Most fluids are non-Newtonian. Complex, non-Newtonian, fluids, comprising of micro-scale entities such as colloids, polymers, cells, or vesicles in a viscous medium, are ubiquitous. Blood, inks, foodstuffs, paints, and personal-care products are just a few examples. While the deformation dynamics, or rheology, of complex fluids under hydrodynamic flows has been well studied, comparatively nothing is known about electrokinetic phenomena (e.g. electro-osmosis and electrophoresis) of complex fluids under applied electric fields. This is surprising, given that electric fields are routinely used to transport, control, and manipulate complex fluids in micro- and nano-fluidic technologies. This project identifies and quantifies electrokinetic effects in complex fluids, including novel flows, interactions, and particle motions, thereby forming the foundation of a new field: electrokinetics in non-Newtonian media. The results of the research will be transformative to current microfluidic technologies that utilize electrokinetic flows and complex fluids, e.g. micro-capillary electrophoresis and lab-on-a-chip separations, and further offer the basis for designing new, previously un-envisioned technologies.
Intellectual Merit
Systematic, complementary experiments and modeling of electrokinetic phenomena in complex fluids, focusing on non-linear electro-osmotic flows; novel electrophoretic particle motions and interactions; and field-directed colloidal assembly are being examined. A key experimental step is the formulation of "non-Newtonian electrolytes" with controllable rheological properties, including viscoelasticity, shear-thinning, and normal stress coefficients. The electro-osmotic flow of non-Newtonian fluids in microfluidic channels is expected to possess non-linear and temporally complex dependencies on applied electric fields. Importantly, the comparison of experimentally measured electro-osmotic flows against computed flow profiles requires care in choosing an appropriate rheological description of the fluid. Modeling work on electrophoresis suggests several novel, experimentally accessible consequences of non-Newtonian rheology, including an explicit dependence of electrophoretic velocity on particle size and shape, and rheology-mediated electrophoretic interactions between colloids. Crucially, all of these effects are absent in Newtonian fluids, illustrating the dramatic influence of complex fluid rheology on electrokinetic phenomena. The knowledge gained from these investigations aids in elucidating the role of viscoelasticity on the single particle dynamics and collective behavior of colloids assembled above electrodes by AC fields.
Broader Impact
This work will furnish an unprecedented understanding of electrically driven flows in complex fluids, offering broad impacts to micro/nano-fluidic technologies that utilize electric fields to transport micro-structured materials. A specific case is capillary electrophoresis for separation of macro- and bio-molecules: Here, the results obtained to-date suggests that the rheology of the continuous phase may provide a route to novel gel-free capillary electrophoresis protocols, due to the explicit dependence of electrophoretic velocity on particle shape and size in a non-Newtonian fluid. The work on particle dynamics above electrodes in AC fields yields new paradigms for directed assembly of colloidal microstructures in viscoelastic fluids. In education, graduate students and undergraduate researchers are receiving cutting-edge experimental and theoretical training in microfluidics, electrokinetics, and complex fluids. A new graduate/upper-undergraduate level course on Micro- and Nano-Scale Fluid Physics will be developed to showcase central themes and results of our research. The visually dramatic nature of non-Newtonian fluid flow is ideally suited to form the scientific core of outreach activities. To this end, a connection to the wider Pittsburgh community is achieved by designing educational modules for K-12th students, made age- and content-appropriate via consultation with dedicated outreach programs at CMU.
