Session: 14-10-02: Micro/Nanofluidics 2025 II

Paper Number: 166752

Electrophoretic Lift in Microflows Enable Colloidal Particle Manipulation 

Accurately controlling the spatial distribution of microscale and nanoscale particles within a fluid flow can facilitate new applications in particle trapping, separations, and sorting for lab-on-a-chip systems. Furthermore, emerging applications utilizing the promise of existing 2D and emerging nanomaterials require new particle assembly techniques to fabricate structures with tunable functional properties at high throughput. Fluid flows in microfluidic channels traditionally operate in the low Reynolds number (Re, which is the ratio of inertial forces to viscous forces) regime, i.e., viscous forces dominate fluid inertia. Therefore, the effect of fluid inertia is typically neglected in these microscale flows. However, the underlying assumption for lack of inertial effects in microscale flows was challenged in the last decade, with demonstrations of unusual particle dynamics, including cross-stream (or perpendicular to the direction of fluid flow) particle migration. These demonstrations showed that, under specific conditions, non-negligible fluid inertia could be used for manipulating microscale particles (typical particle diameter > 1 µm) leading to the development of ‘inertial microfluidics.

Recent experimental observations on combined electrokinetic and shear flows of colloidal suspensions in rectangular cross-section microfluidic channels have shown unusual cross-stream colloidal particle migration and dynamic assembly. Although a new electrophoresis-induced lift force has been postulated to cause the lateral migration of colloidal particles, little is known about how fluid properties and flow conditions impact this force and therefore subsequent colloidal particle migration.We have shown that in combined Poiseuille and electrokinetic flows, particle slip velocity with respect to the fluid can be used to manipulate colloidal particles confined within microchannels. The particle migration was attributed to a new electrophoretic lift-like force that we report here, analogous to the inertial lift forces. The migration of colloidal particles towards the walls occurs for the electric potential gradient and pressure gradient in opposite directions with respect to streamwise flow, a condition we refer to as counter-flow. For dilute colloidal particle suspensions with particle diameters < 1micron, under counter-flow, led to the assembly of distinct colloidal bands within microchannels (100 – 300 wide x 34 deep x 4cm long). Band formation also requires a minimum applied potential threshold at a given shear rate with particles accumulating near the wall while depleting the bulk fluid prior to band formation. Band formation is a function of particle size and volume fraction, particle and channel wall zeta potential, electrolyte concentration, and the minimum electric field thresholds change non-monotonically for particle mixtures. We are able to extract these bands to porous substrates through a continuous flow microfluidic “print-head”. Here, we also discuss the effect of manipulating particle properties and fluid inertia over broad parametric ranges to elucidate robustness of particle migration to and away from microchannel walls, and the formation of particle bands in addition to their extraction and the likely parameters contributing to extracted band structures.

Presenting Author: Shaurya Prakash The Ohio State University

Presenting Author Biography: Shaurya Prakash graduated with a Ph.D. in Mechanical Engineering from the University of Illinois at Urbana-Champaign in 2007. Following his brief stint as an Assistant Professor at Rutgers University, he joined the faculty in the Department of Mechanical and Aerospace Engineering at The Ohio State University in 2009. He is a Fellow of the American Society of Mechanical Engineers (ASME). At Ohio State, he directs the Microsystems and Nanosystems Laboratory, where his team develops novel technologies for applications in healthcare for cancer, wound healing, and infectious disease; and in water purification. He is also a co-director for Ohio State’s Infectious Diseases Institute focusing on Microbial Communities. Dr. Prakash's group addresses fundamental scientific questions towards enabling new technologies that solve problems critical to modern societal needs. Prof. Prakash has also authored a book titled, “Nanofluidics and Microfluidics: Systems and Applications”. He is an Associate Editor for Microfluidics and Nanofluidics and Scientific Reports, both part of the SpringerNature journal collection. His multi-disciplinary research is funded by diverse government and industry sponsors.

Authors:

Shaurya Prakash The Ohio State University