Surface low-energy

In practice, 7s 7sv is negligible as is dys/dT for systems having large contact angles. Also, low energy surfaces have a relatively constant value of dyst/dT = 0.07 0.02 erg cm K , leaving... 

Fig. X-10. The wettability spectrum for selected low-energy surfaces. (From Ref. 155.)...

The interesting implication of Eq. XII-24 is that for a given solid, the work of adhesion goes through a maximum as 7b(a) is varied [69]. For the low-energy surfaces Zisman and co-workers studied, )3 is about 0.04, and Wmax is approximately equal to the critical surface tension yc itself the liquid for this optimum adhesion has a fairly high contact angle. 

It should be loted that with low-energy surfaces the sudden fall in the heat of adsorption is absent. This is illustrated in Fig. 2.15, where the contrast between the behaviour of nitrogen on the carbons (high-energy surfaces) and on the molecular solids (low-energy surfaces) is very clear. 

Fluorochemicals repel both water and oU because they produce an extremely low energy surface (18—26). The effectiveness of the fluorochemicals depends upon uniform surface coverage and orientation of the molecules on the fiber surface so that the perfluoroalkyl chains are directed away from the surface. The result is a GST as low as 5—10 mN /m (dyne/cm). Fluorochemical finishes are often formulated with nonfluorinated resin-based water-repeUent extenders. These water repeUents not only reduce the cost of the finish but may also improve durabUity (27,28). 

Critical surface tension data for low-energy surfaces of varying surface chemistry obtained from Zisman plots... 

To inspect for contaminants, a water break test is frequently employed. Water, being a polar molecule, will wet a high-energy surface (contact angle near 180 ), such as a clean metal oxide, but will bead-up on a low-energy surface characteristic of most organic materials. If the water flows uniformly over the entire surface, the surface can be assumed to clean, but if it beads-up or does not wet an area, that area probably has an organic contaminant that will require the part be re-processed. 

Besides the spontaneous, complete wetting for some areas of application, e.g., washing and dishwashing, the rewetting of a hydrophobic component on a solid surface by an aqueous surfactant solution is of great importance. The oil film is thereby compressed to droplets which are released from the surface. Hydrophobic components on low-energy surfaces (e.g., most plastics) are only re wetted under critical conditions. For a complete re wetting of a hydrophobic oil on polytetrafluoroethylene (PTFE) by an aqueous solution, the aqueous solution-oil interface tension must be less than the PTFE-oil interface tension... 

Shafrin, E.G. and Zisman, W.A. "Constitutive Relations in the Wetting of Low-Energy Surfaces and the Theory of the Retraction Method of Preparing Monolayers," Naval Research Labs Report 5394, Surface Chemistry Branch, Chemistry Division, October 21, 1959. 

Polyfluorinated chemicals now dominate in the fields of oil-repellent and water-repellent finishes. The earlier so-called conventional polyfluorinated products were of the type represented by poly(N-methylperfluoro-octanesulphonamidoethyl acrylate) (10.248) [499]. Such products presented a shield of closely packed fluoroalkyl groups at the fibre-air interface, thus giving low-energy surfaces with excellent oleophobicity. These showed excellent resistance to oil-based stains but were less satisfactory as soil-release agents during... 

Table 10.47 Critical surface energies for low energy surfaces [502]...

Electrochemical deposition of photopolymer films occur on low energy surface states on the metal electrodes. The deposits tend to grow as "pillars" perpendicular to the electrode surface. A smooth coalesced film is observed when... 

The use of biological materials as coatings for piezoelectric crystals was first demonstrated by Shons et al. [237], who immobilized bovine serum albumin (BSA) on a crystal precoated with a 30% solution of Nyebar C, a low-surface energy plastic. The rationale of using this solution as the coating material is that proteins adsorbed on low-energy surfaces retain their antigenic properties. Exposure of the BSA-coated crystal to a solution... 

The value of is very small for low energy surfaces, but it cannot be neglected for fillers, on the contrary can be used for the calculation of the thermodynamic characteristics of their surface. The spreading pressure can be determined from the adsorption isotherm in the following way [74] ... 

High-energy surfaces bind enough adsorbed molecules to make ire significant. On the other hand, ire is negligible for a solid that possesses a low-energy surface. 

A surface energy in the neighborhood of 100 mJ m 2 is generally considered the cutoff value between high- and low-energy surfaces. 

Hard and soft solids are generally classified as having high- and low-energy surfaces, respectively. These criteria must be used cautiously since the mechanical properties of a solid depend on the concentration of defects and dislocations in the bulk. 

Because of the simplification that results from 7 = 0 for low-energy surfaces, they are often chosen as model systems in fundamental research. Even when neglecting 7re is of questionable validity, ysv and ySc are often used interchangeably for lack of suitable data. We see in Chapter 9 —Equation (9.7), for example —how ire may be determined from experimental adsorption data. Otherwise, we generally assume 7re = 0. 

It is informative to apply Equation (60) to low-energy surfaces for two extreme values of 0, 0° and 180°, for which cos 0 is 1 and — 1, respectively. For 0 = 0°, WSL = 2yLV = WAA the work of solid-liquid adhesion is identical to the work of cohesion for the liquid. In this case interactions between solid and solid, liquid and liquid, and solid and liquid molecules are all equivalent. At the other extreme, with 0 = 180°, WSI = 0. In this case the liquid is tangent to the solid there is no interaction between the phases. 

It is not difficult to apply the concept of the dispersion component of y to solid surfaces. In doing this, it is necessary to treat high- and low-energy surfaces differently. We shall not consider solid interfaces in detail our treatment is limited to the following observations. ... 

For low-energy surfaces, ire 0. Manipulation of Young s equation (Equation (44)) generates a relationship that expresses yd in terms of 0 and other experimental quantities. 

Values of yd that have been determined for high- and low-energy surfaces by these two methods are listed in Table 6.4. 

Suppose that the initial film is spread on water that fills to the brim a shallow tray made of some inert material. Rods with low-energy surfaces may then be drawn across the water to adjust the area accessible to the molecules of the monolayer. Figure 7.2a indicates schematically how such an arrangement might appear. In practice, several barriers would be used, first to sweep the surface free of insoluble contaminants and then to confine the monolayer. 

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