Visco-Elastic Dewetting

FIGURE 7.17. Inertial dewetting of water floating on a pool of carbon tetrachloride (CCI4) for three different thicknesses. Left picture e = 0.73 mm, with no ripples and no shock middle picture e = 0.61 mm, with ripples toward center and shock waves directed downstream right picture e = 0.43 mm, with ripples toward the center and the outside, shock waves upstream and downstream (courtesy X. Noblin). 

We are interested now in the opposite limit, namely, the dcwctting of ultra-viscous pastes, with a viscosity roughly a million times greater than that of water, deposited on a low-viscosity liquid or simply suspended in air. 

For the conditions described in this section to apply, the surface viscosity rjs = of the film must be greater than the viscosity of the substrate multiplied by the size of the hole at all times during the dewetting process. If this criterion is not met, the dissipation in the substrate dominates and the opening of the hole (discussed in section 7.2.3) becomes independent of the viscosity of the film. 

When the film is ultra-thin, viscous dissipation becomes dominant. For bare films, the viscous regime should prevail below a thickness 

For soapy water, eo 10 nm. For a paste, on the other hand, eo is measured in kilometers, and we can rest assured that we are then always in the viscous regime. 

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During the subsequent stages of visco-elastic dewetting, a rim developed and its shape changed significantly over time. Initially, the energy supplied by the driving forces was mainly dissipated within the volume of the film in the vicinity of the hole (via deformations in the radial and ortho-radial). The removed material (related to the dewetted distance d) was redistributed within the film over a certain distance Aq, which is characterized by film thickness ho and frictional properties [42, 46, 144] ... 

Dewetting Experiments Using a Visco-Elastic Fluid Polystyrene Films Slightly Above... 

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. 

We want to emphasize that a logarithmic time dependence of d and w, and the corresponding decrease of V, are not expected for a Newtonian liquid [42]. Moreover, our results cover times shorter than the longest relaxation time in equilibrated bulk samples (i.e., the reptation time). Thus, the visco-elastic properties of PS certainly affect our dewetting experiments. Thus, a detailed theoretical model has been developed that takes into account residual stresses, interfacial friction (i.e., slippage), and visco-elasticity [42,44,46],... 

Above results clearly demonstrate that dewetting can be considered as a nanorheology experiment that gives insight into the structure and properties of out-of-equiUbrium PS thin films. We may thus consider dewetting as a rheological probe for the study of the visco-elastic properties of nanoscopic polymer films. 

While we have presented results only for unstable thin liquid films, it should not be ignored that similar instability processes occur also in (visco)-elastic " or even solid films. " We also did not discuss related problems of the consequences of density gradients or the influence of additives. As an alternative to long-range vdW forces, destabilization may also be induced by an electric field. These concepts can also be extended to bilayer systems. " For a review of technological applications of dewetting, see chapter 14. 

Figure 11.2 The shape of the rim during early stages of dewetting of thin visco elastic polymer films on nonadsorbing substrates, (a) The sequence of three fundamentally different shapes of the rim found experimentally (b) a typical optical micrograph (50 x 50 pm ) taken at the edge of the sample obtained after partial...

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