Thermal conductance of hydrophilic and hydrophobic interfaces

Interfaces between water and hydrophilic or hydrophobic surfaces are of great importance for many biological and engineering systems. 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 using self-assembled monolayer. The thermal conductance per unit area of the interface G for hydrophobic Al or Au surface in water is determined to be 37 to 55 MW m$^{-2}$ K$^{-1}$. G for hydrophilic Al or Au surface in water is 150 to 300 MW m$^{-2}$ K$^{-1}$. Our new work on thermal transport complements the extensive research literature on momentum transport at aqueous interfaces: 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 (11-16 nm) is a factor of 3-8 larger than the Kapitza length at hydrophilic interfaces (2-4 nm); a change of terminal group from methyl to hydroxyl increases the Kapitza length by approximately 10 nm.