Hn Stein

Viscoelastic properties of concentrated shear-thickening dispersions

Abstract The viscoelastic properties of electrostatically stabilized concentrated dispersions of silica and of glass particles in a glycerol/water mixture are studied by oscillatory shear measurements. These dispersions are shear-thickening in steady shear flow. At most frequencies the loss modulus is found to dominate the storage modulus. At certain critical combinations of deformation amplitude and frequency the response signal becomes distorted, containing higher harmonics. This phenomenon can be ascribed to flow blockage, closely related to shear thickening in steady shear flow. When plotting the critical deformation against the frequency, three different regions can be detected. At low frequencies there is the steady shear flow limit, at intermediate frequencies double-layer overlap causes nonlinearity in the response signal, and at high frequencies there is a region where the critical deformation for flow blockage is a system-dependent function of the frequency, but is independent of the volume fraction. The magnitudes of the storage moduli are found to be in reasonable agreement with theoretical predictions.

Time‐dependent behavior and wall slip in concentrated shear thickening dispersions

The viscosity of concentrated shear thickening dispersions was measured as a function of shear rate, Couette cylinder size, and time. The level of the low shear rate viscosity, which was found to be independent of system size and time, could be predicted by the equation of Frankel and Acrivos. At shear rates above the critical shear rate for shear thickening in highly concentrated (φ≳0.57) dispersions of monodisperse particles strong viscosity instabilities were detected, together with a dependence on cylinder size. The instabilities are attributed to reversible order–disorder transitions, e.g., from strings to clusters. This dependence on cylinder size is due to wall slip, slipping planes in the dispersion, and even plug flow in the gap. With less concentrated or polydisperse dispersions the effects are much less severe but there is thixotropy, probably due to a reordering of the dispersion.

Computer simulations of shear thickening of concentrated dispersions

Stokesian dynamics computer simulations were performed on monolayers of equally sized spheres. The influence of repulsive and attractive forces on the rheological behavior and on the microstructure were studied. Under specific conditions shear thickening could be observed in the simulations, usually together with a change in the microstructure from ordered layers to a state with large clusters. These clusters are responsible for the high viscosities in shear thickening conditions. The shear thickening results show qualitative agreement with the theoretically expected behavior and with experiments. Van der Waals forces are found to sharpen the shear thickening transitions by inducing even stronger clusters and thus higher viscosities.

Full scale electrokinetic dewatering of waste sludge

Abstract Increasing volumes of waste sludge, an intense environmental awareness and stringent legislation impose increasing demands upon conventional sludge dewatering equipment. In this study, the electrokinetic dewatering of waste slurries is studied. Full-scale electrokinetic facilities were developed that were based on a combination of a gravity-driven thickening belt and a belt press. The method was tested at a drinking water production site. By the use of the electrokinetic facility when drying ‘aluminum-coagulated’ drinking water sludge (i.e. coagulated by the addition of Al-ions), the dry solids content increased by electro-osmosis from 17 to 24% m/m at an additional energy consumption level of 60 kWh per ton dry solids. Additionally, the filter belt fouling was reduced drastically and the loss of solid particles from the cake was almost completely suppressed due to electrophoresis. Corrosion of the anode was effectively suppressed by using Ir2O3-coated titanium plates. Supplementary laboratory experiments suggest that electrokinetic dewatering is also useful in dewatering ‘iron-coagulated’ drinking water sludge, sewage treatment sludge and fresh water dredging sludge. Theoretical analyses indicate that electro-osmosis will contribute to dewatering significantly, at lower and especially at higher volume fractions of solids, provided the slurry particles are of the order of micrometers or smaller. Under such conditions the conventional dewatering is slow due to excessive hydrodynamic resistance.

A radiotracer determination of the adsorption of sodium ion in the compact part of the double layer of vitreous silica

Abstract The amount of adsorbed sodium ions in the compact part of the electrical double layer of the vitreous silica electrolyte interface has been estimated at pH 10 with the aid of a sodium-24 and bromide-82 tracer. Etching experiments show a penetration depth of sodium ions in the solid phase less than 0.3 nm; moreover, a gel layer formation at pH 10 is not observed. This experimental evidence is against the application of the gel layer explanation in case of nonporous silica and in favor of the site-binding model.

The influence of solid particles on foam and film drainage

The effect of hydrophilic glass particles in a cetyltrimethylammonium bromide (CTAB) solution on the drainage and stability of foam and single vertical free liquid films (0.02 M CTAB) in a glass frame was investigated. Large particles (diameter approximately between 1 and 10 ~m), being present in the colored films, retard both foam and film drainage and increase film stability. This is ascribed to dispersion of surface waves by particles extending through the film surfaces. Smaller particles (<1 ~tm) lack this property: they shorten film lifetime in foams, although they have no influence on foam drainage.

The influence of surface-active agents on kaolinite

Abstract The influence of surfactants (CTAB and SDS) on suspensions of monoionic kaolinite (Na + and H + form) was investigated by adsorption, sedimentation, turbidity, electroosmosis, and rheological measurements, at pH = 3.3 and 10.0. Only small differences are found between the Na + and H + forms of the kailinite. The data can be accounted for satisfactorily by a mathematical model based on the DLVO theory for face-face, edge-edge, and edge-face interactions, if some assumptions on the local ψ δ potentials near edge and face type surfaces are introduced.

INTERDIFFUSION OF HYDROGEN AND ALKALI IONS IN GLASS SURFACES

The Boksay-Doremus theory on the interdiffusion of hydrogen and alkali ions in glasses was extended by taking into account the association of hydrogen ions and non-bridging oxygen ions. In the derivation of the interdiffusion coefficient the bulk composition of the glass is taken into account. Theoretical concentration profiles were compared with experimental data, reported in literature, of alkali-alkaline earth-silicate glasses and of a sodium-aluminium-silicate glass. An essential feature of this extended B-D theory is, that the relative order of chemical durability of alkaline earth containing glasses is found by this theory without assuming excessive changes in individual diffusion coefficients.

Hydrodynamic interactions between two identical spheres held fixed side by side against a uniform stream directed perpendicular to the line connecting the spheres’ centres

Abstract Hydrodynamic interactions between two identical solid spheres are investigated numerically using a finite element method. The spheres are held fixed relative to each other with the line connecting their centres normal to the flow direction. The method is applied in three-dimensional, at Reynolds numbers 10, 50 and 5 × 10 −7 . The drag and interaction coefficients of the spheres are calculated as functions of the distance between the two spheres. The results of the calculations show that, for Reynolds number of 50, the two spheres are repelled when the spacing is of the order of the diameter but are weakly attracted at intermediate separation distances. The results agree with experimental and theoretical data reported for different Reynolds numbers.

Efficient dewatering of waterworks sludge by electroosmosis

Abstract Electroosmotic dewatering of water treatment process sludge, in which a relatively high specific resistance causes difficulties with regard to water removal, is described. The dewatering can be both accelerated and made economically more attractive by additives which increase the absolute value of the ζ potential but do not lead to stable suspensions. An increase in ζ potential means an increase in electroosmotic water transport; simultaneously electric charges on the filter repel particles with electrical charges of the same sign, which causes a decrease in the clogging of the filtration material. Best results show an increase in final dry solid content from 4% (by weight) by pressure filtration only, to 16% (by weight) by a combination of electroosmosis with pressure filtration.