Thermal Conductance of Hydrophilic and Hydrophobic Interfaces

Abstract

Using time-domain thermoreflectance, we have measured the transport of thermally excited vibrational energy across planar interfaces between water and solids that have been chemically functionalized with a self-assembled monolayer (SAM). The Kapitza length—i.e., the thermal conductivity of water divided by the thermal conductance per unit area of the interface—is analogous to the “slip length” for water flowing tangentially past a solid surface. We find that the Kapitza length at hydrophobic interfaces (10–12 nm) is a factor of 2–3 larger than the Kapitza length at hydrophilic interfaces (3–6 nm). If a vapor layer is present at the hydrophobic interface, and this vapor layer has a thermal conductivity that is comparable to bulk water vapor, then our experimental results constrain the thickness of the vapor layer to be less than 0.25 nm.