PHYSICAL REVIEW FLUIDS 4, 113902 (2019) Coriolis force-driven instabilities in stratified miscible layers on a rotationally actuated microfluidic platform Saunak Sengupta , 1 Sukhendu Ghosh , 2, 3 Sandeep Saha, 4 and Suman Chakraborty 1 1 Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur- 721302, India 2 Department of Mathematics, Presidency University, Kolkata-700073, India 3 Department of Mathematics, Indian Institute of Technology, Jodhpur-342037, India 4 Department of Aerospace Engineering, Indian Institute of Technology, Kharagpur-721302, India (Received 28 June 2019; published 13 November 2019) Stability analysis of stratified multiphase flow for a spanwise system of rotation plays a pivotal role in micromixing and micromachines. In several such systems, centrifugal actuation is the driving force, which creates a pressure gradient in a rotating channel and Coriolis force enhances mixing in a short span by destabilizing the flow. Here, we focus on the impact of the Coriolis force on a rotating two-fluid flow through a microchannel, which is miscible in nature, having small viscosity difference and thereby forming a thin diffusive interface between fluids due to viscosity stratification. Modal stability analysis is used to estimate the critical flow parameters which are, in turn, responsible for regulating the instability mechanism for different viscosity contrasts and mixed layer thicknesses. Usually, viscosity stratified flow with respect to streamwise disturbance becomes more unstable for a thinner mixed layer. On the contrary, our numerical computation confirms a completely discrepant scenario by considering Coriolis force-driven instability of a miscible flow system on account of spanwise disturbances. Possible physical mechanisms for the same are discussed in terms of base flow pattern and the energy fluctuation between the perturbed and base flow. Comparison of three-dimensional disturbances of the flow field, in both clockwise and anticlockwise directions (for two different viscosity ratios), is executed to provide an insight into the dynamics of the flow system. Distributions of the velocity perturbations display a critical bonding between the vortices near and away from the mixed layer. These vortices are, in turn, responsible for the variation in instability mechanism with respect to different viscosity ratios and rotational directions. DOI: 10.1103/PhysRevFluids.4.113902 I. INTRODUCTION Hydrodynamic stability of stratified flow (variation in density, viscosity, concentration, etc., or a combination thereof) has drawn the attention of numerous researchers due to various industrial, chemical, biological, and geophysical applications (which include lubrication, drag reduction, manufacture of conjugated fibers, polymer melt, co-extrusion processes, filtration processes, etc.). In recent years, bounded or semibounded flows (with or without stratification) in a rotating platform have been receiving progressively increasing attention because of its potential role in portable medical diagnostic devices. A proper understanding of instability mechanism in such flows is critical to several other practical applications, [1] ranging from pumps, vacuum cleaners, jet engines, etc., on one side of the spectrum to the microfluidic mixer on the other side. In a wide variety of fluid flows, the viscosity can vary with space, time, or both, resulting in viscosity stratification, which can have a significant impact on flow instability [2–6]. Such a viscosity stratification can be achieved by various means, for example (a) considering two or more immiscible fluids which are in contact with each other (where slope of velocity profile is 2469-990X/2019/4(11)/113902(26) 113902-1 ©2019 American Physical Society