Changes in contact angle

Figure16.6 (a) Schematicdrawingofexperimental set-upforthe evaluation of the interfacial tension under potential control, (b) Relative change in contact angle as a function ofthe potential after the substrate was inserted into (open circles) and pulled from the nitrobenzene phase. Insets are schematic drawings of the side views of the contact lines. The potential was described with respect to the Au/AuO f reference electrode.

The change in contact angle from 6 to 9 as extrusion commences, for which the corresponding work, is... 

Figure 4. Photographs of bubble profiles superimposed on a voltammogram showing change in contact angle with electrode potential of copper in the presence of 2 X 103 M potassium diethyldithiophosphate as flotation collector. Conditions sweep rate, 50 mV Is pH 9.3 (0.25 M sodium borate). (Reproduced, with permission, from Ref. 1. Copyright 1975, Universita Degli Studi.)...

Urethanation with 100% HMDI Change in contact angles... 

The effect of ultraviolet irradiation in air on the wettability of thin films of amorphous polymers has been studied. With poly(vinyl chloride), poly(methyl methacrylate), poly(n-butyl methacrylate), poly (ethylene terephthalate), and polystyrene the changes in contact angles for various liquids with irradiation time are a function of the nature of the polymer. A detailed study of polystyrene by this technique and attenuated total reflectance spectra, both of which are sensitive to changes in the surface layers, indicates that the contact angle method is one of the most sensitive tools for the study of polymer photooxidation in its early stages. The method is useful in following specific processes and in indicating solvents to be used in the separation and isolation of photooxidation products. 

Literature values of the contact angles for various liquids on four of the five polymers used in this work are given in Table I. These values may be compared with those for our unirradiated polymers. In the case of polystyrene, exhaustive methanol or n-heptane extraction followed by drying did not result in a change in contact angles, and residual extractant could not be observed by means of frustrated multiple internal reflectance (FMIR) spectra of the films. 

That photooxidation is indeed responsible for the observed changes is indicated by a comparison with the results obtained with films irradiated in vacuum. Both polystyrene and poly(n-butyl methacrylate) irradiated in vacuum showed no changes in contact angles after exposures up to 120 min. with poly (ethylene terephthalate), contact angles for all of the liquids tended to increase slightly. 

Techniques for spreading monolayers of polar long chain compounds on mercury in a Langmuir type film balance, and for measuring their surface area-pressure properties, have been described by one of the present authors (3). Using these techniques, it has proved possible to measure continuously the change in contact angle of a water droplet superposed on the monolayer, as the film pressure is controllably varied. This has now been done for monolayers of the normal C12-C20 fatty acids and the normal primary Ci4-Ci8 alcohols on the mercury substrate. 

The transition from one black film type to another for films from non-ionic surfactants does not involve a sharp change in contact angles and in this case no region of metastability is... 

Change in contact angles as a function of storage time was also not studied. Studies on polystyrene indicate that the contact angle does change with time after plasma treatment (11). 

Hansen (1 ) reports a decrease in molecular weight in a helium plasma and Westerdahl ( ) reports a change in contact angle to higher wettability for both helium and oxygen plasmas. 

As the samples were highly oxidized from the beginning, due to the polishing process (17), no real change in contact angle was observed. 

The film surface characteristics were also dramatically changed due to oxygen addition as was seen in the change in contact angle with water, at the peak weight increase at 70 C, films all showed water contact angle reduction of about 20 to 30 percent indicating a drastic reduction of hydrophobicity with the surface formation of carbonyl functionality (see below) which enhanced hydrophilicity. 

Block copolymer surface was also prepared by the copper-catalyzed radical polymerization of MMA from a surface-confined macroinitiator of polystyrene (S-4) obtained by living cationic polymerization although the blocking efficiency was unknown.378 The block copolymerization increased the film thickness by 9 nm and changed the water contact angles. Other monomers such as MA and 2-(7V,7V-dimethylamino)-ethyl methacrylate were also polymerized from S-4.377 The changes in contact angles were observed by treatment of the surface with several solvents and air. 

The results showed that the initial contact angle was < 90°. No attempt was made to obtain measurements for the hnal rapid stages of evaporation where the mixed mode of evaporation occurred. It was also noted that the earlier models = did not distinguish between the two principal radii of curvatnre occnrring at the contact line these two radii do not have the same values. The latter studies, therefore, extended the model to a two-parameter spherical cap geometry, which was able to explain why the experimentally observed change in contact angle should appear linear in time. 

The most common digital microfluidic fluid actuation techniques utilize a combination of strategically placed electrodes and changes in contact angle induced by one of two principles electrowetting on dielectric (EWOD) or dielectrophoresis (DEP). EWOD and DEP can be considered as the low- and high-frequency cases, respectively, of the application of a sufficient electric field to polarizable liquids along the correct axes.7... 

The case of contact angles of solutions is more difficult, but offers more scope for thermodynamic predictions that would be in principle verifiable. In addition to temperature-dependence studies, it is now possible to make independent determinations of changes in y yo and y with solution composition and hence of the expected changes in contact angle. 

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