Resonance frequency of different interfacial modes and steady streaming by a slug trapped at one end of a millichannel

Active micropumping and micromixing using oscillating bubbles form the basis for various Lab-on-chip applications. Acoustically excited oscillatory bubbles are commonly used in active particle sorting, micropumping, micromixing, ultrasonic imaging, cell lysis and rotation. For efficient micromixing, the system must be operated at its resonant frequency where amplitude of oscillation is maximum. This ensures that high-intensity cavitation microstreaming is generated. In this work, we determine the resonant frequencies for the different surface modes of oscillation of a rectangular gas slug confined at one end of a millichannel using perturbation techniques and matched asymptotic expansions. We explicitly specify the oscillation frequency of the interface and compute the surface mode amplitudes from the interface deformation. This oscillatory flow field at the leading order is also determined. The results are compared are compared with the experiments by K. Ryu, S. K. Chung and S. K. Cho, Journal of Association of Laboratory Automation 15(3) 163 - 171. The effect of aspect ratio of gas slug on observable streaming is analysed. The predictions of surface modes from perturbation theory are validated with simulations of the system done in ANSYS Fluent.Comment: Submitted for review in Physical Review

Optimizing performance of liquid-liquid extraction in stratified flow in micro-channels

Several applications such as liquid-liquid extraction in micro-fluidic devices are concerned with the flow of two immiscible liquid phases. Two characteristic flow regimes are observed in these systems: the stratified flow and the slug flow. In this work, two phase (liquid-liquid) stratified flows in a rectangular geometry are first analyzed. The influence of physical properties, in particular the viscosity of the two liquids, on the velocity profiles is determined analytically. The flow profiles are classified in the parameter space of physical properties (viscosity ratio) and operating conditions (flow-rate ratio). Viscosity affects the shapes of the velocity profile and the dispersion of a solute in each phase. The question addressed is: can the viscosity of a fluid be exploited to improve extraction efficiency? This would then give us an extra degree of freedom to control and improve extraction efficiency when there can be more than one possible candidate for extractant. The mass transfer behavior in the liquid-liquid system is numerically simulated using both a finite-difference and a finite-volume method. This helps understanding of the role of various operating conditions as pressure drop, flow rate, etc on the behavior of the system. Our analysis can be used to establish guidelines for carrying out experiments. It is found that the effect of the difference in the shape of the flow profiles on mass transfer is not very significant for some modes of operation. The predictions of our model are compared with experimental results from the literature