Evaporatively controlled growth of salt trees

During the latter stages of the evaporation of ammonium chloride solutions, highly ramified salt structures, centimeters in height, are formed. In contrast with many, if not all, of the commonly discussed branched solidified objects (e.g., dendrites or diffusion-limited aggregation clusters), the porous nature of the salt trees leads to growth in a direction away from the material source owing to a coupling between capillarity and evaporation. A physical mechanism for the phenomenon is reported, growth as a function of external convection is studied and demonstrates growth directed opposite the convective flow, and a method for determining a fractal dimension of the three-dimensional trees is described.