Simulating the Dynamics of Electrowetting: Modeling, Numerics, and Validation

University Of Maryland, College Park, College Park MD

Investigators

Abstract

0754983 Shapiro Electrowetting is a technique for manipulating fluids on the micro-scale. By applying voltages at actuating electrodes, it is possible to (effectively) modify surface tension properties, and to move, split, merge, and mix liquid packets. Applications of electrowetting include re-programmable lab-on-a-chip systems, auto-focus cell phone lenses, and colored oil pixels for laptops and video-speed smart paper. The PIs will develop experimentally validated models that will predict electrowetting dynamics first in two, then in three, spatial dimensions, which will enable next-generation system analysis, design, and control. The models will include the essential bulk-flow physics: surface tension, low-Reynolds fluid dynamics, electrostatics or electrodynamics, as well as critical loss-phenomena such as contact angle saturation and hysteresis. Moving liquid/gas or liquid/liquid interfaces, that can undergo split and merge topology changes, will be tracked by a combination of an implicit finite element (FEM) method, which will allow computation of interface curvature and the resulting surface tension forces naturally, easily, and accurately, and by the level-set method applied only locally at split/merge events, will naturally yield topology changes. This will combine the strengths of FEM (it handles curvature extremely accurately) and the level-set approach (it naturally captures topology changes). FEM will also be used to solve the low-Reynold's Navier Stokes equations, the electrostatic (or electrodynamic) part of Maxwell's equations, and to handle boundary conditions at the moving solid/liquid/gas triple line in a numerically sound manner. Triple line motion/pinning models will be evaluated and compared against electrowetting experiments - this will improve the initial hysteresis model and will incorporate a combined hydrodynamic and averaged molecular-kinetic description from the literature. Intellectual Merit Currently, there are no modeling tools to understand and quantify the dynamic behavior of electrowetting systems. To build such models, the PIs will: 1) include the essential physical phenomena, 2) correctly state the bulk partial-differential-equations (especially the interplay between electrodynamic effects and the resulting fluid forces), 3) use the variational method to recast these equations and then create numerically viable FEM algorithms to solve them, 4) track moving interfaces, that can undergo topological changes, by a combination of the FEM and level-set methods, in a numerically sound manner, 4) include loss-phenomena such as saturation and hysteresis from first-principles and the literature (when possible) or from experimental data (when not), and 5) validate against electrowetting experiments, by isolating and confirming each new part. The merit is in achieving and combining these components. Broader Impact The PIs collaborate with two leading electrowetting groups (at a university and a company), and are about to begin a collaboration with a third (a company). All three groups have expressed a strong need for such a physical-first-principles, experimentally informed, dynamic electrowetting modeling tool. If successful, the results will be used by the electrowetting community to understand, analyze, design, and control next-generation electrowetting systems. The methods developed for tracking 2-phase micro-flow topology changes will be of use in many other micro-fluidic applications.