Suspended-drop method

Figure 4.9 Two modes of liquid phase microextraction, (a) Suspended drop method and (b) membrane loop method.

Liquids have a surface tension with respect to the gaseous phase and an interfacial tension with respect to other liquids. Such interfacial and surface tensions may be measured by a whole series of different methods in the case of low-molar-mass liquids. But polymer liquids have very high viscosities, and so, only a few methods of measurement are suitable. The capillary method and the wire ring method are not suitable since the measured surface tension depends on the speed at which the test is carried out. All of what are known as static methods are suitable, i.e., the suspended drop method and the Wilhelmy plate method. 

In the 1870s, Robert Koch, a Prussian physician, isolated the anthrax bacillus and traced the complete life cycle using suspended-drop culture methods. Koch determined the bacillus could form spores which remained viable, even in hostile environments (Koch, 1876). Louis Pasteur provided demonstration of infectious disease transmission. He inoculated one cohort of cattle with live attenuated vaccine and a control cohort without vaccine. When all animals were injected with virulent anthrax, only the control cattle died. 

Methods used to measure interfacial tension are reviewed by Drelich, Fang, and White [ Measurement of Interfacial Tension in Fluid-Fluid Systems, in Encyclopedia of Surface and Colloid Science (Dekker, 2003), pp. 3152-3156]. Also see Megias-Alguacil, Fischer, and Windhab, Chem. Eng. Sci., 61, pp. 1386-1394 (2006). One class of methods derives intertacial tension values from measurement of the shape, contact angle, or volume of a drop suspended in a second liquid. These metho(Js include the pendant drop method (a drop of heavy liquid hangs from a vertically mounted capillary tube immersed... 

In the case, when the melt reacts with the glass, as for instance the molten alkali metal hydroxides, a suspended cup method (Figure 4.8c) was developed by Aghai-Khafri et al. (1976). Inside the pyrex or quartz liner, a cylindrical silver crucible contained the liquid hydroxide B while the salt or hydroxide A is contained in a small silver cup. This silver cup is held by tongs and could be released by external manipulation of the tongs. The mixing of the two liquids occurs by dropping the silver cup into the crucible. However, the thermal effect connected with the drop of the cup into the liquid A is very small and reproducible. 

In the test method for molecular weight (ASTM D-2503) a small sample of wax is dissolved in a suitable solvent, and a droplet of the wax solution is placed on a thermistor in a closed chamber in close proximity to a suspended drop of the pure solvent on a second thermistor. The difference in vapor pressure between the two positions results in solvent transport and condensation onto the wax solution, with a resultant change in tempera-... 

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. 

Surface tension was determined via the pendant drop method using a Kruss EasyDrop. The liquid was suspended from an 1832 pm needle with equatorial and neck diameters measured for the largest stable drop. The surface tension was calculated using the relationship ... 

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. 

The potential influence of the drop profile by the needle always exists for the drop pickup method. While this method minimizes the kinetic energy gained by the drop during gravity dispensing, it is usually difficult to pick up a suspended drop onto a solid surface with extremely high repellency due to the strong... 

The aperture impedance principle of blood cell counting and sizing, also called the Coulter principle (5), exploits the high electrical resistivity of blood cell membranes. Red blood cells, white blood cells, and blood platelets can all be counted. In the aperture impedance method, blood cells are first diluted and suspended ia an electrolytic medium, then drawn through a narrow orifice (aperture) separating two electrodes (Fig. 1). In the simplest form of the method, a d-c current flows between the electrodes, which are held at different electrical potentials. The resistive cells reduce the current as the cells pass through the aperture, and the current drop is sensed as a change in the aperture resistance. 

Shirato, Gotoh, Osasa, and Usami [J. Chem. Eng. Japan, 1, 164— 167 (January 1968)] present a method for determining the mass flow rate of suspended sohds in a liqiiid stream wherein the liquid velocity is measured By an electromagnetic flowmeter and the flow of sohds is calculated from the pressure drops across each of two vertical sections of pipe of different diameter through which the suspension flows in series. 

---