Directional droplet transport on switchable ratchets by mechanowetting

Materials with a mechanical response to an external stimulus are promising for application in miniaturized cargo and fluid manipulation in microfluidic (lab-on-a-chip) systems and microsystems in general. One of the main challenges in droplet microfluidics is the precise control of the droplet motion, and existing technologies have drawbacks that can compromise the droplet contents. Here, we demonstrate how an on–off switchable ratchet topography combined with a simple actuation strategy can be exploited to accurately manipulate mm-sized droplets. Because of the mechanowetting principle, the three-phase line dynamically attaches to these deforming ratchets, affecting the droplet displacement in a controlled matter. We show that such topographies are capable of transporting droplets over a surface in a stepwise fashion. We calculate the forces generated by the surface using both a theoretical description of the three-phase line and fluid simulations, and we identify the window of applicability in terms of the droplet size relative to the sawtooth dimensions. Our results enable the design of microfluidic systems with deforming wall topographies for controlled droplet manipulation.