Drying transitions

The main conclusion which can be drawn from the results presented above is that dimerization of particles in a Lennard-Jones fluid leads to a stronger depletion of the proflles close to the wall, compared to a nonassociating fluid. On the basis of the calculations performed so far, it is difficult to conclude whether the second-order theory provides a correct description of the drying transition. An unequivocal solution of this problem would require massive calculations, including computer simulations. Also, it would be necessary to obtain an accurate equation of state for the bulk fluid. These problems are the subject of our studies at present. 

Singh S, Houston J, van Swol F, Blinker CJ (2006) Superhydrophobicity drying transition of confined water. Nature 442 526... 

The intrinsic structure of a liquid-vapor interface resembles the surface of a polymer liquid in contact with a nonattractive solid substrate at the pressure where the liquid coexists with its vapor. In the latter case, the system is in the vicinity of the drying transition and a layer of vapor intervenes between the substrate and the polymer liquid. There is, however, one important difference between the vapor-polymer interface and the behavior of a polymer at a solid substrate the local position of the interface can fluctuate. Let us first consider the case where the film is very thick and the solid substrate does not exert any influence on the free surface of the film in contact with its vapor. The fluctuations of the free surface are capillary waves. Neglecting bubbles or overhangs, one can use the position of the liquid-vapor interface, z = h x,y), as a function of the two lateral coordinates, x and y, parallel to the interface to describe the system configuration on a coarse scale. In this Monge representation, the free energy of the interface is given by the capillary-wave Hamiltonian " ... 

This and Eq. (42) are a pair of parametric equations (parameter xq) for a locus of wetting (drying) transitions in the (x, x) plane. This locus is shown as the dashed curve in Fig. 16. 

Figure 7-51. Corrosion potentiai or iron as measured with a Keivinprobe during a dry/wet/dry transition (Stratmann, 1994).

Figure 7-52. Corrosion rate and corrosion potential as measured during a wet/dry transition of an iron surface contaminated with SO2 (Stratmann and Streckel, 1990 b).

Fig. 6.5 Hydrophobic attraction in the model of Lum et al. [30]. The hydrophobic surfaces are surrounded by a thin layer of vapour (or a region of depleted water density due to fluctuations). At some critical separation there is a collective drying transition in the space between the surfaces...

Drying transition of water near hydrophobic surfaces... 

Drying transition may occur in a liquid phase upon heating along the liquid-vapor coexistence curve (see Section 2.1). This transition has drastic effect on the liquid-solid interface above the temperature Tj of a drying transition, the liquid is separated from the solid surface by a macroscopic vapor layer. However, even below Tj and out of the liquid-vapor equilibrium, distant etfect of the drying transition may noticeably affect the liquid density profile. Therefore, it is important to know the temperature of the drying transition of water and its sensitivity to the water-surface interaction. This allows description of the density profiles of liquid water near hydrophobic surfaces at various thermodynamic conditions. 

So, a stable macroscopic vapor layer between a liquid and a solid or its formation via a surface phase transition (drying transition) was never observed experimentally for water and for any other fluid. This well agrees with the theoretical expectations [117, 118, 124] that a drying transition is suppressed by the long-range fluid-surface interaction up to the bulk liquid-vapor critical temperature Tc. Even for the extremely weak fluid-surface interface (liquid neon on cesium), formation of a... 

A critical water-wall interaction was determined by complementary studies of the condensation/evaporation transition in open pores and of the liquid-vapor coexistence in closed pores [30, 32, 205, 208], Such studies were performed for water confined in cylindrical pores of the radius Rp= 12 A. The liquid density in open pore was obtained by direct equilibration in the Gibbs ensemble of confined water and a bulk liquid water at ambient pressure, starting from completely filled pores [208]. The liquid density obtained is a monotonic function of the water-waU potential Uq, and the values obtained at T = 300 K are shown in Fig. 65 (solid squares). In parallel, the density of liquid water at equilibrium with saturated vapor in pore was calculated (Fig. 65, open circles). It also depends on Uo, however, weaker than the density in an open pore. Liquid densities in closed and open pores become equal at Uq — l.Okcal/mol (see crossing point in Fig. 65). If —Uq < l.Okcal/mol, only water vapor is stable in the pore. Accordingly, liquid is stable in the pore, if -Uq >1.0 kcal/mol. Thus, the value of Uq —1.0 kcal/mol separates regimes of capillary evaporation and capillary condensation [32, 208]. This value approximately coincides with the critical water-wall interaction, which provides an absence of wetting or drying transitions up to the liquid-vapor critical point (see Section 2.4). 

---