ISSN Online: 2377-424X
International Heat Transfer Conference 12
Effect of conjoining/disjoining pressures on evaporation from or condensation on coated surfaces
Abstrakt
In the present work we consider first a model for the evolution of a thin apolar liquid film on a coated solid surface under the action of attractive and repulsive molecular forces governed by a 3-4 power-law potential, rather than the Lennard-Jones 3-9 potential employed for an ideal plane interface (molecularly clean and smooth). The model is used for both volatile and non-volatile
isothermal liquid films. It is shown that in the non-volatile case the evolution results in the emergence of static steady states consisting of liquid ridges separated by very thin films. A supercritical bifurcation from the trivial state is shown to be possible in the presence of repulsive forces, while in the presence of only attractive forces the bifurcation is subcritical. In the
evaporative case the long-time evolution of the film is shown to lead to its flattening due to the reservoir effect, and then to its apparent vanishing. Several scenarios for the film disappearance are found. The "kinetic steady state" process of long intervals of near-static states with rapid
transitions to ever-coarser structures thus competes with, and is eventually overwhelmed by, direct transition to dry regions by evaporation. A relationship between the rate of expansion of the dry spot and the apparent contact angle is examined. Also, the dynamics of a condensing apolar
ultrathin liquid film is studied for both horizontal and slightly tilted solid coated surfaces. When condensation is slow, the film on a horizontal substrate passes through the stages of hole opening driven by the reverse reservoir effect, hole closing and eventual thickness equilibration and spatially uniform growth of the condensate. When condensation is faster and resistance to phase change is lower, secondary droplet(s) may emerge within the hole.