Wetting line

When water-wet gas expands rapidly through a valve, orifice or other restriction, hydrates form due to rapid gas cooling caused by adiabatic (Joule-Thomson) expansion. Hydrate formation with rapid expansion from a wet line commonly occurs in fuel gas or instrument gas lines. Hydrate formation with high pressure drops can occur in well testing, start-up, and gas lift operations, even when the initial temperature is high, if the pressure drop is very large. 

Young s equation is the basis for a quantitative description of wetting phenomena. If a drop of a liquid is placed on a solid surface there are two possibilities the liquid spreads on the surface completely (contact angle 0 = 0°) or a finite contact angle is established.1 In the second case a three-phase contact line — also called wetting line — is formed. At this line three phases are in contact the solid, the liquid, and the vapor (Fig. 7.1). Young s equation relates the contact angle to the interfacial tensions 75, 7l, and 7sl [222,223] ... 

At the three-phase contact line the surface tension exerts strong forces on the surface. For instance, if we consider a water drop on a polymer surface, typical contact angles are 90°. The surface tension pulls upwards on the solid surface. If we estimate the wetting line to have a width of 6 = 10 nm, the force F per unit length l can be related to the effective pressure exerted on the solid surface ... 

With 7 = 0.072 Nm-1 and 5 = 10 nm the effective pressure is of the order of P = 72 x 10s Pa. Such a high pressure can change the surface structure, cause mechanical deformation at the moving wetting line [250], and can lead to contact angle hysteresis [251-253], especially on soft polymer surfaces. 

POWER CYLINDERS [H rj WET LINED Cj dry 1 1 UNLINED ELECT. 1 TACHOMETER RAI ENCL. mge MECH. [ 1 VIBR. REED ... 

These authors found further, for the system they investigated, that the dimensionless growth velocity scales with 0. These dependencies are expected from theoretical predictions of wetting line velocities [71]. 

The process of wetting involves replacing the solid/vapour interface (with interfacial tension ygy) with a solid/liquid interface (with interfacial tension yg ). Wetting can be described in equilibrium thermodynamics in terms of the contact angle 0 by Young s equation at the wetting line [5]. 

Fig. 56. (a) Schematic phase diagrams of a semi-infinite lsing magnet in the vicinity of the bulk critical point Tc as a function of temperature T, bulk field H, and surface field Hi. In the shaded part of the plane H 0 the system (for T < rc) is non-wet, while outside of it (For T < Tc) it is wet. The wetting transition is shown by a thin line where it is second order and by a thick line where it is first order. First-order prewetting surfaces terminate in the plane H = 0 at the first-order wetting line. Critical and multicritical points are indicated... 

Plunging application takes place where substrate enters an open pool, pond, puddle, or fountain. Flow around the dynamic wetting line is comparatively unconfined so that the free surface, which is the upstream one, is fairly free to deform into a local meniscus and ultimately to entrain air. Generally, the substrate exits elsewhere and so there is no compact coating bead. 

At the edges of the coated layer its free surface ends in ordinary static contact lines. These lateral contact lines necessarily bend round upstream and connect with the wetting line. Thus at each edge of the layer where it is being delivered to the substrate there must be a curved segment of dynamic contact line, and the apparent slip of the liquid... 

Nevertheless, for most practical surfaces, this is not true and hence the intrinsic contact angle is difficult to determine. The surface heterogeneities could very well be the reason why the wetting line moves in a hesitant jerk or shuffling motion, termed as stick-slip motion [4]. 

Because the measurement of a contact angle must involve some movement of the wetting line, it is possible, or even probable, that the act of spreading of the hquid will displace certain surface equilibria that will not be reestablished over the time frame of the experiment. For example, the displacement of a second fluid may result in the estabhshment of a nonequilibrium situation in terms of the adsorption of the various components at the solid-liquid, solid-fluid 2, and liquid-fluid 2 interfaces. Time will be required for adsorption equilibrium to be attained, and it may not be attained during the time of the contact angle measurement if the transport and adsorption-desorption phenomena involved are slow. The kinetic effect may be especially significant for solutions containing surfactants, polymers, or other dissolved adsorbates. 

A second potential kinetic effect may result from bulk interactions between the surface and the spreading liquid. For example, if the liquid can penetrate the surface (e.g., if the liquid can be absorbed as opposed to adsorbed), the rate of penetration may be so slow that the measured contact angle will not reflect the true equilibrium situation. Likewise, if the liquid swells the surface, the wetting line may lie on a ridge of swollen surface rather than on a flat surface, resulting in an error in 6 (Fig. 17.6). 

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