Gravity-Independent Relaxation Oscillations Enhancing Mixing Performance in a Continuous-Flow Microchannel

Abstract

Continuous-flow devices used in microfluidics and flow chemistry often have a channel width large enough to make simple diffusion mixing ineffective but small enough to use mechanical mixing. Therefore, one must supplement these devices with a specific unit that enhances their mixing performance. In this work, we experimentally and numerically study the self-oscillatory process near an air bubble implanted into an outlet channel of a T-shaped device at some distance from the branching point. If one supplies a non-uniform surfactant solution at the inlet, the solutal Marangoni instability at the liquid–air interface can occur. The excitation of soluto-capillary convection leads to a relatively prompt homogenization of the solution downstream. A feature of the process is that it proceeds in a pulsed manner due to the rapid activation of convection, which mixes the solution near the bubble. This leads to damping of instability, followed by subsequent restoration of the concentration gradient by throughflow. We show that the relaxation process depends on the channel geometry, the flow rate, and the properties of the surfactant, but not gravity. Therefore, one can use this method to enhance mixing in any continuous-flow device that operates in microgravity conditions. The scheme’s crucial advantage is the possibility of easy external mixing control, which is essential for applications. In this work, we study the nonlinear properties of relaxation oscillation and the mixing enhancement by the Marangoni convection. The experimental findings are in good agreement with the numerical results.