Sedimentation field strength

The sedimentation field strength Is the potential difference per unit length In a sedimentation or centrifugation cell. As the contributions of the Interfaclal potential differences at the electrodes should not be Included In. this quantity can only be measured If the exact Identity of the two electrode potentials has been ascertained. 

By analogy to the other electroklnetic phenomena we could define a scalar sedimentation field strength per unit of acceleration, i.e. but this is not... 

Sedimentation Potential The potential difference, at zero current, caused by the sedimentation of dispersed species. The sedimentation may occur under gravitational or centrifugal fields. The potential difference per unit length in a sedimentation potential cell is the sedimentation field strength. 

We have noted that the combination of regular geometry, uniform field strength, and well-defined parabolic flow, make possible a rigorous theoretical treatment to relate particle properties to experimental observations. This theory serves as the basis for particle characterization in sedimentation FFF (2). Here, we will briefly review the main theoretical concepts and some of the equations needed for particle size calculations. 

Specifically, for sedimentation FFF, X relates to d (along with field strength (acceleration) G and the difference bp in density between the particle and the carrier) according to the expression... 

Figure 2. Fractionation of four samples of Dow polystyrene latex beads by sedimentation FFF. The nominal particle sizes are given in the figure. Flowrate = 12 ml/hr, channel thickness w = 0.0127 cm, void volume V° = 2.0 ml, and field strength G = 193.7 gravities. Reproduced with permission from Ref. 20. Copyright 1980 John Wiley.

The final expression, Eq. 8.53, shows how the resolution of two species varies with their average molecular weight Af, the difference Aps in their density, the field strength G, and the density gradient dp/dx. Like Eq. 8.46 for isoelectric focusing, Eq. 8.53 can be used to quantitatively describe the parameters necessary for resolution in isopycnic sedimentation [50]. 

The sedimentation coefficient s> often used to characterize biological macromolecules and particles, is equal to the ratio of the velocity U induced by a sedimentation field to the strength G of that field measured as acceleration. Show that s can be related to the retention parameter A in sedimentation FFF by s = D Gw. 

The solar activity cycles are more complex. The 11 yr sunspot cycle is well known. It is accompanied with charges in the magnetic field strength of Earth. There are fluctuations in the intensity of solar activity on this timescale. As we shall see, periods of fluctuation of 60 yr, 200 yr, and 1,500 yr have been identified through measurements of Be in deep-sea sediments and continental ice sheets. These results also indicate possible variations on the 10 yr timescale as well. 

Fig.l Sedimentation field-flow fractionation fractogram of 0.207- jLm poly (methyl methacrylate) aggregate series from which six cuts were collected and analyzed by electron microscopy. Experimental conditions field strength of 61.6 g and flow rate of 0.84 mL/min. [Reproduced with permission from H. K. Jones et. al. (1988) /. Chromatogr. 455 1 Copyright Elsevier Science Publishers B. V.]... 

Besides being enormously expanded and spheroidal, the mucins also appear very flexible evidence for this derives from the ease with which these molecules are deformed with shear, as shown in streaming-birefringence experiments, from the concentration dependence of viscosity, and the temperature-dependence of sedimentation coeflScients. Mikkelsen and co-workers confirmed these observations by using the dependence on field strength of electric birefringence relaxation-phenomena. Evidence for local flexibility in the carbohydrate side-chains has come from, for example, n.m.r.-spectral studies. ... 

Both Sa and tri are established by logarithmic plots of tr versus d of well-characterized standards such as polystyrene latex beads. The value of tri depends on the field strength and, for sedimentation FFF, also on the particle density. It will be shown later that this dependence makes possible the determination of particle density and porosity values. 

The potential may be obtained from measurements of particle mobility using electrokinetic techniques, such as electrophoresis or sedimentation potential [70]. Electrophoresis, the standard technique for submicrometer particles, is based on the movement of charged particles in response to an applied electrical field. Optical scattering methods are used to measure the distribution of particle velocities for a given field strength, and may then be calculated using the Henry equation. 

Figure 47 Static permittivity vs. time for suspensions containing the particles from Fig. (46) recorded during sedimentation. Different magnetic field strengths were applied, and the volume fraction particles were varied according to the legends. (From Ref. 86.)...

Any field force can be exploited to create conditions for effective action of the steric exclusion mechanism. The only condition is, as mentioned above, that the field strength be high enough to compress all retained species to the accumulation wall. In experimental practice, sedimentation FFF, flow FFF, and thermal FFF are the techniques actually applied in steric mode to separate effectively some particulate species. 

Y. Mori, B. Scarlett, H.G. Merkus, Effects of ionic strength of eluent on size analysis of submicrometre particles by sedimentation field-flow fractionation. J. Chromatogr. A 515, 21-35 (1990). doi 10.1016/S0021-9673(01)89298-0... 

Sedimentation (MICROSCOPIC PROCESS) motion of particles in viscous media due to gravitation or centrifugal fields (also settling), the term refers frequently to the state in which the field forces are counterbalanced by the drag force this state is almost instantaneously achieved in the case of colloidal suspensions the sedimentation of an individual particle depends on its size, the density contrast, the rheological fluid properties, the field strength, and the viscous interactions with other particles. 

A magnetometer measures variations in the earth s magnetic field strength that result from metallic objects (surface or buried), variations in sediment and rock mineral content, and natural (diurnal) variations. Data are used to locate and identify buried objects for cultural, environmental, and archaeological site clearances. 

Figure 13 Sedimentation ratio (Fdcar liquid/Fi ,ai) vs. yield stress at a field strength of 2000 V/mm for samples containing 0.05g/ml of hydrophobically modified saponite (adsorbed surfactant (Ci6)2f>MABr) and different additives in 7-hex.adecanc. The type and concentration of additives are listed in the legend. All samples stood 1 day at rest before the sedimentation ratio was measured. Reproduced with permission from T. Diirrschmidt, and H. Hoffmann, Colloids and Surfaces A, 156(1999)257.

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