(6ly) Recycle, micromixing and particle migration in a microchannel

Lab on chip devices involve flow through channels of very low characteristic length scales. The performance of these devices depends on the residence time i.e. the average time that a fluid element spends in the system. Conventionally, the residence time is increased by increasing the length of the channel or by decreasing the flowrate. Here, we propose a novel technique to increase the residence time by recycling the fluid in the microchannel using electroosmosis. Toward this end, Navier-Stokes equation for the flow field and the Poisson-Boltzmann equation for potential field are solved simultaneously using the finite difference method (FDM) and the minimum threshold value of electric field for inducing recycle flow is determined.

Recycling is then employed as means to enhance mixing of the unreacted reactants. Due to the characteristic low length scales, mixing in a microchannel is limited by diffusion. The extent of mixing can, however, be improved by inducing secondary flows in the system. In our work, we study the enhancement of mixing in pressure-driven flow using a spatially periodic electric field. The stream function formulation is used to solve the Navier-Stokes equation and the Poisson-Boltzmann equation using FDM. The enhanced mixing is quantified using Poincare maps, Shannon entropy, and a scalar species transport equation.

Particle/cell sorting is one of the microfluidic applications, where inertia in the flow is exploited for separation. The inherent nonlinearity due to the presence of inertia in the flow causes lateral migration of the particles, which then self-assemble at certain equilibrium locations. This phenomenon is known as inertial focusing. Inertial focusing can be manipulated by varying the shear gradient profile of the underlying flow. In this work, we study inertial focusing by varying the viscosity. We consider the particle migration in three cases: continuous transverse viscosity variation, a liquid-liquid stratified flow of two miscible liquids, and a liquid-liquid stratified flow of two immiscible liquids. We perform direct numerical simulations, where level set and immersed boundary methods are used to model the interfacial and the particle dynamics. The optimal operating conditions for particle separation are identified.

Keywords: Electrokinetics, Shannon entropy, inertial focusing, liquid-liquid stratified flow, immersed boundary method, and level set method

Publications:

1. Krishnaveni, T., Renganathan, T., Pushpavanam, S., 2017. Recycle Flows in Lab-on-Chip Applications Using Electroosmotic Effects. Eng. Chem. Res. 56, 4145–4155. https://doi.org/10.1021/acs.iecr.6b04942
2. Krishnaveni, T., Renganathan, T., Picardo, J.R., Pushpavanam, S., 2017. Numerical study of enhanced mixing in pressure-driven flows in microchannels using a spatially periodic electric field. Rev. E 003100, 1–15.
3. Krishnaveni, T., Renganathan, T., Pushpavanam, S., Inertial focusing in two dimensional flows with sharp viscosity stratification in a microchannel. http://arxiv.org/abs/1910.09028