Sharif Digital Repository

A novel rotary magnetohydrodynamic (MHD) (RMHD) micropump is presented in this paper in order to both benefit exclusive advantages from the MHD micropumps and eliminate the current obstacles in their implementation. Lorentz force, which is the actuation mechanism in RMHD, is used to propel a mercury slug in a circular microchannel in order to suck the working fluid from the inlet and pump it to the outlet. This idea is integrated with a valve which prevents the working fluid from passing through, while allowing the mercury slug to pass, during each pumping loop. The performance of a fabricated RMHD is evaluated, considering parameters such as pressure difference, running duration, electric... 

A high-order accurate implicit operator scheme is used to solve steady incompressible slip flow and heat transfer in 2D microchannels. The present methodology considers the solution of the Navier-Stokes equations using the artificial compressibility method with employing the Maxwell and Smoluchowski boundary conditions to model the slip flow and temperature jump on the walls in microchannels. Since the slip and temperature jump boundary conditions contain the derivatives of the velocity and temperature profiles, using the compact method the boundary conditions can be easily and accurately implemented. The computations are performed for a 2D microchannel and a 2D backward facing step in the... 

In this study, we simulate the motion and reformation of polymer chain in the nanoscale fluid flow motion of the DPD (Dissipative Particle Dynamics) solvent. The behavior of polymer chain through DPD solvent is studied for 2D and 3D considerations. We implement two body forces of Poiseuille flow and electroosmotic flow to the DPD fluid particles. In case of the electroosmotic flow force, we show that the movement of polymer chain via the electroosmotic phenomenon provides less dispersion than that of the Poiseuille flow for the same polymer chain movement  

The limitations of the microfabrication technology do not allow producing perfectly smooth microchannels. Hence, exploring the influences of roughness on transport phenomena in microtubes is of great importance to the scientific community. In the present work, consideration is given toward the corrugated roughness effects on fully developed electroosmotic flow and heat transfer in circular microtubes. Analytical solutions based on perturbation technique are presented for the problem assuming a low zeta potential under the constant heat flux boundary condition of the first kind. It is revealed that higher values of the corrugation number and relative roughness give rise to smaller Nusselt... 

In the present work, the second law of thermodynamics analysis has been carried out for steady state hydrodynamically and thermally fully developed laminar gas flow in annulus microchannels with asymmetrically heated walls. The rarefaction effects are taken into consideration using first order slip velocity and temperature jump boundary conditions. Viscous heating is also included for both the hot wall and the cold wall cases. Using the velocity distribution obtained in earlier works, the energy equation is solved to get analytically the temperature distribution and consequently to compute the entropy generation rate. The effects of rarefaction and the annulus geometrical aspect ratio on... 

The heat transfer enhancement of natural convection, using electrohydrodynamic technique inside a horizontal enclosure heated from below, is studied numerically. The interactions between electric field, flow field, and temperature field are investigated by CFD methods. The flow and temperature fields are affected by voltage applied at the wire electrodes. For different voltages and number of electrodes, it is noticed that the Nusselt number increased in all cases and the best enhancement is obtained at lower Rayleigh numbers. It is also shown that increasing the number of electrodes doesn't always cause an increase in the heat transfer enhancement. Actually, when the number of electrodes is... 

The purpose of this study is to carry out a numerical investigation of injection type EHD pumps. To this end, the flow is considered laminar, steady and incompressible flow. The flow model is based on the assumptions that the fluid is Newtonian and the fluid properties are constant. The results show that all of the ions emitted from emitter are not collected by next collector electrode, but some of this injected ions move to the previous one, which causes back flow. Although this back flow is smaller than net pumping flow but it would reduce the net flow. Because of existence of this back flow, finding the optimum distance of electrodes from each other is necessary. This study shows that... 

A microfluidic system provides an excellent platform for cellular studies. Most importantly, a three-dimensional (3D) cell culture model reconstructs more accurately the in vivo microenvironment of tissue. Accordingly, microfluidic 3D cell culture devices could be ideal candidates for in vitro cell culture platforms. In this paper, two types of 3D cellular aggregates, i.e., toroid and spheroid, are numerically studied. The studies are carried out for microfluidic systems containing U-shaped barrier as well as microwell structure. For the first time, we obtain oxygen and glucose concentration distributions inside a toroid aggregate as well as the shear stress on its surface and compare its... 

Reinforcement of polydimethylsiloxane (PDMS) was investigated using untreated nanodiamond (uND) and silane functionalized ND (sND) at various degrees of silanization. Scanning electron microscopy revealed an improved dispersion of ND in PDMS by silanization. The incorporation of uND and sNDs enhanced tensile strength up to 45% and 70%, respectively; however, greater improvement was observed for sNDs at very low concentrations, e.g. 0.2 wt%. The much improvement in crosslinking density, analyzed by Flory-Rehner model and Mooney-Rivlin plot, and interfacial interaction, characterized by Kraus and Cunneen-Russell plots, was also observed for sNDs. Interestingly, it was found that the extent of... 

Recently, electrode-based Dielectrophoresis (eDEP) has been used for particle manipulation by means of triangular electrodes. In this theoretical and numerical study, a microchannel with quarter-of-ellipse electrodes is presented and a detailed comparison with triangular electrodes is provided. Electric field, resultant DEP force, and particle trajectories for each microchannel are evaluated by means of COMSOL Multiphysics 4.4. Afterwards, focusing and separation efficiencies of the systems are assessed and compared. Finally, separation efficiency of our proposed model for live and dead cells is compared with that of our previous model published in the literature [1]. It is demonstrated that... 

Protein capturing on polymeric substrate of microfluidic devices is a key factor for the fabrication of immunoassay with high sensitivity. In this work, simple and versatile technique of electrospinning was used to produce electrospun nanofibrous membranes (e.NFMs) with high surface area as a substrate for microfluidic-based immunoassay to increase sensitivity. It was found that the simultaneous use of e.NFM and 1-Ethylethyl-3-(3-dimethylaminopropyl)-carbodiimide/N-Hydroxysuccinimide hydroxysuccinimide as coupling agent has synergic effect on antigen immobilization onto the microchannels. It was found that the oxygen plasma technique for the creation of oxygen containing functional group... 

Microchannels in hydrogels play an essential role in enabling a smart contact lens. However, microchannels have rarely been created in commercial hydrogel contact lenses due to their sensitivity to conventional microfabrication techniques. Here, we report the fabrication of microchannels in poly(2-hydroxyethyl methacrylate) (poly(HEMA)) hydrogels that are used in commercial contact lenses with a three-dimensional (3D) printed mold. We investigated the corresponding capillary flow behaviors in these microchannels. We observed different capillary flow regimes in these microchannels, depending on their hydration level. In particular, we found that a peristaltic pressure could reinstate flow in... 

Background and objective: In this study, the effect of the second excitation frequency mode under different conditions on the fluid streaming and its microparticles displacement is investigated. Methods: For this purpose, some variable parameters such as the particle diameter, microchannel aspect ratio, and applied frequency modes have been selected to study. The resulted acoustic streaming was scrutinized to understand the physics of the problem under different geometrical and input conditions. Finally, the effect of the increasing the microparticle size and aspect ratio of the microchannel, simultaneously, has been evaluated. Results: The results demonstrated that increasing the... 

Since the discovery of inertial focusing in 1961, numerous theories have been put forward to explain the migration of particles in inertial flows, but a complete understanding is still lacking. Recently, computational approaches have been utilized to obtain better insights into the underlying physics. In particular, fundamental aspects of particle focusing inside straight and curved microchannels have been explored in detail to determine the dependence of focusing behavior on particle size, channel shape, and flow Reynolds number. In this review, we differentiate between the models developed for inertial particle motion on the basis of whether they are semi-analytical, Navier-Stokes-based,... 

Microchannels are at the fore front of today's cooling technologies. They are widely being considered for cooling of electronic devices and in micro heat exchanger systems due to their ease of manufacture. One issue which arises in the use of microchannels is related to the small length scale of the channel or channel cross-section. In this work, the maximum heat transfer and the optimum geometry for a given pressure loss have been calculated for forced convective heat transfer in microchannels of various cross-section having finite volume for laminar flow conditions. Solutions are presented for 10 different channel cross sections, namely parallel plate channel, circular duct, rectangular... 

The analytical study of microchannels has been considered as a preliminary approach to alleviate the difficulties which are normally encountered in numerical and experimental studies. Among the analytical solutions, those with high robustness and low complexities are certainly more attractive. In this work, we present a theoretical approach to predict the temperature field in micro-Poiseuille channel flow with constant wall temperature. The use of power series method simplifies the solution in the current analytical approach. The current analytical derivations are examined for channels with both hot-wall and cold-wall conditions. The current solutions agree well with the numerical solutions... 

In this study, the supersonic and choked subsonic flows through micro/nano channels are investigated using direct simulation Monte Carlo (DSMC) method. The supersonic case is simulated at different Knudsen numbers covering slip to transition flow regimes, while the effects of inlet Mach and back pressure are studied in details. The inlet/outlet pressure boundary conditions are suitably implemented benefiting from the basics of characteristics theory. A behavior similar to the one predicted by the Fanno theory is observed here; i.e., the supersonic flow velocity decelerates up to a choking condition where any further increase in Knudsen number is impossible unless strong normal/oblique shocks... 

A new finite-volume-based finite-element method using the quadratic elements is developed in the present study, to analyze the flow in micro and nano sizes with higher-order slip boundary conditions. The method is applied to gaseous flow in micro and nanoscale-channels. The developed method is carried out over a wide range of Knudsen numbers, which cover not only the continuum slip flow regime with 0≤Kn≤0.1 but also it entire the range of transient regime with 0.1