The effect of concentration on the sedimentation rate of uncharged rigid spheres was investigated. Three submicron sizes of silica spheres were prepared according to the method of Stober (1968). The particles were sterically stabilized by chemisorption of stearyl alcohol at their surface by the method developed by van Helden ( 1981 ) . The sterically stabilized silica particles dispersed in cyclohexane are known to behave as hard spheres. Monodisperse gravity sedimentation experiments were carried out for the various particle species over a wide concentration range of 4 = 0.003 to 4 = 0.37, where 4 is the particle volume fraction in cyclohexane. The data for dilute suspensions (4 < 0.03) were found to be well described by Batchelor's equation: U/U,= I -6.554, where U is the sedimentation velocity and U, is the Stokes velocity of a sphere in isolation. The data over the entire concentration range (O< 4 < 0.37) were found to be well described by the equation: U/U, = ( 1 -4)6.55.Bidisperse sedimentation experiments were also carried out, and the dilute data were found to be well represented by Batchelor's 1982 theory f o r polydisperse suspensions. The high concentration data were analyzed in terms of a model that does not distinguish between interactions of like and unlike particles. IntroductionThe sedimentation of a suspension of particles under the action of gravity has been studied extensively because of its importance in practical applications. The problem that has received the greatest attention is the sedimentation of a system of monodisperse spheres at very small particle Reynolds number. Theoretical studies concerned with this problem have been recently reviewed by Davis and Acrivos (1985). Only a few of the most relevant studies will be reviewed here. For a dilute random suspension of monodisperse spheres, Batchelor (1972) derived the following expression for the mean fall velocity U of a particle in the suspension: correct to order 4. The O(4) correction includes only pairwise interactions. For sedimentation at moderate to high concentrations, Reed and Anderson (1980) developed an approximate Correspondence concerning this article should be addressed 10 M. S. Selim pairwise additive theory which reduces the intractable N-body hydrodynamic problem in a multiparticle suspension to appropriate combinations of two-body hydrodynamic interactions. For a monodisperse suspension of rigid particles which interact only through a hard sphere potential, their analysis yieldsIn arriving at this result, the concentration dependence of the pair distribution function was neglected, and instead, the dilute form was used. Consequently, the result overestimates the hindrance effect; giving negative values for the average sedimentation velocity at 4 > 0.546. Glendinning and Russel (1982), applying a method developed by O'Brien (1979), derived a general expression for the sedimentation velocity of a random suspension of spheres which is valid for all volume fractions and arbitrary interaction potentials. In this express...

SYNOPSISIn this study, a novel series of water-soluble hydrophobically modified poly(viny1 alcohol) (PVA) is prepared by chemical modification of PVA, with the objective of investigating the polymer's rheological behavior for enhanced oil recovery applications. The solution viscosity of the polymer obtained is studied with respect to the polymer concentration, temperature, salinity, polymer modification, aging, shear rate, and polymer molecular weight.The solution viscosity of the PVA is greatly enhanced by the modification. The modified PVA exhibits a relatively high salt tolerance, typical of nonionic polymers, in the range of 0-7.0 wt % NaCl concentrations, and the viscosity of the polymer solution is relatively invariant with NaCl above 3.0 wt % NaCl concentration. 'Below 3 wt %, the viscosity shows a maximum then a minimum, an unusual behavior. Generally, the polymer exhibits a n almost constant viscosity at high shear rates and a typical shear thinning behavior at low shear rates. In addition, increasing polymer concentration and molecular weight leads to an increase in the polymer solution viscosity. Moreover, the polymer exhibits smaller solution viscosity at a high temperature, and a slight decrease in viscosity is also exhibited by the modified polymer with aging. Comparison of the viscosities of 18 polymer modifications indicates that the larger the numbers of hydrophobic groups (side chains) in the polymer structure, the smaller the viscosity. Moreover, the longer the hydrophobic groups (side chains) in the polymer structure, the greater the viscosity, if their number is small. 0 1995

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Gravity sedimentation of bidisperse and tridiperse concentrated suspensions have been studied both theoretically and experimentally. Experimental data were collected from batch settling tests using fully‐characterized colored spherical particles. Ten sizes of particles with three different densities were used in the sedimentation experiments. Varying proportions of each size were used and total solids concentration ranged from 13% to 45% by volume. Previously published and newly collected data were compared with predictions from six available models. These models include those of Lockett and Al‐Habbooby (1973), Mirza and Richardson (1979), Masliyah (1979), Reed and Anderson (1980), Selim et al. (1983a, 1983b), and Patwardhan and Tien (1985). Based on the present study and a similar study by Law et al. (1987) on bidisperse suspensions of heavy (sinking) and light (buoyant) particles, it is found that the Selim et al. model represents the data more accurately as compared to the other models.

We investigated the effect of concentration on the Brownian diffusion of uncharged rigid spheres. Monosize silica spheres were prepared according to the method of Stober ( 1968) (1906) and Batchelor ( 19771, (Batchelor, 1976) over the entire concentration range studied.