Neutron scattering study of the freezing of water near a cupric oxide surface

Oscillating heat pipes (OHP) offer promising two-phase heat transfer for a variety of applications, including cooling of electronic devices.$^{\mathrm{2}}$ Recently, it has been shown that a hydrophilic CuO coating on either the evaporator or condenser sections of a flat-plate OHP can significantly enhance its thermal performance.$^{\mathrm{3}}$ This finding has motivated us to assess the strength of the CuO/H$_{\mathrm{2}}$O interaction by investigating the freezing behavior of H$_{\mathrm{2}}$O in proximity to a CuO surface. Using the High-Flux Backscattering Spectrometer at NIST, we have measured the intensity of neutrons scattered elastically from a well-hydrated sample of CuO-coated Cu foils that mimic the oxide surfaces in a flat-plate OHP. We observe abrupt freezing of bulk-like H$_{\mathrm{2}}$O above the CuO surface at 270 K followed by continuous freezing of the interfacial H$_{\mathrm{2}}$O down to 265 K. This freezing behavior is qualitatively similar to that found for water near a zwitterionic single-supported bilayer lipid membrane.$^{\mathrm{3}}$ Further studies are planned to compare the diffusion coefficients of the interfacial water for the coated and uncoated OHPs.$^{\mathrm{2}}$ $^{\mathrm{2}}$F. Z. Zhang \textit{et al}., submitted to J. Heat Transfer. $^{\mathrm{3}}$M. Bai \textit{et al}., Europhys. Lett. \textbf{98}, 48006 (2012); Miskowiec \textit{et al}., Europhys. Lett. \textbf{107}, 28008 (2014).