Nonwetting surfaces

The other type of superhydrophobicity is the Cassie-Baxter form, wherein the liquid does not fully penetrate the surface topography, but instead will sit atop surface asperities and air pockets. For nonwetting surfaces, this is a more likely scenario, since it may be more energetically favorable for the hquid to be in contact with air than a repelhng surface. This type of wetting can be described by... 

Another example of a gas-limited reaction is the hydrogenation of benzene to cyclohexane over Pt/Al203 catalyst [46]. At 76°C and 1 atm, the reaction rate decreased 25% as the liquid rate was increased fourfold. In this case, the higher rate on nonwetted surface was due to a combination of higher hydrogen concentration and diffusion of benzene in the vapor phase. Whether a reaction is gas limited or liquid limited in a trickle bed depends on the relative concentrations of gaseous and liquid reactants, the reaction orders, the diffusion coefficients, and the fraction of the surface that is wetted. A reaction that is gas limited at inlet conditions may become liquid limited at high conversion. 

Equations (IV.38) and (IV.39) are valid under the condition of complete wetting of smooth surfaces, with an angle a 0. For nonwetting surfaces, the right-hand sides of Eqs. (IV.38) and (IV.39) must be multiplied by cos 6 (where 6 is the contact angle) then, for autohesion of particles, = 2nor cos 6 and in the case of particle adhesion. 

Figure 21.25. Classification of standard isotherms (lUPAC [34]). Type I Material that is microporous only, Type II and III, statistical multilayer adsorption on a wetting (II) and a nonwetting surface (III). Type IV (wetting) and V (non-wetting) Mesoporous materials showing a characteristic hysteresis that indicates capillary condensation. VI Stepwise adsorption in a highly ordered material (e.g., graphite).

In these analyses, some assumptions have been made as regards the shape of the drops. The most accurate data were obtained when weight method was used. Different analyses showed that the rate of evaporation was linearly dependent on the radius of the drop. Further, the contact angle of water drop on teflon (i.e., a nonwetting surface) remained constant under evaporation. On the other hand, contact angle decreased as water drop evaporated on glass (i.e., a wetting snrface). 

Kleingartner JA, Srinivasan S, Mabry JM, Cohen RE, McKinley GH (2013) Utilizing dynamic tensiometry to quantify contact angle hysteresis and wetting transitions on nonwetting surfaces. Langmuir 29 13396-13406... 

Normally, CO2 corrosion is possible if water is present in the system. It is expected that the nonwet surface of the pipeline (not wet with water) will have no CO2 corrosion however, this is not the case. In a cross-country pipeline, water condenses as a result of heat loss, and free water is formed. This free water behaves as a mist and wets the dry surface of the pipe. CO2 corrosion of the dry area depends on the rate of water condensation and is defined as a condensation factor. 

The microscopic contour of a meniscus or a drop is a matter that presents some mathematical problems even with the simplifying assumption of a uniform, rigid solid. Since bulk liquid is present, the system must be in equilibrium with the local vapor pressure so that an equilibrium adsorbed film must also be present. The likely picture for the case of a nonwetting drop on a flat surface is... 

The terms wetting and nonwetting as employed in various practical situations tend to be defined in terms of the effect desired. Usually, however, wetting means that the contact angle between a liquid and a solid is zero or so close to zero that the liquid spreads over the solid easily, and nonwetting means that the angle is greater than 90° so that the liquid tends to ball up and run off the surface easily. 

The surface film or sprea ding pressure, FI, is used to account for the change in gas—soHd interaction caused by adsorption of vapor evaporated from the Hquid. A Hquid is called wetting if the contact angle from soHd to Hquid through gas, 65 0 nonwetting if > 90°. Because it is easier to... 

Fig. 2. Problems in wetting A, Hquids that wet the exterior before displacing gas from pores leave gas trapped in the submerged clump B, fully wetted clumps of buoyant particles do not sink C, nonwetting Hquids do not penetrate and displace gas from pores, so clump remains buoyant and caimot submerge and D, foam produced from air is drawn under the surface, sheared into small bubbles, and stabilized by the wetting agent.

Interpretation for irreducible water saturation assumes that the rock is water-wet or mixed-wet (water-wet during drainage but the pore surfaces contacted by oil becomes oil-wet upon imbibition). If a porous medium is water-wet and a nonwetting fluid displaces the water (drainage), then the non-wetting fluid will first occupy the larger pores and will enter the smaller pores only as the capillary pressure is increased. This process is similar to the accumulation of oil or gas in the pore space of a reservoir. Thus it is of interest to estimate the irreducible water saturation that is retained by capillarity after the hydrocarbon accumulates in an oil or gas reservoir. The FFI is an estimate of the amount of potential hydrocarbon in... 

When a drop of liquid is placed on a solid surface, it will displace the gas and spread over the surface. If the contact angle is <90°, the liquid wets the solid (wetting, Figure 18.1a) if the contact angle is >90°, the liquid does not wet the solid (nonwetting, Figure 18.1b). 

A liquid-solid contact angle away from 90° induces the formation of a meniscus on the free surface of the liquid in a vertical tube (the solid phase). In the nonwetting case, the meniscus concaves upwards to the air. The upwards meniscus is the result of a downward surface tension at the liquid-tube interface, causing a capillary depression. In the wetting case, the meniscus has a concave-downward configuration. The downwards meniscus is the result of an upward surface tension at the liquid-tube interface, causing a capillary rise. 

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