Surface spinning drop method

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. 

Spinning Drop Method A method for determining surface or, more commonly, interfadal tension based on measuring the shape of a droplet (or bubble) suspended in the center of a horizontal, cylindrical tube filled with a liquid, as the tube is spinning about its long axis. This method is particularly suited to the determination of very low interfadal tensions. 

Noiiy Ring methods, pendant and spinning drop methods, for surface and interfacial dilational elasticity, thin-film techniques, and surface lateral diffusion when using fluorescence recovery after photobleaching (FRAP) methods. 

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. 

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. 

The interfacial shear viscosities are measured by the deep channel viscous traction surface viscometer (5) at the Illinois Institute of Technology. The oil-water equilibrium tensions are measured by either the spinning drop or the du Nouy ring (6) method. 

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. 

A number of methods are available for the measurement of surface and interfacial tension of liquid systems. Surface tension of liquids is determined by static and dynamic surface tension methods. Static surface tension characterises the surface tension of the liquid in equilibrium and the commonly used measurement methods are Du Notiy ring, Wilhelmy plate, spinning drop and pendant drop. Dynamic surface tension determines the surface tension as a function of time and the bubble pressure method is the most common method used for its determination. 

Many methods for the measurement of surface and interfacial tensions, details of the experimental techniques, and their limitations are described in several good reviews (27-29). Some methods that are used most in emulsion work are the du Nouy ring, drop weight or volume, pendant drop, and the spinning drop. The spinning drop technique is applicable to the very low interfacial tensions encountered in the enhanced oil recovery and microemulsion fields (30). In all cases, when solutions rather than pure liquids are involved, appreciable changes can take place with time at the surfaces and interfaces. 

Perhaps the most striking property of a microemulsion in equilibrium with an excess phase is the very low interfacial tension between the macroscopic phases. In the case where the microemulsion coexists simultaneously with a water-rich and an oil-rich excess phase, the interfacial tension between the latter two phases becomes ultra-low [70,71 ]. This striking phenomenon is related to the formation and properties of the amphiphilic film within the microemulsion. Within this internal amphiphilic film the surfactant molecules optimise the area occupied until lateral interaction and screening of the direct water-oil contact is minimised [2, 42, 72]. Needless to say that low interfacial tensions play a major role in the use of micro emulsions in technical applications [73] as, e.g. in enhanced oil recovery (see Section 10.2 in Chapter 10) and washing processes (see Section 10.3 in Chapter 10). Suitable methods to measure interfacial tensions as low as 10 3 mN m 1 are the sessile or pendent drop technique [74]. Ultra-low interfacial tensions (as low as 10 r> mN m-1) can be determined with the surface light scattering [75] and the spinning drop technique [76]. 

Figure 6.7 Liquid surface tension determination by the spinning drop tensiometer method. A liquid drop (7) is suspended in an immiscible denser liquid (2) in a horizontal transparent tube which can be spun about its longitudinal axis, and the drop (7) elongates from a spherical shape to a prolate ellipsoid with increasing speed of revolution. Later, the drop becomes approximately cylindrical, at very high rotational velocities. A camera with a frame grabber captures the images of the drop inside the transparent tube.

When the value of the interfacial tension is significantly less than 1 mN m 1, then we consider the measurement of ultra-low interfacial tension, which is common in liquid-liquid emulsification processes when effective surfactant solutions are used. The dynamic spinning drop tensiometer method is especially suitable for this purpose. Ultra-low interfacial tension measurement is important in the chemical industry because the cleaning of solid surfaces of dirt, grease, and oil the formulation of stable emulsions the recovery of petroleum, and other applications often rely on lowering the interfacial tension between immiscible liquids to ultra-low values by the use of surfactants. 

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. 

There are many methods available for the measurement of surface and interfacial tensions. Details of these experimental techniques and their limitations are available in several good reviews [101-104]. Table 5 shows some of the methods that are used in petroleum recovery process research. A particular requirement of reservoir oil recovery process research is that measurements be made under actual reservoir conditions of temperature and pressure. The pendant and sessile drop methods are the most commonly nsed where high temperatur pressure conditions are required. Examples are discussed by McCaffery [i05] and DePhUippis et al. [J06]. These standard techniques can be difficult to apply to the measurement of extremely low interfacial tensions (< 1 to 10 mN/m). For ultra-low tensions two approaches are being used. For moderate temperatures and low pressures the most common method is that of the spinning drop, especially for microemulsion research [107], For elevated temperatures and pressures a captive drop method has been developed by Schramm et al. [JOS], which can measure tensions as low as 0.001 mN/m at up to 200 °C and 10,000 psi. In aU surface and interfacial tension work it should be appreciated that when solutions, rather than pure liquids, are involved appreciable changes can occur with time at the surfaces and interfaces, so that techniques capable of dynamic measurements tend to be the most useful. 

Polymer-modified electrodes can be prepared either by direct deposition of polymer onto the surface (via drop-, dip- or spin-coating methods) or by polymerization onto the electrode surface (via chemical, electrochemical or photochemical routes). The simplest method to prepare a polymer-based sensor is by drop-coating a small volume of polymer dissolved in a solvent. With time, the solvent evaporates leaving the polymer adsorbed onto the electrode surface. Dip- and spin-coating methods have also been used to obtain more uniform films. These methods are used when polymers are aheady synthesized and need to be immobilized as they are. In situ polymerization is another effective method to prepare polymer-modified electrodes. For electropolymerization, the electrode is immersed in a monomer solution (e.g., pyrrole, thiophene, phenol, aniline...) and a suitable potential (either cathodic or anodic) is applied to allow the formation of the polymer film on the electrode surface. Photopolymerization is rarer in the case of electrochemical sensors. Nevertheless, poly(vinyl alcohol) functionalized with styrylpyridinium and acrylated polyurethane have been used for the development of electrochemical sensors. 

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