Rotating drop method

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. 

The spinning (rotating) drop method allows one to measure very small values of the interfacial tension at the liquid-liquid interface [23], Let us consider a tube filled with liquid into which a drop of another liquid of lower density is introduced (Fig. 1-16). Upon rotation of the tube around... 

Measurements of interfacial tensions of polymer melts were reviewed by Wu (55), Koberstein (65), and Demarquette (66). The measurements usually need long equilibrium time because of the high viscosities of polymer melts. The measurements can be divided into two groups static methods in which interfacial tension is calculated based on the equilibrium profile of the drops and dynamic methods that study the evolution of fiber or drop profiles with time. Static methods include pendant drop method, sessile drop method, and rotating drop method. Dynamic methods include breaking thread method, imbedded fiber method, and deformed drop retraction method. 

Also called the rotating drop method, this technique sets up a balance between centrifugal and surface tension forces. A small bubble or drop (about 0.2 ml) is placed in a liquid of greater density in a long transparent tube, which is rotated rapidly (Figure 3.13). The bubble (for surface tension) elongates to an extent that depends on the surface tension, density difference and rotational speed. At high... 

Interfacial tension (7,2) between immiscible liquids can, in principle, be measured by the same methods with which liquid/vapor surface tensions are mea-smed (Adamson, 1990). However, espedally for the measurement of fairly small 7,2 values, a few of these methods are better adapted for this particular purpose, e.g., the pendant drop-shape method (see above), the simpler, but less accurate dropnweight method (Adamson, 1990), and a few other methods using deformed interfaces, sudi as the rotating drop method (Vonnegut,... 

There are many classical methods to investigate the chemical reaction and kinetics at the liquid-liquid interface, which include a Lewis cell, a single drop method, and rotating disc method [22]. All of these methods however could not measure both the extraction rate and interfacial concentration of extractant, simultaneously. Modern experimental methods of interfacial reaction can determine the interfacial species, interfacial concentration, and interfacial chirality of an extractant or complex as a function of time. 

O ) polarography uses a dropping mercury cathode and the current is measured 00 the rotating-disc method uses a spinning platinum disc as the cathode and the current in the steady state situation at a series of rotation speeds is measured Hi) the potentiostatic method uses fixed electrodes and the fall of current with time is measured. 

In the rotating-disk method the thickness of the diffusion and reaction layers at the electrode surface is made less than at the dropping mercury electrode surface (Orsega et al., 1982). This reduces the extent of 07... 

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... 

DIN 55992, Part 1, 2—Determination of a parameter for the dust formation of pigments and extenders Part 1 Rotation method/ Part 2 Drop method, DIN German Institute for Standardization, 2005, 1999. 

FIGURE 1.14 Schematic illustration of spinning drop method the equilibrium shape of the rotating drop. 

Equipment available to measure dustiness varies from the drop method (Figure 1.10), Roach dust drop funnel (Lyons Mark 1992), drop test (DIN 5599-2 1999), rotating drum (Lyons Mark 1994), inverse flow (Cowherd et al. 1989) and gas fluidisation (Schofield 1981). 

Only methods based on drop profiles are suitable for both surface and interfacial tension measurements. These include the pendant drop method [125-127], the sessile bubble or drop method [128, 129], and the rotating drop or bubble method [130, 131]. These methods are independent of the solid-Uquid contact angle but require accurate knowledge of the density difference across the interface. The demand of accurate density data becomes even greater when the two phases have similar densities. The rotating drop or bubble method is particularly suited for the determination of very low surface and interfacial tensions. 

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 ... 

For very small values of 7, Vonnegut s (1942) rotating drop (or cylinder) method (which has been further elaborated upon by Princen et ai (1967)) is the most suitable. As a first approximation, the presence of a straight, flat meniscus at the interface between two liquids inside a vertical tube is an indication of a very low interfacial tension between two liquids. All the above methods are also applicable to measuring 7, using hanging drops, in air. 

Vortex formation leads to a considerable drop in mixing efficiency and should be suppressed as much as possible in practical applications to increase the homogenizing effects of mixers. The preferable method of vortex suppression is to install vertical baffles at the walls of the mixing tank. These impede rotational flow without interfering with the radial or longitudinal flow. Figure 11 illustrates such a system. 

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