# Manipulating Pico- to Nanoliter Droplets on Surfaces without Sticking

## TL;DR

This paper introduces a method to manipulate tiny droplets without them sticking to surfaces, enabling precise control in fluidic systems.

## Contribution

A novel coating method using low-surface-energy particles enables nonsticking manipulation of pico- to nanoliter droplets.

## Key findings

- Droplets coated with low-surface-energy particles slide off tilted substrates without sticking.
- The coating reduces interfacial friction, allowing droplet manipulation with subnanonewton forces.
- The method enables complex operations like separation and shape reconfiguration of picoliter droplets.

## Abstract

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.