Abstract

Chaotic Micro-Mixing in Microchannels using Electrokinetic Instability

Supreet Singh Bahga, Assistant Professor, Indian Institute of Technology Delhi

Microfluidic devices enable large scale integration of various bio-chemical laboratory operations on a small microfluidic chip. Among various operations, rapid mixing of reagents with low diffusivities is crucial for assays such as immunoassays and DNA hybridization. Microscale flows are typically characterized by low Reynolds numbers, wherein mixing of two or more fluid streams is relatively slow due to absence of turbulence. One way to overcome this limitation is by using electrokinetic instabilities for active mixing of fluid streams, without any moving parts. Electrokinetic instabilities occur in fluid flow where gradients in electrical properties of fluids, such as conductivity and permittivity, lead to a destabilizing body force. We present a parametric experimental investigation of electrokinetic instability (EKI) in a microchannel flow with orthogonal conductivity gradient and electric field, using time-resolved visualization of a passive fluorescent scalar. This particular EKI has applications in rapid mixing at low Reynolds number in microchannels. We performed a series of experiments by varying the electric Rayleigh number which is the ratio of diffusive and electroviscous time scales. Using dynamic mode decomposition of time-resolved snapshots of EKI, we investigate the spatio-temporal coherent structures of EKI for a wide range of electric Rayleigh numbers. We show that EK instability with orthogonal conductivity-gradient and electric field can exhibit chaotic dynamics via the period doubling route, which can be used for rapid micromixing.