Fluids confined in nanopores exhibit several unique structural and dynamical characteristics that affect a number of applications in industry as well as natural phenomena. Understanding and predicting the complex fluid behavior under nano-confinement is therefore of key importance, and both experimental and computational approaches have been employed toward this goal. It is now feasible to employ both

In this review, we cover recent advances in microfluidic colloid separation enabled by diffusiophoresis. Diffusiophoresis describes the motion of colloidal particles induced by local chemical gradients. Despite its long history, it is only recently that diffusiophoresis has gained a renewed interest in the scientific community. Such a resurgence is, in part, due to the recognition that diffusiophoresis

This paper presents an extensive review of the characteristics of a propeller jet-induced flow field and its impact on scour formation. As the driving force of scour, the jet flow behavior is first explored, including both the free and bounded jets under various boundary conditions. Accordingly, their resulting scour, namely, unconfined and confined scour, are expounded in terms of the temporal and

We investigate experimentally the transitions between systems of two and three satellite vortices found within a shallow layer of water above a rotating disk at the bottom of an open stationary cylindrical tank. We found that these systems of two and three satellite vortices are associated with two fundamental multipolar modes, namely, the quadrupole and hexapole, respectively. The forward and backward

Based on the Helmholtz–Rayleigh minimal dissipation theorem, a theoretical model is proposed to predict both the plateau and peak of pressure in a compression ramp flow with large separation (CRFLS). Since the total dissipation of CRFLS is mainly contributed by the shock waves, the steady flow pattern can be determined by minimizing the shock dissipation among all the possible configurations. The predictions

We show that it is fundamentally inaccurate to apply force-balance models to predict the gravity/buoyancy induced detachment of quasi-steadily growing pendant droplets/bubbles. We solve the governing Young–Laplace (Y–L) equation to clarify that all forces acting on a droplet/bubble always sum up to zero, even prior to or beyond the well-accepted critical volume of detachment. Accordingly, the typical

The cough of a COVID-19 infected subject contaminates a large volume of surrounding air with coronavirus due to the entrainment of surrounding air in the jet-like flow created by the cough. In the present work, we estimate this volume of the air, which may help us to design ventilation of closed spaces and, consequently, reduce the spread of the disease. Recent experiments [P. P. Simha and P. S. M

Due to its biological importance in the fertilization process, the swimming of a solitary spermatozoon has been investigated intensively. However, the elasto-hydrodynamic interactions between spermatozoa remain unclear, and the collective swimming of cells has not been fully clarified. In this study, we numerically investigated pairwise interactions of sperm cells in terms of fluid and solid mechanics

Hemodynamics plays an important role in the cause of atherosclerotic disease. In this work, we simulated the flow fields in six human carotids using the sharp-interface immersed boundary method. We compare the axial velocity, the secondary flow, the vortex structures, the area with reversed flows, and the standard deviation of velocity fluctuations for different carotids. The computed results show

A mathematical model for estimating the risk of airborne transmission of a respiratory infection such as COVID-19 is presented. The model employs basic concepts from fluid dynamics and incorporates the known scope of factors involved in the airborne transmission of such diseases. Simplicity in the mathematical form of the model is by design so that it can serve not only as a common basis for scientific

The diagnosis of sudden cardiac death has progressed with understanding of the rupture of vulnerable stenosis. Fluid dynamic analyses focused on flow-induced deformation and resultant stress on a vulnerable stenosis due to the rupture occurring when stress exceeds its intrinsic strength. The vulnerable stenosis is often observed with another stenosis in a tandem arrangement in the same blood vessel

Chronic rhinosinusitis is a common disease worldwide, and the frequently prescribed nasal sprays do not sufficiently deliver the topical medications to the target sites so that the final treatment in severe cases is surgery. Therefore, there is a huge demand to improve drug delivery devices that could target the maxillary sinuses more effectively. In the present study, different particle diameters

In this numerical study, flow driven by oscillating plates in a channel is investigated at the intermediate Reynolds regime by means of the arbitrary Lagrangian–Eulerian method. The effects of temporal asymmetry, Reynolds number, channel height, phase differences between adjacent plates, and orientation angle on pumping performance, which are unclear under temporally asymmetric linear plate kinematics

Recent experiments conducted in the International Space Station highlight the apparent periodicity of leaf oscillations and other biological phenomena associated with rhythmic variations of lunisolar forces. These events are similar to those occurring on Earth but with greater effects over a shorter period of time. Among the possible disturbances, other than forced or self-existing oscillations, parametric

Although natural convection and thermal creep have been well recognized in the continuum and rarefied regimes, respectively, the study of the competition of them in a wide flow regime is very scarce. From a theoretical point of view, natural convection can be described by Navier–Stokes–Fourier (NSF) equations at the macroscopic level, while thermal creep needs descriptions at the molecular level. Therefore

We propose a novel microfluidic channel wherein three different liquid metal switches can be operated via a single fluidic flow. Liquid metals with movable metallic properties have been used in antenna filters, sensors, etc. In previous works of the liquid metal switch, each switch requires each channel with an inlet and outlet. For instance, three liquid metal switches must employ three channels with

Droplets wetting and impacting on porous substrates play a critical role in various printing processes and industrial applications. However, due to the lack of effective observation inside the pores, the dynamic behavior of the droplet is rather unclear. Here, we used a numerical method to investigate the dynamic behavior of droplets spreading on confined porous substrates with different surface fractions

Mass transport of the Knudsen gas in a porous medium is investigated on the basis of the kinetic theory of gases. First, the mass flow conductance is computed numerically for various porosities and solid grain sizes by stochastic particle simulations (SPS). Then, a kinetic model with a homogeneous scatterer is introduced, which contains the reference Knudsen number as the sole parameter that characterizes

Active particles in diverse circumstances encounter confined channels with asymmetric bounding walls. In the present work, employing the squirmer model, we analyze the trajectory of a single and a pair of active particles in a two-dimensional periodically tapered channel with asymmetric bounding walls through a combined analytical-numerical approach. Assuming Stokes equations for the flow inside the

This paper studies a rotational relaxation process in nitrogen. A series of more than 200 × 106 classical trajectory calculations (CTCs) of collisions of nitrogen molecules were carried out. Based on the CTC results, a collision model accounting for rotational relaxation was proposed and used for three-dimensional event-driven molecular dynamics simulation of a free jet expansion. It was shown that

The present study numerically investigates nonlinear thermal stress in a rarefied gas flow between two coaxial elliptic cylinders with a hot outer wall. Monatomic argon is considered, and isothermal boundary conditions are implemented on the walls. Three different numerical methods are used to solve this problem, i.e., first, the direct simulation Monte Carlo (DSMC) method is used and validated against

Crystal dissolution and precipitation problems often originate in the fields of material sciences, chemical engineering, soil mechanics, hydrology, and other related fields. In this paper, we consider dissolution and precipitation of immobile species (minerals) on the surface of the solid parts and diffusion and reaction of mobile species in the pore space of a porous medium. The mathematical modeling

We introduce Spline Moment Equations (SMEs) for kinetic equations using a new weighted spline ansatz of the distribution function and investigate the ansatz, the model, and its performance by simulating the one-dimensional Boltzmann–Bhatnagar–Gross–Krook equation. The new basis is composed of weighted constrained splines for the approximation of distribution functions that preserves mass, momentum

The kinetic simulations of plume expansion induced by pulsed laser heating of a copper target in a vacuum or low-pressure argon background gas are performed based on the direct simulation Monte Carlo (DSMC) method and ab initio quantum mechanical calculation of interactions between copper and argon atoms. The potential energy curves (PECs) for Cu–Cu, Ar–Ar, and Ar–Cu interactions are obtained in density

In the present paper, a variational method is applied to solve the Boltzmann equation based on the true linearized collision operator for hard-sphere molecules and the Cercignani–Lampis boundary conditions. This technique allows us to obtain an explicit relation between the first- and second-order thermal slip coefficients and the tangential momentum and normal energy accommodation coefficients, defined

The effective permeability of large shale samples provides useful insights into shale gas production. However, its determination can only be achieved through upscaling, since the direct pore-scale simulation of gas flows in large rock samples is not feasible due to the high computational cost and absence of pore connectivity in sample images. Although the Brinkman formulation is widely used in the

In this work, we present the theoretical investigation of the transient rotating electro-osmotic flow of a couple stress fluid in a microchannel, through the Laplace transform technique. The analysis is dependent on the Debye–Hückel linear approximation for electrical potentials. The governing equations of the couple stress fluid are taken to address the flow field in a rotating environment. The mathematical

The Navier–Stokes/Cahn–Hilliard (NSCH) system of equations has been extensively used for investigating the dynamics of two-phase flows of Newtonian fluids. However, the accurate calculation of interfacial tension via NSCH has been perceptibly doubted, and thus, a successive solution of NSCH equations is rarely not accompanied by mesh adaptation techniques and complex numerical schemes. In this work

The dynamics of the interface between two immiscible liquids with a high viscosity contrast is studied experimentally when the liquids are pumped through a radial Hele-Shaw cell. Two cases are considered: a monotonous radial displacement of the viscous fluid, when the classical Saffman–Taylor instability develops, and an oscillatory interface motion due to harmonic flowrate modulation in the absence

The evolution of solid shapes in dissolutive flows is studied using molecular dynamics simulations. The final self-similar structures of the solid are distinct under the convection- and diffusion-dominated conditions. Introducing a dimensionless number, Ds, allows characterizing the relative influence of convection and diffusion on the final structure. When convection dominates, the convergent shape

The aim of this study is to describe the effect of the interface between media with different mechanical properties on the behavior of a gas bubble. The presence of a boundary leads to the breaking of the degeneracy of the surface modes, which are the perturbations of the spherically symmetric shape of the equilibrium bubble. The splitting of the surface modes was determined by using perturbation and

We discuss an electro-osmotic flow near charged porous coatings of a finite hydrodynamic permeability, impregnated with an outer electrolyte solution. It is shown that their electrokinetic (zeta) potential is generally augmented compared to the surface electrostatic potential, thanks to a large liquid slip at their surface emerging due to an electro-osmotic flow in the enriched by counter-ion porous

The impact behavior of a water droplet on small cylindrical superhydrophobic targets is studied numerically and theoretically. A numerical model using the volume of fluid method is developed to simulate the droplet impact process on small cylindrical superhydrophobic targets. The model is verified by comparing the calculated results with the experimental observations in our previous work and reference

The influence of wall slip on the instability of a non-wetting liquid film placed on a solid substrate is analyzed in the limit of negligible inertia. In particular, we focus on the stability properties of the film, comparing the performance of the three lubrication models available in the literature, namely, the weak, intermediate, and strong slip models, with the Stokes equations. Since none of the

A series of three-dimensional numerical simulations have been carried out to examine the characteristics of thermal-solutal Marangoni convection in a rectangular cavity that is subjected to mutually perpendicular temperature and concentration gradients. In the simulations, the thermal Marangoni number MaT is selected as 0, 1, 3, and 7 × 104, but the solutal Marangoni number MaC is varied in order to

The interaction of double-layer density stratified interfaces with initial non-uniform velocity shear is investigated theoretically and numerically, taking the incompressible Richtmyer–Meshkov instability as an example. The linear analysis for providing the initial conditions in numerical calculations is performed, and some numerical examples of vortex double layers are presented using the vortex sheet

Exact solution of the Navier–Stokes equations is used in this study to get a closed form equation for the transient and the steady state displacement flow of two Newtonian iso-viscous fluids in a two-dimensional curved plane channel. First, the displacement flow in a straight plane channel is considered. An available analysis is adapted in order to enhance its capability in explaining more details

The Filament Extension Atomizer™ (FEA) is a unique technology designed for highly viscous or strain-hardening fluids that are otherwise difficult to atomize. The fluid is processed as a thin film between the contact points of two counter-rotating rollers of different materials. As the film is processed beyond the contact point, it is subject to an extensional flow that creates numerous thin filaments

Cylindrical bubbles are frequently accompanied by vortical flows. However, there is a lack of knowledge concerning cylindrical bubble pairs, the simplest interaction unit, compared to the understanding of the interactions between two spherical bubbles or two single-phase vortices. Here, we propose a theoretical model to describe the interaction process of two cylindrical bubbles embedded in two corotating

The linear instability of a surfactant-laden two-layer falling film over an inclined slippery wall is analyzed under the influence of external shear, which is imposed on the top surface of the flow. The free surface of the flow and the interface among the fluids are contaminated by insoluble surfactants. Dynamics of the fluid layers are governed by the Navier–Stokes equations, and the surfactant transport

Viscoelastic fluids have been shown to undergo instabilities even at very low Reynolds numbers, and these instabilities can give rise to a phenomenon called elastic turbulence. This phenomenon, observed experimentally in viscoelastic polymer solutions, is driven by the strong coupling between the fluid velocity and the elasticity of the flow. To explore the emergence of these instabilities in a viscoelastic

We consider the periodic gaits of a microswimmer formed by two rotating cylinders, placed apart at a fixed width. Through a combination of theoretical arguments and numerical simulations, we derive semi-analytic expressions for the system’s instantaneous translational and rotational velocities, as a function of the rotational speeds of each cylinder. We can then integrate these relations in time to

Red blood cells (RBCs) in physiological conditions are capable of deforming and aggregating. However, both deformation and aggregation are seldom considered together when modeling the rheological behavior of blood. This is particularly important since each mechanism is dominant under specific conditions. To address this void, we herein propose a new model that accounts for the deformability of red

The unsteady magnetohydrodynamic flow of viscoelastic fluids through a parallel plate microchannel under the combined influence of magnetic, electro-osmotic, and pressure gradient forcings is investigated. The fractional Oldroyd-B fluid is used for the constitutive equation to simulate the viscoelastic behavior of fluid in the microchannel. Considering the important role of slip boundary condition

A lattice Boltzmann method is developed for the direct numerical simulation of gas, liquid, and solid three-phase flows. The liquid–gas two-phase flow with a high density ratio is solved using a phase-field model where the interface evolution is described by the conservative Allen–Cahn equation, and the dynamics of the solid particle is captured by the momentum exchange method. By distributing the

This investigation considers the effect of density ratio on the modification of coherent turbulent structures in particle-laden channel flows at a shear Reynolds number of Reτ = 180. Direct numerical simulation and Lagrangian particle tracking are used to accurately predict the motion of particles dispersed within the flow at three Stokes numbers, St+ = 0.1, 50, and 92. Particle–fluid coupling is achieved

The effects of the particle collision model in a direct-forcing fictitious domain method on the fluid and particle statistics of a fully developed turbulent channel flow laden with finite-size neutrally buoyant particles are numerically investigated. The particle collisions are described by a combination of the discrete element method and the lubrication force correction model. We first validate our

Superhydrophobic microstructures (100 μm–1 mm) on a solid surface can change the droplet impact dynamics and reduce the contact time, both of which are potentially relevant for various industrial applications. In the study described here, the effects of two superhydrophobic microstructures are compared: a uniformly distributed convex hull structure and a striated structure. Droplet impact dynamics

The determination of the particle dynamics in the human acinar airways having millions of alveoli is critical in preventing potential health problems and delivering therapeutic particles effectively to target locations. Despite its complex geometrical structure and complicate wall movements, the advanced calculation simulations can provide valuable results to accurately predict the aerosol deposition

Several places across the world are experiencing a steep surge in COVID-19 infections. Face masks have become increasingly accepted as one of the most effective means for combating the spread of the disease when used in combination with social-distancing and frequent hand-washing. However, there is an increasing trend of people substituting regular cloth or surgical masks with clear plastic face shields

The ability of many photosynthetic micro-organisms (algae) to detect a localized light source in their environment and move toward (away from) it is known as positive (negative) phototaxis. The phototactic algae absorb the incident light and scatter it isotropically or anisotropically thereafter across the suspension, which is illuminated by both diffuse and collimated irradiation. In this paper, we

Direct consequences stem from the close relation between the recently proposed vortex vector (Rortex) and the swirling strength. It is shown that these vortex-identification methods share some relevant properties: (i) both provide the same and, practically, the largest vortex region, (ii) both allow an unlimited uniaxial stretching described by an axisymmetric strain rate, so the strain-rate magnitude

Floquet stability analysis has been employed to identify modes of three-dimensional (3D) instability in the flow over a circular cylinder of diameter D above a moving wall at a gap height of G = 0.2D, beyond the onset of two-dimensional vortex shedding. Two subharmonic modes (mode C1 and mode C2) and one long-wavelength quasi-periodic mode (mode QPL) are revealed. The characteristics of these 3D modes

Many complex multiphysics systems in fluid dynamics involve using solvers with varied levels of approximations in different regions of the computational domain to resolve multiple spatiotemporal scales present in the flow. The accuracy of the solution is governed by how the information is exchanged between these solvers at the interface, and several methods have been devised for such coupling problems

In this work, the frame-invariant formulation of a new generalized Newtonian fluid (GNF) constitutive equation is proposed. Viscosity is considered as a specific function of the second and third invariant of the strain rate tensor and of the second invariant of the objective velocity gradient. The GNF model was successfully tested using experimental data taken from the open literature for different

Seaweed and fish have slippery outer surfaces because of the secretion of a layer of mucus. The hydrodynamics over a three-dimensional lubricant-infused slip surface that mimics the mucus layers of seaweed and fish was numerically explored. The morphological features of the lubricant-infused surface were designed to mimic such biological mucus storage systems. The lubricant was assumed to fill the

A hybrid computational method was proposed for simulation of biofilm growth processes using a continuum model for transport of water and extracellular polymeric substance (EPS) and a discrete model for simulation of bacterial cells. The current paper focuses on development of accurate models for different forces acting between bacterial cells, which are represented by spherocylinder particles. The

Understanding molecular-scale friction at a liquid–solid interface in a nanofluidic system is essential, as friction affects slip behavior and flow properties at the nanoscale. In this research, we compute the molecular-scale friction at a water–graphene interface, combined with theoretical analysis and Molecular Dynamics (MD) simulation. A solid–solid friction model is modified, regarding a new method

We study the effect of a rigid boundary on the propagation of thermodynamic disturbances in a gas under non-continuum conditions. We consider a semi-infinite setup confined by an infinite planar wall and introduce initial gas disturbances in the form of density and temperature inhomogeneities. The problem is formulated for arbitrary small-amplitude perturbations and analyzed in the entire range of

Modeling dense gas flows inside channels with sections comparable to the diameter of gas molecules is essential in porous medium applications, such as in non-conventional shale reservoir management and nanofluidic separation membranes. In this paper, we perform the first verification study of the Enskog equation by using particle simulation methods based on the same hard-sphere collisions dynamics