Thermal gradient evaporation

In many experiments, strong thermal gradients are present in the liquid which cannot be disregarded (see Section 9.3). On the other hand, below 2.2K, that is in the superfluid state, the thermal conductivity is extremely high (theoretically infinite, see Fig. 2.13), and bubbles do not form in the liquid during the evaporation process, since the temperature gradient is very close to zero. 

Preparation of SHG-Active Composites. A thermal gradient apparatus was utilized to grow aligned crystals from solution (rather than from melt, as described above), by drawing the sample from the cold side (below a temperature at which the solvent evaporates at a significant rate) towards the hot end. 

Figure 2. Methods of achieving supersaturation in high temperature solutions ABC, slow cooling AD, evaporation EF, thermal gradient transport. (From Ref. 2.).

If, on the other hand, the thermal gradient is not applied externally but is created by evaporation, the drops become festooned around their edges, as shown in Figure 10.11. The drops do not, however, grow fingers because no external force is applied to them. 

In the inner region, I, a steep thermal gradient causes a migration of porosity and the formation of columnar grains with a density very close to theoretical. This behavior is thought 4) to be the result of evaporation... 

Finally, it is to be expected that the evaporation coefficient of a very stable compound, such as alumina, which has a large heat of sublimation resulting from the decomposition into the elements, will be low. Since the heat of evaporation must be drawn from the surface, in die case of a substance widr a low thermal conductivity such as an oxide, the resultant cooling of the surface may lead to a temperature gradient in and immediately below the surface. This will lower die evaporation rate compared to that which is calculated from the apparent, bulk, temperature of the evaporating sample as observed by optical pyromeuy, and thus lead to an apparently low free surface vaporization coefficient. This is probably die case in the evaporation of alumina in a vacuum. 

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