Tornadoes in a Microchannel

TL;DR

This study investigates how electric fields induce vortex formation and particle aggregation in microchannel colloidal suspensions, revealing complex vortex behaviors and particle dynamics under specific electrical conditions.

Contribution

It demonstrates the formation of various vortex structures and particle aggregations driven by electric fields in microfluidic channels, providing new insights into electrohydrodynamic phenomena.

Findings

Particles form chains and 3D structures under electric fields

Vortices with stable and varying axes of rotation are observed

Complex vortex interactions including collisions and internal waves occur

Abstract

In non-dilute colloidal suspensions, gradients in particle volume fraction result in gradients in electrical conductivity and permittivity. An externally applied electric field couples with gradients in electrical conductivity and permittivity and, under some conditions, can result in electric body forces that drive the flow unstable forming vortices. The experiments are conducted in square 200 micron PDMS microfluidic channels. Colloidal suspensions consisted of 0.01 volume fraction of 2 or 3 micron diameter polystyrene particles in 0.1 mM Phosphate buffer and 409 mM sucrose to match particle-solution density. AC electric fields at 20 Hz and strength of 430 to 600 V/cm were used. We present a fluid dynamics video that shows the evolution of the particle aggregation and formation of vortical flow. Upon application of the field particles aggregate forming particle chains and three dimensional structures. These particles form rotating bands where the axis of rotation varies with time and can collide with other rotating bands forming increasingly larger bands. Some groups become vortices with a stable axis of rotation. Other phenomena showed include counter rotating vortices, colliding vortices, and non-rotating particle bands with internal waves.