Free-surface microfluidics: Exploring new actuation and sensing methods

In this paper, we present recent advancements in our group on the development of acoustic actuation and electrochemical sensing technologies that can be used for future free-surface microfluidics. We first show that actuation in free-surface microfluidics can be achieved by acoustic radiation forces. Due to the fact that air and water have very different acoustic impedances, the free surfaces act as perfect sound reflectors. The shape of the free surfaces is important in controlling the acoustic radiation forces. We developed two techniques to precisely control the free-surface shapes: edge pinning and biphilic pinning. We also show mat electrochemical sensing inside a free-surface droplet can be achieved by a simple graphene sensor fabricated with a mask-free process using focused ion beam. The experiment shows that the resistance of the sensor decreases linearly with increasing pH values in the free-surface droplets. The novel actuation and sensing methods developed here can be promising candidates for future free-surface microfluidics.