Energy transport at solid-gas,-vapour and-liquid interfaces: the role of adsorption

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As technology has advanced to the nano scale, understanding interfacial phenomena at the molecular level has become more important. Results from a previous Discovery Grant, proposed that vapour adsorbates consist of molecular-clusters, rather than molecular layers, as usually assumed. The latter had led to an embarrassing infinity in the limit of the vapour-phase pressure approaching the saturation-vapour pressure. The molecular-cluster concept was used to derive the Zeta adsorption isotherm (ZAI). With the aid of a recent CRD grant, the ZAI was shown to eliminate the infinity, and to lead to predictions that were strongly supported experimentally. In this proposal, we plan to extend the ZAI to an unexplored area: from equilibrium vapour adsorption to condensation, or in other words, from equilibrium adsorption to steady, non-equilibrium adsorption. We have made a first step by deriving an expression for a non-equilibrium ZAI that can be applied when the vapour phase is supercooled relative to the solid interface. We have conducted a series of steady-state, preliminary experiments to establish the feasibility of our planned experimental procedure for examining the non-equilibrium ZAI experimentally. A smooth silicon surface was exposed to heptane vapour when it was supercooled relative silicon surface. The thickness of the adsorbed film was measured with a interferometer; the vapour temperature was measured with a micro-thermocouple mounted on a positioning micrometer; and the surface temperature was determined using 12 thermocouples implanted in the Si substrate. These measurements indicated an interfacial temperature discontinuity during each steady, condensation experiment. An interfacial temperature discontinuity is now the expected result when a phase change is occurring at an interface, condensation in this case. When the supercooling reached a particular value, a phase transition was observed: the adsorbed vapour phase became ~ 50 times thicker, although not yet an adsorbed liquid. With a solid surface was supercooled relative to a vapour, we propose to conduct to three projects with the objective of investigating interfacial energy transport at solid-fluid interfaces under non-equilibrium conditions. The three projects are entitled:1) Non-equilibrium adsorption and interfacial phase transitions; 2) Interfacial energy transport at solid-vapour interfaces; and 3) Mechanism of nano bubble stability at solid-liquid interfaces. The third project is viewed as a necessary study before examining interfacial energy transport at solid-liquid interfaces.