Surface force sessile drop method

The wettability of diamond with Ni-Mn alloys having Ga, Ge, Sn, Mg, A1 and Si additives was determined by means of the sessile drop method at high pressure (5.5 GPa) and temperature (1400° C) following a special procedure [3,4]. The metal pellet melted and attained its equilibrium form owing to surface forces on the diamond plane surrounded by molten NaCl that gives a quasi-hydrostatic enviroment and suppress oxidation of the metal. 

The pendant drop method and the sessile drop methods for surface tension appear quite different, but are similar in principle. In each case, a drop is formed which either sits on a plane surface (sessile, see Figure 3.11) or hangs as a pendant drop (each can be formed rightside-up or upside-down depending on Ap). In zero gravity, each would form a sphere due to surface or interfacial tension. In the gravitational field, the drop becomes distorted until the gravitational and surface tension forces equal. 

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. 

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). 

The shape adopted by a drop stems from a compromise between the effect of surface tension, which favors a sphere, and gravity (or any other force field), which will cause distortions. For any given situation, analyzing the shape of the drop ought to make it possible to extract the surface tension. Several methods currently used to measure surface (or interfacial) tensions are based on this principle, such as the sessile drop method or the pendant drop method. The latter is the most commonly used. ... 

An alternative method for measuring surface tension is to study the shape of a drop of the fluid, either a drop placed on a surface (a sessile drop) or a drop hanging from a pipette tip (pendant drop). In either case, a motionless droplet of the fluid is imaged. The only forces acting on the drop are gravity and the surface tension force. The result of these forces will determine the drop shape according to the Young-Laplace equation ... 

The Wilhelmy plate method [323,324], the sessile drop method [328,340], and the capillary height method [325-328] measure equilibrium surface tension, if sufficient time is allowed for the adsorption of surfactant molecules at the surface to attain the state of equilibrium. The Wilhelmy plate method measures the force exerted on a vertical plate partially immersed in the liquid (Fig. 9.20). If wetting of the plate is complete, the force, F, is proportional to the surface tension, y, and the circumference, L. of the plate ... 

The surface tension measurement techniques can be divided into the following three categories (i) Force Methods, which include the truly static methods of the capillary rise and Wilhelmy plate methods, as well as the dynamic detachment methods of the Du Nouy ring and drop weight, (ii) Shape Methods, which include the pendant or sessile drop or bubble, as well as the spinning drop methods, and (iii) Pressure Methods, which are represented by the maximum bubble pressure method. These techniques are summarized in the following sections of this chapter. 

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