The thermal boundary conductance between water and self-assembled monolayer was studied using nonequilibrium molecular dynamics simulations. Different thermal transport behaviors were observed for hydrophobic and hydrophilic self-assembled monolayers. In the temperature range between 280 and 340 K, the thermal boundary conductance was found to depend on the temperature for hydrophobic self-assembled monolayers. On the contrary, the difference in thermal boundary conductance at different temperatures was slight for hydrophilic self-assembled monolayers. The correlations in velocity and density between terminal atoms of self-assembled monolayer and water molecules within the interface region were analyzed to understand the mechanism of thermal transport across the interface. The vibrational density of states calculation indicated that the temperature dependence does not originate from the overlap of phonon spectrum. The analysis of radial density distribution revealed that the temperature dependence is mainly attributed to the number of water molecules surrounding the terminal atoms of self-assembled monolayers.

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Thermal boundary conductance is typically positively correlated with interfacial adhesion at the interface. Here, we demonstrate a counterintuitive experimental result in which a weak van der Waals interface can give a higher thermal boundary conductance than a strong covalently bonded interface. This occurs in a system with highly mismatched vibrational frequencies (copper/diamond) modified by a self-assembled monolayer. Using finely controlled fabrication and detailed characterization, complemented by molecular simulation, the effects of bridging the vibrational spectrum mismatch and bonding at the interface are systematically varied and understood from a molecular dynamics viewpoint. The results reveal that the bridging and binding effects have a trade-off relationship and, consequently, that the bridging can overwhelm the binding effect at a highly mismatched interface. This study provides a comprehensive understanding of phonon transport at interfaces, unifying physical and chemical understandings, and allowing interfacial tailoring of the thermal transport in various material systems.

A clear understanding of the wettability of graphene and graphene-coated surfaces is of critical importance for the practical applications of graphene. The present study provides microscopic and thermodynamic perspectives into the wettability of graphene-coated surfaces by molecular dynamics simulations along with free energy calculations utilizing the umbrella sampling. The water droplet adhesion process on graphene-coated surface was characterized by the change in surface area, mean force, and free energy of the droplet. The thermodynamic landscape analysis reveals that the different contributions to the free energies from different underlying substrates induce different entropic resistances from graphene, which leads to the similarity in wettability for graphene-coated silicon and hydroxylated silicon dioxide substrates.

Controlling the thermal boundary conductance (TBC) between copper and carbon crystals is important since it can bottleneck the thermal conductivity when reinforcing copper with carbon-crystals fillers, namely diamond or graphite, to develop heat sinks and spreaders needed for the thermal management. In this work, by using the nonequilibrium molecular dynamics simulation, we show how the TBC of copper/diamond is smaller than that of copper/graphite by an order of magnitude due to poorer overlap of the lattice vibrational spectra. To improve the TBC at the copper/diamond interface, the covalently bonded self-assembled monolayers (SAMs) with different chain lengths are installed at the interface. The TBC is significantly improved and increases with the chain length to approach the value of the copper/graphite interface. The spectral analysis further identifies that this is because the SAMs become softer with increasing chain length due to disordering of the collective SAMs structure, enhancing the spectral transmission in low-frequency vibrational modes with copper. The obtained results are useful to improve the thermal conductivity of metal/carbon-crystal composite materials.