Mechanowetting drives droplet and fluid transport on traveling surface waves generated by light-responsive liquid crystal polymers

Abstract

In nature, capillary forces are often driving microfluidic propulsion and droplet manipulation, and technologies have been developed to utilize these forces in applications such as lab-on-a-chip biosensors and microfluidic systems. At the same time, responsive materials have been developed that can be activated by a variety of external triggers, including light, electric fields, and temperature, to locally deform and create dynamic surface structures, such as traveling waves. Here, we combine these developments into a system that enables capillary-driven droplet transport and fluid propulsion generated by light-induced surface waves in azobenzene-embedded liquid crystal polymers. We demonstrate that the traveling waves are able to efficiently propel fluids by means of mechanowetting. We couple the wave profiles to the fluid simulations using a multiphase computational fluid dynamics approach. We study three different fluid propulsion systems, i.e., peristaltic flow, liquid slug transport, and free-standing droplet transport. The first system operates on a fluid-filled single channel and achieves relative flow speeds of $u/u_{\text{wave}}<0.01$ . In contrast, the slugs and droplets are transported at two orders of magnitude higher speed equal to the wave speed ($u/u_{\text{wave}}=1$ ) by exploiting the mechanowetting effect. We quantify the capillary forces generated by the traveling surface waves. Our method opens new avenues in light-driven (digital) microfluidic systems with enhanced control of fluid flow.