Captive-bubble technique

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],... 

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). 

Zhang, W. and HaUstrom, B. (1990). Membrane characterisation using the contact angle technique, I. Methodology of the captive bubble technique. Desalination 79, 1. 

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. 

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]. 

There are a variety of simple and inexpensive techniques for measuring contact angles, most of which are described in detail in various texts and publications and will be mentioned only briefly here. The most common direct methods (Fig. 17.4) include the sessile drop (a), the captive bubble (b), the sessile bubble (c), and the tilting plate (d). Indirect methods include tensiometry and geometric analysis of the shape of a meniscus. For solids for which the above methods are not applicable, such as powders and porous materials, methods based on capillary pressures, sedimentation rates, wetting times, imbibition rates, and other properties, have been developed. 

Experimentally, contact angle is measured based on geometry of the droplet or the interfacial tension across the contact line using various techniques such as tilting plate, sessile bubble and drop, captive bubble, the Wilhelmy method, and so on. Similarly, in simulation, three methods are generally used to find the contact angle. 

The ADSA technique in the captive bubble mode was used by Cordeiro et al. (2009) to study the thermo-responsive switching of poly(A-isopropylacryl-amide-co-A-(l-phenylethyl) acrylamide) and to investigate the possible impact of electrolytes on wetting behaviour and transition temperature. 

Zhang, W., Wahlgien, M., and Sivik, B. (1989). Membrane characterisation by the contact angle technique, II. Characterization of UF-membranes and comparison between the captive bubble and sessile drop methods to obtain water contact angles. Desalination 72, 263. 

Cain FW, Lee JC. (1985) A technique for studying the drainage and rupture of unstable liquid films formed between two captive bubbles Measurements on KCl solutions./ Coll Interface Sci 106(1) 70-85. 

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). 

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