Bubbles sessile

Fig. 11-16. Shapes of sessile and hanging drops and bubbles (a) hanging drop (b) sessile drop (c) hanging bubble (d) sessile bubble.

The cases of the sessile drop and bubble are symmetrical, as illustrated in Fig. n-16. The profile is also that of a meniscus 0 is now positive and, as an... 

Fig. X-8. Use of sessile drops or bubbles for contact angle determination.

The axisymmetric drop shape analysis (see Section II-7B) developed by Neumann and co-workers has been applied to the evaluation of sessile drops or bubbles to determine contact angles between 50° and 180° [98]. In two such studies, Li, Neumann, and co-workers [99, 100] deduced the line tension from the drop size dependence of the contact angle and a modified Young equation... 

The measurement of contact angles for a sessile drop or bubble resting on or against a plane solid surface can be measured by direct microscopic examination. 

Bubbles or drops which are prevented from moving under the influence of gravity by a flat plate are termed sessile (see Fig. 2.3a and 2.3b). When bubbles or drops remain attached to a surface with gravity acting to pull them away, they are called pendant (see Fig. 2.3c and 2.3d). Floating bubbles or drops, shown in Fig. 2.3e, are those which sit at the interface between two fluids. 

The profiles of pendant and sessile bubbles and drops are commonly used in determinations of surface and interfacial tensions and of contact angles. Such methods are possible because the interfaces of static fluid particles must be at equilibrium with respect to hydrostatic pressure gradients and increments in normal stress due to surface tension at a curved interface (see Chapter 1). It is simple to show that at any point on the surface... 

Fig. 2.3 Shapes of static bubbles and drops (a),(b) sessile (c),(d) pendant (e) floating. (Shading denotes more dense fluid in each case.)...

Several additional points might be noted about the use of the Bashforth-Adams tables to evaluate 7. If interpolation is necessary to arrive at the proper (3 value, then interpolation will also be necessary to determine (x/bl. . This results in some loss of accuracy. With pendant drops or sessile bubbles (i.e., negative /3 values), it is difficult to measure the maximum radius since the curvature is least along the equator of such drops (see Figure 6.15b). The Bashforth-Adams tables have been rearranged to facilitate their use for pendant drops. The interested reader will find tables adapted for pendant drops in the material by Padday (1969). The pendant drop method utilizes an equilibrium drop attached to a support and should not be confused with the drop weight method, which involves drop detachment. 

Provides measuring techniques of contact angle, surface tension, interfacial tension, and bubble pressure. Suitable methods for both static and dynamic inteifacial tension of liquids include du Nous ring, Wilhelmy plate, spinning drop, pendant drop, bubble pressure, and drop volume techniques. Methods for solids include sessile drop, dynamic Wilhelmy, single fiber, and powder contact angle techniques. 

From this fit the surface tension is obtained. The same method is applied with a pendant or sessile bubble. Using a bubble ensures that the vapor pressure is 100%, a requirement for doing experiments in thermodynamic equilibrium. Often problems caused by contamination are reduced. 

Important techniques to measure the surface tension of liquids are the sessile drop method, the pendant or sessile bubble method, the Du-Notiy ring tensiometer, and the Wilhelmy-plate method. 

Alternatively, we can measure the contact angle at the edge of a bubble. This method is called captive or sessile bubble. In this case a bubble is positioned usually at the top of a cell which is otherwise filled with liquid. The method is less sensitive to pollution of the interface. In addition, the vapor phase is automatically saturated. 

Figure 7.6 Sessile drop and sessile bubble on a planar surface.

Contact angles are commonly measured by the sessile drop, the captive bubble, and the Wilhelmy plate method. To characterize the wetting properties of powders the capillary rise method is used. 

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 Young-Laplace equation forms the basis for some important methods for measuring surface and interfacial tensions, such as the pendant and sessile drop methods, the spinning drop method, and the maximum bubble pressure method (see Section 3.2.3). Liquid flow in response to the pressure difference expressed by Eqs. (3.6) or (3.7) is known as Laplace flow, or capillary flow. 

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]. 

One can measure the maximum pressure that can be applied to a gas bubble at the end of a vertical capillary, of radius r and depth t in a liquid, before it breaks away (Figure 3.13) [143], Before break-away the bubble has the shape of a sessile drop and is described by the equation of Bashforth and Adams. The pressure in the tube is the sum of the hydrostatic pressure (Apgt) and the pressure due to surface tension. Equations have been published which allow calculation of surface tension using Bashforth-Adams and density and depth data. 

Hogness,1 Burdon,2 Bircumshaw, and Sauerwald have done a great deal to render accurate measurements possible the best method is probably the maximum bubble pressure method, but the measurement of sessile drops (see Chap. IX), and of drop volumes, are also useful. Metals always have a very high surface tension. Table X gives typical results. 

There are static and dynamic methods. The static methods measure the tension of practically stationary surfaces which have been formed for an appreciable time, and depend on one of two principles. The most accurate depend on the pressure difference set up on the two sides of a curved surface possessing surface tension (Chap. I, 10), and are often only devices for the determination of hydrostatic pressure at a prescribed curvature of the liquid these include the capillary height method, with its numerous variants, the maximum bubble pressure method, the drop-weight method, and the method of sessile drops. The second principle, less accurate, but very often convenient because of its rapidity, is the formation of a film of the liquid and its extension by means of a support caused to adhere to the liquid temporarily methods in this class include the detachment of a ring or plate from the surface of any liquid, and the measurement of the tension of soap solutions by extending a film. 

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