Evaporation of a Liquid Droplet of Aqueous Solutions on Glass Plates and the Patterns of Their Deposits after Evaporationt

Abstract

The evaporation of a droplet of NaC1 aq. solution in various concentrations and of 0.2wt% Na₂SiO₃ on untreated and surface-treated glass plates was investigated through the change of contact angle and the length- of the bottom of a droplet against time and the deposit of the solution droplet after evaporation was observed. The bottom length of a droplet on the untreated glass plate is constant but its contact angle decreases during evaporation. The solute was deposited on the edge of a droplet and showed caldera-like pattern after complete evaporation. The bottom length of a droplet on the surface-treated glass plate is constant, but its contact angle decreases to a certain angle and then the bottom length begins to decrease with a constant contact angle. A single agglomerate of NaCl crystals was deposited after evaporation of a droplet of NaCl aq. solution, and a caldera-like deposit was observed after evaporation of a droplet of Na₂SiO₃ aq. solution. The radius of this calderalike ring was found to be apparently shorter than that of an initial droplet. The mechanism of the evaporation and the pattern of deposit after evaporation are discussed.