The Interaction Between the Substrate and Frost Layer Through Condensate Distribution

摘要

Microscopic observations of frost deposition on a variety of substrates havingdifferent contact angles, (polytetrafluoroethylene PTFE, kapton, glass and others) allowthe quantification of substrate effects on frost structure during inception and growth. Thedeposition of water vapor at the beginning of the frosting process on a clean glasssubstrate is found to be as condensate (condensation frosting) rather than as ice(ablimation frosting) for a substrate temperatures above -33??C and an absolute humidityabove 0.15 g/kg. The inception of "condensation frosting" (the condensation period andearly frost growth period) is further examined microscopically as a function of air andsubstrate temperatures, absolute humidity, and substrate contact angle. The waterdistribution on the substrate at the end of the condensation period is found to be stronglydependent on substrate temperature, humidity ratio, and substrate contact angle. Coldersubstrates result in smaller more uniform droplets and substrates with lower contactangles result in shorter, larger diameter droplets with a larger percentage of the substratecovered. The effective density of the condensate on hydrophobic substrates is found tobe lower than that on hydrophilic substrates. The structure and form of the iceimmediately after freezing is substrate dependent. High-speed imaging of the freezingprocess is used to study the propagation of the freezing front in a droplet. The imagesshow that a protrusion is formed at the top of the droplets during freezing. Fromobservations, this protrusion is hypothesized to result from the convective condition atthe droplet surface and the difference in specific volume between liquid and solid water.Additionally, the apparent ejection of water vapor during freezing of a droplet on a hydrophobic substrate was observed. This ejection of water vapor is thought to be causedby the wanning of the droplet caused by the release of latent heat. In contrast to trendsobserved during the early growth period, the growth rate of mature frost is found todecrease with substrate contact angle while frost density is found to increase. Thisbehavior is explained in terms of the effect of substrate contact angle on the structure andform of the incipient frost, which constitutes the initial condition for further (mature)frost growth. A higher conductivity layer is formed on the hydrophilic than on thehydrophobic substrate. A model relating crystal orientation to conductivity is used tosimulate the frost growth rate and density on the two different substrates and match theexperimental data. Using similar reasoning, the higher conductivity frost formed oncolder substrates is also explained.