Manipulating Pico- to Nanoliter Droplets on Surfaces without Sticking

Droplet manipulation on surfaces is ubiquitous in many industrial fields. Liquid-repellent surfaces are required to facilitate manipulation because sticking restricts droplet motion. Various liquid-repellent surfaces have been used to manipulate microliter droplets. However, classical surfaces suffer from the repellence of pico- to nanoliter droplets. This study demonstrates the nonsticking property of pico- to nanoliter droplets on surfaces when it is coated with low-surface-energy particles with nano-micrometer hierarchy. The dynamic particle coating of ultrasonic-sprayed droplets enables the formation of highly spherical, isolated, particle-coated picoliter droplets. The particle coating changes the solid-liquid interfacial friction to solid-solid interfacial friction and reduces the force required to move the droplet to the subnanonewton range. Consequently, picoliter droplets slide off a tilted substrate without sticking. The coating does not affect the fluid shape reconfigurability of the droplets. This approach facilitates diverse and complex multiway manipulation of picoliter droplets, allowing separation, arrangement, transportation, and shape reconfiguration without sticking. This advances the understanding of droplet behavior at interfaces, and the proposed method may contribute to downsizing fluidic systems.