Manipulating pico- to nanolitre droplets on surfaces without sticking

Many industrial fields require droplets to be manipulated on a surface.1–9 Liquid-repellent surfaces are required to facilitate manipulation because sticking restricts droplet motion. Various liquid-repellent surfaces have been used to manipulate microlitre droplets. However, they are less effective with smaller droplets, which are more sensitive to the chemotopological heterogeneity of the surface, and the reduction of solid–liquid interfacial friction is a limiting factor.10–15 Therefore, a trade-off exists between droplet sticking and droplet volume. Despite previous efforts,16–22 imperfect and non-general nanolitre droplet repellence limits the diversity and complexity of droplet manipulation. This study demonstrates the repellence of pico- to nanolitre droplets coated with low-surface-energy particles with nano–micrometre hierarchy. The coating changes the solid−liquid interfacial friction to solid–solid interfacial friction and reduces the force required to move the droplet. Consequently, picolitre droplets slide off a tilted substrate. The coating does not affect the fluid shape reconfigurability of the droplets. This method can facilitate diverse and complex multi-way manipulation of picolitre droplets, and separation, arrangement, transportation, and shape reconfiguration can be achieved without sticking. This advances the understanding of droplet behaviour at interfaces, and the proposed method may become basic technology underlying downsized fluidic systems.