Gradients in the solid-like surface tension of model tissues, such as cell aggregates result in fast, internal cellular motions. The image shows the velocity (arrows) and vorticity (color) of cellular motions.

We investigate the rise dynamics of a volatile liquid within an open vertical channel, which requires us to consider the effects of capillarity, gravity, viscosity, and evaporation together. Upon quantifying the enhanced evaporation near the solid-liquid-gas contact line and setting up a theoretical model for the rise height, we show how to find an optimal channel dimension that maximizes the equilibrium rise height. This work can lay a foundation for understanding the evaporative capillary rise dynamics occurring in porous media.

A novel deformation-dependent propulsion phenomenon for (blood) cells and soft capsules in oscillating microflows is presented. It enables the separation of cells with different deformabilities, such as healthy and malignant cells, without their labeling or obstacles in a microfluidic device. The propulsion phenomeon is based on a broken time reversal symmetry, which is achieved by a fast forward and slow backward movement of a fluid through microchannels that does not progress on average.

Dense bacterial suspensions display spatiotemporally chaotic flows, termed active turbulence. While tracer trajectories meander diffusively in these flows, we find that highly active suspensions allow for a fundamentally different, persistent motion, aided by emergent streaks in the flow field. This dynamical heterogeneity manifests in faster first-passage times and alters pair-dispersion statistics.

It is hypothesized that soft corals in the family Xeniidae pulse to enhance the photosynthesis of their symbiotic algae. This work seeks to understand this using numerical simulations to quantify the mixing in the fluid flow and directly model the effect of the flow on the photosynthesis. A mathematical model and numerical method are presented in which a chemical concentration is produced on a moving deforming boundary which models the algae photosynthesis on the pulsing corals. Additionally, Poincaré maps are used, taking advantage of the periodicity of the flow to quantify mixing in the fluid. Our results indicate that these corals operate in a parameter regime which optimizes mixing.

Heat release through chemical reactions in turbulence can affect small-scale flow motions, and change the statistics of the enstrophy production as well as thermochemical transport. By using numerical simulations of chemically reacting compressible isotropic turbulence, we found that the strong compression motions cause the destruction of enstrophy by the interaction between the vorticity and strain rate tensor, while strong expansion motions significantly enhance the generation of enstrophy.

Inspired by a rain droplet on a windshield, we theoretically study the dynamics of a partially wetting droplet subject to wind and gravity. Our reduced mathematical model couples the pressure drag induced by the flow separation over the drop and the gravitational force to characterize the drop’s depinning conditions. The results of our lubrication model reveal the limits of up-slope depinning, pinning, and down-slope depinning for varying drop sizes and wind speeds. Our model can be used to identify optimum windshield angles for drop depinning for different ranges of flow speeds.

We derive a new model entirely from first principles to explain the Leidenfrost self-propulsion phenomenon in a quantitative way, where the deformable nature of the liquid has been taken into account for the first time. Our annular ring design enables liquid droplets to reach high terminal velocities, up to 0.42±0.04 m/s, which is potentially beneficial to energy harvesting and flow chemistry applications.

Bubbles encapsulated by an organic bulk phase are widely present in the ocean and many industrial processes. We present experiments on a rising oil-coated compound bubble in quiescent water. By immobilizing the compound bubble surface and increasing the compound bubble effective density, the oil coating causes a smaller bubble shape deformation, changing the wake dynamics and forces acting on the bubble, and thereby its drag coefficient and path oscillation. The compound bubble zigzag oscillation is observed to decay significantly with time above a critical oil volume fraction. These results may provide guidance for simulations and applications of bubbly flows with compound interfaces.

Water harvesting is a critical and very urgent problem for humanity. Inspired by the stenocara beetle, we demonstrate from first principles how heterogeneous wettability surfaces can optimize water collection by varying the area of hydrophilic patches on top of a hydrophobic surface. These theoretical considerations allow determination of the optimal combination of wettability patches for water collection. We have validated these developments with several experiments involving different patch sizes and support inclinations. This new method can be developed for drop storage in biotechnology, for instance, and drop transport applications such as water-harvesting systems.

By employing conformal mapping in conjunction with an experimental approach we investigate electrokinetics in two-dimensional complicated geometries, such as an eccentric annulus, concentric ellipse, and corner geometry. Conformal mapping, a powerful theoretical tool, potentially inspires future work on electrokinetics in complicated geometries, while shedding light on promising applications, including shock electrodialysis, water treatment, and electrophoresis for particle manipulation in microfluidic devices.

An annular dielectric barrier discharge (DBD) plasma actuator has potential flow control applications due to having a simple design, no moving parts, and being light weight and fully electronic with low power consumption. We have experimentally investigated the detailed near-wall flow characteristics generated by the annular DBD plasma actuator. The actuator generates a wall-normal jet on top of a recirculation zone, with characteristics dependent on the actuation parameter i.e., burst frequency and amplitude. Our results can provide guidance for design and process optimization of several flow control applications i.e. lift enhancement, drag reduction, and mixing enhancement etc.

Stratified flows can develop powerful vertical drafts - resulting from a resonant interaction of gravity waves and turbulent motions - when the system explores a certain region of the parameter space which is of geophysical interest. These extreme drafts (detected through the kurtosis of the vertical velocity, $K_w$) do generate turbulence, leading to enhanced local-in-time-and-space kinetic (${\hat{\epsilon }}_V$) and potential energy dissipation. They are needed in order for stratified flows to be as efficient as homogenous isotropic turbulent flow in dissipating the energy.

In the context of CO${}_2$ geological storage in deep saline aquifers, convective dissolution in porous media has been the subject of theory and numerical studies at the Darcy scale. By using both refractive index matching and planar laser induced fluorescence, we measure the onset characteristics (growth rate, wavelength, and flux) of the convective dissolution instability in a three-dimensional granular porous medium located below a gas compartment. The obtained results show large discrepancies with theoretical predictions based on Darcy scale models but are demonstrated to be consistent with a forcing of convection by porosity fluctuations, recently described in a theoretical model.

All objects moving through the atmosphere or the ocean leave behind a fluid dynamic footprint known as a wake. The patterns in this wake are often long-lasting and, moreover, they can contain information about the object that created the wake, e.g., size and speed. In this study, we develop a framework that can identify such information (the “regime”) based on limited measurements in the wake. Tests using data from simulations as well as laboratory experiments show that this approach holds promise in the development of automated data-driven fluid pattern classifiers.

Flow instability of hypersonic and high-enthalpy boundary layers with thermal-chemical nonequilibrium (TCNE) effects is studied for Mach numbers of up to 15. It is quantitatively illustrated that the TCNE effects change the disturbance characteristics predominantly through mean flow modification. In the oblique-mode breakdown case, the intensive energy transfer between the selected modes and their harmonic waves is found to occur where they interact strongly with the mean flow. (L${}_{\mathrm{\Gamma }}$ and N${}_{\mathrm{\Gamma }}$ in the figure are separately the energy transfer through the linear and nonlinear terms in the disturbance energy-norm equation.)

Ultrasonic atomization is increasingly used in industry, and the theory of Faraday instability in this process is investigated and compared with experiments and multiple Faraday wave simulations. For the viscosity-free case, the parametric curve equations explain the excitation and competition mechanisms of the different frequency harmonics and determine the strength of the Faraday instability. In the viscous case, the Faraday instability depends on the wavenumber and viscosity, and the strength of the instability can be obtained from the instability threshold. This study can provide scientific guidance for the process optimization of practical ultrasonic atomization production.

In this work we perform real-time control of forced jets based on a wave-cancellation scheme. Building on previous work on control of stochastic disturbances, here we carefully characterize the performance and limits of this control approach with the onset of nonlinearities. The breakdown of linearity and its impact on control performance are illustrated through a thorough comparison between jets with laminar and turbulent boundary layers.

We experimentally and numerically study the stability of a parallel two-phase flow in a finite length channel. Although the channel is symmetric and the fluids are flowed at the same flow rate, the interface between the fluids becomes unstable locally near the exit junction and prevents complete separation of the two fluids. The instability leads to shedding of droplets of one phase into the other with a period that is inversely proportional to the flow rate. We show how even a small difference in viscosity between the two fluids and the details of the exit geometry can affect this instability.

Once a transparent liquid jet is ablated by an intense nanosecond laser pulse, the blast breaks the liquid column into two sections with a logarithmically growing gap that originates from the blast center. We found that the jet acts like an optical fiber which transmits a few percent of the pulsed optical energy upstream, leading to the material removal of the nozzle orifice. The accompanied thermal explosion forms a sunflower patterned liquid sheet near the nozzle orifice. The present results may provide insights for long range optical energy transmission and liquid-jet guided micromachining applications.

Droplet oscillations of low viscosity liquids spreading over textured surfaces with hysteresis arise from pinning of the three-phase contact line. An improved dynamic contact angle model is proposed here to model pinning, that yields contact angle oscillations in numerical simulation, in agreement with experiments.

Phoretic swimmers reveal complex dynamics ranging from straight trajectories, to stagnation, to chaotic movements mimicking the run-tumble behavior known for living cells, such as bacteria.

Our theoretical study reveals that air pockets inside the grooves of superhydrophobic surfaces may either stabilize the flow of liquid films or lead to film rupture depending on the wetting and geometrical characteristics of the substrate micro-texture.

The hydrodynamics of cable-like bodies play an important role in many biological and mechanical systems. These flows can be accurately modeled using slender-body theory when the body is isolated, thin, and not too coiled, but can be difficult to model outside these limits. In this paper we develop tubular-body theory; a slender-body theory-like method that allows the flow around such bodies to be determined exactly.

The aerodynamic performance of an SD7003 airfoil with active control of the rotating rod through momentum injection into the boundary layer of the suction side is numerically explored at a low Reynolds number of 5000. Striking aerodynamic improvement is observed for a proper parameter combination of rod location and rotational speed. Pressure drop and recovery at the upstream and downstream sides of the rotating rod, respectively, is found to be the primary mechanism for the lift enhancement and drag reduction.

Lubrication flow of a gas in a microscale gap between coaxial circular cylinders with a nonuniform accommodation coefficient is studied on the basis of kinetic theory. The Boltzmann equation is studied analytically using the slowly varying approximation when the dimensionless curvature, namely the gap size divided by the radius of the inner cylinder, is small. It is demonstrated that a nonnegligible effect of small curvature affects the lubrication performance when the Knudsen number is large.

The motion of particles in a channel containing solute is, in part, a result of diffusiophoresis. Previous studies of diffusiophoresis in narrow channels have focused on quasi-one-dimensional dynamics. We consider the two-dimensional dynamics in an early-time regime. In this regime, spanwise variations in solute concentration have not yet diminished, and cross-channel particle migration may be significant.

The impact of nanodroplets has received increasing attention due to their wide applications in nanotechnologies. A phase diagram containing four impact regimes is constructed for the impact of nanodroplets on nanopillared surfaces. Several significant differences in the impact regimes are distinguished between the nanoscale and macroscale. The wetting transition at the nanoscale does not follow the macroscale mechanisms and, hence, a new model is proposed to understand the wetting transition mechanism at the nanoscale.

The interaction between a liquid droplet and a hot granular material is explored in this work. Surprisingly, we found that the droplet deeply penetrated into the hot granular material when the temperature exceeds the boiling temperature of the liquid. The digging speed of the drop decreases with the temperature. A mechanism based on the Leidenfrost effect is proposed considering that the granular material can be modeled as a rough surface that can be eroded when the vapor speed is sufficient.

Using two-dimensional numerical simulations, we study the set of mechanical actions (drag, lift and torque) exerted by a granular medium on a cross-shaped object buried within it. The object has the ability to translate horizontally and rotate at independent imposed velocities. We show that the simulation results are consistent with the Granular Resistive Force Theory often applied in granular locomotion problems.

Settling dynamics of semiflexible Brownian chains illustrate a broad range of configurations depending on the relative ratios of viscous, elastic, and gravitational forces.

Using experiments and numerical simulations, we study the clogging transition of a single pore by colloidal particles. We are able to link the clogging dynamics to the flow decline within the pores formed by the particle accumulation inside the clog. We show that most of the flow decline is due to the reduction of the cross section of the pore as it gets fouled and also to the modification of the flow paths when only few layers of particle have accumulated at the rear of the clog.

To date, it is not known if smooth initial conditions of the Euler equations with finite energy do or do not blow up in finite time. It is shown that under the assumption of spatial anisotropy, an axisymmetric incompressible and inviscid flow potentially presents a finite-time singularity. The singular flow consists of a quadrupolar structure for the vorticity together with a tangential discontinuity of the swirl velocity. On the discontinuity plane, the velocity field becomes a multivalued singularity. This singularity appears to be generic and robust for a wide number of finite energy initial conditions.

A mesoscale ocean eddy can live for years as it propagates across ocean basins, but the water contained within its core may remain for only a fraction of that time. We argue that ocean eddies exhibit multiple timescales: A persistence time associated with the lifetime of the eddy and (multiple) material coherence timescales that measure how leaky the eddy is. Here we study an ocean eddy in the South Atlantic using virtual ocean drifters. Our results suggest that we should revise how we think about ocean eddies to include not only the long-lived coherent eddy core but also the quasi-coherent outer ring, where we speculate that the outer ring is responsible for most of the eddy transport.

In this work, we assess the contributions of large-scale and small-scale Reynolds shear stress events to the skin friction reduction of a turbulent boundary layer modified by a large-eddy break-up device, based on the quadrant analysis of Reynolds shear stress, the Fourier mode decomposition, and an extension of a skin friction decomposition scheme.

It is shown that in driven active matter there is a new universality class for turbulence when the activity coefficient is large. The crossover between standard Kolmogorov turbulence and this new universality class is discussed in detail.

The two-dimensional noise-assisted multivariate empirical mode decomposition (2D NA-MEMD) simultaneously decomposes multiple spatial velocity fields into physically meaningful modes, which are sorted by the inherent scale size and are continuous in time. Spatial features shared by different velocity components are easily detectable by the 2D NA-MEMD, which is beneficial for the inner-outer interaction analysis. The advantage of this approach is demonstrated empirically based on turbulent channel flow data targeting the influence of outer-layer large-scale structures on the near-wall turbulent dynamics.

Using the shell model of turbulence, we show that intermittency with power-law scaling of structure functions can be derived as a consequence of a new (hidden) scaling symmetry of equations of motion. In this description, anomalous exponents are given by Perron-Frobenius eigenvalues of linear operators based on the self-similar statistics.

Analytical wind turbine wake models are prone to failure when the turbine induction factor increases, as they do not account for the increasingly important effects of low wake pressure. This work proposes a novel analytical wake model which incorporates the effect of wake pressure in its predictions. Comparison with high-fidelity wind turbine simulations shows that the newly developed model is comparable to conventional ones at low induction factors, but continues to be accurate at higher induction factors where existing models break down.

Bundles of quantized vorticity are believed to be the building block of the quasiclassical regime of quantum turbulence. We show that these bundles approach and separate from one another obeying a fundamental scaling law which has been observed in reconnections of single quantized vortices and classical vortex tubes.

The interaction of an array of finite span synthetic jet actuators with a massively separated flow was explored experimentally over a cantilevered, swept, and tapered model having a deflected control surface. The interaction of one synthetic jet with the local flow led to a flow field associated with formation and advection of vortex rings that bent into the flow, resulting in flow reattachment in their trajectory. Under specific conditions, when multiple jets were actuated, their interaction was either constructive or distractive, depending on the separation severity and spanwise flow magnitude. This work improves control of three-dimensional flow separation using distributed actuators.

We show helicity segregation due to Ekman pumping in rotating convective flows with gravity orthogonal to rotation. We observe cyclonic flows in the bulk and helical flow driven by Ekman pumping near the boundaries normal to the rotation axis. This leads to helicity being negative (positive) near the wall with outward normal parallel (anti-parallel) to the rotation vector.