Results of Contact Angle Measurements

It is clear from our discussion of contact angle hysteresis that there is some degree of variability in reported contact angle values. The data collected in Table X-2, therefore, are intended mainly as a guide to the type of behavior to be expected. The older data comprise mainly results for refractory and relatively polar solids, while newer data are for polymeric surfaces. 

There is appreciable contact angle hysteresis for many of the systems reported in Table X-2 the customary practice of reporting advancing angles has been followed. 

As a somewhat anecdotal aside, there has been an interesting question as to whether gold is or is not wet by water, with many publications on either side. This history has been reviewed by Smith [119]. The present consensus seems to be that absolutely pure gold is water-wet and that the reports of non wetting are a documentation of the ease with which gold surface becomes contaminated (see Ref. 120, but also 121). The detection and control of surface contaminants has been discussed by White [121] see also Gaines [122]. 

A major contribution to the rational organization of contact angle data was made by Zisman and co-workers. They observed that cos 6 (advancing angle) is usually a monotonic function of 7l for a homologous series of liquids. The proposed function was 

The critical surface tension concept has provided a useful means of summarizing wetting behavior and allowing predictions of an interpolative nature. A schematic summary of 7 values is given in Fig. X-10 [123]. In addition, actual contact angles for various systems can be estimated since )3 in Eq. X-38 usually has a value of about 0.03-0.04. 

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Section 4.1 briefly describes some of the commonly employed experimental tools and procedures. Chaudhury et al., Israelachvili et al. and Tirrell et al. employed contact mechanics based approach to estimate surface energies of different self-assembled monolayers and polymers. In these studies, the results of these measurements were compared to the results of contact angle measurements. These measurements are reviewed in Section 4.2. The JKR type measurements are discussed in Section 4.2.1, and the measurements done using the surface forces apparatus (SFA) are reviewed in Section 4.2.2. 

Table 10.5. Results of Contact Angle Measurements toward Hexadecane/Aif... 

Figure 16 shows result of contact angle measurements on PTFE powders having different irradiation doses in comparison to nonmodified (0 kGy-irradiated) PTFE powder. The horizontal line indicates the contact angle of a typical commercial... 

Results of contact angle measurements also support this conclusion. Subsequently, the introduction of APTES moieties by... 

Contact angles were measured on plasma polymers deposited from numerous hydrocarbon monomers of different structure containing triple bond, olefinic double bonds, aromatic and aliphatic structures. The results of contact angle measurements and evaluated surface energy properties for these polymers are summarized in Table II, column A. The data for plasma polymers from acetylene, ethylene, and hexane indicate that monomer unsaturation does not change substantially the dispersion component but increases the polar component to a considerable extent as in the case of acetylene. This, undoubtedly, is due to the high concentration of radicals in PP-AC and resulting rapid formation of carbonyls and... 

From the result of contact angle measurements, it was expected that there would be no significant difference between PAS and SILASTIC 500-1 in the BSA adsorption and the cell adhesion. In the BSA adsorption test, this expectation seemed to hold. In the cell adhesion experiment, however, the PASs, which showed almost the same protein adsorption behavior, exhibited different cell adhesiveness. The number of adherent cells on PAS-41 and 71 were drastically decreased. Conversely, the numbers of the adherent cells on... 

Table 3.5. Minimum and maximum values of contact angle measured for some pure metals on alumina. Results come from data compilations given in references.

Any discussion leading to the interpretation of contact angle measurements, such as that presented below, presupposes the existence of a body of reliable experimental data. Such data are now available, due to the efforts of Zisman and his collaborators at the Naval Research Laboratory. This situation is the result of solving a number of problems connected with the preparation of suitable solid surfaces. These problems had presented major difficulties in the field of contact angle studies ever since its inception and hence had prevented any substantial progress. 

It has been found that the liquid surface tension values measured from sessile drops are less accurate than those obtained from pendent drops. This is a consequence of the assumption that the drop shape is axisymmetric. While the shape of sessile drops is very sensitive to even a very small surface imperfection, such as roughness and heterogeneity, axisymmetry is enforced in the case of a pendent drop through the circularity of the capillary orifice supporting the drop, thus resulting in more reliable surface tension results. The contact angle measurement, on the other hand, is less sensitive to geometrical imperfections than the surface tension measurement. 

As lubricants, low-surface-tension perfluorinated liquids (e.g., DuPont Krytox oils) that are nonvolatile and are immiscible with both aqueous and hydrocarbon liquids can be used. Kim et al. (2012) reported the development of lubricant infosed structured surfaces fabricated on aluminum substrates. Polypyrrole was initially electrodeposited on the surface of aluminum samples, which was then followed by the subsequent fluorination of the structured coating and infiltration with lubricant. The resulting water contact angle measurements showed that these surfaces were characterized by low contact angle hysteresis (A0= str,R H7°-115° = 2°) and... 

The technique of contact mechanics has also been applied to the direct mechanical determination of solid-fluid interfacial energies, and the results compare favorably with those obtained by contact angle measurements [19]. 

The surface forces apparatus (Section 2.3) enables the estimation of a surface energy term, Fq (Eq. 9), providing sufficiently smooth surfaces can be produced. In recent years Chaudhury, Pocius and colleagues have made a valuable contribution to the field of adhesion by developing the technique to study energies of adhesion and of surface energies of polymers [81-85]. These SFA results provide alternatives to values based on traditional destructive tests or contact angle measurements. 

Induction period measurements can also be used to determine interfacial tensions. To validate the values inferred, however, it is necessary to compare the results with an independent source. Hurley etal. (1995) achieved this for Cyanazine using a dynamic contact angle analyser (Calm DCA312). Solid-liquid interfacial tensions estimated from contact angle measurements were in the range 5-12 mJ/m which showed closest agreement with values (4—20mJ/m ) obtained from the log-log plots of induction time versus supersaturation based on the assumption of — tg. 

Synthesis of siloxane-urethane copolymers from various hydroxyalkyl-terminated PDMS oligomers and aliphatic diisocyanates, such as tetramethylene- and hexame-thylene diisocyanate and HMDI was reported 333,334). Reactions were conducted either in chloroform or 1,4-dioxane and usually low molecular weight, oily products were obtained. No data were available on the molecular weights or the thermal and mechanical properties of the copolymers obtained. These products were later cross-linked by a peroxide. Resulting materials were characterized by IR spectroscopy and water contact angle measurements for possible use as contact lenses. 

Contact angle measurements were obtained using a goniometer, measuring the advancing angle from 2 to 20 microliter drop sizes, of purified water upon polymer films at room temperature. Films were cast on metal plates and allowed to dry slowly from chloroform solutions. Several spots were measured on each film and the results averaged. 

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