Physical Review Fluids

The interaction between particles and turbulence poses significant challenges for understanding and modeling practically relevant flows, for example volcanic ash clouds or microplastics transport in the oceans. The interaction depends on many parameters, such as particle size distribution relative to turbulence scale, density, volume fraction, turbulence intensity and particle shape. The role of shape anisotropy and turbulence modulation are particularly important for practical particle-laden flows, where the particles are non-spherical and the flows usually occur at large Reynolds numbers.

Now Cannon, Olivieri and Rosti have performed new two-way coupled simulations of particle laden flows that compare (isotropic) spherical particles with (anisotropic) fibers over a large range of turbulence intensities to study the turbulence modulation as Reynolds number becomes large. While particles reduce turbulence intensity relative to the single-phase case at all Reynolds numbers, the fibers produce a more significant reduction effect than the spheres. This effect is analyzed scale by scale and discussed in relation to anomalous dissipation, intermittency and changes in the local vorticity and flow structures. The authors propose a lower Re number bound for the emergence of modulation and discuss the returning to single-phase behavior as the Reynolds number becomes large. This work provides a framework for modeling complex, real-world flows such as volcanic ash clouds and oceanic microplastic transport.

When gas is injected into a viscous liquid the gas-liquid interface is hydrodynamically unstable, leading to the formation of long slender protrusions of the gas into the liquid, known as viscous fingering. The instability occurs in geological processes, such as soil drainage or during CO2 sequestration, as well as in other natural and industrial processes. The formation of viscous fingers has been studied for decades, with the capillary number of the interface identified as the key parameter governing the instability, assuming that the liquid and gas are both incompressible.

Now Cuttle, Morrow, and MacMinn have used laboratory experiments, theory, and numerical simulations to examine the effect of gas compressibility on viscous fingering. Using a syringe to inject air into silicone oil confined between two circular glass plates, the researchers varied the air compressibility, parametrized by a dimensionless compressibility number, by using syringes of different sizes. Conducting experiments and simulations over a wide range of capillary and compressibility numbers demonstrated that increasing the compressibility number systematically delays the onset of fingering at high capillary number. The results show that compressibility controls the time-dependent injection rate and can be used as a control parameter for viscous fingering.

The distribution of bubble sizes in a turbulent fluid such as a breaking wave has a critical scale below which the number of bubbles of a given size decreases more gently. Understanding this distribution and the physics of bubble fragmentation which feeds into it are important for quantifying the exchange of mass in both natural and industrial processes. For example, up to 40% of the CO2 transfer from the atmosphere to the ocean is driven by bubble fragmentation.

Rivière, Ruth, Mostert, Deike, and Perrard provide a theoretical explanation of the size distribution below the critical scale as due to capillary effects which set the small bubble production rate. They validate the theoretical predictions with experiments and numerical simulations. A physical explanation for the full bubble size distribution when large air cavities break apart in turbulent flow now includes classic inertial turbulence breakup above the critical scale and capillary driven fragmentation below it.

Chemically active droplets are remarkably simple synthetic microswimmers that, as their name suggests, obtain the energy needed for motion from either chemical energy (stored inside) or from reactions between chemicals on the surface and the surrounding medium. Studies of these droplets, which are a class of active matter, show that they can swim close and parallel to a rigid boundary, but that phenomenon has not been well-studied to date.

Desai and Michelin investigated the rules that govern the emergence of motion in active droplets close to a wall. The model droplet constantly emits a reactive solute and is heavier than the surrounding fluid, and the researchers describe the effects of changing convection and diffusion rates for the system when the system was perturbed. They note that when active droplets are closer to the wall, the growth rate of the perturbations increases, which leads to more motion. In other words, surprisingly, proximity to the wall promotes the swimming of the active droplet.

When a volatile liquid falls on a hot surface that’s well above the liquid’s boiling point, it can levitate on a vapor film that forms through the quick evaporation at the bottom of the droplet. Scientists have been captivated by this phenomenon, called Leidenfrost drops, for more than 260 years. Recent experiments have shown that drops at rest begin dancing as soon as they’re deposited, self-rotating and moving in the absence of any external force.

Brandão and Schnitzer used a two-dimensional model of these Leidenfrost wheels to investigate the theory behind these translational and rotational dynamics. Their new model brings together equations that describe the drop’s motion, the flow of the vapor, and the deformations of the interface between vapor and liquid. Previous analytical models have predicted only symmetric outcomes, but the new work predicts that above a critical radius threshold, symmetry breaks and the system resolves into steady states of moving or rolling—in agreement with the experimental observations. The model also predicts that the initial acceleration of the droplet is proportional to an angle found in the interface between the liquid and vapor.

Insects, arachnids, and other critters scramble effortlessly up walls and across ceilings, held fast by adhesion forces that scientists don’t completely understand. A deeper understanding of the phenomenon could promote the development of new, bio-inspired adhesives.

Many investigations have modeled this adhesion using two rigid surfaces separated by a drop of liquid. Here, Butler, Box, Robert, and Vella offer a fresh take. Inspired by the observation that many insects have deformable footpads, the researchers investigate how deformability affects adhesion. In theory and in experiments, they demonstrate that flexibility offers significant advantages: Their model reveals that when one surface is sufficiently deformed to touch the other, it can produce an adhesion force that holds the system together orders of magnitude greater than the force between two rigid surfaces.

Viscous fluids pack a mighty punch: They drive natural phenomena like intrusions of magma in the Earth’s crust and the spreading of cracks at the base of glaciers, as well as industrial processes like fracking, or hydraulic fracturing of shale. Previous research has examined how fluids deform their vessels—like blood reshaping capillaries as it flows—as well as how adhesion affects elasticity—as in stretchable electronic devices worn on the skin.

Ball and Neufeld bring those two areas together to investigate the effect of flow on deformation by injecting glycerol beneath a plastic sheet adhered to a substrate. The paper describes experimental and theoretical evidence for two regimes of spreading, with one dominated by adhesion and the other by viscosity.

Vortices that form, separate, and reconnect in fluids—like tornadoes (in the air) or whirlpools in water—have long fascinated physicists. These phenomena have been studied in ordinary fluids as well as in superfluids, which flow with zero viscosity, and numerical simulations of reconnecting vortices in superfluid Helium date back at least to the early 1990s.

Here, Villois, Proment, and Krstulovic investigate universal aspects of the process of reconnection vortices in superfluids by studying a range of initial configurations and using a tracking algorithm to reconstruct the vortex filaments. During reconnection, they observed that vortex lines always approach and separate according to the same time scaling law; in addition, the vortex curvature exhibits self-similar behavior near the reconnection point.

Colloidal dispersions are often applied as liquid and left to dry; they’re particularly useful in applications like coating other surfaces. But understanding the physics of that drying process is tricky: It combines aspects of colloidal physics with transport phenomena like convection and diffusion.

Although studies of colloidal drying have arrived at a common description, here, Loussert, Bouchaudy, and Salmon take a closer look at how electrostatic interactions influence the process for highly charged colloids. The researchers sandwiched a drop of gel between two circular wafers—protected from direct contact by a PDMS layer—and used Raman microspectroscopy to measure the spatial distribution of colloid concentrations during the drying of the drop. They found a diffusion coefficient an order of magnitude higher than the Stokes-Einstein estimate. Fluorescence imaging of tracers embedded in the gel revealed two distinct drying regimes.