Surfaces nonwetted

There is another characteristic to be considered in this type of loading. The surface properties of the material are quite significant. If the water does not wet the surface, the tendency would be to have the droplets that do not impact close to perpendicular bounce off the surface with considerably less energy transfer to the surface. Nonwetting coatings reduce the effect of wind and rain erosion. 

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. 

Separation of milled solid materials is usually based on differences in their physical properties. Of the various techniques to obtain ore concentrates, those of froth flotation and agglomeration exploit differences in surface activities, which in many cases appear to involve the formation of complexes at the surface of the mineral particles. Separation by froth flotation (Figure 4) depends upon conversion of water-wetted (hydrophilic) solids to nonwetted (hydrophobic) ones which are transported in an oil-based froth leaving the undesired materials (gangue) in an aqueous slurry which is drawn off from the bottom of the separator. The selective conversion of the ore particles to hydrophobic materials involves the adsorption of compounds which are usually referred to as collectors. 4... 

Groendes and Mesler (1982) studied the saturated film boiling impacts of a 4.7 mm water droplet on a quartz surface of 460 °C. The fluctuation of the surface temperature was detected using a fast-response thermometer. The maximal temperature drop of the solid surface during a droplet impact was reported to be about 20 °C. Considering the lower thermal diffusivity of quartz, this temperature drop implies a low heat-transfer rate on the surface. Biance et al. (2003) studied the steady-state evaporation of the water droplet on a superheated surface and found that for the nonwetting contact condition, the droplet size cannot exceed the capillary length. 

Another very complicated problem where the approach to equilibrium with time after a quenching experiment is described by an asymptotic law is the owth of wetting layers, in a situation where thermal equilibrium would require the surface to be coated with a macroscopically thick film, but is initially nonwet. For a short-range surface potoitial as discussed in section 3.5, analytical theories predict for a non-conserved density a growth of the thicknm of the layer according to a law f(t) oc In t, and this has in fact been observed by simulations . In the case where the surface potential decays with stance z from the surface as z, the prediction for the thickness l(t) is for the nonconserved case and... 

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... 

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