Droplet friction on nature-inspired liquid repellent surfaces

Great progress has been made in the development of nature-inspired superhydrophobic surfaces since the Lotus Effect, well known now, was first reported last century. Micro-structured surfaces that enable water droplets to exhibit large contact angles are no longer a rarity. However, large static contact angles do not guarantee minimal droplet friction for some of the micro-structured surfaces. Instead, dynamic contact angles (advancing and receding contact angles) are more important to the evaluation of droplet friction on micro-structured surfaces. This is because droplet movement on micro-structured surfaces is characterised by energy dissipation that is largely related to contact angle hysteresis. On the one hand, several approaches have been shown to decrease droplet friction on micro-structured surfaces. This mainly includes (a) the use of hierarchical structures and (b) decreasing the size of surface features constituting the liquid repellent layer. On the other hand, the development of experimental methods has enabled new understanding of droplet friction on micro-structured surfaces. For example, one can now directly synchronise the measurement of dynamic contact angles, contact line motion and droplet friction. It is envisioned that the new insight into droplet friction will lead to new breakthroughs in the design of micro-structured surfaces with superhydrophobic/superamphiphobic properties.