The Spinning Drop Method

An equihbrium shape is reached at any given speed of rotation. At moderate speeds of rotation, the drop approximates to a prolate ellipsoid, whereas at very 

When the shape of the drop approximates a cyhnder, the interfacial tension is given by the following expression [15]  

At the S/L interface, the main interest is in determining the following parameters  

The general equation for calculating the amount of surfactant adsorbed onto a solid adsorbent from a binary solution containing two components (surfactant component 1 and solvent component 2) is given by [16]  

When the liquid phase is a dilute solution of surfactant (component 1) that is much more strongly adsorbed onto the solid substrate than the solvent (component 2), then n Axj = Awj where A j =change in number of moles of component 1 in solution, 0, and Xj 1. In this case. Equation (5.39) reduces to 

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The AOS and IOS surfactants discussed in this section are laboratory-prepared samples. The experiments were carried out by Borchardt [40] who used the spinning drop method throughout. Experimental details are given in the footnotes of the tables that follow. 

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. 

In the spinning drop method, an oil droplet of approximately 1 fxL is placed in a glass capillary (2 mm i.d.) full of aqueous solution. The glass tube is sealed and spun horizontally around its axis at 5000 to 10,000 rpm. Centrifugal force will cause the lighter phase (usually the oil) to form an... 

The spinning drop method was first proposed by Vonnegut in 1942 and... 

The measurement of interfacial tension (IFT) was performed through the spinning drop method as described by Cayias (11). 

Figure 4.9. Schematic representation of the inteifacial tension measurements by the spinning drop method ffi is the angular velocity, and d the deformed drop diameter.

In the spinning drop method, a droplet of polymer-1 immersed in polymer-2 is submitted to extensional deformation through the use of centrifugal forces [Elmendorp and De Vos, 1986]. The extent of deformation is controlled by the rotational speed imposed on the system contained... 

The interfacial tension may be determined to within about 1% accuracy with the spinning-drop method (127, 128). It is an absolute and static method that requires only small samples and, in contrast to most other methods, does not depend on the wettability of a probe, such as a ring or Wilhelmy plate. The stabilizing surfactant is commonly used at concentrations in the bulk continuous phase that are far above the critical micelle concentration (erne). This ensures that the concentration remains above the erne after adsorption on to the vastly extended interface has taken place, which is clearly needed to maintain emulsion stability. It is tempting, therefore, to assume that the interfacial tension in the finished emulsion equals that between the unemulsified bulk phases and that it remains constant when a mother emulsion is diluted with continuous phase in order to create a series of emulsions in which only O is varied (67). This may be a reasonable assumption when a pure surfactant is used, but there is evidence that this may not be so when impure commercial surfactants or surfactant mix-... 

Evidence of the interfacial activity of diblock copolymers at the interface between two immiscible homopolymers, rather than between a single homopolymer and air, has been presented by Gaillard et al. who used the spinning drop method to measure... 

The spinning drop method is commonly used in petroleum-related applications for determining the interfacial tension at the crude oil/aqueous solution interface in the presence of different surfactants. 

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. 

Figure 11.5. Schematic of the spinning drop method note that the white region in the centre represents the less dense fluid, which is surrounded by the more dense fluid (shaded region)...

One encounters the following difficulties in the interpretation of the data from the experiments with interfacial dilatation. As discussed in Ref. 58, the shear viscosity, T jh, does not influence the total stress, 67, only for interfacial flow of perfect spherical symmetry. If the latter requirement is not fulfilled by a given experimental technique, its output data will be influenced by a mixture of dissipative effects (not only -r d but also -qsh and tr). The apparent interfacial viscosity thus determined is not a real interfacial property insofar as it depends on the specific method of measurement. For example, the apparent interfacial viscosity measured by the capillary-wave methods [189-196] depends on the frequency the apparent interfacial viscosity measured by the Langmuir trough method [197,198] is a sum of the dilatational and shear viscosities ("q + -q h) for the methods employing nonspherical droplet deformation, like the spinning-drop method [199-201], the apparent surface viscosity is a complex function of the dilatational and shear interfacial viscosities. 

These problems are partially overcome by dynamic techniques such as the spinning drop method [77, 78]. 

The spinning drop method determines the cylindrical profile of a polymer drop dispersed in a denser polymer matrix under constant rotation, thus yielding a balance between centrifugal and interfacial forces. With a rotational speed high enough to yield a drop length > 4 times the diameter the interfacial tension is expressed by ... 

The spinning drop method [348-350] is used to determine inteifacial tensions between two liquids. A capillary tube is mounted in a chamber leaving the ends open (Fig. 9.24). The chamber and the tube are filled with the heavier of the two liquids and the capillary is rotated at a high speed (about 2000 rpm). A drop of the other liquid having a lower density is placed into the capillary. The drop moves into the center of the capillary tube and usually assumes the shape of a cylinder with curved edges. The radius of the drop is measured using a camera or a microscope ... 

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