Dewetting controlled

In biology, dewetting controls the dynamics of adhesion on wet substrates (mushroom spores, living cells). 

Spreading dynamics plays a key role in numerous applications. However, controlling the dewetting of liquids may be potentially more important for some industrial uses. 

However, on rigid substrates, the growth of dry zones is accompanied by a rim of excess liquid with width X (Fig. 10). As the dewetting proceeds, X increases. For short times and < K, the growth of dry patches is controlled only by surface tension forces and the dewetting speed is constant. A constant dewetting speed of 8 mm-s has been measured when a liquid film of tricresyl phosphate (TCP) dewets on Teflon PFA, a hard fluoropoly-mer of low surface free energy (p. = 250 MPa, 7 = 20 mJ-m ). 

The theory of viscoelastic braking in liquid spreading exposes the various possibilities that may exist for controlling wetting or dewetting speeds by changing solid rather than liquid properties. Applications may exist in the fields of contact lenses, printing, and vehicle tire adhesion. 

Karthaus, O., Mikami, S. and Hashimoto, Y. (2006) Control of droplet size and spacing in micronsize polymeric dewetting patterns./. Colloid. Interface Sci., 301, 703-705. 

Kurimura, S. and Nakamura, R. (2005) Control of crystal morphology in dewetted films of thienyl dyes. Appl. Phys. A, 80, 903-906. 

Modern coating technologies require increasingly thinner polymer films. This requirement is opposed by the surface pressure and the chain elasticity. Below a certain equilibrium thickness, the film is either metastable or even unstable and tends to break into droplets regardless of the chemical structure of the substrate [321, 322]. Anomalous wetting behaviour was observed for amphiphilic polymer films whose stability is controlled by the orientation of the surface active moieties [323,324]. All these phenomena belong to the dewetting problem. 

Surface treatment and substrate patterning can be employed to further reduce feature resolution. Eor example, drop spread can be controlled and significantly reduced by prepatterning the substrate with "dewetting patterns" that confine the ink to specific regions on the sirrface. Alternatively, Smith et al achieved printed lines 5-30 microns thick by creating channels in the substrate and then filling them with inkjetted ink. 

The dense arrangement of the chains points to the action of other forces that can control the molecular conformation of adsorbed macromolecules in addition to the interaction with the substrate. Capillary forces and dewetting during evaporation of the solvent can cause condensation and dense packing of the molecules in monolayer patches,147 148 150 For large molecules with a diameter of ca. 5 nm, capillary forces are in the range of a few nN, which is sufficient to... 

We are interested in the following questions How does a polymer move or flow on its own monolayer, or more generally, how do macromolecules slide past each other What are the consequences of the autophobic behavior between grafted and free polymers for dewetting What is the value of the friction coefficient at such polymer-polymer interfaces After a brief description of our experimental conditions, we present the experimental results, which are discussed in light of the theory described above. Finally, we show what we can learn about molecular parameters controlling interfacial properties. 

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