Thickness Of Fluid Layer Research Articles

The effect of a normal electric field on wave propagation on a fluid film

Long-wavelength, small-amplitude disturbances on the surface of a fluid layer subject to a normal electric field are considered. In our model, a dielectric medium lies above a layer of perfectly conducting fluid, and the electric field is produced by parallel plate electrodes. The Reynolds number of the fluid flow is taken to be large, with viscous effects restricted to a thin boundary layer on the lower plate. The effects of surface tension and electric field enter the governing equation through an inverse Bond number and an electrical Weber number, respectively. The thickness of the lower fluid layer is assumed to be much smaller than the disturbance wavelength, and a unified analysis is presented allowing for the full range of scalings for the thickness of the upper dielectric medium. A variety of different forms of modified Korteweg-de Vries equation are derived, involving Hilbert transforms, convolution terms, higher order spatial derivatives, and fractional derivatives. Critical values are identified for the inverse Bond number and electrical Weber number at which the qualitative nature of the disturbances changes.

Porous squeeze-film flow

The squeeze-film flow of a thin layer of Newtonian fluid filling the gap between a flat impermeable surface moving under a prescribed constant load and a flat thin porous bed coating a stationary flat impermeable surface is considered. Unlike in the classical case of an impermeable bed, in which an infinite time is required for the two surfaces to touch, for a porous bed contact occurs in a finite contact time. Using a lubrication approximation an implicit expression for the fluid layer thickness and an explicit expression for the contact time are obtained and analysed. In addition, the fluid particle paths are calculated, and the penetration depths of fluid particles into the porous bed are determined. In particular, the behaviour in the asymptotic limit of small permeability, in which the contact time is large but finite, is investigated. Finally, the results are interpreted in the context of lubrication in the human knee joint, and some conclusions are drawn about the contact time of the cartilage-coated femoral condyles and tibial plateau and the penetration of nutrients into the cartilage.

SIMULATION OF SELF-ASSEMBLIES OF COLLOIDAL PARTICLES WITH DIFFERENT SIZES BY USING A LATTICE BOLTZMANN PSEUDO-SOLID MODEL

Lattice Boltzmann pseudo-solid model (LB-PSM) is a newly proposed method for the efficient simulation of colloidal particles' motion in single or multiphase fluids. In this study, it is applied to the study of self-assemblies of colloidal particles with two different sizes, partially immersed in a fluid layer on a substrate. Simulation results show that the separation of different particles can emerge as the ratio of particles radii increases. Also, the influence of the thickness of fluid layer on both the assembling process and the assembled structures is investigated.

Theory of lubrication due to polyelectrolyte hydrogels with arbitrary salt concentration and degree of compression

A Poisson-Boltzmann equation solution is used to determine the thickness of a thin fluid lubricating layer predicted to separate two polyelectrolyte hydrogels in contact for arbitrary salt concentration as a function of applied load and fixed charge and salt concentration. We consider loads ranging from 1 Pa, at which the thin fluid layer thickness is of the order of micron, up to loads of the order of a MPa, at which it is estimated to be of the order of an angstrom. This allows us to predict the thickness of this layer over the wide range of loads that can occur in various applications of hydrogels.

Transient electro-osmotic and pressure driven flows of two-layer fluids through a slit microchannel

By method of the Laplace transform, this article presents semi-analytical solutions for transient electroosmotic and pressure-driven flows (EOF/PDF) of two-layer fluids between microparallel plates. The linearized Poisson-Boltzmann equation and the Cauchy momentum equation have been solved in this article. At the interface, the Maxwell stress is included as the boundary condition. By numerical computations of the inverse Laplace transform, the effects of dielectric constant ratio ɛ, density ratio ρ, pressure ratio p, viscosity ratio µ of layer II to layer I, interface zeta potential difference \(\Delta \bar \psi\), interface charge density jump Q, the ratios of maximum electro-osmotic velocity to pressure velocity α, and the normalized pressure gradient B on transient velocity amplitude are presented. We find the velocity amplitude becomes large with the interface zeta potential difference and becomes small with the increase of the viscosity. The velocity will be large with the increases of dielectric constant ratio; the density ratio almost does not influence the EOF velocity. Larger interface charge density jump leads to a strong jump of velocity at the interface. Additionally, the effects of the thickness of fluid layers (h1 and h2) and pressure gradient on the velocity are also investigated.

Free vibration and dynamic response of a fluid-coupled double elliptical plate system using Mathieu functions

A three-dimensional analytic model involving products of angular and radial Mathieu functions is developed for the exact free vibration analysis and transient acousto–structural response of two parallel elliptical plates, coupled with an internal bounded inviscid and compressible fluid medium, and under general external transverse loads of arbitrary temporal and spatial variations. Extensive numerical data are presented in an orderly fashion for the first ten symmetric/anti-symmetric system natural frequencies as a function of fluid layer thickness parameter for selected plate aspect ratios. Also, the occurrences of frequency veering phenomena between various modes of the same symmetry group and the interchange of associated mode shapes in the veering region are noted and discussed. Moreover, selected fluid-coupled structural deformation mode shapes are presented in vivid graphical form and the issue of mode localization is examined. The Laplace transform with respect to the time variable is subsequently invoked and a linear system of coupled algebraic equations is ultimately obtained, which is truncated and then solved by implementing Durbin's Laplace inversion algorithm accompanied with special solution convergence enhancement techniques for eradication of spurious oscillations (Gibbs' phenomenon). Numerical simulations are conducted for the displacement time histories of water-coupled double aluminum plates of selected aspect ratios and fluid depths, subjected to external loads of practical interest (i.e., an impulsive point load, a concentrated pulse load, and a uniformly distributed blast load). Validity of the results is established through computations made by using a commercial finite element package as well as by comparison with the data available in literature.

How does the angiotensin II type 1 receptor ‘trump’ the type 2 receptor in blood pressure control?

A kinetic model for the binding of angiotensin II (Ang II) to AT1 receptors (AT1Rs) in arterioles did suggest a novel mechanism of association rate amplification and facilitated Ang II diffusion in vivo. To examine how this mechanism, acting on AT1R, will affect the stimulation of AT2R. The model distinguishes between the diffusion of plasma Ang II across the endothelium layer (thickness 10(-4) - 5 × 10(-4) cm) into the vascular smooth muscle (VSM) layer (5 × 10(-4) cm), and the diffusion of tissue Ang II from perivascular interstitium (thickness of micromilieu fluid layer at abluminal VSM surface 10(-6) - 10(-5) cm, i.e. 1 to 10 times the glycocalyx). Thus, Ang II concentration [Ang II] is taken to be 0 at the abluminal and adluminal VSM cell surfaces, respectively. Tissue Ang II is defined as originating from local generation and/or from the capillary circulation. [Ang II]/AT1R and [Ang II]/AT2R occupancy curves for the two directions of diffusion are constructed from the model-based calculations. Ang II, at 10(-15)-10(-13) mol/ml (~1-100 pg/ml), is much less likely to react with vascular AT2R than AT1R, though it has similar affinity for the receptor types. With plasma [Ang II] = 10(-15)-10(-13) mol/ml, AT2R occupancy is less than 10% of maximum on endothelium, and virtually 0 on VSM, whereas AT1R occupancy on VSM is virtually 0 at plasma [Ang II] < 10(-14) mol/ml, and between 0 and 30% at plasma [Ang II] = 10(-13) mol/ml. With tissue [Ang II] = 10(-15)-10(-13) mol/ml, VSM AT2R occupancy is close to 0, whereas VSM AT1R occupancy is 40-60% in the absence of endocytotic AT1R down-regulation, and up to 70-90% in its presence. The threshold concentration of Ang II needed for response is much higher for AT2R than for AT1R. Plasma Ang II rather than tissue Ang II is the agonist of AT2R, and the reverse applies to AT1R. Thus, AT2R stimulation may come into play only at unusually high circulating levels of Ang II.

Small Pleural Effusion Versus Physiologic Pleural Fluid

Small Pleural Effusion Versus Physiologic Pleural FluidIgor Kocijancic1 and Ksenija Vidmar2Audio Available | Share

Two-phase fluid flow in a porous tube: a model for blood flow in capillaries

A theoretical study of blood flow, under the influence of a body force, in a capillary is presented. Blood is modeled as a two-phase fluid consisting of a core region of suspension of all erythrocytes, represented by a micropolar fluid and a plasma layer free from cells modeled as a Newtonian fluid. The capillary is modeled as a porous tube consisting of a thin transition Brinkman layer overlying a porous Darcy region. Analytical expressions for the pressure, microrotation, and velocities for the different regions are given. Plots of pressure, microrotation, and velocities for varying micropolar parameters, hydraulic resistivity, and Newtonian fluid layer thickness are presented. The overall system was found to be sensitive to variations in micropolar coupling number. It was also discovered that high values of hydraulic resistivity result in an overall slower velocity of the micropolar and Newtonian fluid.

The effect of head orientation on subarachnoid cerebrospinal fluid distribution and its implications for neurophysiological modulation and recording techniques

Gravitational forces may lead to local changes in subarachnoid cerebrospinal fluid (CSF) layer thickness, which has important implications for neurophysiological modulation and recording techniques. This study examines the effect of gravitational pull associated with different head positions on the distribution of subarachnoid CSF using structural magnetic resonance imaging. Images of seven subjects in three different positions (supine, left lateral and prone) were statistically compared. Results suggest that subarachnoid CSF volume decreases on the side of the head closest to the ground, due to downward brain movement with gravity. These findings warrant future research into currently unexplored gravitation-induced changes in regional subarachnoid CSF thickness.

Linear stability of natural convection in a multilayer system of fluid and porous layers with internal heat sources

The onset of thermal natural convection in a horizontal multilayer system consisting of a homogeneous porous layer sandwiched between two fluid layers has been simulated by an accurate numerical method. The porous and fluid layers include uniform heat sources. Flow in the porous medium has been governed by Darcy–Brinkman’s law. On the other hand, the Navier–Stokes equations with the Boussinesq approximation have ruled over the clear fluid layers. The lower and upper rigid surfaces are assumed to be fixed at the equal temperatures TL and TU. The eigenvalues and eigenfunctions of the linear stability analysis have been solved by utilizing the compound matrix method (CMM). The CMM reaches accurate results in a very efficient manner. Moreover, the method removes the stiffness from the equations of the stability system. The results indicate that the onset of convection and the nature of convection cells depend on the relative depths of layers. It has been observed that the thickness of the lower fluid layer increases the critical Rayleigh number of the upper fluid layer and stabilizes it.

Finite Element Vibration Analysis of a Magnetorheological Fluid Sandwich Beam

This study investigates the dynamic properties of a sandwich beam with magnetorheological (MR) fluid as a core material between the two layers of the continuous elastic structure. Timoshenko beam theory has been employed to derive the governing differential equation of motion in the finite element form for the transverse vibration response of MR fluid sandwich beam. The validation of the developed finite element formulation is demonstrated by comparing the results in terms of natural frequencies evaluated with those of available literature. Various parametric studies are also performed in terms of variations of the natural frequencies and loss factor as functions of the applied magnetic field and thickness of the MR fluid layer for various boundary conditions.

The Onset and Nonlinear Regimes of Convection in a Two-Layer System of Fluid and Porous Medium Saturated by the Fluid

The onset of convection and its nonlinear regimes in a heated from below two-layer system consisting of a horizontal pure fluid layer and porous medium saturated by the same fluid is studied under the conditions of static gravitational field. The problem is solved numerically by the finite-difference method. The competition between the long-wave and short-wave convective modes at various ratios of the porous layer to the fluid layer thicknesses is analyzed. The data on the nature of convective motion excitation and flow structure transformation are obtained for the range of the Rayleigh numbers up to quintuple supercriticality. It has been found that in the case of a thick porous layer the steady-state convective regime occurring after the establishment of the mechanical equilibrium becomes unstable and gives way to the oscillatory regime at some value of the Rayleigh number. As the Rayleigh number grows further the oscillatory regime of convection is again replaced by the steady-state convective regime.

VIBRATION STUDY OF MAGNETORHEOLOGICAL FLUID FILLED SANDWICH BEAMS

The concept of vibration mitigation in flexible structures using smart fluids has attracted researcher’s interest in the last few years. Significant research work has been done on the structures with electrorheological fluids, but there has been a little work with the magnetorheological fluids. This study investigates the vibration controllability of MR fluid filled cantilever beams under externally applied magnetic fields. The adaptive structures were fabricated by adding a MR fluid layer between the adjacent elastic layers and the properties of two different types of sandwich beams were studied. In the first type, free and forced vibration responses of aluminium beam with partial treatment were evaluated and compared with fully filled beam. The controllability of vibration response was observed in terms of variations in vibration amplitudes and shifts in magnitudes of the resonant natural frequency. In second type, a composite cylindrical sandwich cantilever beam with end plate was fabricated. It consisted of outer stainless steel hollow pipe with end mass, inner wooden rod and an embedded MR fluid. Inner rods with different diameters were used to vary the thickness of MR fluid layer. Forced vibration response of such beams exhibited increased damping under increased magnetic fields.

Throughflow Effects on Penetrative Convection in Superposed Fluid and Porous Layers

The effect of vertical throughflow on the onset of penetrative convection simulated via internal heating in a two-layer system in which a layer of fluid overlies and saturates a layer of porous medium is studied. Flow in the porous medium is governed by Forchheimer-extended Darcy equation, and Beavers–Joseph slip condition is applied at the interface between the fluid and the porous layers. The boundaries are considered to be rigid, however permeable, and insulated to temperature perturbations. The eigenvalue problem is solved using a regular perturbation technique with wave number as a perturbation parameter. The ratio of fluid layer thickness to porous layer thickness, ζ, the direction of throughflow, and the presence of volumetric internal heat source in fluid and/or porous layer play a decisive role on the stability characteristics of the system. In addition, the influence of Prandtl number arising due to throughflow is also emphasized on the stability of the system. It is observed that both stabilizing and destabilizing factors can be enhanced because of the simultaneous presence of a volumetric heat source and vertical throughflow so that a more precise control (suppress or augment) of thermal convective instability in a layer of fluid or porous medium is possible.

Interferometer and analysis methods for the &lt;italic&gt;in vitro&lt;/italic&gt; characterization of dynamic fluid layers on contact lenses

The anterior refracting surface of the eye when wearing a contact lens is the thin fluid layer that forms on the surface of the contact lens. Under normal conditions, this fluid layer is less than 10 μm thick. The fluid layer thickness and topography change over time and are affected by the material properties of the contact lens and may affect vision quality and comfort. An in vitro method of characterizing dynamic fluid layers applied to contact lenses mounted on mechanical substrates has been developed by use of a phase-shifting Twyman-Green interferometer. This interferometer continuously measures light reflected from the surface of the fluid layer, allowing precision analysis of the dynamic fluid layer. Movies showing this fluid layer behavior can be generated. Quantitative analysis beyond typical contact angle or visual inspection methods is provided. Different fluid and contact lens material combinations have been evaluated, and variations in fluid layer properties have been observed. This paper discusses the interferometer design and analysis methods used. Example measurement results of different contact lens are presented.

Hilbert diagnostics of Rayleigh-Benard convection in fluids

An combined optical and thermal imaging experimental system was designed to investigate Rayleigh-Benard convection of a fluid in a layer with two rigid isothermal boundaries and a free upper boundary under steady and unsteady thermal boundary conditions. The fluid surface structure was visualized using methods of reflected-light Hilbert optics. Noncontact control of the fluid layer thickness was performed using a specially designed remote meter based on an MBR-1 microscope with a smooth focusing unit based on the Meyer mechanism. The evolution of the dynamic structure of the surface and temperature field of the fluid being analyzed were studied experimentally, and the existence of flows in the form of two-dimensional rolls with axes of rotation parallel to the lateral boundaries (the walls of the cavity) was confirmed. It is shown that the highly viscous fluid flow has a thermal gravitational nature. Correspondence is found between the evolution of the thermograms and Hilbert schlieren patterns of surface structures in different modes of Rayleigh-Benard convection.

Theory of the effects of surface roughness on fluid lubrication of hydrogels

Multi-length scale roughness can have important effects on the fluid lubrication of two hydrogel coated surfaces, both in the nonequilibrium situation while the gels are being compressed under an applied load and when the two gels are in equilibrium. For the nonequilibrium case, fluid is expelled from hydrogels coatings on surfaces as they are compressed under an applied load. The model improves upon the picture put forward by Ateshian to describe such a mechanism of the dynamical lubrication of cartilage by including the effects of surface roughness on fluid flow through the interface. The model introduces a length scale h0 ∼ (kηsR2/w)1/3 to describe this, where k is the permeability, ηs is the fluid viscosity, R is the radius of the gel interface and w is the gel thickness. As the mean asperity height at the interface gets smaller compared to h0, the expelled fluid can support a larger fraction of the applied load. As it becomes large compared to h0, however, more of the load is supported by the asperities. When a hydrogel is in equilibrium, it is shown that for sufficiently smooth surfaces, even at a length scale comparable to that of the gel mesh size, the fluid layer thickness will remain larger than a polymer radius, and hence even at this length scale, the gel interface is separated by a thin fluid lubricating layer.

Fully Developed Hydrodynamic and Thermal Transport of Combined Pressure and Electrokinetically-driven Flow in a Microchannel With Three Immiscible Fluids

Thermally and hydrodynamically fully developed combined pressure-driven and electroosmotic flow through a channel with three immiscible fluids has been simulated for isoflux wall boundary conditions. Closed form expressions have been developed for velocity and temperature profiles and Nusselt number. The results indicate strong effects of fluid layer thickness, force fields and boundary conditions.

The electrohydrodynamics of superimposed fluids subjected to a nonuniform transverse electric field

This study aims to investigate electrohydrodynamics of two superimposed fluids that are confined between a pair of two-dimensional flat plates and are exposed to a sinusoidal electric field in zero gravity. The goal is to identify the parameters that affect the flow structure and interface deformation using a simple closed form solution. The governing electrohydrodynamic equations are solved analytically for Newtonian and immiscible fluids in the framework of leaky-dielectric theory and in the limit of small electric field and fluid inertia. A detailed analysis of the electric and flow fields is presented and it is shown that the electric field induces sinusoidal electrical stresses at the interface, which lead to periodic convection cells. The parameters affecting the sense of flow circulation and strength are investigated and it is shown that the former depends on the relative magnitude of the electric permittivity and conductivity ratios while the latter is controlled by the relative thicknesses of the fluid layers and the ratio of the electric conductivities and viscosities of the fluids. The maximum flow strength is achieved at a relative thickness that is set by the competition between the electric and hydrodynamic effects. For small deformation, the distortion of the interface is examined using a normal stress balance at the interface, and it is shown that the degree of interface deformation scales with the square of the amplitude of the electric potential nonuniformity, while its wavenumber is twice that of the imposed potential nonuniformity. Furthermore, a zero-deformation curve is found, which delineates the region in the permittivity-conductivity space according to the sense of interface deformation. The results show that for certain ranges of fluid layer thicknesses and permittivity ratios, the interface will remain flat, despite the action of the nonuiform field.