Particle migration is the underlying mechanism for continuous-flow focusing, trapping, and sorting of various types of particles in microfluidic devices. We present in this work an experimental investigation of the cross-stream migration of spherical polystyrene particles in a combined pressure- and electric field-driven flow of viscoelastic fluid through a straight rectangular microchannel. We find that particles migrate toward the centerline of the channel if they are leading the hydrodynamic flow due to positive electrophoresis. Conversely, lagging particles due to negative electrophoresis migrate toward the walls of the channel. These migrations are found to be exactly opposite to those in a Newtonian fluid in our control experiment. We attribute this phenomenon to the shear-induced extra lift in a viscoelastic fluid that has been recently predicted to arise from the nonlinear coupling of the electrophoretic particle motion with the local flow field. The effects of hydrodynamic flow rate and electric field magnitude on the bidirectional particle migrations are studied, which can both be reasonably explained using the theoretical formula of the electrophoretic motion-induced extra lift in a viscoelastic shear flow.

The quasistatic growth of an air bubble nucleated on an inclined plate is investigated. Three substrates (Teflon, treated glass, and glass) are used to study the influence of the wetting conditions of the plate. The plate has been drilled to allow air injection and nucleation of the bubbles on the substrate. High-speed shadowgraphy visualizations in two perpendicular planes are used to record the evolution of the bubble shape during its growth. Experiments are conducted in quiescent water for different slopes of the plate. The bubble shape is extracted from image processing and several geometric parameters are determined. These geometric parameters are used to evaluate the forces acting on the bubble using a point-force approach. It appears that the classical expressions of the forces are valid for small slopes of the surface (less than ${15}^{\circ })$, while the force balance is no longer verified for higher slopes. This result is linked to an elongation of the bubble foot that has to be taken into account in the modeling of the capillary force. Thus an expression for the capillary force is proposed and the force balance is used to predict the bubble detachment diameter.

We present an extensive numerical and experimental study, investigating a three-dimensional (3D) granular flow of elongated particles down an inclined plane. Similarly to sheared systems, the average particle orientation is found to enclose a small angle with the flow direction. In the bulk, this behavior is independent of the shear rate. At the surface, however, the particles move in more dilute conditions, and the average orientation strongly depends on the shear rate. A systematic numerical study varying the particle aspect ratio and the plane inclination reveals that the particle size perpendicular to the flow direction, $d_{\mathrm{eff}}$, is an appropriate length scale to define an effective inertial number $I_{\mathrm{eff}}$, which fully captures the impact of the particle shape on the system's rheology. Like in the case of spheres, density and friction result in well-defined functions of the effective inertial number $I_{\mathrm{eff}}$. Thus, we quantify and explain the dependence of the rheological parameters on the aspect ratio, based on the micromechanical details.

In this study, the impact of a liquid drop onto a thin liquid film of a different fluid is investigated experimentally using a high-speed video system and analysed theoretically to obtain expressions for predicting splashing thresholds. The study focuses on impact conditions leading to one of four outcomes: deposition, corona without splash, corona splash, and partial rebound. In addition to the conventional influencing parameters, which are usually described by combinations of impact Weber and Reynolds numbers and dimensionless film thickness, also the ratio of the drop and film liquid viscosities have been systematically varied over a wide range. The results of the theoretical analysis, in good agreement with the experimental observations, show that the well-known $K$ number determines the splashing threshold only if the viscosity of the film is much larger or much smaller than the drop viscosity. If the drop and film viscosities are comparable, a critical modified $K$ number is introduced, which is a function of the viscosity ratio.

We study hydrodynamic fluctuations in a compressible and viscous fluid film confined between two rigid, no-slip, parallel plates, where one of the plates is kept fixed while the other one is driven in small-amplitude, translational displacements around its reference position. This jiggling motion is assumed to be driven by a stochastic, external, surface forcing of zero mean and finite variance. Thus, while the transverse (shear) and longitudinal (compressional) hydrodynamic stresses produced in the film vanish on average on either of the plates, these stresses exhibit fluctuations that can be quantified through their equal-time, two-point, correlation functions. For transverse stresses, we show that the correlation functions of the stresses acting on the same plate (self-correlators) as well as the correlation function of the stresses acting on different plates (cross correlators) exhibit universal, decaying, power-law behaviors as functions of the interplate separation. At small separations, the exponents are given by $-1$, while at large separations, the exponents are found as $-2$ (self-correlator on the fixed plate), $-4$ (excess self-correlator on the mobile plate), and $-3$ (cross correlator). For longitudinal stresses, we find much weaker power-law decays in the large separation regime, with exponents $-3/2$ (excess self-correlator on the mobile plate) and $-1$ (cross correlator). The self-correlator on the fixed plate increases and levels off upon increasing the interplate separation, reflecting the nondecaying nature of the longitudinal forces acting on the fixed plate.

Rogue waves are localized large-amplitude events in various nonlinear wave systems. For these uncertain rare events, whether precursors exist and how they can be identified are challenging open issues. In this work, these issues are experimentally addressed from the wave-particle interaction view, using the information of the surrounding waveform preceding the rogue wave in a Faraday wave system exhibiting disordered oscillon pattern. It is found that oscillon peak height is uncorrelated with the high-order orientational symmetries of its prior surrounding waveform. The preceding waveform with high angular average and the low standard deviation of the ridge height surrounding the center oscillon is the key precursor for the strong inward water focusing. It leads to the subsequent rogue wave (oscillon with extreme peak height) generation, after one external driving period ${\tau }_0$. The rapid increase of the angular average rescaled by the standard deviation of the surrounding ridge height also serves as a good earlier precursor several ${\tau }_0$ before rogue wave generation.

In this work we investigate buoyant particle dynamics in the ocean mixed layer (OML) under a purely convective regime. We focus on noninertial particles that are lighter than the surrounding seawater (thus, buoyant), which is a useful configuration when representing oil, microplastic debris, and other buoyant materials that do not necessarily exhibit strong inertial effects. Our main goal is to understand and describe the physical mechanisms that control the buoyant particles' surface concentration under such conditions, specifically the preferential concentration effects that arise independently of inertia (rather than the well-known centrifuging mechanism for heavy particles). In our investigation we use large-eddy simulation to model the particle dispersion in the OML in which the evolution of the particle field is simulated using an Eulerian approach. We find that in addition to the preferential concentration effect that clusters particles into convergence regions on the surface (which is a well-known and straightforward effect on free surfaces), there is a secondary effect for highly buoyant particles that drives them into vorticity-dominated regions. We explain this effect as the advection of buoyant particles by persistent vortices in the flow, which turns out to be the dominating mechanism controlling the surface particle distribution. Highly buoyant particles are trapped in the interior of the vortices (at the surface), which favors clustering in vorticity-dominated regions, while for particles with low buoyancy this effect is negligible.

The slip boundary conditions for the compressible Navier-Stokes equations for a polyatomic gas are derived from kinetic theory using the ellipsoidal statistical model of the Boltzmann equation for a polyatomic gas. The analysis, which follows the previous work by the present authors and others for a monatomic gas [Aoki et al., J. Stat. Phys. 169, 744 (2017)], is based on the Chapman-Enskog expansion and the analysis of the Knudsen layer adjacent to the boundary. The resulting slip boundary conditions are presented with explicit slip coefficients for some typical polyatomic gases.

Numerical simulations are performed to study the effect of particle size and density on dust dispersion behind a moving shock. The numerical model used in this paper takes into account multiple particle types with a binning approach, and each bin of particles has its own characteristic uniform particle size and density. The model solves one set of governing equations for the gas phase and $M$ sets of governing equations for the $M$ particle types. Equations for each bin of particles are coupled with the gas flow as well as all other particle types. The specific conditions simulate a Mach-1.4 shock passing over a dust layer containing two uniformly mixed particle types. The background gas condition is at 67 kPa and 295 K. The dust layer is 1.27 cm thick and consists of spherical particles with selected sizes and densities. Preliminary one-dimensional computations of a shock passing through a dilute particle curtain containing different particle types suggest that particles with different sizes or densities behave differently and can separate into different clouds. Particles with larger inertia require a longer relaxation time to the postshock condition. The two-dimensional calculations show that larger particles are lifted to a higher level than smaller particles. In regions near the shock front, larger particles experience a smaller drag force, pushing the particles into the dust layer, and a larger lifting force, pulling the particles into the air. In addition, lighter particles are lifted higher than heavier particles due to a smaller inertia. Particle size plays a significant role on dust dispersion, while particle density has only a minor effect.

We analyze the angular dynamics of triaxial ellipsoids in a shear flow subject to weak thermal noise. By numerically integrating an overdamped angular Langevin equation, we find the steady angular probability distribution for a range of triaxial particle shapes. From this distribution we compute the intrinsic viscosity of a dilute suspension of triaxial particles. We determine how the viscosity depends on particle shape in the limit of weak thermal noise. While the deterministic angular dynamics depends very sensitively on particle shape, we find that the shape dependence of the intrinsic viscosity is weaker, in general, and that suspensions of rodlike particles are the most sensitive to breaking of axisymmetry. The intrinsic viscosity of a dilute suspension of triaxial particles is smaller than that of a suspension of axisymmetric particles with the same volume and the same ratio of major to minor axis lengths.

Active matter systems display a fascinating range of dynamical states, including stationary patterns and turbulent phases. While the former can be tackled with methods from the field of pattern formation, the spatiotemporal disorder of the active turbulence phase calls for a statistical description. Borrowing techniques from turbulence theory, we here establish a quantitative description of correlation functions and spectra of a minimal continuum model for active turbulence. Further exploring the parameter space, we also report on a surprising type of turbulence-driven pattern formation far beyond linear onset: the emergence of a dynamic hexagonal vortex lattice state after an extended turbulent transient, which can only be explained taking into account turbulent energy transfer across scales.

The flow of an electrically conducting fluid driven by a traveling magnetic field imposed at the end caps of a cylindrical annulus is numerically studied. At sufficiently large magnetic Reynolds number, the system undergoes a transition from synchronism with the traveling field to a stalled flow, similar to the one observed in electromagnetic pumps. An unusual type of boundary layer is identified for such electromagnetically driven flows that can be understood as a combination of Hartmann and Shercliff layers generated by the spatiotemporal variations of the magnetic field imposed at the boundaries. An energy budget calculation shows that energy dissipation mostly occurs within these boundary layers and we observe that the ohmic dissipation $D_{\eta }$ always overcomes the viscous dissipation $D_{\nu }$, suggesting the existence of an upper bound for the efficiency of electromagnetic pumps. Finally, we show that the destabilization of the flow occurs when both dissipations are nearly equal, $D_{\nu }\sim D_{\eta }$.

Flow visualization using particle image velocimetry (PIV) and particularly particle tracking velocimetry (PTV) has been applied to thermal counterflow in He II for nearly two decades now, but the results remain difficult to interpret because tracer particle motion can be influenced by both the normal fluid and superfluid components of He II as well as the quantized vortex tangle. For instance, in one early experiment it was observed (using PTV) that tracer particles move at the normal fluid velocity $v_n$, while in another it was observed (using PIV) that particles move at $v_n/2$. Besides the different visualization methods, the range of applied heat flux investigated by these experiments differed by an order of magnitude. To resolve this apparent discrepancy and explore the statistics of particle motion in thermal counterflow, we apply the PTV method to a wide range of heat flux at a number of different fluid temperatures. In our analysis, we introduce a scheme for analyzing the velocity of particles presumably moving with the normal fluid separately from those presumably influenced by the quantized vortex tangle. Our results show that for lower heat flux there are two distinct peaks in the streamwise particle velocity probability density function (PDF), with one centered at the normal fluid velocity $v_n$ (named G2 for convenience) while the other is centered near $v_n/2$ (G1). For higher heat flux there is a single peak centered near $v_n/2$ (G3). Using our separation scheme, we show quantitatively that there is no size difference between the particles contributing to G1 and G2. We also show that nonclassical features of the transverse particle velocity PDF arise entirely from G1, while the corresponding PDF for G2 exhibits the classical Gaussian form. The G2 transverse velocity fluctuation, backed up by second sound attenuation in decaying counterflow, suggests that large-scale turbulence in the normal fluid is absent from the two-peak region. We offer a brief discussion of the physical mechanisms that may be responsible for our observations, revealing that G1 velocity fluctuations may be linked to fluctuations of quantized vortex line velocity, and suggest a number of numerical simulations that may reveal the underlying physics in detail.

The dynamics of molecularly thin graphene sheets in transient flows are important to understand their behavior in suspension and during processing. Scattering dichroism is used to evaluate changes in the orientation distribution function in dilute suspensions. To evaluate if the graphene sheets behave as flexible sheets or as more rigid flat particles, the results are compared with numerical computations of the single-particle Smoluchowski equation for flat spheroidal particles. In particular, the evolution of the orientation angle in oscillatory flows as a function of increasing amplitude is studied. The results show that even when taking an average rotational diffusivity, the results for the polydisperse graphene sheets show all the hallmarks of the motion of rigid disks, including a frequency doubling of the time-dependent orientation signal. The results indicate that the motion of functionalized graphene sheets in suspension is consistent with flat rigid objects.

This paper considers the linear stability of a liquid metal flow driven by superimposed alternating and static axial magnetic fields in the floating zone configuration. A simple model is constructed that assumes a slight axial variation of the flow-driving radial magnetic force and a low-to-moderate skin effect. This force drives two symmetrical flow tori. Formation of the field-parallel layer is observed for a strong static field. In this regime the instability sets in as a standing azimuthal wave around the circumference of the cylindrical melt volume near its midplane. The length of this wave scales with the thickness of the parallel layer. The instability criterion may be formulated in terms of an interaction parameter reaching its critical value at around 525.

Artificial microswimmers are a new technology with promising microfluidics and biomedical applications, such as directed cargo transport, microscale assembly, and targeted drug delivery. A fundamental barrier to realizing this potential is the ability to independently control the trajectories of multiple individuals within a large group. A promising navigation mechanism for “fuel-based” microswimmers – for example, autophoretic Janus particles – entails modulating the local environment to guide the swimmer, for instance by etching grooves in microchannels. However, such techniques are currently limited to bulk guidance. This Rapid Communication will argue that by manufacturing microswimmers from phoretic filaments of flexible shape-memory polymer, elastic transformations can modulate swimming behavior, allowing precision navigation of selected individuals within a group through complex environments.

This paper presents a global hydrodynamic stability analysis of flow fields in a backward-facing step combustor, assuming weakly nonparallel flow. The baseline experiments in a “long” combustor of length of 5.0 m shows the presence of two combustion instability states characterized by coherent low- and high-amplitude acoustic pressure oscillations. The analysis is performed for propane-air mixtures at three values of $\varphi =0.63$, 0.72, and 0.85, which correspond to quiet, low-amplitude and high-amplitude instability states in the long combustor experiments. Base flow velocity and density fields for the hydrodynamic stability analysis are determined from time-averaged particle image velocimetry measurements made after the length of the duct downstream of the step has been shortened to eliminate acoustic pressure oscillations. The analysis shows that the shear layer mode is self-excited for the $\varphi =0.72$ case with an oscillation frequency close to that of the long combustor's fundamental acoustic mode. We show from an analysis of the weakly forced, variable density Navier-Stokes equations that self-excited hydrodynamic modes can be weakly receptive to forcing—suggesting that the low-amplitude instability in the long combustor is due to semi-open loop forcing of heat-release oscillations by the shear layer mode. At $\varphi =0.85$, the analysis shows that the flow is hydrodynamically globally stable but locally convectively unstable. Spatial amplification of velocity disturbances by the convectively unstable flow causes high-amplitude combustion instability in the long combustor case. These results show that combustion instability can be sustained by two different mechanisms by which acoustic and hydrodynamic modes being either strongly coupled result in fully closed loop forcing, or weakly coupled result in semi-open loop forcing of the flame by a self-excited hydrodynamic mode.

As computing power has grown and turbulent flows at increasing Reynolds numbers are being computed by direct numerical simulations, conventional assumptions on adequate spatial and temporal resolutions are being continually challenged. We perform a systematic study of the resolution effects via numerical simulations at various spatial and temporal resolutions to clarify the proper scaling of dissipation and enstrophy (vorticity squared). Results show that inadequate resolution in space and/or time leads to overestimation of the likelihood and intensity of extreme fluctuations in dissipation and enstrophy. In order to capture rare events accurately, for instance, one needs to have not only grid resolutions that are increasingly smaller fractions of the Kolmogorov scale as the Reynolds number increases but also the Courant number that becomes increasingly smaller than that assumed to be adequate previously. Some comparisons are made with results obtained from an alternative approach where a stricter criterion for truncation in wave-number space allows aliasing errors to be removed completely. In contrast to prior work, the present data do not support the notion that dissipation and enstrophy probability density functions (PDFs) approach each other in the far tails at high Reynolds number. However the two PDFs are remarkably similar in form, being well described by stretched exponentials.

Using a dynamic surface force apparatus (SFA), we demonstrate that the notion of slip length used to describe the boundary flow of simple liquids is not appropriate for viscoelastic liquids. Rather, the appropriate description lies in the original Navier's partial slip boundary condition, formulated in terms of an interfacial friction coefficient. We establish an exact analytical expression to extract the interfacial friction coefficient from oscillatory drainage forces between a sphere and a plane, suitable for dynamic SFA or atomic force microscopy noncontact measurements. We use this model to investigate the boundary friction of viscoelastic polymer solutions over 5 decades of film thicknesses and 1 decade in frequency. The proper use of the original Navier's condition describes accurately the complex hydrodynamic force up to scales of tens of micrometers, with a simple Newtonian-like friction coefficient that is not frequency dependent and does reflect closely the dynamics of an interfacial depletion layer at the solution-solid interface.

The settling of colloidal particles with short-ranged attractions is investigated via highly resolved immersed boundary simulations. At modest volume fractions, we show that intercolloid attractions lead to clustering that reduces the hinderance to settling imposed by fluid back flow. For sufficient attraction strength, increasing the particle concentration grows the particle clusters, which further increases the mean settling rate in a physical mode termed promoted settling. The immersed boundary simulations are compared to recent experimental measurements of the settling rate in nanoparticle dispersions for which particles are driven to aggregate by short-ranged depletion attractions. The simulations are able to quantitatively reproduce the experimental results. We show that a simple, empirical model for the settling rate of adhesive hard-sphere dispersions can be derived from a combination of the experimental and computational data as well as analytical results valid in certain asymptotic limits of the concentration and attraction strength. This model naturally extends the Richardson-Zaki formalism used to describe hindered settling of hard, repulsive spheres. Experimental measurements of the collective diffusion coefficient in concentrated solutions of globular proteins are used to illustrate inference of effective interaction parameters for sticky, globular macromolecules using this empirical model. Finally, application of the simulation methods and empirical model to other colloidal systems are discussed.