DETERMINING THE PHASE-TRANSITION OF CONDENSATE FORMED BY HETEROGENEOUS NUCLEATION OF CONDENSABLE VAPORS ONTO A COLD SUBSTRATE

The heterogeneous phase transition of four organic vapors and water va por on a cold substrate was studied by either slowly cooling a substra te or by accumulating vapor under reduced-pressure conditions. At low vapor pressures a solid condensate formed directly on the substrate. A t higher vapor pressures, liquid condensate usually appeared first, fo llowed either by 1) subsequent formation of solid crystals independent ly from the liquid phase (subsequent solidification) or 2) rapid freez ing of the liquid condensate (subsequent freezing). Subsequent freezin g occurred in a wider temperature range than did the subsequent solidi fication. A new phase transition model is proposed, based on the equil ibrium phase diagram and the measured relationship between critical va por pressure and temperature for solid condensate formation. The propo sed model explains the experimental results quite well. Liquid condens ate always forms at critical supersaturation ratios close to unity. Th e critical vapor pressure and temperature at the boundary separating t he formation of the solid condensate from formation of the liquid cond ensate, as determined by the proposed model, agrees well with the expe rimentally determined values.