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Three-dimensional molecular dynamics (MD) simulation is conducted to understanding the dynamics processes of atomic-scale friction, which takes place in the columnar approach-separation model. A cylinder-cylinder sliding simulation model with tangential stress for Cu/Cu is built. Embedded atom potential (EAM) is employed for simulation. Based on the simulation result and the adhesion theory, the temperature change, friction force evolution and microstructures are systematically analyzed. The simulation results show that the temperature is related to the friction force, both increased during the transient sliding period. The friction force curve is analyzed by real contact area with tangential stress. Fluid mechanics is implanted to explain the decrease of friction force. An epitaxial bcc (body center cubic) copper layer is formed inducing adhesion during sliding. The rough peaks break causing the atoms leaving from interface.

Micro-flow field is a preeminent stratagem for improving Proton exchange membrane fuel cells (PEMFC) output performance. Elucidating the droplet dynamics behavior in the micro-channel and its impact on water management is essential. This work has investigated that, for the same radius droplets, the maximum driving pressure difference of droplets will increase by 560% approximately in micro-channel due to the matched characteristic dimension through theoretical analysis based on the fluid mechanics model. The micro-flow field ( 0.5 m m × 0.5 m m channel) in PEMFC can promote the maximum power density by about 48% compared to the normal-flow field ( 1 m m × 1 m m channel) in PEMFC by experiments under the same operating conditions and membrane electrode assembly. Simultaneously, PEMFC with the micro-flow field can obtain the most excellent performance without humidifying the air. Its self-humidification characteristic makes it possible to cancel the humidification equipment in the auxiliary system in the future.

The interfacial dynamics of a water droplet subjected to SAW (surface acoustic wave) was investigated using a high-speed video camera analysis system. The SAW device was fabricated to generate high frequency vibration for the droplet excitation. The water droplet showed bursting into a fine spray of smaller droplets and ultra-fine mist at the higher applied voltage to the SAW device. The appearance of droplet interface varied with elapsed time of surface acoustic wave excitation. At the initial stage after SAW excitation, capillary surface waves were observed on the droplet interface. Subsequently, surface disintegration due to fine spray and mist ejection was observed. The details of surface response of water droplet on the SAW device were revealed experimentally.

In living cells, biomacromolecules are exposed to a highly crowded environment. The cytoplasm, the nucleus, and other organelles are highly viscous fluids that differ from dilute in vitro conditions. Viscosity, a measure of fluid internal friction, directly affects the forces that act on immersed macromolecules. Although active motion of this viscous fluid - cytoplasmic streaming - occurs in many plant and animal cells, the effect of fluid motion (flow) on biomolecules is rarely discussed. Recently NMR experiments that apply a shearing flow in situ have been used for protein studies. While these NMR experiments have succeeded in spectroscopically tracking protein aggregation in real time, they do not provide a visual picture of protein motion under shear. To fill this gap, here we have used molecular dynamics simulations to study the motion of three proteins of different size and shape in a simple shearing flow. The proteins exhibit a superposition of random diffusion and shear-flow-induced rotational motion. Random rotational diffusion dominates at lower shear stresses, whereas an active "rolling motion" along the axis of the applied flow occurs at higher shear stress. Even larger shear stresses perturb protein secondary structure elements resulting in local and global unfolding. Apart from shear-induced unfolding, our results imply that, in an ideal Couette flow field biomolecules undergo correlated motion, which should enhance the probability of inter-molecular interaction and aggregation. Connecting biomolecular simulation with experiments applying shear flow in situ appears to be a promising strategy to study protein alignment, deformation, and dynamics under shear.

The paper considers the dynamics of the plasma generated by the explosive-emission cells of the cathode spot of a vacuum arc in a magnetic field. It is shown that the expansion of the (high-density) plasma in a transverse magnetic field may cause asymmetry in the plasma density distribution at the cathode spot boundary. The asymmetry, in turn, increases the probability of the ignition of new explosive-emission cells in the region of a stronger magnetic field in the 'anti-amperian' direction of BxI. The disturbed plasma density distribution estimated in the MHD approximation is presented. In addition, the velocity of the 'retrograde' spot motion (ignition of new explosive-emission cells) in the stronger field region is estimated as a function of the external magnetic field strength. The velocity estimates (a few to tens of m/(s T)) are shown to agree with experimental data

In this study, an in vitro device that mimics the oral phase of swallowing is calibrated using in vivo measurements. The oral flow behavior of different Newtonian and non-Newtonian solutions is then investigated in vitro, revealing that shear-thinning thickeners used in the treatment of dysphagia behave very similarly to low-viscosity Newtonian liquids during active swallowing, but provide better control of the bolus before the swallow is initiated. A theoretical model is used to interpret the experimental results and enables the identification of two dynamical regimes for the flow of the bolus: first, an inertial regime of constant acceleration dependent on the applied force and system inertia, possibly followed by a viscous regime in which the viscosity governs the constant velocity of the bolus. This mechanistic understanding provides a plausible explanation for similarities and differences in swallowing performance of shear-thinning and Newtonian liquids. Finally, the physiological implications of the model and experimental results are discussed. In vitro and theoretical results suggest that individuals with poor tongue strength are more sensitive to overly thickened boluses. The model also suggests that while the effects of system inertia are significant, the density of the bolus itself plays a negligible role in its dynamics. This is confirmed by experiments on a high density contrast agent used for videofluoroscopy, revealing that rheologically matched contrast agents and thickener solutions flow very similarly. In vitro experiments and theoretical insights can help designing novel thickener formulations before clinical evaluations.

In this paper, we present a mathematical model describing the effect of polar lipids, excreted by glands in the eyelid and present on the surface of the tear film, on the evolution of a pre-corneal tear film. We aim to explain the interesting experimentally observed phenomenon that the tear film continues to move upward even after the upper eyelid has become stationary. The polar lipid is an insoluble surface species that locally alters the surface tension of the tear film. In the lubrication limit, the model reduces to two coupled non-linear partial differential equations for the film thickness and the concentration of lipid. We solve the system numerically and observe that increasing the concentration of the lipid increases the flow of liquid up the eye. We further exploit the size of the parameters in the problem to explain the initial evolution of the system.

The recent progress in bioengineering has created great interest in the dynamics and manipulation of long, deformable macromolecules interacting with fluid flow. We report experimental data on the cross-flow migration, bending, and buckling of extremely deformable hydrogel nanofilaments conveyed by an oscillatory flow into a microchannel. The changes in migration velocity and filament orientation are related to the flow velocity and the filament's initial position, deformation, and length. The observed migration dynamics of hydrogel filaments qualitatively confirms the validity of the previously developed worm-like bead-chain hydrodynamic model. The experimental data collected may help to verify the role of hydrodynamic interactions in molecular simulations of long molecular chains dynamics.

Cavitation and bubble dynamics are important topics of fluid mechanics. Generally, cavitation phenomenon plays a negative role in the hydraulic machineries (e.g. hydroturbines, pumps, space engines, and marine propellers) through various kinds of mechanisms. First, the cavitation could cause serious damage on the surfaces of the materials (e.g. patterned erosions with pits) during its final collapse near the boundary. Second, flow inside/outside the machines will be further deteriorated by the vortices generated by the cavitating flows. Third, significant unstable flow could persist for a long time due to the generation of cavitation. As a result, both the efficiency and stable operation of the machines will be challenged by the cavitation.

This article addresses the boundary layer flow and heat transfer in third grade fluid over an unsteady permeable stretching sheet. The transverse magnetic and electric fields in the momentum equations are considered. Thermal boundary layer equation includes both viscous and Ohmic dissipations. The related nonlinear partial differential system is reduced first into ordinary differential system and then solved for the series solutions. The dependence of velocity and temperature profiles on the various parameters are shown and discussed by sketching graphs. Expressions of skin friction coefficient and local Nusselt number are calculated and analyzed. Numerical values of skin friction coefficient and Nusselt number are tabulated and examined. It is observed that both velocity and temperature increases in presence of electric field. Further the temperature is increased due to the radiation parameter. Thermal boundary layer thickness increases by increasing Eckert number.

The aim of this study is to present an exact analysis of combined effects of radiation and chemical reaction on the magnetohydrodynamic (MHD) free convection flow of an electrically conducting incompressible viscous fluid over an inclined plate embedded in a porous medium. The impulsively started plate with variable temperature and mass diffusion is considered. The dimensionless momentum equation coupled with the energy and mass diffusion equations are analytically solved using the Laplace transform method. Expressions for velocity, temperature and concentration fields are obtained. They satisfy all imposed initial and boundary conditions and can be reduced, as special cases, to some known solutions from the literature. Expressions for skin friction, Nusselt number and Sherwood number are also obtained. Finally, the effects of pertinent parameters on velocity, temperature and concentration profiles are graphically displayed whereas the variations in skin friction, Nusselt number and Sherwood number are shown through tables.

This paper describes osmotically-driven pressure generation in a membrane-bound compartment while taking into account volume expansion, solute dilution, surface area to volume ratio, membrane hydraulic permeability, and changes in osmotic gradient, bulk modulus, and degree of membrane fouling. The emphasis lies on the dynamics of pressure generation; these dynamics have not previously been described in detail. Experimental results are compared to and supported by numerical simulations, which we make accessible as an open source tool. This approach reveals unintuitive results about the quantitative dependence of the speed of pressure generation on the relevant and interdependent parameters that will be encountered in most osmotically-driven pressure generators. For instance, restricting the volume expansion of a compartment allows it to generate its first 5 kPa of pressure seven times faster than without a restraint. In addition, this dynamics study shows that plants are near-ideal osmotic pressure generators, as they are composed of many small compartments with large surface area to volume ratios and strong cell wall reinforcements. Finally, we demonstrate two applications of an osmosis-based pressure generator: actuation of a soft robot and continuous volume delivery over long periods of time. Both applications do not need an external power source but rather take advantage of the energy released upon watering the pressure generators.

Fish can sense a wide variety of sounds by means of the otolith organs of the inner ear. Among the incompletely understood components of this process are the patterns of movement of the otoliths vis-à-vis fish head or whole-body movement. How complex are the motions? How does the otolith organ respond to sounds from different directions and frequencies? In the present work we examine the responses of a dense rigid scatterer (representing the otolith) suspended in an acoustic fluid to low-frequency planar progressive acoustic waves. A simple mechanical model, which predicts both translational and angular oscillation, is formulated. The responses of simple shapes (sphere and hemisphere) are analyzed with an acoustic finite element model. The hemispherical scatterer is found to oscillate both in the direction of the propagation of the progressive waves and also in the plane of the wavefront as a result of angular motion. The models predict that this characteristic will be shared by other irregularly-shaped scatterers, including fish otoliths, which could provide the fish hearing mechanisms with an additional component of oscillation and therefore one more source of acoustical cues.

In modern industry there is an increasing number of situations where particulate and granular materials are handled in bulk, and there is a greater awareness than ever before of the importance of safety and efficiency in processing and handling such materials. Designers and plant operators, perhaps schooled in traditional fluid mechanics involving only liquids and gases, thus have a considerable task in understanding and predicting the unfamiliar flow characteristics of bulk solids.