A droplet resembles a ball. As a flying golf ball lands, it instantaneously rebounds, a scenario similar to the impact of insecticide droplets falling on the surface of leaf crops. Such bouncing makes it difficult for the insecticide to be delivered onto the plant. In diverse processes such as fertilisation, insecticides and cooling, liquid delivery is compromised by the super-repellency of receiving surfaces, causing issues such as pesticide overuse, soil contamination and water resource waste.
A mechanical engineering research team at the Faculty of Engineering of the University of Hong Kong (HKU) has developed a conceptually different 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. The findings have been published in the journal Nature Communications.
The paper’s first author, Dr. Xing Han, a postdoctoral fellow at the HKU Department of Mechanical Engineering said that analogically, the ultrathin liquid overlayer turns the elastic golf ball into a balloon filled with water. As the water balloon hit the ground, the water whirls and flows, consuming the kinetic energy of the water balloon. In this way, the balloon will not rebound easily.
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.
Currently, special polymers are added to the droplet to modify the liquid’s interfacial properties. Such polymers act as superglue to pin the droplet onto the surface. They will however 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 wide diversity, depending on the liquid type.
Dr Han explained that the footprint of the team’s method can be made to be minimal, circumventing surface contamination and toxification. The technology enables multifunctional and dynamic control of droplets that impact different types of nonwetting 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 spray cooling. The research team estimates that the technology can enhance the cooling rate of spray cooling by fourfold, i.e., 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 make it less easy to vaporise, and hence more readily 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. Using the technology, the water is firmly deposited onto the pot, allowing fast cooling.” Dr Han said.
The research 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, Professor Liqiu Wang, Chair Professor of Thermal-Fluid Sciences and Engineering, from the Department of Mechanical Engineering said.
