Breakthrough in droplet manipulation

Date:
October 4, 2021
Source:
The University of Hong Kong
Summary:
Researchers have made a key breakthrough in droplet
manipulation. They have discovered an innovative way to navigate
liquids on a surface in the absence of external force or energy.



FULL STORY ========================================================================== Researchers in the Department of Mechanical Engineering at the
University of Hong Kong (HKU) have made a key breakthrough in droplet manipulation. They have discovered an innovative way to navigate liquids
on a surface in the absence of external force or energy.


========================================================================== Droplet resembles a ball. In-plane droplet control is similar to
snooker where the balls are directed to move along desired trajectory,
a feature highly valued for thermal management, desalination, materials self-delivery, and numerous other applications.

Conventionally, researchers fabricate chemical wetting gradient or
asymmetric microtextures to drive droplet into motion, similar to
designing a conveyor belt to transport the balls. For the first time, RGC postdoctoral fellow Dr TANG Xin, Postdoctoral fellow Dr. LI Wei, and Chair Professor of Thermal-Fluid Sciences and Engineering WANG Liqiu from the
HKU Department of Mechanical Engineering discovered that when a cold/hot
or volatile droplet is liberated on a lubricated piezoelectric crystal
(lithium niobate) at ambient temperature, the droplet instantaneously
propels for a long distance (which can be ~50 times the droplet radius)
in furcated routes. Depending on the crystal plane that interfaces with
the droplet, the self-propulsion can be unidirectional, bifurcated,
and even trifurcated.

The discovery has been published in Nature Nanotechnology in an article
titled "Furcated Droplet Motility on Crystalline Surfaces." "This is
an unforeseen phenomenon with far-reaching implications. Droplets
with a temperature difference mild at 5 DEGC on a surface can undergo self-sustained propulsion. Imagine placing a ball on a perfectly
leveled and smooth table, instead of remaining static, the ball rolls by itself. Even more surprising is that the ball only automatically rolls
towards certain definite directions," said Professor Wang Liqiu.

The researchers have found that the intrinsically orientated liquid motion
is fueled by cross-scale thermo-piezoelectric coupling which is caused by
the anisotropy of crystal structure. This resembles that a smooth table is atomically arranged in an unusual way such that a symmetric heat source
can produce asymmetric electric field that drives a ball into motion in
a direction determined by the cutting direction of the table surface.

"The work enables an innovative way to deliver and transport liquids
with controllability, versatility and performance, and provides clues
for solving some long-standing challenges such as anti-icing, defrost
and antifog in humid environments," said Dr Tang Xin.

As a droplet strikes a supercooled substrate such as that of an airplane
wing and power cable, it rapidly freezes and adheres to the surface. In
this case, the spontaneous electric force generated by the crystal
can perturb the nucleating droplet, potentially decreasing interfacial
adhesion and delaying detrimental ice accretion.

Self-propulsion will also upgrade the performance of dropwise condensation
by removing growing condensate from the surface, the thermal barrier,
and thus potentially providing a very promising solution to droplet manipulation in space where gravity-assisted droplet shedding is absent.

Moreover, the furcated routes may be selectively chosen by adding external disturbances such as subtle electric fields. In this way, the surface can
act as a two- or three-way planar valve to deliver droplets containing information, chemical or biological payloads.

"Clearly, this novel approach to liquid manipulation works for a wide
variety of liquids and piezoelectric crystals, hence opening opportunities
for further research, and new materials and technologies development,"
said Dr Li Wei.

Please click here for a video to illustrate the self-propulsion of
different liquids on a lithium niobate surface, and depending on the
crystal plane that interfaces with the droplet, the self-propulsion can
be unidirectional, bifurcated, and even trifurcated.

========================================================================== Story Source: Materials provided by The_University_of_Hong_Kong. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Xin Tang, Wei Li, Liqiu Wang. Furcated droplet motility on
crystalline
surfaces. Nature Nanotechnology, 2021; DOI:
10.1038/s41565-021-00945-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211004104300.htm

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