Gowranga Hanasoge, Srinivas Kumar. Formation of multiple micro-vortices in regions of ion concentration polarization. Retrieved from https://doi.org/doi:10.7282/T3CJ8BSH
DescriptionIon transport in nano-scale channels is of interest due to their perm-selective behavior, occurring due to the overlap of electric double layers. This leads to a variety of interesting electrokinetic phenomenon in regions around the nano-pores including, ion concentration polarization, field amplified sample stacking and formation of micro vortex instability among others. Incorporating nano-pores within microfluidic systems in functional lab-on-a-chip devices has resulted in techniques for better sample handling, fluid manipulation, and mixing. Effects of concentration polarization on local hydrodynamics was studied experimentally near a nano-porous membrane incorporated in a Micro-channel in this thesis. Along with the formation of depletion and pre-concentration regions in the setup, we observe additional effects of field amplification on local hydrodynamics. We report the direct observation of a primary micro-vortex instability in the depletion region of ICP, along a plane perpendicular to the substrate. This hydrodynamic effect is directly attributed to the non-uniform electro osmotic slip along the micro channel’s bottom wall due to field amplification. In addition to the primary vortex, we also note the formation of a secondary, tertiary and subsequent vortices along the length of the depletion region. A physical model is proposed which considers the two dimensionally varying concentration profile in the depletion region to account for the formation of multiple vortices. The mixing effect of the fast moving primary vortex in a two dimensionally varying concentration profile changes the local concentration in the depletion region. This in turn affects electro-osmotic slip velocity, leading to the formation of multiple vortices. We perform numerical analysis on simpler, slipping wall models, which show excellent agreement with the experimental results.