When a liquid droplet is put onto a surface, two situations distinguishable by the contact angle may result. If the contact angle is zero, the droplet spreads across the surface, a situation referred to as complete wetting. Second, if the contact angle is between 0° and 180°, the droplet does not spread; a situation called partial wetting. A wetting transition is a surface phase transition from partial to complete wetting. We present experiments that show three different ways in which the wetting transition can come about.

We give a phenomenological overview of recently discovered complex wetting states in simple liquid mixtures relevant to both fundamental research and industrial applications such as oil recovery, Alkanes on water show a sequence of two wetting transitions, from partial wetting to 'frustrated-complete wetting', and finally to complete wetting: a first-order thin-thick transition between a microscopic and a mesoscopic adsorbed alkane film is followed by a long-range critical wetting transition to a macroscopic wetting layer. The existence of the new 'frustrated-complete wetting' state follows from a competition between short-range and long-range components of the intermolecular forces, the latter opposing wetting. The effective long-range forces between interfaces consist of Debye dipolar and London dispersion contributions, which can also be in mutual competition. The London component is ultimately responsible for the frustration preventing complete wetting at ambient temperatures and pressures.

We apply the phenomenological wetting theory of Cahn to fluids with van der Waals forces, and in particular to the wetting of pentane on water. Taking into account explicitly the long-range substrate–adsorbate interaction allows us to reproduce the experimentally observed critical wetting transition, which arises from the vanishing of the Hamaker constant at T≈53°C. This transition is preceded by a first-order transition between a thin and a thick film at a (much) lower temperature. If long-range forces are neglected, this thin–thick transition is the only wetting transition and critical wetting is missed. Our study focuses on the development of useful theoretical tools, such as phase portraits and interface potentials adapted to systems with van der Waals forces.

The wetting properties of oil on water are relevant to the industrial problem of oil recovery from porous reservoirs. The location of wetting phase transitions, a subject of fundamental interest for experimentalists and theorists, is also of practical importance for petroleum engineering because the hydrodynamics of oil flow is different on either side of the transition. This seminar treats the application of the phenomenological wetting theory of Cahn to fluids with van der Waals forces, and in particular to alkanes on water. Including the long-range substrate-adsorbate interaction in the theory allows one to reproduce the experimentally observed sequence of two wetting-like transitions. A critical wetting transition, which arises from the subtle action of the van der Waals forces, is preceded by a first-order transition between a thin and a thick film at a lower temperature. The three wetting regimes separated by these two transitions presumably lead to three different oil recovery efficiencies.

This paper presents recent experimental and theoretical results concerning the wetting behavior of n-alkanes on water as a function of thermodynamic conditions (i.e., temperature, pressure, etc.). The transition from lenses to a macroscopically thick film, that takes place when the temperature is increased, occurs for n-alkanes on water in a manner very different from that encountered in other fluid systems. For n-pentane on water, ellipsometric measurements reveal that the growth of the pentane layer to a macroscopically thick film occurs in a continuous manner, for a temperature (≈53°C) corresponding to a change in the sign of the Hamaker constant. A theoretical approach based on the Cahn–Landau theory, which takes into account long-range (van der Waals) forces, enables us to explain the mechanism of this continuous wetting transition. This transition is preceded (at a lower temperature) by a discontinuous transition from a thin film (of adsorbed molecules) to a thick (but not macroscopically thick) film. The latter transition was not visible for pentane on water (it should occur below the freezing temperature for water), but we expect to observe it for longer alkanes (e.g., hexane) on water. Work is underway to examine the wetting behavior of oil/brine systems more representative of reservoir conditions.

We report an ellipsometry study of the wetting of hexane on water. By adding salt to the water, we are able to tune the Hamaker constant of this system. This allows us to demonstrate, for the first time, that two rather than one wetting transitions can exist in a single system. Upon increasing the temperature, a discontinuous (first-order) transition from a microscopic film to a mesoscopic film occurs, followed by a continuous (critical) wetting transition that leads to a thick adsorbed film. The latter is due to the Hamaker constant which changes sign with temperature. The first-order transition temperature changes by the same amount as the critical wetting temperature upon changing the Hamaker constant.

Abstract The method of parachors is widely used in conventional thermodynamic codes and reservoir simulators to calculate oil/gas interfacial tensions of complex hydrocarbon mixtures. In the low-to-moderate interfacial tension regime, a value p≈11/3 has previously been shown to be the "best" parachor exponent. This exponent is a critical exponent and its value is consistent with the values of critical exponents characterizing the liquid/vapor critical behavior. Therefore parachors may be viewed as critical amplitudes. By using critical scaling theory, parachors are related to other critical amplitudes and critical parameters that describe the bulk thermodynamic behavior of fluids. A simple expression relating the parachor of a pure compound to its critical temperature Tc, critical pressure Pc and acentric factor ω is proposed: P= (0.85-0.19ω)Tc12/11 /Pc9/11 where the parachor P is in units of (dyn/cm)3/11cm3/mol, Tc in K and Pc in MPa. This equation matches (within experimental error) the known parachor values of normal fluids (e.g. alkanes, aromatics, CO2, N2, H2S, etc…).