Enhancing liquid delivery on non-wetting surfaces

In diverse processes such as fertilisation, insecticide treatment and cooling, liquid delivery is compromised by the super-repellency of the receiving surfaces, causing droplets of liquid to ‘bounce’ off these surfaces like a golf ball bouncing along a fairway. This results in issues such as pesticide overuse, soil contamination and water waste.

Now, mechanical engineering researchers at The University of Hong Kong (HKU) have developed a strategy to enhance liquid disposition by overlaying impacting droplets with a tiny amount of lubricant (less than 0.1 vol% of the droplet), to modify their interfacial properties for easy manipulation. As published in the journal Nature Communications, the overlayers suppress the out-of-plane rebounds by slowing the departing droplets through viscous dissipation and sustaining the droplets’ in-plane mobility through self-lubrication — a preferential state for scenarios such as shedding of liquid in spray cooling and repositioning of droplets in printing.

“Analogically, the ultrathin liquid overlayer turns the elastic golf ball into a balloon filled with water,” said Dr Xing Han, first author on the paper, from the HKU Department of Mechanical Engineering. “As the water balloon hits the ground, the water whirls and flows, consuming the kinetic energy of the water balloon. In this way, the balloon will not rebound easily.”

Currently, special polymers are added into the droplet to modify the liquid’s interfacial properties. Such polymers act like superglue to pin the droplet onto the surface; however, they will also mix with the content and change the nature of the droplet. The new method allows a thin layer of lubricant to be formed on the droplet, while leaving its content intact. The choices for the overlayers can be of a wide diversity, depending on the liquid type.

“The footprint of our method can be made to be minimal, circumventing surface contamination and toxification,” Dr Han said.

“Our method enables multifunctional and dynamic control of droplets that impact different types of non-wetting surfaces. The droplet’s post-deposition state can be switched between immobilised and sliding on the surface by tuning the overlayer volume, enabling rich fluid controls on repellent surfaces, including superhydrophobic, superomniphobic and superheated types, similar to depositing golf balls on all kinds of terrain.”

Another potential application of the new technology is in spray cooling. The researchers estimate that the technology can enhance the cooling rate of spray cooling fourfold, meaning only one-fourth of the amount of water or time is required to obtain the same cooling effect. Water droplets overlaid with liquids of a higher boiling point will be less easy to vaporise, and hence more readily able to come into direct contact with the overheated surfaces — whereas in conventional water spraying, vaporised water will form a barrier inter-facially to prevent more water from reaching the heated surfaces to substantially retard the cooling process.

“Imagine cooling a hot pot by spraying water,” Dr Han said. “Using the technology, the water is firmly deposited onto the pot, allowing fast cooling.”

“This is especially important for application in major catastrophic events involving high temperatures such as fire or overheating at power plants, and in other industrial applications,” concluded Liqiu Wang, Chair Professor of Thermal-Fluid Sciences and Engineering, from the Department of Mechanical Engineering.

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