Fluid Mixing Control inside a Y-shaped Microchannel by Using Electrokinetic Instability

An experimental study was conducted to further our understanding about the fundamental physics of electrokinetic instability (EKI) and to explore the effectiveness to enhance fluid mixing inside a Y-shaped microchannel by manipulating convective EKI waves. The dependence of the critical voltage of applied static electric field to trig EKI to generate convective EKI waves on the conductivity ratio of the two adjacent streams was quantified at first. The effect of the strength of the applied static electric field on the evolution of the convective EKI waves and fluid mixing process were assessed in terms of scalar concentration fields, shedding frequency of the convective EKI waves and scalar mixing efficiency. The effectiveness of manipulating the convective EKI waves by introducing alternative electric perturbations to the applied static electric fields was also explored for the further enhancement of the fluid mixing process inside the Y-shaped microchannel. INTRODUCTION Two-fluid mixing is an essential process for many microfluidic or “lab-on-a-chip” devices. Various biomedicial and biochemical processes, such as DNA purification, polymerase chain reaction (PCR), enzyme reaction, and protein folding, involve the mixing of two fluids. The performance of such processes depends heavily on the mixing effectiveness and rapidness of the samples and reagents. However, effective mixing of two fluids inside microchannels could be challenging since turbulence is usually absent due to the low Reynolds numbers of the microflows in nature. Therefore, the studies aimed to develop novel techniques and methodologies to enhance diffusion-dominated fluid mixing processes and to increase the interfacial contact surface area between the adjacent streams inside microchannels is very important and necessary for improved performances of microfluidic or “labon-a-chip” devices. In recent years, extensive studies have been conducted to develop novel techniques and methodologies to enhance fluid mixing inside microchannels. Several innovative concepts of “micro-mixers” have been proposed through those studies. In general, the proposed “micro-mixers” can be categorized into two groups: passive mixers and active mixers [1]. Passive mixers do not require external energy; the enhanced mixing process relies entirely on the augmentation of diffusion or chaotic advection through special geometrical design of microchannels. In contrast, active mixers usually rely on adding external energy to introduce disturbances to enhance fluid mixing. Relying on generating external disturbances in terms of temperature [2], pressure [3, 4], electrohydrodynamics [5], dielectrophoretics [6], magnetohydrodynamics [7] as well as acoustics [8], several kinds of active micro-mixers have been proposed to effectively enhance fluid mixings in microchannels. In this study, we report an experimental study to explore the effectiveness of achieving fluid mixing control/enhancement inside a Y-shaped microchannel with an active control method of using electrokinetic instability. Electrokinetic instability (EKI) occurs when two streams with different electric conductivities meet in a microchannel under a static electric field as shown schematically in Fig. 1. If the strength of the applied static electric field exceeds a certain threshold value, the flow instability of adjacent streams could be observed in a sinuous form along the downstream [9, 10]. The conductivity gradient subject to an external electric field has been suggested as a source of electrical charges and the Coulombic force acts to generate additional body force [11]. Relevant to the mechanism of electrokinetic instability, Hoberg & Melcher [11] showed that the interface of miscible fluids with conductivity gradient becomes unstable under a normal