The mechanism underlying the stability of bulk nanobubbles has been a long-standing and contentious topic, with recent studies emphasizing the role of external pressure in nanobubble dynamics. However, there remains a lack of experimental evidence to unravel the complexities of bulk nanobubble behavior under elevated external pressures. In this study, we investigated the effect of external pressure on the stability of bulk hydrogen nanobubbles in both water and methanol liquid media using a nanobubble experimental system, with the external pressure up to 0.9 MPa. Our results show that nanobubble concentration increases linearly with pressure, and the nanobubbles exhibit longer lifetimes under high-pressure storage conditions, indicating enhanced stability. Notably, the effect of external pressure on surface charge is negligible, ruling out the hypothesis that surface charge dominates nanobubble stability. All-atom molecular dynamics (MD) simulations were conducted to elucidate the nanobubble evolution under varying external pressures. It demonstrates that elevated external pressure does not diminish the internal-external pressure differential. Increased supersaturation, reduced gas molecular diffusion, and reinforced hydrogen bond network were identified as primary factors for the stability enhancement at high external pressures. This study provides insights and experimental findings regarding the impact of external pressure on the dynamic behaviors of bulk nanobubbles.

A capillary jet plunging into a quasi-2D slab of monodisperse foam of the same solution is studied experimentally. We show that the jet can have a drastic impact on the foam. At small speeds it inflates the channels separating the bubbles. At intermediate speeds, impact with channels creates very small bubbles. At higher speeds the jet action reverses: it now thins the liquid film up to the point of rupture, which can induce bubble coalescence. We study the mechanisms behind these processes using an elementary foam made of three soap films in contact through a Plateau border. We obtain scaling laws ruling the jet-foam interaction which transfer quantitatively from the elementary foam to the 2D foam.

Opposition control (OC), which suppresses drag-inducing motions near the wall by local blowing and suction, has been shown to be effective in reducing drag in wall-bounded turbulence at low Reynolds numbers ($\text{Re}$). However, its performance at high $\text{Re}$ remains an open question. Here we investigate OC at high $\text{Re}$ using direct numerical simulations of turbulent channel flow at friction Reynolds numbers (${\text{Re}}_{\tau }$) ranging from 200 to 2000, delineating its limitations and identifying the physical mechanisms responsible. The principal finding is that drag reduction ($\mathcal{DR}$) decreases with increasing ${\text{Re}}_{\tau }$, consistent with similar low $\text{Re}$ results reported in the literature. For the detection plane at $y_d^{+}=15, \mathcal{DR}$ decreases from $22.3\%$ at ${\text{Re}}_{\tau }=200$ to $18.1\%$ at ${\text{Re}}_{\tau }=2000$. In addition, the optimal $y_d^{+}$ moves slightly inward as ${\text{Re}}_{\tau }$ increases, in line with the inward shift of the near-wall peak of the streamwise vorticity fluctuation. Using previously proposed models, we predict that an approximately $15\%$ drag reduction can be obtained at high Reynolds numbers, say, ${\text{Re}}_{\tau }\ge {10}^4$. Interestingly, the root-mean-square wall-normal velocity fluctuation at the wall ($v_{\mathrm{rms},w}^{\prime +}$) to achieve the maximum $\mathcal{DR}$ is around 0.25 for all ${\text{Re}}_{\tau }$ cases. Further statistical analysis reveals that the decrease in $\mathcal{DR}$ at high ${\text{Re}}_{\tau }$ is due to a reduced ability to suppress large-scale turbulence near the wall, whose contribution continuously increases due to the enhanced modulation and penetration effect of the outer large-scale and very large-scale motions. Coherent structure analysis confirms the inability of OC to control outer large-scale structures and shows the strengthening of near-wall streamwise vortices with increasing ${\text{Re}}_{\tau }$. These findings suggest that future efforts should explore advanced approaches such as hybrid control strategies incorporating phase-lag formulations or complementary actuation targeting outer-layer motions, to extend the effectiveness of OC at high $\text{Re}$.

In fluid dynamics experiments, flake-based flow visualization captures flow structures through light reflection from suspended flat tracers. However, the correspondence between visualization patterns and underlying flow physics remains unclear when the flows are unknown a priori. Inspired by the introduction of spin variable in quantum mechanics, we introduced the orientation variable into fluid dynamics and adapted the time-dependent Fokker-Planck framework to reveal the connection between experimental flake visualization patterns and virtual orientation probability fields from an Eulerian perspective. As a first example in which a dimensionless parameter distinguishes the dependency on the initial condition, we illustrated an analytical solution of the orientation probability in a rotating wave flow. With the inclusion of the diffusion term in the governing equation, the probability converges, beyond simple isotropization, to the flow-determined state with spatially varying anisotropy, eliminating dependency on initial conditions. As a second example, we solved the orientation probability field in the axisymmetric state in spherical Couette flow, demonstrating independence from initial conditions consistent with experimental reproducibility. An asymmetric pattern in experimental images, unexplained by the dynamics of the tracer orientation, was successfully recovered from the unique solution of the proposed equation.

Understanding and controlling particle transport in porous media is of widespread interest across diverse practical applications. Here, we report geometry-mediated accumulation effect on particle transport under dilute conditions driven by the interplay between particle lagging and viscosity effect, offering an alternative perspective for preferential flow control in porous media. Pore-to-throat velocity variations can trigger strong local accumulation of microgel particles in the absence of clogging effects, which is anomalous given the low Stokes number. Based on volume-averaged equations for two-phase flow, we theoretically elucidate the competition between the drag force representing particle-fluid interactions and an additional resistance force representing interparticle interactions, which governs particle accumulation and becomes significant only with concentration-sensitive viscosity. Differing from shear-induced migration driven by multibody hydrodynamic or collision interactions at higher concentrations, such accumulation occurs only in the presence of geometry variations, highlighting its promising impact on suspension flow in porous media. A new dimensionless number is proposed and validated by numerical simulations in typical pore-throat geometries to generally describe the triggering criterion and accumulation intensity. Investigations in heterogeneous porous media reveal varying accumulation patterns under various injection conditions, which are predictable applying theoretical descriptions. Unexpected preferential flow control performances arise from lateral flow reallocation, controlled by the formation and distribution of localized intense accumulation zones. Our findings provide insights into particle transport mechanisms and flow control strategies in microchannels and porous media.

It seems to be common sense that decreasing the pore size of a mesh surface, thereby enhancing the capillary pressure resisting the liquid penetration, is the only way to suppress its penetration. Here, we demonstrate that a macro-ridge created on a mesh surface increases its dynamic pressure threshold for liquid penetration by a droplet by up to 130%. This phenomenon is attributed to the symmetry and synchronicity breaking of the flow-focusing process during the retraction stage of droplets. Based on multiple flow-focusing events of separate liquid films at different sites that take place synchronically and independently, experiments and analyses are conducted to capture the timescale, magnitude, and the resultant pressure of the droplet jump-off force imparted onto ridged substrates, despite the variations in droplet impact speed, liquid viscosity, and the ridge morphology.

Liquid films play a crucial role in bubble lubrication and related engineering applications, and also serve as a soft nanofluidic channel with potential applications for neuromorphic computing. We previously demonstrated that an external electric field can modulate film thickness, thereby creating a resistance-switchable system that resembles a soft memristor and is relevant to bubble lubrication. However, the electrohydrodynamics of liquid films under an external electrical field remained unclear. In this work, we developed an analytical model of liquid-film dynamics, incorporating Maxwell stress, disjoining pressure, and Laplace pressure at a surfactant-covered liquid-vapor interface. We derived the equilibrium film thickness under an electric field to explain the observed conductance rectification, which arises from film thickening or thinning under different bias polarities. Furthermore, we analyzed the electrohydrodynamic response of the film under a triangular voltage sweep, revealing frequency-dependent current hysteresis and the effects of voltage amplitude. This study offers theoretical insights into the electrohydrodynamics of the liquid film for applications in soft memristors and bubble lubrication.

Sessile ciliates attach to substrates and generate feeding currents to capture passing particulates and dissolved nutrients. Optimal ciliary activity that maximizes nutrient flux at the cell surface while minimizing the rate of hydrodynamic energy dissipation is well characterized in uniform nutrient fields. However, it is unclear how ciliary motion should change when nutrients are nonuniform or patchy. To address this question, we modeled the sessile ciliate and feeding currents using the spherical envelope model, and used an unsteady advection-diffusion equation to describe the nutrient scalar field. In the absence of flows, we introduced an analytical solution to the diffusive nutrient uptake in linear concentration gradients and found no advantage over uptake in uniform concentration fields. With ciliary activity driving feeding currents, we used a combination of spectral and finite difference method to solve for the unsteady nutrient concentration. We found that when the axis of symmetry of the ciliary motion is aligned with the concentration gradient, nutrient uptake at the cell surface increases steadily over time, with the highest uptake achieved by the treadmill ciliary motion, which is optimal in uniform fields as well. The associated nutrient uptake in concentration gradients scales with the square root of the product of time and Péclet number. In patchy environments, optimal ciliary activity depends on the nature of the patchiness. Our findings highlight strategies that enable sessile ciliates to thrive in environments with fluctuating nutrient availability.

We study the stretching of an elastic dumbbell in a turbulent flow, with the aim of understanding and quantifying the effect of hydrodynamic interactions (HIs) between the beads of the dumbbell. Adopting the Batchelor-Kraichnan model for the flow, we derive a Fokker-Planck equation and solve it analytically to obtain the probability distribution of the dumbbell's extension. Using different formulations of the HI tensor, we find that HIs preferentially enhance the stretching of stiff dumbbells, i.e., those with a small Weissenberg number. We also evaluate the averaging approximations commonly used to simplify the description of HI effects; the consistently averaged approximation shows that HIs result in a less-pronounced coil-stretch transition in chaotic flows. Finally, we confirm the relevance of our analytical results by a comparison with Brownian dynamics simulations of dumbbells transported in a direct numerical simulation of homogeneous isotropic turbulence.

A framework is developed to study the deformation of a spherical poroelastic particle immersed in an unbounded, three-dimensional Stokes flow. The flow is driven by imposing a steady far-field condition, and the particle is modeled using a two-phase approach, where a deformable solid skeleton is fully saturated by the surrounding viscous fluid. Slip is permitted on the interface between the poroelastic particle and the surrounding Stokes flow. We consider the regime in which the ratio of typical viscous fluid stress to elastic stiffness is small, leading to small deformations and a decoupling of the fluid and solid problems. The traction exerted by the fluid on the particle and the Darcy pressure within are computed and used to formulate a purely solid-mechanics problem for the equilibrium particle deformation. To demonstrate the method, two example far-field flow profiles (shear flow and Poiseuille flow) are considered. Closed-form solutions for the translational velocity, rotation, and surface deformation of the particle are presented and analyzed as functions of the particle's permeability, slip, and Poisson's ratio. We show that the rotation of the particle is not impacted by its poroelasticity and that the Poisson's ratio plays a key role in selecting the dominant mechanism of particle deformation. For incompressible particles, the shear traction exerted by the fluid on the particle drives the deformation, causing the deformation to decrease with slip. For compressible particles, the Darcy pressure in the particle drives the deformation, and the deformation increases with slip.

The effect of compressibility on drag reduction (DR) by spanwise traveling transversal surface waves (STTSW) is investigated in turbulent boundary layer (TBL) flow over a flat plate. Wall-resolved simulations are performed at Mach numbers $M_1=0.2$ and $M_2=0.7$ over a range of actuation parameters, i.e., wavelength, period, and amplitude. The results based on 36 cases simulated for each Mach number show that DR is more pronounced at higher Mach numbers with a maximum reduction of 19.2% at $M_1=0.2$ compared to 27.5% at $M_2=0.7$ for identical actuation parameters. This enhanced DR at $M_2=0.7$ is mainly due to the formation of spanwise shock waves at higher phase velocities. These spanwise shock waves are generated by the actuated wall deformation that redistributes the pressure field. Upstream of the shock, in the supersonic flow region, the development of streamwise vortices is weakened, resulting in reduced turbulent mixing and dissipation. Downstream of the shock, the turbulent structures are reorganized due to the compression wave leading to locally high wall-shear stress. Integrating the wall-shear stress distribution over the complete surface wave shows that the spanwise shock waves cause significant friction drag reduction. However, the pressure variations upstream and downstream of the shock wave require an increased actuation power, which results in negative net power saving at $M_2=0.7$.

In this study, we investigate the Stokesian hydrodynamics of rigid circular and spherical particles near a plane wall within a heterogeneous viscous environment. We focus on the resistance problem to evaluate the hydrodynamic forces and torques acting on the particle. The viscosity field is assumed to vary spatially, comprising a background linear profile and a disturbance field. We derive analytical expressions for the circular particle using reciprocal theorem in the limit of small viscosity gradient, and later validate them against finite-element-based numerical simulations. For the spherical particle, only numerical results are shown. We consider two typical background viscosity variations: with transverse (wall-perpendicular) and longitudinal (wall-parallel) gradients. The findings reveal that viscosity gradient induces novel hydrodynamic couplings in near-wall particle motion. Specifically, within transverse gradient, force-rotation and torque-translation cross-couplings are evident in both wall-parallel translating and near-wall rotating circular and spherical particles. In two dimensions, the leading-order dynamics are uncoupled, and these terms solely dictate the hydrodynamics, whereas in three dimensions they oppose the existing leading-order couplings. Wall-perpendicular particle translation, however, remains uncoupled. In longitudinal gradient, wall-parallel translation and near-wall rotation of a circular particle develops force-force and force-torque couplings, respectively, while wall-normal translation acquires both; for a spherical particle, all translation and rotation modes exhibit couplings. We qualitatively attribute these to the viscosity gradient-induced asymmetries in near-wall traction distributions around the particle, resulting from associated pressure and shear-rate profile modifications. Overall, this study provides fundamental insights into the combined influence of wall and viscosity gradient on particle hydrodynamics. These couplings are also directly manifested in particle trajectories, so the results may have potential implications for particle transport, sorting, and separation in microfluidic devices.

We examine rectilinear two-dimensional miscible viscous fingering in response to both injection and withdrawal of low-viscosity fluid using numerical simulations. We consider sinusoidal injection-withdrawal cycles. In the injection period, the input flow rate increases initially before decreasing; we perform a linear stability analysis and examine the development of nonlinear fingering regimes. The instability can progress through the regimes of independent finger growth, coalescence and interaction, the emergence of a single finger, and finally shutdown. Whether or not substantial fingering occurred during the injection controls the dynamics in the withdrawal period. The amount by which fingers are reversed is somewhat insensitive to the specific nonlinear fingering regime that was attained. At the end of injection, the fingers generally have a tip with a localized peak in concentration of the low-viscosity fluid. This region then changes direction upon withdrawal. The low viscosity tip retraces the original finger because flow there is preferential as the viscosity is lower relative to the ambient fluid. However, a “reverse viscous finger” emerges because this tip has even lower viscosity than the fluid within the body of the original finger, leading to an unstable concentration gradient at the edge of the reverse finger. Where the original fingers merged, the reverse finger tends to split. This reversal can substantially reduce the mixing length during the withdrawal period. The analysis is extended to the case of multiple cycles.

Characterization of the turbulent flow field in a constant-pressure, dual-chamber, expanding flame apparatus is reported, with the flow field measured by a high-resolution particle image velocimetry system. Eight symmetrically located fans with rotation speed range of 3000 ∼ 7000 rpm generate nearly homogeneous and isotropic turbulent flow with fluctuation velocity linearly ranging from 0.70 to 1.77g m/s, and the integral length scale maintaining about 10 mm. The homogeneity and isotropy of turbulence are verified by statistically analyzing the probability density functions of the instantaneous velocity. Turbulent flame speeds for ethylene/air and ammonia/methane/air mixtures were measured over a range of the turbulent Reynolds number ($R{e}_T=u^{\prime }{R}_f/S_u^0{\delta }_f$) from 29 to 2706, and from 69 to 2560, respectively. An extended turbulent flame speed scaling correlation is proposed, applicable to both the corrugated flame and thin reaction zone regimes. The extended model accounts for the geometry and history effects of the interaction between turbulence and flame propagation. For ethylene/air and ammonia/methane/air turbulent flame speeds, the Mean Absolute Percentage Error between the experimental and predicted values is 7.8% and 8.5%, respectively, which shows higher accuracy compared to existing scaling models.

During the inertial collapse of a cavitation bubble, potential energy is converted into kinetic energy of the liquid drawn by the collapsing bubble. This kinetic energy concentrates into the bubble as internal energy and is eventually redistributed as the bubble rebounds. An important outcome of this process is the shock emitted during collapse. In the incompressible limit, the bubble dynamics and energy transfer during collapse are well understood and effectively governed by a single parameter, the driving pressure ratio. However, inertial collapse is characterized by high bubble-wall velocities and accelerations, and therefore compressibility effects become important when analyzing energy balance. The present article examines the role of compressibility on energy concentration and release during the inertial collapse of a spherical gas cavity from a theoretical standpoint. This work improves both the methods for and the understanding of this problem in several ways. First, a correction is made to the calculation of radiated energy due to compressibility effects, thus enabling more accurate predictions of energy redistribution at high driving pressures than existing methods. Second, closed-form expressions are provided for the energy and size of the bubble at collapse (i.e., at minimum volume) in terms of the parameters governing the problem, which take into account the effects of compressibility. Finally, we develop an analytical expression relating the shock pressure to the parameters governing the problem. Based on this analysis, we find that energy concentration during collapse obeys one of two scenarios: Up to a driving pressure ratio of approximately 100, $70–90\%$ of the initial energy is concentrated into the bubble at the instant of collapse, whereas beyond that pressure ratio, more than 60% of the initial energy is radiated, thus reducing the efficiency of the energy concentration process.

Cells and other soft particles are often forced to flow in confined geometries in both laboratory and natural environments, where the elastic deformation induces an additional drag and pressure drop across the particle. In contrast with other multiphase flows, the physical parameters that determine this additional pressure are still not known. Here, we start by measuring the pressure drop across a single spherical hydrogel particle as it flows in a microfluidic comparator. This pressure is found to depend on the amount of confinement, elastic modulus, fluid viscosity, and velocity. A force balance model for the particle is then proposed by incorporating these factors and relying on simulations of bead geometry and lubrication flow considerations. The final model collapses the force measurements onto a single scaling law spanning several decades, while providing physical insights by recalling elements from classic multiphase flows and contact mechanics.

Aortic valve replacement is a key surgical procedure for treating aortic valve pathologies, such as stenosis and regurgitation. The precise placement of the prosthetic valve relative to the native aortic annulus plays a critical role in the postoperative hemodynamics. This study investigates how the positioning of a biological prosthetic valve—either intra-annular (within the native annulus) or supra-annular (slightly downstream, in the widened portion of the aortic root)—affects cardiac fluid dynamics. Using high-fidelity numerical simulations on a patient-specific left heart model derived from CT imaging, we simulate physiological flow conditions to isolate the impact of valve placement. Unlike previous clinical studies that compare different patients and valve models, our approach evaluates the same valve in both positions within a single virtual patient, ensuring a controlled comparison. Key hemodynamic parameters are assessed, including transvalvular pressure drop, effective orifice area, wall shear stress, and hemolysis. Results reveal that supra-annular implantation offers significant advantages: lower pressure gradients, larger orifice area, and reduced shear-induced stress. Furthermore, hemolysis analysis using advanced red blood cell stress models indicates a decreased risk of blood damage in the supra-annular configuration. These findings offer valuable insights to guide valve selection and implantation strategies, ultimately supporting improved patient outcomes.

We analyze the impact of gas rarefaction on the two-dimensional aerodynamic ground effect over a flat plate. Focusing on highly rarefied flow conditions, we formulate the free-molecular problem based on the collisionless Boltzmann equation and the Maxwell boundary conditions. A semianalytical solution is derived, where specular and diffuse surface reflections are studied separately. The calculated ballistic field is compared with direct simulation Monte Carlo computations at finite Knudsen numbers to test its validity and breakdown with decreasing rarefaction. The specific effect of ground reflections is illustrated through comparison with the nonconfined (in the absence of ground) flow field. The results indicate that the ground invariably increases the aerodynamic loading on the plate and shifts the maximum lift value to lower angles of attack compared with the nonconfined configuration. While the ground may yield a negative contribution to the lift in the continuum (ideal-flow) limit, its relative difference compared with the nonconfined setup is found significantly larger and consistently positive at high Knudsen numbers.

The spontaneous (so-called Quincke) rotation of an uncharged, solid, dielectric, spherical particle under a steady uniform electric field is analyzed, accounting for the inertia of the particle and the transient fluid inertia, or “hydrodynamic memory,” due to the unsteady Stokes flow around the particle. The dynamics of the particle are encapsulated in three coupled nonlinear integro-differential equations for the evolution of the angular velocity of the particle, and the components of the induced dipole of the particle that are parallel and transverse to the applied field. These equations represent a generalization of the celebrated Lorenz system. A numerical solution of these ‘modified Lorenz equations’ (MLE) shows that hydrodynamic memory leads to an increase in the threshold field strength for chaotic particle rotation, which is in qualitative agreement with experimental observations. Furthermore, hydrodynamic memory leads to an increase in the range of field strengths where multistability between steady and chaotic rotation occurs. At large field strengths, chaos ceases, and the particle is predicted to execute periodic rotational motion.

A spatiotemporal linear stability analysis is performed on a confined two-dimensional shear flow to investigate the effects of compressibility on its absolute stability characteristics. The unsteady perturbation is decomposed into varicose and sinuous modes. As the shear Mach number increases from low subsonic to low supersonic values, the varicose mode under very strong or very weak confinement stabilizes, while that under moderate confinement destabilizes. Within the subsonic and low supersonic regime, the sinuous mode of strongly confined and weakly confined jets destabilizes with the increase of the shear Mach number, while that of moderately confined jets stabilizes. The sinuous mode of a subsonic wake, however, is stabilized as the shear Mach number increases. At low supersonic shear Mach numbers, insensitivity of both the varicose and sinuous modes to confinement is observed for certain ranges of parameters. At high supersonic shear Mach numbers, a new absolutely unstable varicose mode that remains stable at low shear Mach numbers is identified. In particular, this mode is shown to dominate the long-time impulse response under certain conditions. A clear characterization of these stability behaviors is important in applications such as high-speed jets in rocket and aeroacoustic facilities.