Critical Thickness for Dewetting

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. 

We will deal primarily with the dewetting of metastable films. We will also discuss the case of spinodal dewetting (of more restricted practical interest), which is limited to very thin films (e 10 nm) and to ultra-pure conditions to avoid the nucleation of dry zones at specks of dust. 

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Suppose now that we deposit a thick film, the thickness of which we gradually reduce by suction with a pipette (or with a pump if we are dealing with a porous material). A hole created in the film will close up when e > e - When < c < a hole will remain stable since the angle 9 associated with e via equation (7.1) is between On and 6a and the contact line can neither advance nor recede. When e < en, a hole will open up spontaneously since 6 < 6n- It therefore follows that the critical thickness for dewetting on a dirty or rough surface is en-... 

The first interpretation of this newly identified short-time breakup of thin nematic films was put forward by van Effenterre et al. [73] and is rooted in basic thermodjmamics, i.e., in the biphase coexistence of the two (meta)stable phases taking part in the discontinuous transition. However, as pointed out in detail by Ziherl and Zumer [74], the explanation misses two critical points First, that the isotropic-nematic coexistence can only exist in the temperature range where both phases are thermodynamically metastable or stable, and second, that the nematic director is only distorted in thick enough films (i.e., above the critical thickness dc = [Ai — A2I). Therefore, the dewetting process in thin LC films is still an open question for both theoretical and experimental studies. 

Considerable interest still exists in the application of fluorine-containing cyclophosphazenes in lubricant technology. Recent advances in the use of N3P3(OC6H4F-4) (OC6H4CF3-3)6-n (n 2 code name X-IP) as lubricant either by itself or as an additive to perfluoropolyethers (PFPE) have been reviewed. Addition of X-IP to PFPE films reduces the critical dewetting thickness on amorphous nitrogenated carbon compared to that of neat PFPE. The influence of X-IP on the stabilization of the PFPE lubricant for the slider/disk interface in hard disk drives has been studied. Micro-phase separation of X-IP... 

All the results obtained to this point correspond to liquid films with thickness h hp, stated, for these films, the dewetting capillary number does not depend on h. But when h becomes comparable to the critical value he, gravitational effects decrease the dewetting velocity in the following way... 

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