Researchers from the University of Hong Kong’s (HKU) Department of Mechanical Engineering have made a significant breakthrough in droplet manipulation. They’ve devised a novel method for navigating liquids on a surface without the use of external force or energy.
A droplet looks like a ball. In-plane droplet control is comparable to snooker, in which the balls are controlled to follow a specific path, and is a feature that is highly prized in thermal management, desalination, materials self-delivery, and a variety of other applications.
To propel droplets into motion, researchers traditionally use chemical wetting gradients or asymmetric micro-textures, analogous to building a conveyor belt to carry the balls.
The HKU Department of Mechanical Engineering’s RGC postdoctoral discovered for the first time that when a cold/hot or volatile droplet is liberated on a lubricated piezoelectric crystal (lithium niobate) at ambient temperature, the droplet instantly propels for a long distance (which can be 50 times the drop Self-propulsion can be unidirectional, bifurcated, or even trifurcated depending on the crystal plane that interfaces with the droplet.
In an article titled “Furcated Droplet Motility on Crystalline Surfaces,” the discovery was reported in Nature Nanotechnology. According to Professor Wang Liqiu, this is an unexpected occurrence with far-reaching repercussions. Self-sustained propulsion can be achieved by droplets with a temperature difference of 5°C on a surface. Consider placing a ball on a perfectly leveled and smooth table; instead of remaining stationary, the ball begins to roll. Even more shocking is the fact that the ball only rolls in one direction at a time.
The inherently oriented liquid motion is fueled by cross-scale thermo-piezoelectric interaction, which is induced by crystal structural anisotropy, according to the researchers. This is similar to a smooth table being atomically structured in an odd way so that an asymmetric heat source can produce an asymmetric electric field that propels a ball in a path dictated by the table surface’s cutting direction.
According to Dr. Tang Xin, the research permits a unique technique to carry and transport liquids with controllability, variety, and performance, as well as clues for overcoming some long-standing difficulties including anti-icing, defrost, and anti-fog in humid situations.
When a droplet hits a supercooled substrate, such as an aviation wing or a power line, it freezes quickly and sticks to the surface. In this instance, the nucleating droplet may be perturbed by the spontaneous electric force generated by the crystal, potentially reducing interfacial adhesion and delaying harmful ice accretion.
By removing growing condensate from the surface, the thermal barrier, self-propulsion will improve the performance of dropwise condensation, potentially providing a very promising solution to droplet manipulation in space where gravity-assisted droplet shedding is absent.
Furthermore, external disturbances such as modest electric fields can be used to preferentially choose the furcated paths. The surface can then be used as a two- or three-way planar valve to transmit droplets containing data, chemicals, or biological payloads.
Dr. Li Wei remarked that the revolutionary approach to liquid manipulation works for a wide range of liquids and piezoelectric crystals, which opens up new potential for research and the creation of new materials and technologies.
