Pumping of electrolyte with mobile liquid metal droplets driven by continuous electrowetting: A full-scaled simulation study considering surface-coupled electrocapillary two-phase flow.

With the excellent merits of both solid conductors and rheological fluids, liquid metal (LM) provides new opportunities to serve as flexible building blocks of miniaturized electronic and fluidic devices. The phenomenon of continuous electrowetting (CEW) has been long utilized for actuating LM contents in buffer medium, wherein an externally imposed voltage difference is responsible of manipulating the interfacial tension of deformable LM droplets. CEW effectively lowers the surface tension at the LM/electrolyte interface by driving bipolar counterions to the surface of conducting droplet. Since surface tension coefficient relies sensitively on the local voltage drop across the induced double layer, an electric-analogy Marangoni effect occurs even under a rather weak electric field in the presence of a surface gradient of the interfacial tension. CEW of LM routinely induces unidirectional pumping of electrolyte in the direction of applied electric field, with LM droplet translating oppositely within the device channel. Although this subject has received great attention from the microfluidic society in the past decade, previous reports concerned either the individual delivery of the suspension medium or the transport of LM droplet. Starting from this point, we offer herein a fully coupled physical description of two-phase flow dynamics occurring in CEW. The proposed simulation model successfully incorporates the synergy of the interfacial electrokinetic momentum transfer, surface tension on a curved surface, contact angle at the three-phase contact line as well as the gravity force density. The spatial-temporal motion of the contact interface is traced instantly with a moving mesh approach. By direct numerical simulation, the importance of the direct-current bias, additional alternating-current forcing, droplet size, initial ion adsorption in the process of CEW is addressed. Additionally, it is discovered that increasing the number of LM droplet is more cost-effective than enhancing the volume of a single drop in terms of achieving an improvement of the resulted electrocapillary pump performance, while the translational speed of the discrete droplet carrier does not make an observable change in response to a variation in the drop number. These results prove invaluable in terms of an elaborate design of smart on-chip electrokinetic frameworks embedding flexible LM contents in modern micro-total-analytical systems.