Surface force drop weight method

There are numerous other methods for measuring surface tension that we do not discuss here. These include (a) the measurement of the maximum pressure beyond which an inert gas bubble formed at the tip of a capillary immersed in a liquid breaks away from the tip (the so-called maximum bubble-pressure method) (b) the so-called drop-weight method, in which drops of a liquid (in a gas or in another liquid) formed at the tip of a capillary are collected and weighed and (c) the ring method, in which the force required to detach a ring or a loop of wire is measured. In all these cases, the measured quantities can be related to the surface tension of the liquid through simple equations. The basic concepts involved in these methods do not differ significantly from what we cover in this chapter. The experimental details may be obtained from Adamson (1990). 

Ring method — Method to determine the - interfacial tension in liquid-gas systems introduced by Lecomte du Noiiy [i]. It is based on measuring the force to detach a ring or loop of a wire from the surface of a liquid. The method is similar to the -> Wilhelmyplate method when used in the detachment mode [ii]. See also -> electrocapillarity, -r electrocapillary curve, -> Gibbs-Lippmann equation, - Wilhelmy plate (slide) method, - drop weight method, - Lippmann capillary electrometer. 

A rather complex theory of the drop weight method, which makes it possible to tabulate the data required in order to determine the surface tension, has been worked out in some detail [27]. In the first (roughest) approximation, it can be assumed that, at the moment of detachment the gravity force acting on a drop, P, is balanced by the surface tension forces,... 

The drop-weight method depends, as do all of the detachment methods, on the assumption that the circumference times the surface tension is the force holding two parts of a liquid column together. When this force is balanced by the mass of the lower portion, a drop breaks off (Fig. 18.4a) and... 

To solve these problems, we sometimes use the drop-weight method see Fig. 17.5. In this method we allow drops to fall slowly (one every 2 to 5 min) from the tip of a burette or hypodermic needle. The drops are caught and weighed. If the liquid wets the burette perfectly, then at the instant that the drop breaks away its weight must be exactly equal to the surface force holding it up, or... 

It is a well-known fact that when a drop is formed at a nozzle under pseudostatic conditions, its size is determined solely by the balance of interfacial tension and gravity or buoyancy forces. This forms the basis of the drop-weight method of determination of surface and interfacial tension. 

If one considers a system consisting of water (with or without added electrolyte) + oil + surfactant (with or without a cosurfactant) at equilibrium, there will most likely be present more than two phases (due to the formation of emulsion or microemulsion). The determination of the interfacial tension, Yij> between the two liquid phases is, therefore, of much importance, in order to understand the forces which stabilize these emulsions or microemulsions. The interfacial tension can be measured by using a variety of methods, as described in detail in surface chemistry text-books (1-3). If the magnitude of yij is of the order of few mN/m (=dyne/ cm), then the methods generally used are Wilhelmy plate method or the drop volume (or weight) method (1-4). However, in certain systems ultra-low (or low) interfacial tensions have been reported. Since these low values are reported to be essential in order to mo-... 

Maximum weight method The detachment method is based upon the following to detach a body from the surface of a liquid that wets the body, it is necessary to overcome the same surface tension forces that operate when a drop is broken away. The liquid attached to the solid surface on detachment creates the following surfaces ... 

This is one of the many detachment methods of which the drop weight and the Wilhelmy slide methods are also examples. As with all detachment methods, one supposes that, within an accuracy of a few percent, the detachment force is given by the surface tension multiplied by the periphery of the surface (liquid surface) detached (from a solid surface of a tubing or ring or plate). This assumption is also found to be acceptable for most experimental purposes. Thus, for a ring, as illustrated in Figure 2.12,... 

For this reason, the surface tension has units of either energy per unit area or force per unit length. The surface tension of a liquid is measured in one of several ways capillary rise, ring detachment, or drop weight. Each method of surface tension measurement is outlined in Figure 9.2. With capillary rise, the fluid is suspended by the surface... 

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. 

The second method to be discussed here is the determination of surface tension by measuring the weight of falling mercury drops. To a first approximation the weight of the drop (corrected for the weight of solution displaced by it) is proportional to the surface forces retaining it... 

Several convenient ways to measure surface tension involve the detachment of a solid from the liquid surface. These include the measurement of the weight in a drop falling from a capillary and the force to detach a ring, wire, or thin plate from the surface of a liquid. In this section we briefly describe these methods and their use. 

The second method is to calculate a stress that is appropriate to a particular situation of interest. An example of this would be the stress acting on a drop of material due to its own weight as it rests on a support medium. The force of gravity tends to make the drop spread out into a film, while its surface tension tends... 

The interfacial tension of C02-saturated triglycerides in a triglyceride-saturated CO2 atmosphere was measured using the pendant drop method introduced in 1938 by Andreas et al. [8]. In this method, a drop is suspended on a capillary in a gas or liquid atmosphere. Its shape results from the balance of gravitational and interfacial forces. As the drop minimises its surface by taking on spherical shape, the pressure inside the drop increases as a result of the curved interface. At the same time, the spherical shape is distorted under the weight of the drop. This results in pear-shaped droplets, which are typical for this method. 

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