Surface captive bubble

Fig. 7. The concept of contact angle with a captive bubble in an aqueous medium, adhering to a hydrophobic sofld P is the three-phase contact point. Here, the vector passes through P and forms a tangent to the curved surface of the air bubble. The contact angle 0 is drawn into the Hquid.

Some of the commonly used techniques for measuring contact angle [215, 216, 217] are the sessile drop method, captive bubble method and Wilhelmy plate method. These techniques have been extensively used and well documented for characterisation of modified PE surfaces [218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230] for various applications. Whitesides et al. [231 ] studied the wetting of flame-treated polyethylene film having ionisable organic acids and bases at the polymer-water interface. The effect of the size of substituted alkyl groups in amide and ester moieties on the surface hydrophilicity was also studied [232]. The biocompatibility of the polyethylene film surface modified with various water-soluble polymers was evaluated using the same technique [233]. The surface properties of hy-perbranched polymers have been very recently reported [234]. 

The captive drop and captive bubble methods, variations of the sessile drop method, have been developed for the determination of very low values of surface or interfacial tension [140,141], including at elevated temperature and pressure [141]. 

The captive bubble method was applied to quantify the wettability of the resist in contact with water, with surfactant solutions of different concentration and with water after contact with the surfactant solution. The wafer piece is mounted with the photoresist layer down in a cuvette filled with the solution of interest. Through a small hole in the wafer an air bubble is placed under the photoresist surface. The shape of the drop is analyzed while its volume is slowly increased and decreased and the contact angle of the bubble is computed. It has to be converted into the water contact angle by subtracting its value from 180°. 

Surface Tension Measurement. The surface tension of the surfactant solution was determined by means of the Dynamic Contact Angle Tester FIBRO DAT 1100 (FIBRO Systems, Sweden) using the pendant drop method. It was also an output of the ADSA captive bubble contact angle measurements with surfactant solutions. 

Marmur A. (1998) Contact angle hysteresis on heterogeneous smooth surfaces theoretical comparison of the captive bubble and drop methods. Colloids Surf A 136 209-215. 

A number of systems such as sessile drop, captive bubble, hanging, or pendent drop in which a drop or bubble is kept in position on a surface belong to this category. 

Three methods are usually used to calculate contact angle—Wilhelmy plate method, sessile drop method [33], and captive bubble method [34]. Sessile drop method is the most commonly used method for biomedical polymers. In this method, about 3 pi of a liquid droplet is placed on the polymer surface and images of the drop are acquired about 30 s of equilibration of the drop. Interface energy between the solid sample snrface and hqnid can also be calculated using the Young s eqnation ... 

Altogether, it may be clear that as long as wetting is not free of hysteresis one should take extrane care in further interpretation and analysis of the contact angle. As an alternative for a sessile drop of liquid, a captive gas (air) bubble may be used to probe the wettability of a solid surface. See Figure 8.5. Analysis and interpretation of the contact angle of the captive bubble are, mutatis mutandis, analogous to those for the sessile drop. 

A distinction can be made among the available methods between static and dynamic contact angle determination methods. In the case of a static determination the contact angle of a drop with an immobile solid/liquid/gas interface is determined microscopically (sessile drop). In the captive bubble method the contact angle of an air bubble, which is located under the solid surface in contact with the liquid, is determined. In contrast to the sessile drop method, in the captive bubble method the contact angle is measured at a completely wet surface. 

The IR-ATR spectra of the untreated and modified PES as well as that of the PHEMA reference surface are shown in Fig. 15. The spectra of the modified PES surface shows the superposition of the bands of the pure PES and PHEMA. The surface tension of the PES surface modified with the hydrogel was investigated by means of the captive bubble technique by measurement of the air/water and octane/water contact angle (Fig. 16). The polar and dispersive components of the surface tension, yP and were calculated from the contact angles determined [114],... 

Regions of practically immobile states of a meniscus are shown in Fig. 25 by arrows on the pressure axis for solution concentration Co = 5 x 10 (curves 3) and 5 x 10 M (curves 4). This makes it possible to assess static values of contact angles. Because of small hysteresis (the regions shown by arrows are short) the mean value of static contact angle is equal to 40° for C = 5 X 10 M and 36° at 5 x 10 M. The calculated values are close to those measured using captive bubbles [45] and differential ellipsometry method [46] on quartz surface for the same solutions. 

Figure 6.2 Forces acting on a captive bubble on an inclined surface, (a) Vertical net force and its tangential and normal components, (b) Lateral adhesion (static friction) force.

J. Drehch, J. D. Miller, and R. J. Good, The effect of drop (bubble) size on advancing and receding contact angles for heterogeneous and rough sohd surfaces as observed with sessile-drop and captive-bubble techniques, J. Colloid Interface Set, 179,37-50 (1996). 

It has been demonstrated that the potential at which the contact angle measurements indicate that abraded mineral surfaces become hydrophobic in the presence of a thiol can be somewhat higher than the value at which chemisorption commences. It was considered that this reflects the presence of a significant induction time in establishing a captive bubble on such a surface. Energy barriers in particle-to-bubble attachment... 

Equilibrium contact angles can be measured very simply from the profiles of liquid drops (Figure 2a) or bubbles (Figure 2b) resting on a plane surface. These methods are known as the sessile drop and captive bubble methods respectively. The contact angle may be measured indirectly by... 

Schurch, S., F. H. Y. Green et al. 1998. Formation and structure of surface films Captive bubble surfactom-etry. Biochim Biophys Acta Mol Basis Dis 1408(2-3) 180-202. 

The precise structure of the film is still in question. The results obtained from the nonaqueous techniques indicate that the fluid lining the alveolus is continuous, varies in thickness from 0.09 pm over protruding features to 0.14 pm over relatively flat areas (74) and may have a variable number of lipid layers at its surface (69). This concept is supported by surface-activity studies, using the captive bubble surfactometer, which indicate that the film can form a reservoir of surface-active material (77). 

Putz G, Goerke J, Clements JA. Surface activity of rahhit pulmonary surfactant subfractions at different concentrations in a captive bubble. J Appl Physiol 1994 77 597-605. 

An alternate method frequently mentioned in the literature is the so-called captive bubble technique [42]. Experimentally, an air bubble of known volume is produced at the tip of a microsyringe. This bubble is then injected into a tank containing the test liquid. The test surface is positioned on the top of the test liquid and 2-3 mm above the injected air bubble. A captive bubble is formed when the air bubble floats upward and be captured by test surface. Contact angle 6, formed at the three phase contact line, follows the same Young s equation in Eq. (2.1) and can be calculated from the drop profile of the bubble in a manner similar to the sessile drop method. A schematic of the captive bubble method is shown in Fig. 2.11. 

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