Energy-efficient generation of controlled vortices on low-voltage digital microfluidic platform.

Generating controlled vortices in a sessile surface droplet configuration in an energy efficient manner is an outstanding research problem of interdisciplinary relevance, having implications in widely varying areas ranging from biomedical diagnostics, thermal management to digital microfluidic technology. Here, we experimentally and theoretically demonstrate a simple yet energy efficient strategy for generating controlled vortices inside a surface droplet, by deploying interacting electrical and thermal fields over inter-digitated electrodes on an electrically wetted platform. Unlike the traditional electrically driven mechanisms, this strategy involves significantly low voltage (≤ 10 V) to induce rotational structures inside the droplet, by exploiting the strong spatial gradient of electrical properties on account of the prevailing thermal field as attributable to intrinsically induced Joule heating effects. Our experiments demonstrate that fluid velocities typically of the order of mm/s can be generated inside the droplet within the standard regimes of operating parameters, bearing far-reaching consequences towards enhancing internal mixing in multifarious droplet based microfluidic applications. An inherent integrability with the existing electrowetting on dielectric platforms renders the process ideal to be used in conjunction with digital microfluidic technology. [ABSTRACT FROM AUTHOR]