Dewetting substrates

A prototypical form of the dependence of the interface potential, g, on temperature or strength of the attraction between liquid and substrate is sketched in Figure 10. In this figure we assume that Ap = 0, that is, liquid and vapor coexist. At weak attraction or low temperatures, T interface potential exhibits a sharp minimum at a very small film thickness, It. In the case that the liquid dewets from the substrate, h characterizes the residual amoimt of liquid on the dewetted substrate areas. It is a microscopic value, which can be vanishingly small for strongly first-order wetting transitions. The minimal value of the interface potential sets the contact angle. 

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. 

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. 

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. 

FIG. 16 SPFM image of a droplet formed as a result of dewetting of Zdol on an amorphous hard carbon substrate film. No layering around the drop was observed. (From Ref. 70.)... 

Spontaneous dewetting takes place by the formation of a droplet connected to the second layer. As a function of time, the droplets increase in volume while the area covered by the second layer decreases. In cases where the bare substrate was partly exposed, the diffusion and aggregation of second-layer molecules into droplets preserved the exposed regions of the substrate, as shown in Figure 20. 

FIG. 20 Dewetting of Zdol-TX on a nonunifomly covered surface. As in the previous figure, bare substrate regions are exposed (darker areas). Only material in layer 2 dewets to form a droplet (bright area). Diffusion of molecules from layer 2 occurs on top of layer 1 and does not fill the uncovered, possibly contaminated, substrate regions. (From Ref. 70.)... 

In this chapter, we will review the consequences of solid deformation in the kinetics of the spreading of a liquid on a soft material, in both wetting and dewetting modes. The influence of solid deformation induced by the liquid surface tension will be shown in the case of a liquid drop placed on a soft elastomeric substrate and in the case of an unstable liquid layer dewetting on a soft rubber. The impact of solid deformation on the kinetics of the wetting or dewetting of a liquid will be analyzed theoretically and illustrated by a few concrete examples. The consequences of solid deformation in capillary flow will be also analyzed. 

IV. DEWETTING DYNAMICS HARD VERSUS SOFT SUBSTRATES... 

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 technique used to study dewetting dynamics on materials consists of making a flat, smooth elastomer surface. A hquid puddle is deposited within a 50-mm-diameter ring of 0.1-mm-thick plasticized adhesive paper adhering to the substrate. The adhesive paper acts as a spacer. A microscope slide is drawn over the liquid to obtain a liquid film of ca. 0.1-mm thickness. At this thickness, the liquid film is unstable, being much less than the equilibrium value, of ca. 1.5 mm calculated from Eq. (29). Nucleation of dry patches... 

Some comparative dewetting experiments were conducted on a fluoropolymer. Teflon PFA (du Pont de Nemours and Co.) representing a relatively rigid substrate (p, = 250 MPa) with similar surface characteristics (surface free energy ys = 20 mJ m ). 

We therefore have qualitative evidence for the dependence of the dewetting speed on the elastic properties of the substrate. Dependence of wetting on the elastic modulus was previously suggested in the case of thin substrates [31], It may be conjectured that cross-linking affects the surface properties of the elastomer and, therefore, wettability. However,... 

The values of Go and y are known and for the elastomer of Young s modulus of 2.1 MPa, [/o = 8 X 10 mm-s [12]. We can then evaluate 8 at ca. 20 mn. This value is perhaps a little high but of the same order of magnitude as earlier estimated [6]. Thus, despite some necessary approximations and simplifying hypotheses, we arrive at a semiquantitative explanation of the relationship between dewetting and therefore, presumably, wetting speed and the molecular structure of the elastomeric substrate. 

In the wetting and dewetting kinetics studies described earlier, the solid substrate was a flat and smooth surface. However, the sohd deformation due to the action of the vertical component of the hquid surface tension may be expected to act in any geometry. For example, viscoelastic braking is involved in the sliding of a liquid drop on a tilted rubber track [32],... 

One possibility to prepare regular dewetted patterns on rough substrates is by pattern transfer. A regular pattern is formed on mica, on which then another substrate is placed. The pattern is released from the mica and fixed on the other substrate at the original droplet positions [49]. 

Para-t-butylphenyl glycidyl ether BPGE had a similar viscosity and C/O ratio as those of NA and had the best properties of the photocurable epoxies that were surveyed, but this monomer dewetted from Si substrates immediately after spin coating and formed a puddle at the substrate center. Other monofunctional epoxies exhibited the same behavior. Mixtures of BPGE with multifunctional aromatic epoxies wetted Si substrates and could be used as planarizing layers. 

Epps TH, Delongchamp DM, Fasolka MJ, Fischer DA, Jablonski EL (2007) Substrate surface energy dependent morphology and dewetting in an ABC tiiblock copolymer film. Langmuir 23 3355-3362... 

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