Fluid flows in a microchannel with highly ordered laminar fashion. For this reason, two different fluid streams cannot mix easily, or it takes a very long time. The problem becomes intense for large molecules such as peptides, proteins, and nucleic acids during rapid mixing for biochemical applications in a microfluidic device. Many researchers tried to solve this problem by applying an electric potential. In this work, a numerical simulation was performed on a 2D micromixer. Four symmetric electrodes were placed on the wall of a straight microchannel of width 19 μm. The electroosmotic slip velocity boundary condition was used to create the turbulence on the laminar fluid stream. It was found that this model creates a well-mixed flow at the channel outlet. Then the input parameters were changed to compare the mixing performance in terms of concentration distribution at the channel outlet. Channel width, inter-electrodes gap, the magnitude of electric potential, frequency of the electric potential and asymmetricity of the electrodes were changed and results were compared. An experimental micromixer like the numerical model was fabricated by dc magnetron sputtering machine. Four gold electrodes (thickness, 120 nm) were sputtered on top of a silicon substrate. The value of the input parameters was chosen based on the results obtained from the numerical simulation. To test the mixing functionality of our device the movement of tracer particles was tracked down on the zone surrounded by four electrodes. The micro-PIV (Particulate Image Velocimetry) system was used to analyze the movement of the tracer particles and visualize the flow field in the mixing zone. The magnitude of the AC electric potential and frequency was changed to find out the optimum input parameters for the micromixer. These results could play an important role to design and improve a micromixer design using an AC electric field. A micromixer has many potential applications in biology (DNA analysis, enzyme Screening), chemistry (synthesis, polymerization) and detection (drug discovery, diagnosis).

Sensing and detecting micro particles require a bulk fluid motion towards the sensing element in order to get a desirable response from the sensing element. Specially for low-concentrated fluid suspension response time is very long. So both for detection and sensing mechanism if the fluid flow is guided at a reasonable speed and at a low voltage and relatively low frequency which is suitable for bio-particles; the sensing mechanism can be enhanced largely. But sometimes it is required to re-accumulate or recombine the fluid. Previously parallel plate configuration was used to concentrate particle, but this is for the first time a V-shaped electrode pattern used to guide the bulk flow for concentration purpose. The V-shaped electrode set-up was made by following an unconventional way using sputtering machine which was cheaper than the conventional Photolithography method. AC-Electroosmosis from planar electrodes is a strong mechanism for creating micro-flows from several hundred microns away from the electrode surface. The mechanism for the AC Electroosmotic fluid flow is based on Capacitive charging which causes due to the generation of counter-ions at the electrode-electrolyte interface and Faradaic charging which is generated by the accumulation of co-ions. These two different methods are responsible for a converging and diverging surface flow of the fluid particles. At lower voltage capacitive charging method plays a significant role and most of the applied voltage drops occur at the electrical double layer but up to a certain voltage level Faradaic charging method takes over and starts dominating. The induced flow velocity by both methods has different relationship with the applied voltage. In this experiment Electrical Impedance Spectroscopy (EIS) was used to determine the suitable frequency range for the application & 2.12Vrms was used initially which is a very low voltage. An equivalent circuit for the setup was analyzed. Finally, an analysis was made on this setup using conductive fluid to observe the AC Electrothermal (ACET) effect. In our experiment the goal was to get an optimum velocity for concentration at low voltage and low frequency also to observe the guiding direction of the fluid flow in order to find a way to focus the fluid flow towards the desired direction.

In this work, we applied Taguchi Signal-to-noise (S/N) analysis to investigate the effect of varying three process parameters, namely — sputtering power, working pressure and Ar gas flow rate on the surface, morphological and electrical properties of the RF sputtered SiO2 over Si substrate. We also inspected the contribution of a particular process parameter on these properties by applying Analysis of Variance (ANOVA). SiO2 thin films were fabricated over Si substrate using RF magnetron sputtering system. Three sets of inputs for the three mentioned process parameters were chosen; for power, we chose 100W, 150W and 200W; 5mTorr, 10mTorr and 15mTorr were chosen for pressure and three Ar gas flow rate levels at 5, 10 and 15 sccm were selected. By performing Taguchi L9 orthogonal array, nine combinations of sputtering parameters were prepared for depositing SiO2/Si Thin films. The surface morphological and electrical properties (resistivity per unit area and capacitance per unit area) of the sputtered samples were therefore inspected by analyzing the Taguchi design of experiment. Signal-to-noise (S/R) analysis presents how the properties were affected by the variation of each process parameter. ANOVA analysis showed that sputtering power and working pressure are the two dominant process parameters contributing more to surface morphological and electrical properties. A regression model for surface roughness of the SiO2/Si thin film samples was also derived. The electrical properties of the SiO2/Si thin films, however, didn’t show linear properties.

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