Retardation of freezing of precooled, impinged water droplets on glass surfaces with microgrooves and silane coating.

Freezing impinged water droplets on glass surfaces cause serious problems such as reduced visibility of traffic lights and surveillance cameras. Droplets in the air associated with these issues are often at subzero temperatures. However, experimental results on the freezing of precooled impinged droplets are limited. In this study, we measured the freezing of precooled and impinged water droplets on cold glass surfaces. Two types of lattice-patterned microscale grooves were formed on glass surfaces to reduce the contact area of droplets and growth of frosts, which contributed to droplet freezing. In addition, the surfaces were coated with a silane coupling agent to further reduce the contact area. We analyzed the images of droplets captured using a high-speed video camera. The results of the linear relationships between the frozen droplet height, freezing front velocity, and freezing time (for the impinged droplets) indicated that the grooves and coating were effective in retarding the freezing of impinged droplets. This retardation was more evident for frost-free glass surfaces, and it was less evident for precooled droplets. Moreover, a simple heat transfer analysis was conducted to effectively estimate the overall heat flux and freezing front velocity. The sublimation of frost (adjacent to the impinged droplets) and supercool elimination of the precooled droplets significantly contributed to the heat flux and caused an increase in the freezing front velocity.