Wetting forced

Consider first a solid plate partially immersed in a liquid, as we are about to pull it out. If the liquid is wetting (5 0), the surface of the bath connects with the solid with a zero angle, which creates a meniscus in the vicinity of the line of contact. We have already discussed in chapter 2 some- of the characteristics of this meniscus, including its height, which is of the order of the capillary length [equation (2.20)]. 

If the film is being pulled slowly enough, the film it drags along is thin since it would not exist at all absent the drag effect (ignoring the possible 

FIGURE 5.10. Plate being pulled out of a pool of wetting liquid. The plate drags a liquid film along with it. 

The solid/liquid interface, because the associated boundary condition is actually responsible for the liquid coating. By virtue of its viscosity, the liquid in the vicinity of the solid moves at the same velocity as the solid and is being dragged by it, 

The liquid/vapor interface, which is being distorted by the film despite the opposing action of the surface tension of the liquid. Of course, gravity causes the film to flow downwards as well and is therefore also opposed to its movement, although we will see that, when the so-called capillary number is small, the contribution of this force is negligible when compared to that of the surface tension. 

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Experiments on forced wetting showed that, in general, the apparent contact angle depends not only on the speed v but also on the viscosity 77, and the surface tension 7l of the liquid. 

Theoretical considerations by Clarke and co-workers (Clarke, 1987 Clarke etal., 1993) show that an equilibrium film thickness arises from the competition between attractive dispersion forces determined by the dielectric properties of the grains and repulsive disjoining forces which can be steric forces and/ or double-layer forces. Wetting will occur when the solid-solid boundary energy, yb, is less than that of the wetted boundary, 2y, where y is the liquid-solid interfacial energy (Clarke, 1985), provided that there is a suitable source of liquid, for example as a consequence of liquid-phase sintering at high temperatures. 

Figure 2.12. Effect of velocity parameter Ui /

Figure 5.5. Forced wetting. Trends in the dependence of the contact angle on the rate of displaeement.

R.A. Hayes and J. Ralston, J. Colloid Interface Set 159 (1993) 429, collected literature examples of forced wetting and presented them (their table 1). In their note Langmuir lO (1994) 340 the same authors Illustrated fig. 5.5 for the systems water and water-glycerol on poly (ethylene terephtalate). 

In this connection we should also recall that in forced wetting the angle becomes rate dependent, as sketched in fig, 5,5,... 

Mutatls mutandis, the approach can also be applied to LjLjS interfaces. For several practical purposes such measurements are relevant, for instance in detergency and in the flooding of oil fields. However, for forced wetting the droplet method is not so suitable. 

No precursor films are expected for partially wetting liquids. For such systems, spreading can be achieved only by applying an external force, leading to forced wetting. Then, entirely different mechanisms prevail. 

The presence of a permanent contact during friction (induced by the applied normal load), can explain these results. This elastic contact acts like a forced wetting and therefore compensates for the low adhesive contact of PDMS 6. Moreover, for PDMS 6, the lower content of free and pendant chains induces a more direct and efficient contact with the glass substrate. The crosslinked network will then be directly constrained during friction, thus increasing the bulk dissipation. 

In this classical forced wetting situation, it has been shown, by expressing a force balance on the triple line, that this contact line becomes unstable above a critical value 0 of the putting velocity, associated with a constant capillary number Ca = vlm lYvsi (with t 15-20 a numerical factor [52, 53]). This in-... 

Viscous dissipation in the non-wetting phase (air in the classical forced wetting problem) is usually neglected, but has to be considered here, because of the significance of the liquid viscosity i/l with respect to i/ak- Indeed, both viscous forces in the wetting phase (air) comer Fair(u) = (3f i/air/i t - 0dl) and in the non-wetting one (liquid) Fl Cri v are of the same order (C 1 a numerical factor), because a = 3f l air/ L F... 

Forced wetting at a solid/air interface is a classical phenomenon and was described in chapters 5 and G. We have seen that when a plate (or fiber) is drawn out of a non-wetting liquid bath at increasing velodiy b the plate remains dry up to a threshold velocity IT, and it is wetted by a film of... 

We focus here on forced wetting of a liquid at the interface between a hard solid and a soft one. This situation has begun to be understood only very recently, even though it controls the deposition of films by soft iniplemeiits (paints, printing, etc.), as well as the loss of adhesive contacts (aquaplaning). 

We can now express the competition between forced wetting of the contact and its dewetting. We have seen that the initial dewetting velocity is related to the thickness e of the film through the relation ... 

Role of Nudeators in Forced Wetting Cerenkov Wake... 

Variable temperature Scanning Force Microscopy of mixed polystyrene (2000 - 100000 g/mol) and poly (methylmethacrylate) (100000 g/mol) thin films was used to probe mechanical properties such as surface stiffness and pull-off forces. Adhesion data can be explained by the molecular properties of the constituents. The adhesion of Polystyrene samples was measured by force distance curves and using the Pulsed Force Mode. It can be shown that surface tension is not the dominant part of the tip-surface interaction, but the mechani cal properties of the material will influence the measured adhesive force. Wetting of the tip by polymer molecules at higher temperatures due to increasing mobility is one possible model. 

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