Gradient Quasi-Liquid Surface Enabled Self-Propulsion of Highly Wetting Liquids

Self-propulsion of highly wetting liquids is important in heat exchanger, air conditioning, and refrigeration systems. However, it is challenging to achieve such a spontaneous motion as these liquids tend to wet all the surfaces due to their ultralow surface tensions. Despite that extensive asymmetric surface structures and gradient chemical coatings are developed for directional droplet transport, they will be flooded and covered by these liquids. Here, this challenge is addressed by creating a gradient quasi-liquid surface to achieve the self-propulsion of droplets with surface tensions down to 10.0 mN m⁻¹. Such a surface engineered by tethering flexible polymers with gradient grafting density shows ultralow contact angle hysteresis (<1^o) to highly wetting liquids. Thus, the surface can simultaneously provide sufficient driving forces through the gradient wettability and negligible retention forces through the slippery boundary lubrication for spontaneous droplet movement. Moreover, continual self-propulsion of tiny droplets is achieved by spraying highly wetting liquids in simulated condensation conditions and demonstrates that adding temperature gradient can further accelerate the self-propulsion. The study provides a new paradigm to promote passive removal of highly wetting droplets, leading to potential impacts in enhancing condensation heat transfer regardless of surface orientations.