Wetting of low energy surfaces

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. 

Shafrin, E.G. and Zisman, W.A. (1960) Constitutive relations in the wetting of low-energy surfaces and the theory of the retraction method of preparing monolayers. /. Phys. Chem., 64, 519-24. 

Unfortunately, the effect of constitution on either y o or y is still unknown, and neither quantity can be studied until a satisfactory experimental method for measuring it has been found. However, neither quantity appears to be a simple function of the constitution of the solid and liquid phases. A more precise theory of wetting of low-energy surfaces should at least include y. Probably the lateral spread in the data points of our graphs of cos 9 vs. given solid surface... 

Relatively few data are available for the wetting of low-energy surfaces by liquid metals. Reliable contact angles are available, however, for mercury (y v =485 dynes per cm.) on three different surfaces. When plotted as a function of (y y - y )j their data points suggest that a linear limiting relation also characterizes the wetting properties of this liquid metal. 

Churaev N, Sobolev V. (2007) Wetting of low-energy surfaces. Adv Colloid Interface Sci 134-135 15-23. 

RE Johnson Jr, RH Dettre. Wetting of low energy surfaces. In JC Berg, ed. Wettability. Surfactant Science Series, vol. 49. New York Marcel Dekker, 1993, pp 1-73. 

Aqueous solutions of certain trisiloxane surfactants wet rapidly low-energy surfaces (water contact angle > 90°) [1]. The spreading rate of a superspreader solution significantly exceeds that expected for a purely liquid diffusion controlled process [2, 3]. 

Another process that promotes chemical adhesion is the pre-treatment of low energy surface materials. This can be considered as a dry priming process, as no wet chemicals are used. Corona discharge treatment, dielectric barrier discharge, flame and Plasma treatment are widely used techniques in Pre-treatments of polyolefins. This reactive surface allows chemical bonding with the applied adhesive. 

The most common inspection of actual bonding surfaces is the water-break test. Water, being a polar molecule, will wet a high-energy surface, such as a clean metal oxide, but will not wet a low-energy surface, such as most organic materials and will bead up. As a part is removed from a rinse bath or spray, if the water flows uniformly over the entire surface, the surface is clean. However, if the water beads up or does not wet an area, that area likely has an organic contaminant that must be cleaned. 

Fluorinated surfactants can reduce the surface tension of water and organic solvents and aid wetting of low-energy plastic surfaces. For example, a nonionic fluorinated surfactant, Fluorad FC-740, can lower the surface tension of some low-polarity solvents to 20-24 mN/m and facilitate wetting of plastic surfaces which might be contaminated with silicones, oil, or grease [8]. 

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... 

Zisman discovered that there is a critical surface tension characteristic of low-energy solids, such as plastics and waxes. Liquids ihat have a lower surface tension than the solid will spread on that solid, while liquids with a higher surface tension will not spread. Examples of critical surface tension values for plastic solids in dynes per cm are "Teflon/ 18 polyethylene, 31 polyethylene terephthalate, 43 and nylon, 42-46. As one indication of the way this information can be used in practical applications, one can consider the bonding of nylon to polyethylene. If nylon were applied as a melt to polyethylene, it would not wet the lower-energy polyethylene surface and adhesion would be poor. However, molten polyethylene would spread readily over solid nylon to provide a strong bond. 

The surface of PTFE material is smooth and slippery. It is considered to be very low energy surface with yc = 18.5 dyne/cm (mN/m)33 and can be, therefore, completely wetted by liquids with surface tensions below 18 mN/m, for example, solutions of perfluorocarbon acids in water.34 The PTFE surface can be treated by alkali metals to improve this wettability and consequently the adhesion to other substrates,35 but this increases its coefficient of friction.36... 

FEP resins have a very low energy surface and are, therefore, very difficult to wet. Surface preparation for improved wetting and bonding of FEP can be done by a solution of sodium in liquid ammonia or naphthalenylsodium in tetrahydrofurane,44... 

Wagner, R., Richter, L., Weissmuller, J., Reiners, J., Klein, K.D., Schaefer, D. and Stadtmuller, S. (1997) Silicon-modified carbohydrate surfactants 4. The impact of substructures on the wetting behaviour of siloxanyl-modified carbohydrate surfactants on low-energy surfaces. Appl. Organometallic Chem., 11(7), 617-32. 

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