Nanoscale liquid-vapor interfaces and their role in microbubble formation.

摘要

The study of liquid-vapor interfaces plays a critical role in modern sciences and technologies. Most studies treat nanoscale liquid-vapor interfacial regions as single dividing surfaces in which abrupt changes of material properties occur. Such a treatment occasionally introduces large errors when the characteristic length of the system is comparable to interface thickness. The objective of this work is to conduct a detailed investigation on nanoscale liquid-vapor interfacial regions and then to apply gained insight to study the mechanism of microbubble formation.; Molecular Dynamics (MD) simulation has a unique capability of capturing detailed molecular-level information. Coupled with statistical thermodynamic models, MD simulation is applied to study both planar and spherical interfaces. Simulation on atomically thin liquid films reveals a new “interface overlapping” phenomenon, in which as film thickness decreases, the two interfacial regions of a liquid film merge together. The interface merge has a significant effect on stability conditions of liquid films, and has been overlooked in all classical studies. MD simulation on spherical interfaces is to calibrate Tolman's equation, which describes the curvature effect on surface tension. Results indicate that surface tension of droplets follows prediction of Tolman's equation very well whereas surface tension of bubbles does not.; A new scenario is proposed to describe microbubble formation on a smooth surface. Classically, bubble nucleation on a solid substrate has been described as heterogeneous nucleation from cavities. For a heater with a smooth surface, the cavity sizes are on nanometer scale and will require high temperature to activate. As the heater temperature increases, all liquid molecules adjacent to the heater surface will be vaporized and a vapor film forms over the heater. Bubbles will form as the vapor film breaks. The criterion for bubble formation is then correlated with vapor film stability, which in turn is determined by vapor film dimensions, and therefore the heater size and temperature. Two sets of experiment are carried out to verify this scenario: experiments on microbubble formation during Joule heating confirm the heater size effect; and time-resolved images of bubble evolvement when the heater is subject to short-pulsed laser radiation reveal the existence of vapor films.