The interest on shedding and coalescence of sessile droplets arises from the importance of these phenomena in various scientific problems and industrial applications such as ice formation on wind turbine blades, power lines, nacelles, and aircraft wings. It is shown recently that one of the ways to reduce the probability of ice accretion on industrial components is using superhydrophobic coatings due to their low adhesion to water droplets. In this study, a combined experimental and numerical approach is used to investigate droplet shedding and coalescence phenomena under the influence of air shear flow on a superhydrophobic surface. Droplets with a size of 2 mm are subjected to various air speeds ranging from 5 to 90 m/s. A numerical simulation based on the Volume of Fluid method coupled with the Large Eddy Simulation turbulent model is carried out in conjunction with the validating experiments to shed more light on the coalescence of droplets and detachment phenomena through a detailed analysis of the aerodynamics forces and velocity vectors on the droplet and the streamlines around it. The results indicate a contrast in the mechanism of two-droplet coalescence and subsequent detachment with those related to the case of a single droplet shedding. At lower speeds, the two droplets coalesce by attracting each other with successive rebounds of the merged droplet on the substrate, while at higher speeds, the detachment occurs almost instantly after coalescence, with a detachment time decreasing exponentially with the air speed. It is shown that coalescence phenomenon assists droplet detachment from the superhydrophobic substrate at lower air speeds.

Icing on aerodynamic surfaces occurs due to the accumulation of rain droplets when the surrounding temperature is below the freezing temperature. It is well known that icing phenomenon alters the aircraft aerodynamic forces and may cause serious damage. Therefore, studying water droplet behavior, such as shedding and coalescence serves as the primary step which can lead to understanding the fundamental physics of aircraft icing. Hence, in this study an experimental approach is used to investigate the shear-driven droplet shedding and coalescence on a hydrophilic substrate which can serve as the building block for the formation of rivulets.

Non-Newtonian fluid can be encountered in many applications of Microdevices. In this study, two-dimensional non-Newtonian simulations of a viscous micropump were performed. The viscous micropump consists of a rotating cylinder located eccentrically inside a microchannel. When the cylinder rotates, a net force is transferred to the fluid due to the unequal shear stresses on the upper and lower surfaces of the cylinder, thus causing the fluid to displace. Non-Newtonian fluid is predicted by Navier Stokes equations and proposed by a modified Bingham model to describe the fluid flow

High speed gas flows through two-dimensional microchannels have been investigated using the Direct Simulation Monte Carlo (DSMC) method, where the pressure boundary condition has been implemented using the theory of characteristics as an alternative to the vacuum boundary. Two species, nitrogen and helium, have been used to conduct the flow simulations. It was found that the pressure boundary condition cannot only predict the flow with exit-plane pressure equal to the back pressure, which the vacuum boundary condition fails to do, but can also simulate the flow with expansion waves outside the channel. Therefore, it is considered to be more appropriate. Two inlet Mach numbers, 4.15 and 3.39, have been employed for the nitrogen flow cases with an inlet Knudsen number (Kn) of 0.062. It has been shown that for cases with an inlet Mach number equal to 4.15, the back pressure only has an effect on flow in the latter half of the microchannel, where the wall heat flux can be enhanced by increasing the back pressure. At an inlet Mach number of 3.39, the wall heat flux has the same trend as that in the higher Mach number case, though its magnitude is considerably lower. In addition, no significant effect of a step change in wall temperature distribution on the total heat exchange between the wall and the bulk flow was detected for the same inlet Mach number and back pressure.

Flows in uniform, parallel, and series microchannels have been investigated using the direct simulation Monte Carlo (DSMC) method. For the uniform microchannel cases, at higher pressure ratio, mixed Kn-regime flows were observed, where the Knudsen number (Kn) varies from below 0.1 to above 0.1. Also, the higher pressure ratio makes the flow accelerate more as the flow develops through the uniform microchannel. In order to examine the heat transfer characteristics between the wall and the bulk flow, a linear temperature distribution was imposed on the wall. Most of the wall heat flux occurs within the channel entrance region while it remains a constant with a slight magnitude along the rest of the channel wall. For the series microchannel cases, the computational domain was established by adding three surfaces and excluding one region from the rectangular domain. Diffuse effects were observed near the interface of the two segments, where the flow upstream the interface can be either heated or cooled by the flow downstream depending on their temperature difference. In addition, the effect of the gas species was investigated by conducting the simulation using helium and argon respectively. It can be found that the speed of the gas with lighter molecular mass is much higher than that of the heavier gas. The computational domain of the parallel microchannel was established similarly to that of the series microchannel. Under a certain pressure ratio, more pressure drop occurs in the parallel parts as the gap height increases. The recirculation phenomenon was observed after the gap wall between the two parallel parts and was evaluated quantitatively in the present study by defining a parameter called the developing coefficient. The gap height between the two parallel parts has only slight effect of the flow development.

Increasing efforts are being directed towards applying the technologies of micro-fluidic, to the development of micro-devices for a wide range of applications such as medical, biological and related technologies. In the present study, the transient performance of the micro-pump will be investigated numerically. The micro-screw pump operation depends on the surface sweep forces. It consists of a screw placed inside a micro-channel. When the screw rotates, a net force is transferred to the fluid due to the differential pressure on the depth of the thread and pressure gradient along the screw axis, thus causing the fluid to displace. The present numerical investigation is a comparative study of transient flow behavior in a micro-channel with rotating screw with different cross-sectional geometries. The effect of screw thread, pitch, Reynolds number and pump load were studied. The thread was determined to be the geometrical parameter that affected the transient performance of the micro-pump the most significantly. The development of the drag, lift and moment coefficients was studied for different geometrical and flow conditions. This work provides crucial fundamentals for different applications.

Since all industrial applications of crude oil–polymer emulsions demand emulsions transportation, it is important to study the flow characteristic in term of apparent yield stress. The yield stress measurements of crude oil–polymer emulsions were carried out using RheoStress RS100 under controlled stress mode. Controlled stress rheometers provide the most direct technique for the measurement of yield stress. The yield stress measurements were carried out for crude oil–Alcoflood polymer emulsion over the range of 0–75% by volume of crude oil concentration and 0–104 ppm of polymer concentration. Three different Alcoflood polymers were employed in this investigation. These are AF1235, AF1275, and AF1285. The yield stress measurements of polymer aqueous solutions and crude oil–polymer emulsions are extensively investigated. Casson Model can be used to predict the apparent yield stress for either Alcoflood polymer aqueous solutions or crude oil–Alcoflood polymer emulsions. The increase of polymer concentration improves the initial flow resistance for the three tested polymer materials. At a polymer concentration beyond 103 ppm, the apparent yield stress raises strongly with polymer concentration. For polymer concentration below 103 ppm, the three polymer materials show close values of the apparent yield stress. For polymer concentration higher than 103 ppm, the polymer material of AF1275 causes higher apparent yield stress than AF1285 and AF1235 in order. Non-linear three dimensional model is provided to predict the apparent yield stress of crude oil–Alcoflood polymer for a wide range of crude oil and Alcoflood polymer concentrations.

The viscous micropump consists of a cylinder placed eccentrically inside a microchannel, where the rotor axis is perpendicular to the channel axis. When the cylinder rotates, a net force is transferred to the fluid because of the unequal shear stresses on the upper and lower surfaces of the rotor. Consequently, this causes the surrounding fluid in the channel to displace toward the microchannel outlet. The simplicity of the viscous micropump renders it ideal for micropumping; however previous studies have shown that its performance is still less than what is required for various applications. The performance of the viscous micropump, in terms of flow rate and pressure capabilities, may be enhanced by implementing more than one rotor into the configuration either horizontally or vertically oriented relative to each other. This is analogous to connecting multiple pumps in parallel or in series. The present study will numerically investigate the performance of various configurations of the viscous micropumps with multiple rotors, namely, the dual-horizontal rotor, triple-horizontal rotor symmetrical dual-vertical rotor and eight-shaped dual-vertical rotor. The development of drag-and-lift forces with time, as well as the viscous resisting torque on the cylinders were studied. In addition, the corresponding drag, lift, and moment coefficients were calculated. The flow pattern and pressure distribution on the cylinders' surfaces are also included in the study. Results show that the symmetrical dual-vertical rotor configuration yields the best efficiency and generates the highest flow rate. The steady-state performance of the single-stage micropump was compared to the available experimental and numerical data and found to be in very good agreement. This work provides a foundation for future research on the subject of fluid phenomena in viscous micropumps.

The design of a novel micro-screw pump for viscous fluid is described. The device consists of a rotating screw in the centre of the channel, connected with a shaft and micro motor. The objective of this research is to investigate the effect of using various screw geometries on the pump performance. Theoretical analysis by finite volume simulations is carried out to study the influence of pitch, diameter of the screw and the thread (flight depth) to evaluate the optimal dimensions for the pump and to obtain the maximum flow rate. When the screw rotates, a net force is transferred to the fluid due to the differential pressure on the depth of the thread and pressure gradient along the screw axis, thus causing the fluid to displace. The three-dimensional simulations indicate a gradual increase of the average velocity with increasing the screw diameter. The maximum average velocity can be obtained when the ratio between the pitch and screw diameter (pi/d) is 0.6. Effective pumping is achieved by increasing the thread and pitch at maximum screw diameter. The numerical simulation has been validated experimentally.

Complex liquids can be encountered in many applications of microdevices. In the present study, the performance of microscrew pump using complex liquid is investigated numerically. The microscrew pump operation depends on the surface sweep forces. It consists of a screw placed inside a microchannel. When the screw rotates, a net force is transferred to the fluid due to differential pressure on the depth of the thread and pressure gradient along the screw axis, thus causing the fluid to displace. Three-dimensional complex liquid simulations of micropump were performed.The effect of screw pitch, thread, Reynolds number and pump load on the micropump performance has been studied. The simulations of complex liquids indicate that the highest bulk velocity is achieved with high thread depth at low Reynolds number. However, effective pumping is accomplished at low Reynolds number, high pressure load and high thread depths. (c) 2005 Elsevier Ltd. All rights reserved.

Paper coating colours exhibit complex interfacial and rheological properties resulting from the interaction between paper coating colors and yellowing inhibitors, which are added to coating colors to increase the stability of paper brightness. The zeta potential of ground calcium carbonate and kaolinite particles was measured. It was found that the zeta potential increased with the percentage of added inhibitor. Moreover, the measurements revealed the influence of adsorption of inhibitors on ground calcium carbonate and kaolinite. Rheological properties of inhibitors were measured and their effect on the rheological properties of coating formulations was investigated. The elastic character of the colour containing inhibitors was more pronounced than the viscous character. The elastic modulus and viscosity were strongly influenced by the inhibitor used. Measurements of water retention of coating suspensions showed that the inhibitor (radical scavenger) reduced the water retention of coating colors.

In the present study, the transient performance of the viscous micropump will be investigated numerically. The viscous micropump's operation depends mainly on viscous forces and can operate in any situation where viscous forces are dominant. All the micropump calculations are reported in nondimensional quantities, which allows for the prediction of the micropump performance, regardless of the dimensions or the fluid that is used. The effect of the microchannel height, rotor eccentricity, Reynolds number, and pump load on the transient performance of the viscous micropump has been studied in detail. The steady state performance was compared with the available experimental data and was found to be in a very good agreement. The rotor eccentricity was determined to be the parameter that affected the transient performance of the micropump the most significantly. This work provides a foundation for future research on the subject of fluid phenomena in viscous micropumps.

This is an investigation of the effect of paper-yellowing inhibitors on the rheological, colloidal, and interfacial properties of paper-coating liquids and the associated changes in the liquid surface microstructure. In addition to rheological measurements, we measured the zeta potential and imaged the surface microstructure of coating liquids by transmission electron microscopy (TEM) using an advanced Pt/C replica technique. The zeta potential is related to the concentration of added inhibitors. The images reveal interparticle structuring with increasing concentration of inhibitors. The structuring is related to the interaction between the coating liquids and the inhibitors. It was also found that the viscosity and the elastic modulus increased with inhibitor concentration. The significant changes in mixture properties due to the additives show the importance of the rheological and surface characterization of liquids and the ensuing effect on the corresponding engineering process.

This is an investigation of the effect of paper-yellowing inhibitors on the viscosity-temperature dependence; scatter particle size, and the associated changes in the Surface microstructure. The temperature has no effect on the qualitative viscosity behavior of the coating formulation including inhibitors. Particles size of pigment increases with the combination of UV absorber and radical scavenger. Transmission Electron Microscopy shows the microstructure of increasing agglomerations with added inhibitors to coating mixture. Numerical 3D simulation of the flow in the coating nip of metering size press taking inertia forces into consideration depicts vortex formations on the surface of the coater's rolls.

A theoretical analysis for the onset of gas pull-through (entrainment) during discharge from a stratified two-phase region through two vertically aligned side branches has been developed in this paper. Initially, a simplified point-sink model was developed, which was then followed by the acquisition of a more accurate finite-branch model. The prediction of the critical height at the onset of gas entrainment was found to be a function of the corresponding Froude number of each branch (Fr1 and Fr2), as well as the vertical distance between the centerlines of the two branches (L/d). The predicted values of the critical height were found to be consistent with the corresponding experimental data for different values of Fr1, Fr2 and L/d. From the basis of the present models, it was found that by increasing the flow through the lower branch, the critical height increases for all values of Fr1 and L/d. Furthermore, by increasing the vertical distance between the two branches, the effect of the lower branch on the determination of the critical height was decreased.

The structure and surface pressure of compressed monolayers consisting of silica nanoparticles at the water–air interface have been studied by means of molecular dynamics computer simulations. The simple “hexagonal array of monodisperse particles” model overestimates the range of the repulsive interparticle potentials between the nanoparticles in a monolayer. On the basis of the results of the simulation we proposed a method to assess the error of the estimation. We also investigated the relevance of the rate of compression in terms of the structure formation and the simulated surface pressure–area isotherms, and considered a possible collapse mechanism based on the different potential energies of the ejected particles.

The viscous micropump consists of a cylinder placed eccentrically inside a microchannel, where the rotor axis is perpendicular to the channel axis. When the cylinder rotates, a net force is transferred to the fluid due to the unequal shear stresses on the upper and lower surfaces of the rotor. Consequently, this causes the surrounding fluid in the channel to displace towards the microchannel outlet. The simplicity of the viscous micropump renders it ideal for micro pumping, however, previous studies have shown that its performance is still less than what is required for various applications. The performance of the viscous micropump, in terms of flow rate, pressure head and efficiency, may be enhanced by implementing more than one rotor into the configuration. The present study will numerically investigate the performance of various configurations of the viscous micropumps with multiple rotors, namely the dual-horizontal rotor, the triple-horizontal rotor, the symmetrical-dual-vertical rotor, and the 8-shaped dual-vertical rotor. The development of drag force with time, as well as the viscous resisting torque on the cylinders were studied. In addition, the corresponding drag and moment coefficients were calculated. Results show that the symmetrical-dual-vertical rotor configuration yields the best efficiency, and generates the highest flow rate. The steady state performance of the single-stage micropump was compared with the available experimental and numerical data, and was found to be in very good agreement. This work provides a foundation for future research on the subject of fluid phenomena in viscous micropumps.

A theoretical investigation has been conducted for the prediction of the critical height at the onset of gas entrainment during single discharge from a stratified, two-phase region, through a branch installed on an inclined flat wall. The predicted critical height at the onset of gas entrainment was proven to be a function of Froude number (Fr) and density ratio of the interface fluids. Three different experimental data sets at wall inclination angles of zero, 45 and 90 degrees (i.e. side, inclined and bottom branches) were used for comparisons. A good concurrence was illustrated between the experimental and theoretical values.

A theoretical analysis for the onset of gas pull-through (entrainment) during discharge from a stratified two-phase region through two vertically aligned side branches has been developed in this paper. Initially, a simplified point-sink model was developed; this was followed by the acquisition of a more accurate finite branch model. The predicted value of the critical height at the onset of gas entrainment was found to be a function of each branches’ corresponding Froude number (Fr1 and Fr2), as well as the ratio between the vertical distance connecting the centerlines of the two branches and the diameter of the branches (L/d). The predicted values of the critical heights were found to be consistent with the corresponding experimental data at different values of Fr1, Fr2 and L/d. From the basis of the present models, it was established that when increasing the flow through the lower branch, the critical height increased for all values of Fr1 and L/d. In addition, it was found that increasing the vertical distance between the two branches, reduced the effects of the lower branch on the determination of the critical height.

A theoretical investigation has been conducted for the prediction of the critical height at the onset of gas entrainment during single discharge from a stratified, two-phase region through a side branch with a finite diameter. Two different models have been developed, a simplified point-sink model and a three-dimensional finite-branch model. The two models are based on a new criterion for the onset of gas entrainment. The results of the predicted critical heights at the onset of gas entrainment showed that the finite-branch model approaches the physical limits at low Froude numbers. However, as the values of the Froude number increased, the predictions of both models eventually converged to the same value. Based on the results of the models, the critical height corresponding to the onset of gas entrainment was found to be a function of Froude number and fluid densities. The results of both models are compared with available experimental data. The comparisons illustrate a very good agreement between the measured and predicted values.