Dewetting

Dewetting refers to the spontaneous withdrawal of a liquid film from a hostile surface, for instance, water on a hydrophobic solid. 

As we step out of a shower, we generally use a good absorbing towel to dry ourselves and we may grab a hair dryer to restore a fluffy hairdo. But if we pay a bit of attention, we realize that our skin actually dries spontaneously. Areas free of water form and expand. The process involves nucleation and growth of dry regions. That is the method ducks rely on when they emerge from a pond. Their feathers are extremely hydrophobic and dewet almost instantly. In short, there are three ways to restore a dry state  

capillary suction (tissue paper, bath towels, and other porous media)  

dewetting, if th surface is hydrophobic either naturally or through an appropriate surface treatment. 

Although de wet ting is an ubiquitous phenomenon—we can observe it everyday on a windshield, in a glass of water, or while casually taking a bath—the mechanics governing the retreat of liquid films began to be understood only recently. 

The molecular-kinetic theory predicts a maximum wetting speed vmax and a minimum dewetting speed vmin. At speeds larger than vmax gas bubbles form. This was indeed observed. For water this value is vmax 5 — 10 m/s. 

De Gennes and Cazabat proposed an alternative continuum model [290]. They describe the spontaneous spreading of a liquid by a competition between the driving force, which is the disjoining pressure in the precursor film and the core region, and the friction between layers of liquid with the solid [287], 

In many applications, films on solid surfaces are only metastable or they are stable only from a certain thickness. Example are metal films prepared by evaporation [291] and many polymer films. Since most paints and coatings contain a substantial amount of polymer, this is a highly relevant case. 

How can we produce polymer films First, the polymer is dissolved in a suitable, volatile solvent. Then two methods are widely used  

When the polymer wets the solid (0 = 0), polymer films are thermodynamically stable. For 0 0 the films are only metastable. When a thin, metastable film is heated above the glass transition temperature, holes start to form spontaneously, usually at small defects. The holes increase in size until only a network of polymer lines is formed which eventually breaks up into individual droplets (Fig. 7.19) [150], The film stability of films with a thickness of 1-100 nm is determined by long-range surface forces, mainly van der Waals forces [151,268, 294,295], 

As the speed increases further, e V) decreases again and a third regime is entered, in which only the hydrodynamic limiting layer is laid down, of thickness 

Below ec, there are two dewetting mechanisms (a) a macroscopic film is metastable and dewets by nucleation and growth of dry zones (b) a microscopic film is unstable and spontaneously breaks into a multitude of droplets. Capillary waves are amplified and this mechanism is called spinodal decomposition, by analogy with what happens in phase transitions. 

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Very small sessile drops have a shape that depends on the line tension along the circular contact line if large enough it induces a dewetting transition detaching the drop from the surface [84]. 

The unstable situation caused when a spread him begins to dewet the surface has been studied [32, 33]. IDewetting generally proceeds from hole formation or retraction of the him edge [32] and hole formation can be a nucleation process or spinodal decomposition [34]. Brochart-Wyart and de Gennes provide a nice... 

The speed of wetting has been measured by running a tape of material that is wetted either downward through the liquid-air interface, or upward through the interface. For a polyester tape and a glycerol-water mixture, a wetting speed of about 20 cm/sec and a dewetting speed of about 0.6 cm/sec has been reported [37]. Conversely, the time of rupture of thin films can be important (see Ref. 38). 

Eqs. 4 and 6 enable the extent of contact between a liquid adhesive and a solid substrate to be gauged. Some consequences are shown in Table 1 where the concept of the reduced spreading coefficient S/yw, employed by Padday [10], was used to clarify the situation. As is readily seen, if S is positive, the liquid at equilibrium will be spread completely over the solid, but if S/yi is less than —2, spontaneous dewetting will occur. 

This interaction energy is reversible because removal of the wetting liquid from the surface only requires the disruption of these interaction sites. Solidification of the liquid into an adhesive changes the requirements for dewetting, however. 

Yamamoto and Minamizaki [159] disclose the use of a curable silicone based release agent blended with resin particles which swell or are soluble in organic solvent. Coatings made with such blends can be written on with solvent based inks. For example, an addition cure silicone network containing 20 wt% 0.1 p,m diameter PMMA particles exhibited both good writeability (no ink dewetting and smear free) and a low release force of 10 g/cm for a PSA tape. 

In cases when the two surfaces are non-equivalent (e.g., an attractive substrate on one side, an air on the other side), similar to the problem of a semi-infinite system in contact with a wall, wetting can also occur (the term dewetting appHes if the homogeneous film breaks up upon cooHng into droplets). We consider adsorption of chains only in the case where all monomers experience the same interaction energy with the surface. An important alternative case occurs for chains that are end-grafted at the walls polymer brushes which may also undergo collapse transition when the solvent quality deteriorates. Simulation of polymer brushes has been reviewed recently [9,29] and will not be considered here. 

Fig. 33(a,b) shows a series of snapshot pietures as a result of a eomputer experiment probing the kineties of dewetting. The loeal darkness of eaeh snapshot indieates the loeal eoverage of the substrate surfaee. Coverage fluetuations (white spots) appear rather early and get rapidly amplified. The substrate regions, eovered with polymer, have very irregular surfaee initially and are eonneeted with many weak links later, these hnks disappear, and the droplets of adsorbed polymer eompaetify, a pattern similar to spinodal deeomposition. 

In order to eharaeterize the dewetting kineties more quantitatively, the time dependenee of the average thiekness of the film and the deerease of adsorbed fraetion Fads(0 with time (Fig. 34) are monitored. The standard interpretation of the behavior of sueh quantities is in terms of power laws, ads(0 with some phenomenologieal exponents. From Fig. 34(a), where sueh power-law behavior is indeed observed, one finds that the exponent a is about 2/3 or 3/4 for small e and then deereases smoothly to a value very elose to zero at the eritieal value e k. —. 2 where the equilibrium adsorbed fraetion F s(l l) starts to be definitely nonzero. If, instead, one analyzes the time dependenee of — F ds(l l) observes a eollapse... 

A. Milchev, K. Binder. Dewetting of thin polymer films adsorbed on solid substrates A Monte Carlo simulation of the early stages. J Chem Phys 705 1978-1989, 1997. 

G. Reiter. Dewetting of thin polymer films. Phys Rev Lett 62 75-78, 1992. 

G. Reiter. Unstable thin polymer films rupture and dewetting process. Langmuir 9 1344-1351, 1993. 

K. R. Shull, T. E. Karis. Dewetting dynamics for large equilibrium contact angles. Langmuir 70 334-339, 1994. 

A. Faldi, R. J. Composto, K. I. Winey. Unstable polymer bylayers Morphology of dewetting. Langmuir 77 4855 861, 1995. 

G. Henn, D. J. Bucknall, M. Stamm, P. Vanhoorne, R. Jerome. Chain end effects and dewetting of thin polymer films. Macromolecules 29 4305 313, 1996. 

G. Gompper, M. Hennes. Layering, dewetting, and first-order wetting in ternary amphihilic systems. J Chem Phys 2871-2880, 1994. 

Here rj is the viscosity of the dewetting liquid. Note that a relaxational term proportional to a has been added, with fi(j)) being the chemical potential of the vapor. This term alone guarantees that a homogeneous liquid film will relax to its equilibrium value hooip) by evaporation or condensation. For h = hooip) this term vanishes. 

An important consideration is the effect of filler and its degree of interaction with the polymer matrix. Under strain, a weak bond at the binder-filler interface often leads to dewetting of the binder from the solid particles to formation of voids and deterioration of mechanical properties. The primary objective is, therefore, to enhance the particle-matrix interaction or increase debond fracture energy. A most desirable property is a narrow gap between the maximum (e ) and ultimate elongation ch) on the stress-strain curve. The ratio, e , eh, may be considered as the interface efficiency, a ratio of unity implying perfect efficiency at the interfacial Junction. 

Adicoff, Dynamic Mechanical Behavior of Highly Filled Polymers Dewetting Effect , Rept No NWC-TP-5486 (1971) 11) H. Yasu-... 

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