The mechanisms of frost formation on a semipermeable membrane

Abstract

In this paper, the process of frost formation on a semipermeable membrane, which allows water vapour to pass through the membrane, is studied. The frosting process is analysed experimentally on a semipermeable membrane at different water vapour transfer rates (zero, low and high) and an impermeable plastic sheet. Through photographs of the frosting process, a delay in freezing of dropwise condensates on a semipermeable membrane is reported for the first time. Furthermore, it is shown that the frost spreads on a semipermeable membrane slowly because of a water vapour transfer through the semipermeable membrane. While the impermeable surfaces (impermeable plastic sheet and membrane at zero water vapour transfer condition) are covered with a layer of frost after only 5 min; after 2 h of an experiment, a mixture of water droplets and frost is observed on the surface of the membrane. Also, the delay in freezing of dropwise condensation can be increased as the water vapour transfer rate through the membrane increases. Moreover, the shape and structure of the frost layer are remarkably different on the semipermeable membrane from that on the impermeable surface. The frost layer on the membrane is similar to a cluster of irregular hexagonal ice crystal columns that are connected to each other with horizontal growth of ice crystals, while the frost layer on the impermeable surface is a bushy layer of ice crystals. The delay of frosting on a semipermeable membrane has many benefits for the field of membrane fabrication for low-temperature applications.