Sliding layers, suspensions

For very low particle concentrations, the shearing of colloidal crystals produces only a weak stress above that of the solvent. However, in more concentrated suspensions, the shear viscosity and normal stress differences have been found to have quite unusual behavior, which can, in part, be explained by (a) the formation of sliding layers and (b)... 

Hoffman [1972, 1974], Strivens [1976], van de Ven [1984, 1985], Tomita et al. [1982, 1984], and Otsubo [1994] reported pseudoplastic/dilatant flow of concentrated suspensions of uniform and polydispersed spheres. A dramatic change in light diffraction pattern was systematically observed at the shear rate corresponding to the onset of dilatancy. Van de Ven and his collaborators demonstrated that, depending on concentration and shear rate, the distance between the sliding layers of uniform spheres in a parallel plate rheometer can vary by as much as 10%. 

Shear thinning of concentrated suspensions is typical for submicron particles dispersed in a low viscosity Newtonian fluid.At low shear strain rates. Brownian motion leads to a random distribution of the particles in the suspension, and particle collision will result in viscous behavior. At high shear strain rates, however, particles will arrange in layers, which can slide over each other in the direction of flow. This results in a reduced viscosity of the system in agreement with the principles of shear thinning. A pro-noimced apparent yield stress can be found for shear thinning suspensions, if the Brownian motion is suppressed by electrostatic repulsion forces, which result in three-dimensional crystal-like structures of the particles with low mobility. 

It has been shown (Adler et al. 1990) from a rigorous asymptotic, lubrication-theory analysis that lubrication concepts cannot lead to a singular behavior of the viscosity of a spatially periodic suspension in which layers of particles slide past one another. This means that the use of Eq. (9.3.8), for example, which employs lubrication concepts to characterize suspension viscosity is limited to suspensions where particle layering does not take place, for example, where the microstructure is random. 

On the other hand, the influence of the state of hydration on the photocycle of bacteriorhodopsin is remarkable. In order to study its effect, thin layers of purple membrane were prepared by drying concentrated suspensions of purple membranes in water on a glass sUde at room temperature and atmospheric pressure. Variable hydration of the thin layers was obtained by equilibrating the preparations with various relative humidities produced by saturated salt solutions. The average thickness of the preparation was 1-3 m as determined by scanning electron microscopy. The glass slide containing the preparation was inserted into a cuvette suitable for spectroscopy and equilibrated with the required specific humidity before analysis. 

Morita and Haruta [457] have used a simple glass wash-bottle to spray a suspension of 10 g silica gel G (Firm 88) in 30 ml water. They coated simultaneously 21 microscope slides (25 x 75 mm) which they had placed in three parallel rows of seven, a few millimetres apart and forming a rectangle. According to Bbkersky [54], layers 0.25 to 1.1 mm thick can be prepared with the spraying technique. Thicknesses of ca. 1 mm are realised by spraying coat by coat at two minute intervals. 

Newitt et al. (1962) conducted speed measurements of a slurry mixture in a horizontal pipe. In the case of light Plexiglas pipe, zircon or fine sand did not result in local slip particles and water moved at the same speed. However, for coarse sand and gravel, they observed asymmetric suspension and a sliding bed. They also observed that in the upper layers of the horizontal pipe, the concentrations of larger particles were the same as for finer soUds, but were marked by differences in the magnitude of the discharge rate of the lower layers. 

The success of the thin layer wicking procedure rests on the ability to fabricate a series of glass slides covered with uniform thin layers of finely powdered sample. This is usually possible for materials which form stable or nearly stable suspensions in water (the most common mediiun for minerals). To ensure that the powder remains in suspension, the liquid should be stirred with a magnetic stirrer during the process. The typical glass microscope slide will support approximately 5 ml of aqueous suspension. The number of slides to be prepared for each sample is determined by the number of liquids used and the number of replicates desired. The replicates (more than one slide with the same liquid) allow an estimate of the uniformity of the sample procedure. The alkanes listed in Table 6.1 can be used to determine the pore radius R (Eq. 6.20). For such spreading liquids, the Washburn equation can be written as ... 

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