2014-05-07
10:15 at HCI J 574

Heat and Mass Transfer through Interfaces of Nanosized Bubbles and Droplets

Oivind Wilhelmsen

Department of Chemistry, Norwegian University of Science and Technology, Trondheim, Norway

Bubbles and droplets are found everywhere, as precipitating rain or in your soda. They are all born at the nanoscale, where their sizes are comparable with the interfacial width. Consequently, their behavior in the initial growth phase depends strongly on the properties of the interface. An introduction is given to how curvature dependent interfacial resistances to heat and mass can be calculated for nanoscopic bubbles/droplets with density functional theory. First, we discuss how bubbles and droplets can be stabilized in closed systems, to properly examine their properties. Using the first approximation to density functional theory, the square gradient model, interface resistances are calculated for bubbles/droplets with radii down to 2 nm. We present interface resistances which vary continuously with curvature, from negative (bubbles) to zero (planar interface) to positive (droplet) values. The interface resistances of 2 nm radii bubbles/droplets are in some cases one order of magnitude different from those of the planar interface. Molecular dynamics simulations and experiments indicate that the thermal resistance of a droplet decreases with its size. We use local resistivity functions qualitatively consistent with non-equilibrium molecular dynamics simulations, where the peak in resistivity is closer to the vapor-phase. Remarkably, density functional theory then predicts thermal interface resistance of droplets to decrease with droplet size, similar to experiments. Moreover, the framework also predicts curvature to have the opposite effect on interface resistances of bubbles. We believe that this explains why thermal conductivity is enhanced with decreasing particle size in nanoparticle suspensions, while the thermal conductance decreases with smaller pores in nanoporous materials. The importance of including the heats of transfer through the interface is emphasized, and we show that they depend much less on curvature than the interface resistances.