Research progress and outlook of the effect of wetting transition on the freezing of impinging droplets on superhydrophobic surfaces

Freezing of impinging droplets widely appears in industrial applications, such as energy power, electric communications, aerospace, and many others, which seriously interferes with equipment operation. Due to this fact, how to inhibit the impinging droplet freezing is of great significance in practical applications. It is worth recalling that the droplet bounces off surfaces as it impacts on superhydrophobic surfaces, and hence the impinging droplet freezing is thoroughly inhibited by the rebound characteristics of superhydrophobic surfaces. However, superhydrophobicity is the basis for droplet rebound. As the Cassie-Wenzel irreversible wetting transition is triggered by droplet impact, this irreversible wetting transition causes the failure of superhydrophobicity such that droplets adhere to and freeze on cold superhydrophobic surfaces. As a result, from the viewpoint of wetting transition, how to enhance the rebound characteristics of superhydrophobic surfaces is essential to inhibit the impinging droplet freezing. This paper provides a review on the influence of wetting transition on the impinging droplet freezing on superhydrophobic surfaces, ranging from its effect on dynamic behaviors, its effect on freezing forms to energy paths and transition mechanisms for impact-induced wetting transition. Based on this review, it is found that as droplets impact on superhydrophobic surfaces, it can bounce off only when the Cassie-Wenzel irreversible wetting transition is not triggered, or Cassie-Wenzel-Cassie reversible wetting transition is triggered. Secondly, when the impinging droplet adheres to the cold superhydrophobic surface, the impinging droplet freezes in the form of the Wenzel state as the Cassie-Wenzel irreversible wetting transition is thoroughly triggered, in the form of the partial Wenzel state as the Cassie-Wenzel wetting transition or the Wenzel-Cassie dewetting transition is partially triggered, and in the form of the Cassie state as the Cassie-Wenzel-Cassie reversible wetting transition is triggered. Compared with the partial Wenzel state droplet and the Cassie state droplet, the freezing delay time is less for the Wenzel state droplet because the solid-liquid contact area is larger for the Wenzel state droplet than that for the partial Wenzel state droplet and the Cassie state droplet. Thirdly, on the superhydrophobic surfaces at room temperature, the energy paths and transition mechanisms have been revealed for the impact-induced Cassie-Wenzel and Wenzel-Cassie wetting transition. However, on cold superhydrophobic surfaces, these energy paths and transition mechanisms are absent, which should be revealed by researchers. Based on the above summary, the impinging droplet freezing can be thoroughly prevented on cold superhydrophobic surfaces as the Cassie-Wenzel irreversible wetting transition is inhibited or the Cassie-Wenzel-Cassie reversible wetting transition is promoted. The outlook is provided in the end of this review, which includes that by the design of surface topology, inhibiting the irreversible wetting transition and promoting the reversible wetting transition that is based on the energy paths and transition mechanisms of wetting and dewetting transition on cold superhydrophobic surfaces.