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We use the Zeta adsorption isotherm and propose a method for determining the conditions at which an adsorbed vapour becomes an adsorbed liquid. This isotherm does not have a singularity when vapour phase pressure, P(V), is equal to the saturation-vapour pressure, Ps, and is empirically supported by earlier studies for P(V) < Ps. We illustrate the method using water and three hydrocarbon vapours adsorbing on silica. When the Zeta isotherm is combined with Gibbsian thermodynamics, an expression for γ(SV), the surface tension of the solid-vapour interface as a function of x(V)(≡P(V)/Ps) is obtained, and it is predicted that adsorption lowers γ(SV) from the surface tension of the substrate in the absence of adsorption, γ(S0), to that at the wetting condition. The wetting hypothesis indicates that γ(SV) at wetting, x, is equal γ(LV), the surface tension of the liquid-vapour interface. For water vapour adsorbing on silica, adsorption lowers γ(SV) to γ(LV) at xVW equal unity, but for the hydrocarbons heptane, octane and toluene adsorbing on silica xVW is found to be 1.40, 1.30 and 1.32 respectively.

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This paper presents a series of molecular dynamics simulations of the evaporating process of an argon droplet on heated substrates and the energy transport mechanism through the solid–liquid interface. Results indicate that the mass density through the liquid–vapor interface decreases sharply when the evaporation is in the steady state. Meanwhile, there is an adsorption layer in the form of clusters at the solid–liquid interface, which has a higher mass density than the droplet inside. Furthermore, the wetting property of the solid substrate is related to the system’s initial temperature and the solid–liquid potential energy parameter. The contact angle decreases with the increase of initial temperature and solid–liquid potential energy parameter. During the accelerated evaporation process, small part of energy transports into the liquid in the perpendicular direction to the solid–liquid interface and most of the energy transports along the parallel direction to the solid–liquid interface in the adsorption layer to the three-phase contact line. The heat-transfer process from the solid substrate to the droplet inside is hindered by the Kapitza resistance at the solid–liquid interface, no matter the solid substrate is hydrophilic or hydrophobic. Meanwhile, the Kapitza resistance gradually increases with the increase of the initial temperature and decreases with the increase of the solid–liquid energy parameter.

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In order to understand the fundamental characteristics of the forced flow driven by iso-and counter-rotation of a shallow pool and a disk on the free surface, we conducted a series of unsteady three-dimensional numerical simulations in a shallow pool. The ratio of the disk to pool radius is R s = 0.3 and the aspect ratio of pool is H = 0.06. The results indicated that the forced flow driven by disk and pool rotation is axisymmetric and steady at the small rotation Reynolds number. However, when Reynolds number exceeds a critical value, the flow will undergo a transition to three-dimensional oscillatory flow, which is characterized by the velocity fluctuation waves traveling in the azimuthal direction. The propagating direction and the velocity of the waves depend on the rotation rates and directions of the disk and pool. Besides, the critical conditions for the onset of the oscillatory flow were determined. The details of the flow fields were discussed and the mechanism of the flow pattern transition was also exhibited.