How Wetting and Adhesion Affect Thermal Conductance of a Range of Hydrophobic to Hydrophilic Aqueous Interfaces

We quantify the strength of interfacial thermal coupling at water-solid interfaces over a broad range of surface chemistries from hydrophobic to hydrophilic using molecular simulations. We show that the Kapitza conductance is proportional to the work of adhesion—a wetting property of that interface—enabling the use of thermal transport measurements as probes of the molecular environment and bonding at an interface. Excellent agreement with experiments on similar systems [Z. B. Ge et al., Phys. Rev. Lett. 96, 186101 (2006)] highlights the convergence of simulation and experiments on these complex nanoscopic systems.

Figure 1

Snapshots of water droplets on self-assembled monolayers presenting nonwetting ($-{\mathrm{CF}}_3$), partially wetting ($-{\mathrm{CONHCH}}_3$), and wetting ($-\mathrm{OH}$) head groups to water. The plot shows the relationship between $\cos (\theta )$ measured in simulations for droplets of water containing 2176 molecules and in experiments [6] on similar surfaces. Two open circles show limiting macroscopic extrapolations for $-{\mathrm{CF}}_3$ and $-{\mathrm{OCH}}_3$ surfaces obtained from several drop-size-dependent simulations.

Figure 2

(a) A snapshot of the system used in thermal transport simulations (water: red and white; surfactant tails: cyan; head groups: green; sulfur atoms: yellow). Heat source is at the center of the water phase, i.e., at the edge of the simulation box, and the heat is removed from sulfur atoms (yellow) at the center. The inset shows density profiles of water and SAM (for two chemistries) in arbitrary units: SAM densities $-{\mathrm{CF}}_3$ (green) and $-\mathrm{OH}$ (blue); vicinal water densities $-{\mathrm{CF}}_3$ (red) and $-\mathrm{OH}$ (black). No vapor formation near SAM is evident. (b) Steady-state temperature profile for hydrophilic $-\mathrm{OH}$ (SAM)-water interface. (c) Same as in (b) but for hydrophobic $-{\mathrm{CF}}_3$-water interface.

Figure 3

Heat flux as a function of the temperature drop at hydrophobic and hydrophilic interfaces. The linear relationship indicates that despite large thermal fluxes involved, the system is within the linear response regime. The horizontal dashed line shows the thermal flux employed in simulations.

Figure 4

Interfacial thermal conductance as a function of [$1+\cos (\theta )$]. Simulations results (circles) are in good agreement with experimental data [10] (triangles) and show the direct proportionality of interfacial thermal conductance to the work of adhesion.