Sessile bubble method

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. 

Measurements on molten metals. The maximum bubble pressure method has proved one of the most satisfactory, but sessile drops, and drop-volumes have also been used with success.2 The principal difficulty lies in the proneness of metals to form skins of oxides, or other compounds, on their surfaces and these are sure to reduce the surface tension. Unless work is conducted in a very high vacuum, a freshly formed surface is almost a necessity if the sessile bubble method is used, the course of formation of a surface layer may, if great precautions are taken, be traced by the alteration in surface tension. Another difficulty lies in the high contact angles formed by liquid metals with almost all non-metallic surfaces, which are due to the very high cohesion of metals compared with their adhesion to other substances. 

Sessile Bubble Method See Sessile Drop Method. 

Methods. Interfacial tension was measured by the sessile bubble method. Density difference between demixed phases was measured by float method and differential re-fractometory. Details of the experimental methods have been described elsewhere. The volume ratio of separated phases was also measured to obtain the coexistence curve. The critical point was determined on the coexistence curve as a point at which the volume ratio was unity. The detailed method for ternary systems will be described elsewhere. 

Figure 11.4. Schematics of the various shape methods (a) the pendant drop method (b) the sessile drop method (c) the pendant bubble method (d) the sessile bubble method. In all cases, the shaded areas represent the denser fluids, while the white areas represent air (in the case of liquid-air interfaces) or the less dense fluid (in the case of liquid-liquid interfaces)...

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... 

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. 

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. 

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. 

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]. 

The bubble injection method of contact angle measurement utilizes a sessile bubble below the surface that is immersed in water. It is nearly impossible to measure the sessile bubble contact angle on top surface of a sample immersed in water, of which contact angle is to be measured, because the buoyancy works in the direction to lift the bubble. Although the two methods should yield the identical contact angle, the values obtained by the two methods could deviate significantly depending on the solubility of air in water and the perturbability of the surface by water as described in the previous chapters. 

The static methods are based on studies of stable equilibrium spontaneously reached by the system. These techniques yield truly equilibrium values of the surface tension, essential for the investigation of properties of solutions. Examples of the static methods include the capillary rise method, the pendant and sessile drop (or bubble) methods, the spinning (rotating) drop method, and the Wilhelmy plate method. 

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 ... 

Pendant or Sessile Drop Method The surface tension can be easily measured by analyzing the shape of a drop. This is often done by optical means. Assuming that the drop is axially symmetric and in equilibrium (no viscous and inertial effects), the only effective forces are gravity and surface or interfacial forces. In this case, the Young-Laplace equation relates the shape of the droplet to the pressure jump across the interface. Surface tension is, then, measured by fitting the drop shape to the Young-Laplace equation. Either a pendant or a sessile drop can be used for surface tension measurement. The pendant drop approach is often more accurate than the sessile drop approach since it is easier to satisfy the axisymmetric assumption. Similar techniques can be used for measuring surface tension in a bubble. 

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. 

The capillary rise method provides very accurate values of interfacial tension if used carefully. The capillary tubes must not deviate significantly from a circular shape, must be of known and uniform radius, and must be carefully aligned in the vertical position. They must be cleaned or otherwise prepared to ensure that the contact angle is zero. Finally, Equation 1.64 must be corrected in experiments of high precision to accoimt for meniscus deviation from the hemispherical shape due to gravity effects. Equation 1.56 describes the meniscus shape in this case, since the situation is basically the same as that for a sessile bubble. Hence the... 

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. 

Modern methods of measuring the surface tension include the pendant drop method, the sessile drop method, and others (7,8,15). These methods depend on the shape of a drop of the polymer or a bubble in it, and on the balance of surface tension and gravitational forces see Figure 12.3 (8). 

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. 

Methods based on the shape of static drops or bubbles include the pendant drop method and the sessile drop or bubble method. The general procedure is to make certain measurements of the dimensions or profile. It is accurate to a few tenths of a percent. 

Mack, G.L., Lee, D.A., 1936. The determination of contact angles from measurements of the dimensions of small bubbles and drops. II. The sessile drop method for obtuse angles. J. Phys. Chem. 40, 169-176. 

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