A novel micromixer is presented in this study and the effect of DC or AC electric field on mixing efficiency is investigated numerically. Four types of AC waveforms are considered to explore the flow characteristic and mixing efficiency. The velocity field, concentration field and the mixing efficiency are analyzed in details. The results demonstrated that a pair of vortices with opposite rotating directions is generated when DC or AC voltages is applied to the electrode plates planted within the walls. The generated vortices greatly enhance the mixing of incoming fluids with different concentrations. The mixing efficiency firstly rises with time and then reaches a relative stable periodic state under different potential waveforms. As the voltage applied on the plates increases, the mixing efficiency is improved obviously. The mixing efficiency under full-wave AC signal is the highest, and it is up to 95.44% when the applied potential is 3 V and frequency is 5 Hz.

In the present study, mixing of two fluids of different viscosities in a micromixer with oscillating stirrer was simulated by MRT-LBM and the effect of aspect ratio (AR) of stirrer on mixing efficiency was analyzed. The simulation was performed at Re=50, Sc=10. The results showed that at low values of amplitude (K), at low values of Strouhal number (St), increase of AR causes increase in mixing efficiency and at other values of St it decreases and then increases. At intermediate values of K, at low and intermediate St, mixing efficiency decreases with the increase of AR and at high values of St, it first decreases and then increases. At high values of K, at low and high St, increasing AR causes no improvement in mixing efficiency and at intermediate values of St, mixing efficiency decreases and then increases. In this research, the best mixing efficiency for viscosity logarithmic ratio (R) =2 is at AR=0.7, St=1, K=0.5. The effect of AR and St on mixing efficiency at different R was analyzed and results revealed that for two fluids of different viscosities, at very low values of St, mixing efficiency increases with the increase of R and at others decreases considerably. In general, for two fluids of different viscosities, stirrer with low values of AR has better mixing efficiency.

Micromixers are one of the most crucial components of Lab-On-a-Chip devices with the intention of mixing and dispersion of reagents like small molecules and particles. The challenge facing micromixers is typically insufficient mixing efficiency in basic designs, which results in longer microchannels. Therefore, it is desirable to increase mixing efficiency, in order to decrease mixing length, which enables miniaturization of Lab-On-Chip devices. This study investigates two different designs of a passive T-shaped micromixer employing several rectangular obstacles and grooves to monitor mixing efficiency with geometry change, while keeping the Reynolds number under 2. The mixing performance of these geometries is studied by numerical study and it was implemented in COMSOL Multiphysics environment. It was clarified that T-shaped micromixer with obstacles and grooved micromixer improved mixing efficiency of the basic design by 37.2% and 43.8%, respectively. Also, it was shown that this increase in mixing efficiency was due to the development of transversal component of flow caused by the obstacles and grooves.

In this study, effects of zeta potential distribution and geometrical specifications are numerically investigated on mixing efficiency in electroosmotic flows. Considered geometries include straight, converging, diverging, and converging-diverging microchannels. Electroosmotic flow simulations are conducted based on the N-S and Nernst-Planck equations for momentum and ionic charges distributions, respectively, by lattice Boltzmann method. Numerical simulations are validated against available analytic electroosmotic flow solutions in homogeneous straight channels, and then flow patterns and mixing performances in the presence of non-uniform zeta potential distributions are investigated in search for enhanced mixing performances. Numerical results indicate that converging channel leads to a sizable increase in mixing efficiencies, while the flow rate decreases at the same time. In contrast, diverging channels increase the flow rate, while decrease the mixing efficiency. Therefore, it is expected to achieve a balance between the mixing efficiency and mass flow rate using converging-diverging geometries. Numerical results indicate that mixing efficiency of about 90% can be reached with a converging-diverging microchannel with a reasonable decrease in mass flow rate as compared to its geometrical diverging-converging counterpart channel.

The mixing of Newtonian and non-Newtonian fluids in a magnetic micro-mixer was studied numerically using  ferrofluid. The mixing process was performed in a three-dimensional steady-state micro-mixer. A magnetic source was mounted at the entrance of the micro-channel to oscillate the magnetic particles. The effects of electric current, inlet velocity, size of magnetic particles, and non-Newtonian fluid were examined on the mixing efficiency. It was demonstrated that the mixing efficiency would increase with applied current and the size of magnetic particles. The inlet velocity has an inverse effect on the enhancement of the mixing efficiency. It is found that electric currents of 0A and 50A would lead to the mixing efficiency of 10% and 83%, respectively. In addition, the results of the present work revealed that the mixing efficiency of a non-Newtonian fluid (blood) is smaller than that of a Newtonian one.

In the present paper, a numerical simulation is performed to analyze aspect ratio effects of a rectangular stirrer along with two other parameters, frequency and amplitude of stirrer on the mixing efficiency inside a straight microchannel by LBM. Results showed that in a low frequency amplitude, AR variations are not effective on the efficiency in low values of St, however, in the intermediate values, the efficiency increases with the increase of AR. Moreover, in high values of St, the rise in AR up to 0.5 leads to the efficiency reduction, but further increase of AR, increases the mixing efficiency too. In a larger amplitude than the previous case, in high values St, increasing AR up to 0.7 rises efficiency, but further increase of AR, decreases the mixing efficiency. In low and intermediate values of St, the mixing performance in low values of AR has been more efficient than high values of AR. Furthermore, it was shown that in a low frequency, the mixing efficiency decreases and then increases with rise in AR for all values of K so that the closer to 1 the AR, the better the mixing efficiency. In a higher frequency, the efficiency decreases and then increases with the growth in AR in small value of K, while this trend become inverse in large values of K. Generally, in intermediate values, the larger and nearer to 1 the AR, the larger the mixing efficiency.

In this paper, the mixing efficiency of the ejector in the ejector expansion refrigeration cycle with condenser outlet split using R134a as refrigerant is investigated. Considering the effects of non-equilibrium expansion of the flow in the primary nozzle, a correlation has been derived with the aid of available experimental results to correct the mass flow rate resulted from the homogeneous equilibrium model. Two thermodynamic models, one based on the mixing pressure correction and the other based on the secondary flow velocity correction, have been compared. According to the experimental results, in the model based on the mixing pressure correction, mixing efficiency is in the range of 0.09 to 0.32 and in the model based on the secondary flow velocity correction, mixing efficiency is in the range of 0.03 to 0.3. However, in thermodynamic analysis of the ejector expansion refrigeration cycle, the mixing efficiency is considered in the range of 0.7 to 0.9. The results of thermodynamic analysis based on the mixing efficiency obtained from the experimental data showed that there are temperature ranges for condenser, low-temperature evaporator and high-temperature evaporator in which the ejector is not able to supply secondary flow. However, in the same temperature ranges, thermodynamic analysis with mixing efficiencies in the range of 0.7 to 0.9 shows that the ejector has the ability to supply secondary flow.

In this study, effects of zeta potential distribution and geometrical specifications are investigated on mixing efficiency in electroosmotic flows. Flow geometry in this research is a series of converging-diverging microchannels with different diverging ratios. Governing equations including the Navier Stokes equation for fluid flow and the Poisson-Boltzmann equation for internal electrical field are solved numerically in a two-dimensional domain by using the lattice Boltzmann method. Numerical simulations are validated against available analytic solutions for electroosmotic flow in homogeneous straight channels. The response surface methodology (RSM) is then employed to investigate relationship between flow variables and consequently to optimize mixing efficiency and flow rate of the channel. Results indicate that increasing the zeta potential ratio and diverging ratio, leads to increased value of flow rate, while meanwhile it decreases the mixing efficiency. Zeta potential pattern does not affect flow rate considerably, but its effects on mixing efficiency is noticeable. Furthermore, it is found that mixing efficiency and flow rate are more sensitive to zeta potential ratio than diverging ratio. At last, optimum parameters are determined by RSM which are 0.5 for zeta potential ratio, 0.6 for diverging height, and pp-nn pattern for zeta potential distribution, all associated to simultaneously maximized flow rate and mixing efficiency.

In this paper, the mixing efficiency in electroosmotic flow inside a micromixer is simulated numerically for different states of non-uniform wall Zeta potential. The geometry of flow is a two-dimensional channel between two parallel plates and the flow is assumed to be incompressible, steady and laminar. The governing equations, including a Laplace equation for the distribution of external electric potential, a Poisson equation for the distribution of electric double layer potential, the Nernst-Planck equation for the distribution of ions concentration, the species convection-diffusion equation, the modified Navier-Stokes equations for the fluid flow field, have been solved using the finite volume numerical method. In order to validate the numerical results, the analytical results of an ideal electroosmotic flow in where throughout the walls are charged is compared with the obtained numerical results. The numerical results show that, by linear-ascending, linear-descending and parabolic changes of the wall Zeta potential at the middle length of the microchannel, the mixing efficiency increases compared to a constant Zeta potential. For the cases of linear changing of Zeta potential, the mixing efficiency increases to 86% and for parabolic change of Zeta potential the mixing efficiency increases to 75%, while the Zeta potential is constant at middle length the maximum of mixing efficiency increases to 64%. In the case that only the upper wall at middle length is charged, the results show that a vortex region is created in the flow. This vortex region causes a maximum (100%) mixing efficiency.

In this paper, an electrokinetic micromixing system with conductive mixing-enclosure is proposed. The simulated micromixer can be fabricated easily and accordingly, it can be used in the microfluidic systems effectively. The mixing process is intensified by controlling the geometry of the mixing-enclosure and electric field strength. The effects of different parameters including existence of mixing-enclosure, horizontal and vertical sizes of mixing-enclosure, orientation angle of mixing-enclosure, and electric field strength on the mixing index are studied. The mixing efficiencies and mixing lengths of current electrokinetic micromixer and those previously proposed by other researchers are compared. The results showed that the mixing efficiency can be enhanced significantly as a micromixer with conductive mixing-enclosure is employed. As an advantage of the proposed micromixer, the maximum mixing efficiency does not change by boosting the electric field strength in the range of 100 V.cm-1 to 200 V.cm-1, while the mixing time diminishes as the electric field strength increases in this range. For the conductive mixing-enclosure with the orientation angle of 45°, maximum mixing efficiency of 96.6% is achieved by exerting electric field with strength of 75 V.cm-1. The current electrokinetic micromixer has superior mixing efficiency and mixing length as compared with other electrokinetic micromixers.