The Study Of Superhydrophobic Surface In Anti-Dew And Anti-Icing Properties

The application of superhydrophobic surface (SHS) in anti-dew and anti-icing has attracted extensive attention in recent years. However, when the environment humidity is higher and the surface temperature is low, condensed water droplets may push the embeded air out of the micro/nanostructure gaps, resulting in Wenzel adhesion. It is well known that it is Cassie drops that could slide away SHS easily. Thus fabricating SHS on which the condensate drops could hold Cassie state during condensation prodecure is the key step in achieving anti-dew and even anti-icing functions. In our earlier studies, we have shown that SHS with high-aspect-ratio nanostructures could endow Cassie condensation and form coalescence induced spontaneous motion phenomenon of condensate droplets.In first part of this study, SHSs with and without microflowers on nanosheet structures were fabricated on copper substrates by different chemical etching process and same fluoridation treatment. Their superhydrophobic stability and spontaneous motion phenomenon during water vapor condensation were studied by water contact angle measurement and optical microscopy observation. The results showed that the SHS without microflowers had better superhydrophobic stability and more obvious spontaneous motion phenomenon than those with microflowers. This study would be helpful in designing SHS with anti-fogging and anti-icing function.In seond part of this study, copper SHSs with jumping condensation phenomenon were selected as substrate for observing condensation icing at-10℃by eyes and microscope. The results showed that when the surfaces were placed upward (-10℃), significantly delayed interdrop freezing phenomenon was found. Some discrete ice crystals firstly appeared on the SHS, and then eventually proliferated on the entire surface through interdrop freezing and direct vapor-ice deposition mechanism. However, when the SHS was placed downward (-10℃), delayed frosting phenomenon also appeared but little interdrop freezing phenomenon was found. The ice crystals emerged at the hydrophilic edges of the SHS firstly and then spread slowly to the surface center by direct vapor-ice deposition. The frosting front looks like sword and the propagating seems hanging in the air. This demonstrated that the mechanism of delayed frost growth on SHSs with jumping condensation is not limited by interdrop freezing but also includes direct vapor-ice deposition. The key approach in delaying frost growth on SHS should be retarding or restraining initial ice crystal formation.In third part of this study, the SHSs with finer nanostrctures were fabricated by chemical etching and fluoride treatment on copper and aluminum surface, respectively. Water vapor condensation trials results show that the SHSs with finer nanosrructure exhibited more obvious anti-dew character, e.g., after a period of condensation, there is lots of "drying" area on them. On aluminum SHSs with submicron rectangle microstructures, typical self-propelled motion or jumping condensation occurred. However, on aluminum SHS with coral like micro/nano-structures, vapor nucleation occurred tardily, randomly and sparsely. Different with continuous, homogeneous self--propelled motion or "jumping" on copper SHS with relatively larger nanoribbon structures, the condensate droplets on copper SHS with finer nanosheet structures appear homogeneously at beginning stage but quickly coalesce into several larger drops and stop motion or "jumping". Much higher vapor-liquid nucleation energy barrier caused by much finer nanostructures should be responsible for such more obvious anti-condensation property. These studies would be helpful in designing new SHSs and moving their application in anti-icing, antifogging, air humidity control, et al.