Pseudo-Partial Wetting

What is the contact angle 6e of the drop with the substrate Based on equation (4.11), the equilibrium condition of the horizontal forces is 7 + 7SZ, + P(em) + errJi em) = JSL + 7COS0E. Since Il(em) = 0, the equation 

The equilibrium between a drop and a film prevails as long as there exists an ample supply of liquid. Should the available supply be insufficient to cover the entire substrate with a film of thickness then a drop placed down will spread completely (hence the name pseudo-partial wetting). 

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Pseudo-partial wetting if 5 > 0 and P < 0. A macroscopic droplet with a finite contact angle can form on top of a flat film, while a microscopic droplet will spread to form a pancake or a diluted gas. 

The dendrimer droplets were used as a tool to examine surface heterogeneity on the nanoscale [319]. Figure 32 shows two pairs of SFM images of the droplets deposited on mica (a,b) and a copolymer film (c,d), respectively. On mica, the hydroxyl terminated dendrimer showed the autophobic or pseudo-partial wetting behaviour as the droplets sit on a thin film of the same material [312,320]. 

FIGURE 4.6. Energy P(e) for (a) bare plastic (partial wetting) and (b) plastic coated with a film of gold (pseudo-partial wetting). 

If fi > 0 and 5 < 0, then an oil drop will form a lens, which may be in equilibrium with the original air-foaming liquid surface (i.e., partial wetting) or that surface after incorporation of material from the oil drop (i.e., pseudo-partial wetting). However, formation of oil lenses on the equilibrium liquid surface does not preclude the possibility that the initial spreading coefficient S is positive. If 5 > 0 and S < 0, then pseudo-partial wetting will occur and the oil drop may, for example, spread to form a duplex film, which is, however, unstable. That film will disproportionate to form oil lenses in equilibrium with oil-contaminated air-water surfaces The spreading of a drop of benzene on distilled water to form lenses in equilibrium with a benzene monolayer exemplifies this type of behavior. 

In the case of pseudo-partial wetting, a condition of stable equilibrium with respect to an oil lens can exist at a film thickness if the disjoining pressure curve is such that we can set = -H owC e) in the approach to the primary maximum on the disjoining pressure isotherm as shown in Figure 3.10a. Here A, B, C, and E represent the indicated areas in the isotherm. The integral in Equation 3.27 with the upper limit set at H owl e) is then equal to the generalized equilibrium spreading... 

From oil contaminated monolayer to thin oil film Pseudo-partial wetting... 

If > 0, then the oil will initially spread over the air-foaming liquid surface as a duplex film, which subsequently disproportionates to form a film in equilibrium with oil lenses because that film has a lower film tension than either the duplex film or the uncontaminated air-foaming liquid surface. If, however, both < 0 and S < 0 where A > A , then we will still have Aofw > 0 so that the pseudo-partial wetting film forms by direct spreading from an oil drop. Here we... 

Complete and Pseudo-Partial Wetting Behavior of Hydrocarbons on the Surfaces of Aqueous Surfactant Solutions... 

FIGURE 3.16 Effect of increasing salinity on pseudo-partial wetting film thickness of decane on aqneons 3 mM AOT solntion illustrating a maximum that correlates with a minimum in oil-water interfacial tension, Oqw- ( ) Film thickness, (h) log(interfacial tension), . (After Kellay, H. et al., Phys. Rev. Lett., 69, 1220, 1992 and KeUay, H. et al., Adv. Colloid Interface Sci., 49, 85, 1994 from the review of Aveyard, R., CUnt, J.H., J. Ghent. Soc. Faraday Trans., 91, 2681, 1995. Reprodnced hy permission of The Royal Society of Chemistry.)... 

Even on micellar solutions of this compound, the PDMS oil forms lenses with zero spreading pressure. Similar behavior is seen with submicellar solutions even up to air-water surface tensions of 45 mN m. Since the oil spreads on water, this suggests a possible first-order wetting transition between partial wetting and pseudo-partial wetting at even higher air-water surface tensions. Rather surprisingly then, this means that no penetration of the monolayer of this silicone surfactant by the PDMS must occur. 

Denkov et al. [90] have also shown that the presence of a spread layer of a PDMS oil of viscosity 1000 mPa s on micellar AOT solution reduces the applied capillary pressure for pseudoanulsion film rupture from 28 to 19 Pa. As we have seen in this case, pseudo-partial wetting is involved where a thin oil film is in equilibrium with oil lenses. 

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