Experimental investigation of water droplet impact and freezing on micropatterned stainless steel surfaces with varying wettabilities

Abstract

Ice formation and accumulation on surfaces can cause operational risks and failure to various engineering structures and systems. Liquid repellent surfaces could reduce ice accretion and improve asset integrity and safety in harsh environments. This article examines how surface wettability affects water droplet impact and freezing on a surface. It presents experimental results of water droplet impact and freezing on horizontal and inclined stainless-steel surfaces with varying wettabilities achieved through microscale texturing and coating. The water droplets are at 5 °C before impacting on the target surfaces and the diameter of the droplets ranges from 1.80 mm to 4.11 mm. The static contact angles of water droplets on the surfaces range from 77° to 145° and the surface temperatures vary from −10 °C to −13 °C. The droplet impact speed ranges from 0.77 m/s to 1.17 m/s. In addition, all experiments are conducted under approximately the same room air temperature and relative humidity of 15 °C and 30% respectively. It is found that the surface and water droplet properties, along with its impact velocity, affect the droplet dynamics after impacting on the surface, which in turn affects the heat transfer and the freezing process. The results demonstrate that water droplets spread less and oscillate longer on more hydrophobic surfaces, which leads to a delay of freezing and significantly longer total icing time due to the lower heat transfer rate caused by less contact between the liquid and the surface. A two-stage droplet freezing process is observed through high-speed imaging. A small tip forms at the top of the droplet when it is completely frozen. The contribution of droplet volume (larger droplets need a longer freezing time) is found to be more significant than the effects of droplet dynamics (larger contact areas cause a shorter freezing time). On an inclined surface, the droplet shows a gliding and stretching process. The total icing time decreases at higher inclinations due to the increased contact areas. This paper demonstrates an icing delay with water repellent surfaces on engineering metals such as stainless steels.