Two-fluid mixing is an essential process for many microfluidic or "lab-on-a-chip" devices. Effective mixing of two fluids inside microchannels could be very challenging since turbulence is usually absent due to the nature of low Reynolds numbers of microflows. In the present study, a parametric study is carried out to elucidate underlying physics and to quantify the effectiveness of manipulating Electro-Kinetic-Instabilities (EKI) to actively control/enhance fluid mixing inside Y-shaped microchannels. Epi-fluorescence imaging technique is used to conduct qualitative flow visualization and quantitative scalar concentration field measurements to quantify the fluid mixing process inside the Y-shaped microchannels in terms of scalar concentration distributions, shedding frequency of the EKI waves and scalar mixing efficiency. The effects of the relevant parameters, such as the conductivity ratio of the two mixing streams, the strength of the applied static electric fields, the frequency and amplitude of the applied alternating perturbations, and micro-structures inside the microchannels on the evolution of the EKI waves and resultant fluid mixing process are investigated systematically.

Micro-flows and heat transfer process inside small surface droplets have many interesting applications associated with microfluidics such as DNA molecule imaging, micro-pumps, and ink-jet printing. The second component of present study is to investigate the unsteady flow and heat transfer phenomena inside small surface droplets over a solid substrate at different temperature levels. Particle Image Velocimetry (PIV) technique is used to quantify the dynamics of the evaporation process and surface-tension induced Marangoni flows inside the small surface droplets. Molecular Tagging Thermometry (MTT) technique is used to map the transient temperature distributions inside the droplets to quantify the unsteady heat transfer process. The effects of the substrate temperature on the evaporation process, surface-tension induced Marangoni flows and micros-scale heat transfer process inside the surface droplets are quantified in detail.