Freezing propagation between condensed droplets: Visualization and formulation of governing equation from ice bridge growth velocity

Abstract

Condensation frosting causes performance degradation and other problems in various fields, such as wind power generation and heat exchangers. During the initial stage of condensation frosting, freezing propagation from frozen to supercooled water droplets is often observed, the dynamics of which must be clarified to solve these problems. A frozen droplet forms a protrusion to an adjacent supercooled droplet, known as an ice bridge; the growth velocity of the ice bridge is essential for discussing freezing propagation. In this study, we fabricated wettability-patterned surfaces on silicon wafers to obtain droplets with controlled spacing and size and visualized and evaluated the ice bridge formation process under reproducible conditions, and the local velocity change in the ice bridge growth was measured for the first time. Experiments showed the local velocity of the ice bridge growth increased as the tip of the ice bridge approached the adjacent supercooled droplet. Simple model equations for the freezing propagation velocity and ice bridge growth were proposed. The proposed model agreed well with the measured velocity and did not include any fitting or geometric parameters of the ice bridge or droplets, such as volume or shape.