensions. For example, metallodielectric Janus spheres are propelled by the induced-charge electro-osmotic flow occurring on their metallic side, the most common case in electrokineti…

[Phys. Rev. E 109, 045109] Published Tue Apr 30, 2024

### Enhanced diffusiophoresis in dead-end pores with time-dependent boundary solute concentration

Author(s): Robben E. Migacz, Morgan Castleberry, and Jesse T. Ault

The diffusiophoretic velocity of a particle depends nonlinearly on solute concentration. We examine the implications of this nonlinearity in a dead-end pore, which is a geometry found both in nature and in microfluidic devices. We define the efficiency of injection and withdrawal processes, then describe particle dynamics with a step-like change in solute concentration at the pore inlet (commonly used in experiments) and with time-dependent boundary solute concentration. We show that particle migration becomes linear with slow transitions in solute concentration and demonstrate changes in particle dynamics with oscillatory solute concentration at the pore inlet.

[Phys. Rev. Fluids 9, 044203] Published Tue Apr 30, 2024

### Flow rate–pressure drop relations for shear-thinning fluids in deformable configurations: Theory and experiments

Author(s): SungGyu Chun, Evgeniy Boyko, Ivan C. Christov, and Jie Feng

The flow rate–pressure drop relations for laminar flow of Newtonian fluids in common geometries are well understood. However, a complete understanding of how the interplay between shear-thinning rheology and wall compliance sets the flow rate–pressure drop relation for a deformable configuration is still lacking. Here, we provide detailed quantitative comparisons between theory and experiments for the flow rate–pressure drop relation for Newtonian and shear-thinning fluids in two common deformable configurations: a rectangular channel and an axisymmetric tube. Such a comparison is of fundamental importance since it provides insight into the adequacy of the constitutive model used.

[Phys. Rev. Fluids 9, 043302] Published Thu Apr 25, 2024

### Robust microstructure of self-aligning particles in a simple shear flow

Author(s): Neeraj S. Borker, Abraham D. Stroock, and Donald L. Koch

A self-aligning particle (SAP) attains near perfect alignment with the fluid lamellae of a low Reynolds number simple shear flow without application of external torques in contrast with the continuous rotation exhibited by most rigid bodies including thin fibers and disks. We characterize the robustness of the flow alignment of SAPs to secondary perturbations such as flow disturbances, Brownian motion, inter-interparticle interactions, and the presence of a wall using dynamic simulations of ring-shaped SAP geometries. The robust flow alignment of SAPs provides an alternative route to access highly aligned microstructures that are inaccessible to suspensions of traditional particle geometries.

[Phys. Rev. Fluids 9, 043301] Published Wed Apr 24, 2024

### Flow induced by the rotation of two circular cylinders in a viscous fluid

Author(s): E. Dormy and H. K. Moffatt

Stokes flow driven by rotation of two parallel cylinders inside a cylinder of large radius R0 is investigated, and the flow in this triply-connected domain is determined numerically, with particular focus on the narrow-gap situation, when the local behavior is well described by lubrication theory. The asymptotic situation for infinite R0 is inferred (i) when the cylinder axes are unconstrained, and (ii) when they are held fixed. Contributions to the far-field are identified: a torquelet and a radial quadrupole in the counter- and co-rotating cases, respectively. In the former case, when the cylinders make contact (zero gap) a contact force acting on the cylinder pair is identified.

[Phys. Rev. Fluids 9, 044102] Published Mon Apr 22, 2024

### Sedimentation of a single soluble particle at low Reynolds and high Péclet numbers

Author(s): Nan He, Yutong Cui, David Wai Quan Chin, Thierry Darnige, Philippe Claudin, and Benoît Semin

We report experiments on the dissolution of a single particle during its sedimentation in a quiescent aqueous solution in the regime of low Reynolds and high Péclet numbers. We use butyramide, a chemical which does not change the density of water when it dissolves. The particle shrinks at a rate independent of its initial radius, in agreement with the model that we derive assuming Stokes drag and a mass transfer rate given by Levich (1962). This model becomes quantitative when including two correction factors to account for the non-sphericity of the particle and for the inclusions of air bubbles inside the particle.

[Phys. Rev. Fluids 9, 044502] Published Mon Apr 22, 2024

### Straight to zigzag transition of foam pseudo-Plateau borders on textured surfaces

Author(s): Alexis Commereuc, Sandrine Mariot, Emmanuelle Rio, and François Boulogne

The structure of liquid foams follows simple geometric rules formulated by Plateau 150 years ago. On smooth surfaces, the foam liquid channels, also called pseudo Plateau borders, are straight between vertices. We demonstrate experimentally that on rough surfaces and under some conditions that we establish, the bubble footprint exhibits a morphological transition. The footprint can adopt a zigzag shape between vertices. We rationalize the number of zigzag segments by a geometric distribution describing the observations made with the footprint perimeter and the mesh size of the asperities.

[Phys. Rev. Fluids 9, L041601] Published Mon Apr 22, 2024

### Direct comparison of density-driven convective mixing in a three-dimensional porous medium using experiments and simulation

Author(s): Rebecca Liyanage, Xiaojing Fu, Ronny Pini, and Ruben Juanes

We examine how fluids mix in three-dimensional (3D) porous materials due to differences in density which represent one mechanism of underground carbon dioxide storage. The experiment closely matched the simulation in terms of the patterns and speed of mixing. Interestingly, the experiment reveals columnar plumes self-organizing into a reticular pattern, previously seen only in 3D simulations. Results demonstrate quantitative matching over time in concentration, variance, scalar dissipation rate, and dissolution flux. A new relation between dissipation rate and flux is established, highlighting a 30% higher flux in 3D versus 2D systems, affirming prior estimations.

[Phys. Rev. Fluids 9, 043802] Published Thu Apr 18, 2024

### Instability and trajectories of buoyancy-driven annular disks: A numerical study

Author(s): G. Corsi, P. G. Ledda, G. Vagnoli, F. Gallaire, and A. De Simone

Seed dispersal strategies exemplify the role of morphology in defining falling paths. Here, macroscopic geometry effects are systematically studied by considering trajectories of annular disks. Via linear stability analysis, we identify the stability boundary for vertical fall, with a non-monotonic behavior of the critical falling velocity with the hole size, before increasing for large holes. Nonlinear simulations confirm linear analyses and suggest strategies for annular seed release at different heights, with the emergence of paths possibly beneficial for controlled positioning or advantageous for covering large lateral distances, depending on the hole size and disk weight.

[Phys. Rev. Fluids 9, 043907] Published Wed Apr 17, 2024

### Extended spreading of saline droplets upon impact on a frosty surface

Author(s): Hao Zeng, Feng Wang, and Chao Sun

The impact and freezing process of a water droplet are studied experimentally, incorporating the presence of frost and salt. A distinct transition of the spreading dynamics is observed, altering from the well-known 1/2 inertial scaling law to a 1/10 capillary-viscous scaling law. By considering the effect of impact inertia, partial-wetting behavior, and salinity, the mechanism of this transition is elucidated, and a unified model for predicting the droplet arrested diameter is proposed.

[Phys. Rev. Fluids 9, 044001] Published Tue Apr 16, 2024

### Extremely large water droplet impact onto a deep liquid pool

Author(s): Sandip Dighe, Dilip Kumar Maity, Jeffrey N. Fonnesbeck, Som Dutta, and Tadd Truscott

Most studies of droplet impact on liquid pools focus on droplet diameters up to the capillary length (0.27 cm). We break from convention and study extremely large water droplets (1 to 6 cm diameter) falling into a pool of water. We demonstrate that the depth and width of the cavity formed by large d…

[Phys. Rev. E 109, 045107] Published Tue Apr 16, 2024

### Vortex polarization and circulation statistics in isotropic turbulence

Author(s): L. Moriconi, R. M. Pereira, and V. J. Valadão

We carry out an in-depth analysis of a recently introduced vortex gas model of homogeneous and isotropic turbulence. Direct numerical simulations are used to provide a concrete physical interpretation of one of the model's constituent fields: the degree of vortex polarization. Our investigations she…

[Phys. Rev. E 109, 045106] Published Mon Apr 15, 2024

### Degrees of freedom and the dynamics of fully developed turbulence

Author(s): Diego Donzis and Shilpa Sajeev

Turbulent flows comprise a multitude of scales which make them extremely difficult to analyze and simulate. In this work, we study homogeneous isotropic turbulence using a novel approach which solves the Navier-Sokes equations on a reduced set of modes that are sampled stochastically. The complementary set are solved using trivial dynamics. This method, called Selected Eddy Simulations, is able to capture broad dynamics of turbulence with just 10% of resolved modes, suggesting the turbulence attractor may be smaller or more robust to modeling than previously thought. This result also holds promise for developing alternative low-cost numerical approaches to study turbulent flows.

[Phys. Rev. Fluids 9, 044605] Published Mon Apr 15, 2024

### Electrocapillary, thermocapillary, and buoyancy convection driven flows in the Melcher-Taylor experimental setup

Author(s): Alexander Yu. Gelfgat and Gerrit Maik Horstmann

Electrocapillary flows in a classical experiment of Melcher and Taylor are studied. The computed streamlines qualitatively represent the experimental image. With the increase of electrocapillary forcing, the main circulation localizes near a boundary with a larger electric potential. When a dielectric liquid is replaced by a poorly conducting one, the system becomes non-isothermal owing to the Joule heating, and the flow is driven also by buoyancy and thermocapillary convection. The results show that consideration of the two-phase model is mandatory. The Lippmann equation, connecting electrically induced surface tension with nonuniform surface electric potential, is numerically verified.

[Phys. Rev. Fluids 9, 044101] Published Fri Apr 12, 2024

### Turbulence spreading by resonant wave-wave interactions: A fractional kinetics approach

Author(s): Alexander V. Milovanov and Jens Juul Rasmussen

This paper is concerned with the processes of spatial propagation and penetration of turbulence from the regions where it is locally excited into initially laminar regions. The phenomenon has come to be known as “turbulence spreading” and witnessed a renewed attention in the literature recently. Her…

[Phys. Rev. E 109, 045105] Published Thu Apr 11, 2024

### Turbulence modulation by suspended finite-sized particles: Toward physics-based multiphase subgrid modeling

Author(s): S. Balachandar, C. Peng, and L.-P. Wang

The presence of a dispersed phase substantially modifies small-scale turbulence. Here we present a comprehensive mechanistically based model to predict turbulence modulation, the predictions of which, compared with particle-resolved simulations and experiments, is shown.

[Phys. Rev. Fluids 9, 044304] Published Thu Apr 11, 2024

### Particle hydrodynamics in acoustic fields: Unifying acoustophoresis with streaming

Author(s): Xiaokang Zhang, Jake Minten, and Bhargav Rallabandi

Acoustic fields are used in numerous applications to induce movement of suspended particles. We develop a rigorous theory that systematically unifies inviscid acoustophoresis with viscous streaming. The theory connects particle motion to a generalized form of the secondary radiation force, which depends on the Stokes layer thickness around the particle, and the contrast of density and compressibility between the particle and the fluid. We identify a reversal of particle motion when inertial and viscous forces are comparable, validated with numerical solutions. This has significant implications for applications involving particle sorting or focusing based on size or material properties.

[Phys. Rev. Fluids 9, 044303] Published Wed Apr 10, 2024

### Microscopic self-dynamics in liquid Ne-${\mathrm{D}}_{2}$ mixtures: Quantum features and itinerant oscillators reexamined

Author(s): Daniele Colognesi, Ubaldo Bafile, Eleonora Guarini, and Martin Neumann

In this paper, we report the results of a centroid molecular dynamics (CMD) study of the canonical velocity autocorrelation functions (VACFs) in liquid Ne−D2 mixtures at a temperature of T=30K and in the full D2-concentration range (0%≤xD2≤100%). This binary system was selected because of its modera…

[Phys. Rev. E 109, 045104] Published Tue Apr 09, 2024

### Transient energy growth in channel flow with compliant walls

Author(s): Frédéric Alizard, Benoît Pier, and Smail Lebbal

Shear flows in contact with compliant boundaries exhibit a rich dynamics involving traveling-wave-flutter, Tollmien-Schlichting, as well as divergence instabilities. In this context, maximum transient growth effects are the result of optimal energy exchanges during the fluid-structure interaction process. The present investigation studies the detailed contribution of the different interacting modes. In particular, it is found that the optimal gain may be associated with a large-amplitude oscillatory behavior and that wall-compliance enhances this phenomenon.

[Phys. Rev. Fluids 9, 043905] Published Tue Apr 09, 2024

### Lagrangian coherent structures control solute dispersion in heterogeneous poroelastic media

Author(s): Junhong Wu, Daniel Lester, Michael G. Trefry, and Guy Metcalfe

This study focuses on how Lagrangian coherent structures (LCSs) control solute dispersion in heterogeneous poroelastic media (HPM). We show how interactions between medium compressibility, conductivity heterogeneity, and periodic forcing give rise to complex flows and diverse LCS types (KAM islands, chaotic saddles, etc) that have profound impacts on diffusive solute transport (main image) that do not arise in the steady counterpart (inset). Strongly anomalous transport impacts both spatial moments and residence time distributions and persists at low Péclet numbers. This study reveals the complex transport phenomena that can arise in HPM and shows how LCSs govern solute dispersion.

[Phys. Rev. Fluids 9, 044501] Published Tue Apr 09, 2024