A single grain of salt, placed on the surface of everyday hand sanitizer, slowly bores a hole through it, leaving a milky “comet trail” in its wake. The fall speed does not depend on the size of the salt grain, but does depend on salt type, and reveals subtle aspects of collapsing microgels.

When a flexible elastic filament is attached to a Quincke rotating sphere, an elasto-electro-hydrodynamic instability leads to self-propulsion of the composite system.

A suspension of rigid particles in plane Couette flow with elastic boundaries is simulated numerically. Reduction in apparent viscosity for the more deformable boundaries is modeled as a concentration reduction as the volume for particles expands in amount equal to the boundary elastic deformation.

A leukocyte model is used to show how different flow shear rates affect bending deformation of the microvilli and adhesive bond forces.

The “regularized ringlet,” which is the fundamental solution to axisymmetric Stokes flow driven by a ring of smoothed point forces, is derived and used to model cytosolic flow in the growing pollen tube.

We examine the effects of locally altering the tip region of a periodic flapping fin system with a low order robotic model and load, motion, and wake measurements. This actuation modifies propulsive performance and unveils the importance of the phase difference between the fin and the tip.

The impact of the Lewis number on ultrafast premixed flame acceleration in channels equipped with comb-shaped arrays of tightly spaced obstacles attached to the walls is studied by the computational simulations of the reacting flow equations, including fully compressible hydrodynamics.

The application of an iterative deconvolution modeling framework for the unresolved stresses in turbulent and reacting flows is demonstrated using high-fidelity direct numerical simulation data of premixed rod-stabilized V-flames.

A turbulent premixed flame anchored by a solid wall can reach a statistically stationary state. Overall the configuration is counterflowlike. Flame-wall interactions under such conditions are key to understanding combustion in a confined space and in developing wall flame models. A numerical study is presented.

Experiments and three-dimensional numerical simulations of yield stress fluids in heterogeneous porous media show that, as a result of this disorder, the fluid flows through a single channel near a critical pressure. Beyond that, the number of open channels increases with the pressure applied.

The image shows the flow field around a sphere that sediments and rotates simultaneously in a viscoelastic liquid.

Microfluidic experiments and simulations reveal an anomalous percolation transition for the flow of a yield stress fluid through a random porous medium. The evolution of the fluidized network results in a highly nonlinear flow conductivity and reveals a novel dispersion mechanism.

High-speed ultrasound imaging is used to reconstruct the three-dimensional flow field of dense cornstarch suspensions under tilted impact. Experiments show how jamming fronts propagate and get deformed in a nonaxisymmetric manner under different impact angles.

An investigation finds that instability in gravity-driven thin viscoelastic films can be substantively enhanced or suppressed by sinusoidal wall topography with an appropriately chosen wavelength. Topography also introduces finer temporal scales in the film dynamics, and simulations produce a broad range of nonlinear states.

When a poroelastic spherical drop is under a uniaxial extension flow or a shear flow, the elastic network deforms in response to the external fluid viscous stress. A small-deformation analysis is conducted to investigate effects of interfacial slip and permeability on flow around the drop.

The collapse of a spherical cloud composed of 12 500 gas bubbles in water is studied. Wave propagation is captured and compared to existing reduced order models. Microjet formation is quantified and a dependence between microjet velocity magnitude and the strength of the collapse wave is identified.

A numerical study of the collapse of a vapor bubble finds that a small amount of noncondensable molecules affects the temperature profile inside the collapsing vapor bubble, preventing shock wave generation inside the collapsing bubble.

The ion partitioning effect due to polyelectrolyte layer (PEL)-electrolyte permittivity contrast in soft microchannels leads to depletion of electrolyte ions within the PEL. This effect, surprisingly, magnifies the electroosmotic flow rate because of the space charge increase outside the PEL.

Linear instability analysis, performed on the mean flow of a fully turbulent jet, provides quantitatively correct predictions of the energetic coherent turbulent structures found in experiments. It is concluded that the most energetic structures in jet turbulence are governed by linear dynamics.

An investigation of the phase relationship between the Kelvin-Helmholtz mode and its subharmonic mode finds that it has important implications for vortex pairing and mixing in the flow by ensuring the instability. Results are consistent with laboratory observations for suppression of the subharmonic mode near an unfavorable phase.

Experiments find that vortex merging occurs naturally in a laminar separation bubble. Acoustic forcing at the subharmonic and fundamental vortex shedding frequency promotes and inhibits merging, respectively. In all cases, merging occurs in the aft portion of the bubble in a spanwise nonuniform manner.

Bypass transition in boundary layers is governed by streak instability. An investigation of the effect of oscillatory wall-control on bypass transition shows that transition delay is due to streak attenuation via the modification of the eigenfunction of the Orr-Sommerfeld operator.

Group theory analysis of the Rayleigh-Taylor instability with an acceleration varying in time finds that the early time behavior depends on whether the acceleration decays faster or slower than $1/t^2$.

With a cylindrical and incompressible model of an impulsively accelerated conducting fluid interface, effects of normal or azimuthal magnetic fields on interface growth rates are studied. In the normal case growth decays at late times and in the azimuthal one a growth perturbation oscillates.

A dissolution-driven density instability with reaction $A+B\to C$ is simulated in both homogeneous and heterogeneous media at pore scale. Six types of fingering scenarios are found in each medium, and it is shown that media with large pore size in the top layer can enhance the storage of solute $A$ in the host fluid.

Coupled Faraday waves can develop under weightlessness on liquid-vapor bands induced by vibrations. The instability appears above a threshold that is determined by theoretical analysis and numerical simulation. It compares well with sounding rocket experiments in CO${}_2$ near its critical point.

An air bubble expanding into a viscous liquid in the small gap between parallel plates deforms into highly branched, continuously evolving interfacial fingering patterns. By separating the plates following a power law in time, it is found that the interface can also form self-similar fingering patterns.

An analysis of the drop-size selection mechanism in a locally heated silicon-in-silica coaxial fiber is presented. Surprisingly, it is found that the size of the silicon drops is independent of the linear stability properties of the system and is instead set by nonlinear effects.

A study of the impact of liquid drops on millimeter-scale hairy surfaces finds that the behavior of the impacting drops depends on the amount of kinetic energy dissipated through the hairs via a balance of inertia, viscosity, and surface tension.

Complex actuation of a caudal fin can generate turning moments enabling underwater swimmer navigation. A numerical study explores the hydrodynamics resulting from two actuation strategies leading to pitching and yaw moments.

A molecular-kinetic model, based on thermally activated processes, is developed for the slip of a simple fluid on a smooth solid boundary. Navier slip is recovered at low shear-rates. Predictions at higher shear-rates are tested against experiments and MD simulations.

Holographic cinematography is used to measure rotational and translational dynamics of inertial fibers in isotropic turbulence. Results show that for the present parameter range, translational motion is similar to that for spheres. In contrast, tumbling rates peaked at an intermediate Stokes number.

A numerical study of sheared suspensions of frictional and frictionless rigid spheres finds that macroscopic friction in granular suspensions is nearly independent of interparticle contact friction, and besides contact lubrication, the viscous contribution of the pore fluid is negligibly small.

The small-scale flow topologies in droplet-resolved direct numerical simulations of isotropic turbulence are classified using tensor invariants. The results show that when approaching the droplet surface, the flow topologies fundamentally change from those found in isotropic turbulence to boundary-layer-like structures.

Numerical simulations of turbulent flow past a sphere immersed in decaying turbulence, free to rotate around a fixed center, are presented. This arrangement allows the same simulations with and without the sphere, thus permitting the identification of the flow features that most affect the sphere rotation.

In many geophysical situations, variable bathymetry can affect surface transport and mixing. Experiments show that the effect of bathymetry is to produce a porous transport barrier leading to asymmetric transport. These results may help to explain the dynamics of phenomena such as ocean dead zones.

Chaotic mixing of yield stress fluids is considered in a novel static mixer that imparts uniformly efficient mixing due to the underlying topology of the device. Experimental and computational studies indicate that mixing is largely insensitive to flow dynamics, boundary conditions, and fluid rheology.

Statistical properties of an incompressible passive vector in isotropic turbulence are compared with the velocity and passive scalar in order to explore the physics behind their differences and similarities.

A theoretical framework, the H-theory, is applied to describe the skewed non-Gaussian distribution of velocity increments as a weighted mixture of asymmetric Gaussians. The weighing distribution is obtained from a hierarchical stochastic model of intermittency. Good agreement with direct numerical simulation data is found.

Machine-learning-based turbulence generators are developed to generate unsteady inflow conditions for turbulent flow simulations. The proposed method can provide inflow conditions with reasonable turbulence statistics without spurious periodicity and at a low computational cost.

Snapshot of vortex structure in a shear-thickening fluid in fully developed turbulence. Colors from blue to red go from low to high vorticities. Relative to Newtonian fluids, shear-thinning fluids adjust to have an augmented dissipation inside vortices, the opposite of shear-thickening fluids.

Lagrangian statistics and pair dispersion induced by an isolated pulse of a small jellyfish are quantified with 3D particle tracking velocimetry. Lagrangian velocity shows more intense mixing in the radial direction and reveals three stages dominated by flow acceleration, mixing, and dissipation.

In a Lagrangian point of view, anisotropy in small time scales of turbulent flow is not yet well understood. Using Particle Tracking Velocimetry over nearly a decade in Taylor-scale based Reynolds numbers, we theoretically predict anisotropic acceleration correlation times of tracer particles in turbulent flow.

Very-large-scale motions contribute significantly to the kinetic energy in turbulent pipe flow. An analysis of the scale energy budget reveals a similar production mechanism compared to the small scales, but the interscale energy exchange is found to be quite different.

The horizontal vibrations of an interface between miscible fluids trigger a parametric instability, leading to frozen wave patterns. Theory and numerical simulations reveal that the mixing layer size varies as the square of the forcing amplitude in the turbulent regime.

Experiments using a cyber-physical hydrofoil, tested over a wide range of stiffness and damping coefficients, reveal that the optimal response is achieved at the structural resonant frequency where the leading-edge vortex formation is synchronized with the foil pithing and heaving.

When a partially submerged vertical cylinder is moved at constant speed through water, the motion induces a turbulent wake, free-surface deformations up to the rupture, and air entrainment within the flow. Two modes of air entrainment are uncovered and the critical velocity is compared to a model.

Imagine a thin liquid film that must cover the substrate it spreads over before solidifying: tilting the substrate is useful to get a little push from gravity, but obtaining a uniform film thickness is challenging. Here, an adjoint method finds time-dependent motions that improve uniformity.

A pair of distributed vortices in stratified flow disintegrate sooner than a traditional vortex pair and form multiple coherent vortex structures.

In the low viscosity liquid and zero thickness vibrating elastic sheet, turbulent behavior is observed. We find the same Kolmogorov spectrum cascade for energy redistribution per scale $E_k\sim P^{2/3}{k}^{-5/3}$ in both, apparently, completely disparate physical situations.