Viscous Dewetting

The experiment itself consists of depositing a small puddle of water at the center of the ring and to spread it with a circular rod (a pencil works nicely). This produces a circular film that fills the inside of the wetting ring. The film formed in this way is metastable. With the help of a pipette 

FIGURE 7.4. Experimental demonstration of dewetting A drop (a) or a puddle (b) is spread as a film inside a wetting ring (c). The liquid is removed by suction to nucleate a dry zone that subsequently expands (d). 

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Assuming that the Laplace pressure is the same everywhere within the rim (i.e., the same curvature is assumed to exist everywhere in the rim) requires that dyn 0- For thin films in the nanometer range and a size of the rim in the range of micrometers, the logarithmic factors at positions R and F are similar. Consequently, a highly useful relation is obtained between dynamic and equilibrium contact angles for the case of viscous dewetting (with 4>) [136] ... 

The ideal experiment described above leads to the laws of viscous dewetting illustrated in Figure 7.5 ... 

Cracks at fillets, liner separation, viscous deformation, dewetting (blanching)... 

The highly asymmetric shape of the rim suggests that the polymer does not flow like a liquid. For a viscous fluid we would expect that equilibration of the Laplace pressure (which is proportional to curvature) within the rim is fast, i.e., fast with respect to the shortest possible experimental time scale. This would lead to a more symmetric shape. In our experiments, the asymmetric shape of the rim represents a characteristic feature of dewetting of high molecular weight visco-elastic fluids at temperatures close to Tg [17, 37, 38, 41-48]. Typical examples showing the evolution of the shape of the rim are given in Fig. 15. 

In the intermediate case (ii) corresponding to classic dewetting on a solid substrate (Poiseuille flow), we find that viscous dissipation generally predominates. In contrast, for low-viscosity liquids, such as water, dewetting very hydrophobic surfaces (large angles 6 ), rapid inertial dewetting can be achieved... 

In the case (m) of intercalated films, the rim must distort the soft material around it. Consequently, it becomes rather flattened and viscous dissipation still predominates. It is no longer localised only in the wedge fronts, but occurs throughout the volume. It increases with the size of the rim, whereas the driving force 5 remains constant the dewetting speed thus decreases in time. Experiments on intercalated Aims are currently underway at the Institut Curie in Prance (Martin). 

FIGURE 7.1. The dewetting process has been studied in three different geometries (a) supported films resting on a solid substrate. The process controls the spontaneous drying out of liquid films without the intervention of heat, as well as the stability of liquid films (b) inserted films, sandwiched between a rubber-like material and a solid, with applications to the process of adhesion onto a wetted solid surface and to the stability of the lachrymal film between a soft contact lens and the cornea (c) suspended viscous films (in air or in a liquid with low viscosity) this configuration controls the stability of polymer and glass foams as well as that of emulsions. 

In each of these cases, we will study the stability conditions of liquid films whose thickness ranges from a nanometer to a millimeter. We will also describe the dynamics of the dewetting process for a wide variety of liquids, ranging from water to ultra-viscous pastes, the viscosity of which is millions of times that of water. We will deal with three different dynamical regimes ... 

We will explain some rather unusual phenomena. As an example, dewet-ting at high speed can generate shock waves Likewise, in liquid films millions of times more viscous than water, holes can open up at such high velocities ( m/s) that high-speed cameras are required to capture the phenomenon. 

The second is the friction force Fy exerted by the solid on the liquid in motion. The viscous dissipation is dominated by friction in the liquid wedges that form the boundaries of the ridge, and this force does not depend on the size of the ridge. Fy is proportional to the velocity, and imposing Fy = Fm thus leads to a dewetting velocity that is constant in time (law 1). 

R( (ion has studied the dewetting of ultra-viscous PDMS on ideal surfaces (Figure 7.9). She showed that on such smooth and passive surfaces, a highly viscous fluid slides. 

Experiments with liighly viscous PDMS on silanized silicon wafers have shown that, for thick films (e > 10 pm), the dewetting is of the classic type and obeys the law i oc <. On the contrary, for microscopic films (e < 1 pm), C. Redon found a dependence of the type R [Pg.168]

C. Andrieu was the first to examine the dewetting of water on plastic. One conclusion of this study was that, upon decreasing the thickness e, the velocity diverges as 1/y/e instead of approaching a limiting value independent of e as would happen in a viscous regime. 

When i e 1, dewetting is viscous and we then have Fm = Fy. This limit corresponds to the analysis in section 7.2 where we have neglected the inertial term d MV)/dt. When > 1, dewetting is inertial and Fy is negligible. The second term in equation (7.48) no longer depends on V. The ridge of mass M(t) = peR advances at a constant velocity V that is the solution of equation (7.48). That velocity is... 

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