The wetting of droplets plays a critical role in engineering applications. Intensive study on nano-droplets is of great significance in revealing the wetting characteristics of droplets. The wetting behavior of ethanol/water mixture nano-droplets on a smooth substrate was investigated through molecular dynamics simulation in this paper. Five nano-droplets of the ethanol/water mixture with different mass fractions placed on a copper surface with different surface energies were simulated to equilibrium at 298.15 K. The contract angle, wetting limit (the minimum and the maximum surface tension of droplets make the surface completely wetted or completely non-wetted by the droplet), and density distribution at the solid–liquid interface of the nano-droplet were analyzed. The effects of ethanol concentration and solid–liquid interaction (corresponding to the droplet’s surface tension and the substrate’s surface energy, respectively) on droplet wettability were intensely discussed. Results revealed that the contact angle of nano-droplets decreased with the increase in ethanol concentration and the interaction between droplets and the substrate. In addition, the critical and ultimate surface tension increased with the increase in the substrate surface energy. The peak density values of the droplets were proportional to the interaction between the droplets and substrate and not related to the droplet concentration. The research also indicated that solid substrates with different surface energies and droplets with various components affected the contact angle of droplets in different ways: the former not only increased the surface tension of droplets at the solid–liquid interface but also increased the separation energy of solids and droplets, while the latter only had a great influence on the surface tension of droplets at the solid–liquid interface.

Many fields would greatly benefit from the realization of the manipulation of droplet impact behavior by an asymmetric surface structure, such as self-cleaning, anti-icing, inkjet printing, etc. However, research on the prediction of the impact of the dynamics of small-volume droplets on the asymmetric superhydrophobic surface has been insufficient. In this study, a superhydrophobic curved micropillar array surface with controllable bending angles induced by a magnetic field was prepared. The impact and rebound behaviors of the nanoliter droplets with diameters of 100–300 μm were investigated. The experimental results showed the positive correlation between the threshold Weber number of the impact morphology transition of the droplet and the inclination angle of the micropillar. In addition, the restitution coefficient, which measures the degree of energy loss during the impact process, showed a nonmonotonic dependence on the Weber number. A critical velocity model of the impact morphology transition of the droplet on the curved micropillar array surface and a prediction model of the restitution coefficient of the droplet in different impact morphologies are suggested. Our findings will help in the design of a functional surface for manipulating the impact behavior of the droplet.

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Droplet regulation has significant application potential in many fields; however, conventional controlling methods make it difficult to effectively control low surface tension droplets. Inspired by the Leidenfrost phenomenon, a pseudo-Leidenfrost system was established innovatively through micro-airflow rather than evaporated vapor to lift a droplet. Both experimental and numerical studies were carried out to investigate the pseudo-Leidenfrost effect of the FC3283 (perfluorotripropylamine) droplet. By FC3283, it is an extremely low surface tension working medium with thermal stability at room temperature. The oscillation of the droplet in the vertical direction was analyzed by tracking the position of the droplet centroid. The velocity of micro-airflow and pressure distributions at the bottom surface of the droplet, which were similar to the Leidenfrost phenomenon, were revealed. The mechanical analysis of the FC3283 droplet in a pseudo-Leidenfrost period was analyzed. Besides, the pseudo-Leidenfrost phenomenon of FC40 [FC-40 FluorinertTM Electronic Liquid] droplets with various Weber number was investigated. Weber number conditions for droplets triggering the pseudo-Leidenfrost phenomenon were revealed. The results showed that the motion of pseudo-Leidenfrost droplets in a period could be divided into three stages: falling, hovering, and rising. In the hovering stage, the Laplace force played an important role, which was the main reason for the rebound of the droplet, while the role of the aerodynamic force was to keep the droplet on the surface of the gas film. The Weber number had a significant influence on the pseudo-Leidenfrost phenomenon: droplets with a small Weber number tended to be absorbed by the micropores, while a too large Weber number would cause droplets to suspend or even leave. This study is helpful for controlling low surface tension droplets and laying a foundation for the transportation of low surface tension droplets.

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