Study of the Influence from the Disjoining Pressure and the Liquid Conduction Resistance to the Thin Film Evaporation

Thin film evaporation in a meniscus is critical to the design of heat pipes and applications related to desalination, lubrication, and medical technologies. Theory suggests that with smaller film thickness, the local evaporation mass flux will increase due to increasing interfacial temperature. The rise of mass flux is followed by a sharp decrease to zero in the non-evaporating region, due to interfacial forces that inhibit the escape of vapor molecules. With novel atomistic simulations and thermal experimental approaches, we studied the local evaporation profile in the meniscus region. The interfacial force, named disjoining pressure, is studied through molecular dynamics (MD) simulations. The first-ever validation of the Hertz-Kundsen-Schrage relation with disjoining is performed using MD simulations and good agreement has been achieved. The experimental efforts to determine the local evaporation mass flux across the meniscus is also conducted to study the influence from the liquid conduction resistance. We use frequency domain thermoreflectance (FDTR), a non-contact laser-based method with micron-scale lateral resolution. A two-step workflow is developed to extract evaporation properties. In step one, a neural network (NN) model is trained on a series of representative finite element (FE) simulations to emulate the FDTR experiment, accounting for the non-uniformity of the meniscus thickness. In step two, the trained NN model is applied on the FDTR signals for property extraction. The maximum evaporation heat transfer coefficient across the meniscus is found to be 1.0 +0.5 −0.4 MW/m²-K, with a spatially weighted substrate temperature rise of 20 K. In the end, a technique proposed to reduce the uncertainty caused by the laser spot radius in FDTR with a reference sample is also discussed.