Optimizing contact angle changes for droplet actuation by optoelectrowetting (OEW): A numerical multi-parameter analysis

Abstract

With the transition of fluid-capillary-based “Lab on a chip 1.0″ concepts in analytical chemistry to “Lab on a chip 2.0″ approaches relying on distinct fluid droplets (“digital microfluidics”, DMF), the need for reliable methods for droplet actuation has increasingly come into focus. One possible approach is based on “electrowetting on dielectric” (EWOD). This technique has the disadvantage that any possible desired later positions of the droplets on the chip have to be defined prior to chip realization because one of the EWOD electrode layers has to be structured accordingly. “Optoelectrowetting” (OEW) goes a step further in the sense that the later droplet positions do not have to be known before, and none of the electrode layers has to be structured. Instead, the electrical parameters of the layer sequence can be altered locally by an impinging (and movable) light spot. Although some research groups have succeeded in demonstrating OEW actuation of droplets, the optimization of the relevant parameters of the layer sequence and the droplet – at least half a dozen parameters altogether – is tedious and not straight-forward. In this contribution, for optimization purposes, the equations governing OEW are revisited and altered again, e.g., by numerical implementation of the experimentally well-known saturation of the contact angle change. Additionally, a Nelder-Mead algorithm is applied to find the parameters, on which the optimization has to focus to maximize contact angle changes and, thus, mechanical forces on the droplets. The numerical investigation yields diverse results, e.g., the finding that the droplet’s contact area on the dielectric layer has a strong influence on the contact angle change and the question whether the droplet is pulled or pushed. Moreover, the interplay between frequency and amplitude of the applied rectangular alternate voltage is important for optimization.