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Ever more powerful and densely packed chips for applications like cryptocurrency mining and artificial intelligence generate such enormous heat fluxes that designers are pivoting from gas to liquid cooling to forestall damage from thermal runaway. Even with optimal flow patterns however, the intrinsic thermal boundary resistance at the liquid/solid (L/S) interface poses an additional source of thermal impedance. There is a lingering misconception in the field that the higher the liquid contact density, the more frequent the L/S collision rate and the smaller the thermal slip length. Here we present an insightful counterexample based on non-equilibrium molecular dynamics simulations of a classical liquid confined between different facets of a face centered cubic crystal held at different temperature. We have conducted a comprehensive study to quantify thermal exchange and propagation across the interface by varying the L/S interaction energy, L/S repulsive distance, facet orientation, thermal flux and local temperature with particular emphasis on the properties of the liquid contact layer (i.e., liquid monolayer adjacent to the solid surface). Numerous static and dynamic quantities characterizing the contact layer reveal the ways in which long range order, anisotropy of the L/S potential and correlated motion act to reduce the thermal slip length. Systems with the smallest thermal slip length exhibit two distinct features: 2D caged motion with string-like alignment of liquid particles unlike that observed in glassy systems and larger non-ergodicity parameter but shorter, not longer, caging times. These simulations have revealed two master curves which help unify the various influences at play. The first relation directly links the thermal slip length to the temperature modified 2D static structure factor representing long-range order in the contact layer. The second relation directly links the thermal slip length to the temperature modified dominant frequencies of the first solid and liquid layer as extracted from the density of states. These correlations, which represent power law dependencies, offer a new paradigm for the design of L/S interfaces to maximize thermal exchange across a classical L/S interface. |
