Suspensions Of Spherical Silica Particles Research Articles

The coherent shear wave in suspensions

We consider a disordered suspension of spherical silica particles in water. For a particle size of a few hundred nanometres and concentration (volume fraction) around 0.15 to 0.2, experiments conducted in the MHz range have shown that the non ideal nature of water must be taken into account for the "longitudinal" coherent wave attenuation to be understood, because of wave conversions, from longitudinal to shear and then back to longitudinal, occurring at each pair of scattering events. We are interested here in the properties of the "shear" coherent wave that are given by the expansion of its squared wavenumber, around that in the absence of particles, in powers of the concentration. At 1 MHz and a particle radius of 0.05 μm, we show that convergence of the modal series involved in that expansion may be reached after three terms: we use ten terms subsequently. We study the evolution of both the effective shear velocity and attenuation with concentration, as well as that of the effective shear viscosity deduced therefrom.

Phenomenological modeling of the response of a dense colloidal suspension under dynamic squeezing flow

The split Hopkinson pressure bar experimental technique is used to evaluate the squeezing flow response of a concentrated, discontinuously thickening colloidal suspension of spherical silica particles loaded at high stresses/strain rates. These results provide insight into the transitional behavior of these materials, as well as the post-transitional response under compressive loading. A method of analyzing the strain and strain rate dependent behavior is presented to identify modes of material response (viscous, elastic, etc.). Experimental results are presented as stress–strain–strain rate plots and a surface fitting approach is used to develop a phenomenological model describing the overall response. From this model, it is possible to identify regions of elastic and viscous behavior using a gradient analysis approach. It was found that, after an initial period of viscous deformation, the suspension behaves like a viscoelastic material – this regime corresponds well with transition in which large clusters of particles percolate. This is followed by a third, viscous regime in which the material undergoes viscous deformation. At the highest stresses, a plateau region of plastic deformation has been identified. This approach and the conditions under which it may be applied are described in detail in the paper.

Quantification of Microstructures in Stable and Gelated Suspensions from Cryo-SEM

It is shown that the microstructures of concentrated suspensions can be analyzed in a quantitative way from cryo-SEM images of high-pressure frozen samples, both in the electrostatically stabilized and in the flocculated state. Suspensions of spherical silica particles (40 vol%) in an aqueous solution were used. The average particle radius was 525 nm with a polydispersity of below 7%. The suspensions were high-pressure frozen, which resulted in a quenching of the configuration without apparent change in volume or crack formation. After fracturing the samples at liquid nitrogen temperature, the fracture surface was etched by controlled sublimation of the frozen aqueous phase, coated with 8 nm of platinum, and examined by stereo-cryo-SEM. The 3-dimensional positions of all the visible particles were determined from the SEM images. Assuming an isotropic particle configuration in the sample before cracking, it is possible to extract the 3-dimensional pair correlation function from the particle positions on the fracture surface. A comparison to recent results from Brownian Dynamics simulations shows good agreement between our experiments and the simulations.

Nonlinear rheology of concentrated spherical silica suspensions: 3. Concentration dependence

Nonlinear rheology was examined for concentrated suspensions of spherical silica particles (with radius of 40 nm) in viscous media, 2.27/1 (wt/wt) ethylene glycol/glycerol mixture and pure ethylene glycol. The particles were randomly and isotropically dispersed in the media in the quiescent state, and their effective volume fraction φeff ranged from 0.36 to 0.59. For small strains, the particles exhibited linear relaxation of the Brownian stress σB due to their diffusion. For large step strains γ, the nonlinear relaxation modulus G(t,γ) exhibited strong damping and obeyed the time-strain separability. This damping was related to γ-insensitivity of strain-induced anisotropy in the particle distribution that resulted in decreases of σB/γ. The damping became stronger for larger φeff. This φeff dependence was related to a hard-core volume effect, i.e., strain-induced collision of the particles that is enhanced for larger φeff. Under steady/transient shear flow, the particles exhibited thinning and thickening at low and high γ˙, respectively. The thinning behavior was well described by a BKZ constitutive equation using the G(t,γ) data and attributable to decreases of a Brownian contribution, σB/γ˙. The thickening behavior, not described by this equation, was related to dynamic clustering of the particles and corresponding enhancement of the hydrodynamic stress at high γ˙. In this thickening regime, the viscosity growth η+ after start-up of flow was scaled with a strain γ˙t. Specifically, critical strains γd and γs for the onset of thickening and achievement of the steadily thickened state were independent of γ˙ but decreased with increasing φeff. This φeff dependence was again related to the hard-core volume effect, flow-induced collision of the particles enhanced for larger φeff.

Nonlinear rheology of a concentrated spherical silica suspension:

Time-dependent nonlinear flow behavior was investigated for a model hard-sphere suspension, a 50 wt% suspension of spherical silica particles (radius = 40 nm; effective volume fraction = 0.53) in a 2.27/1 (wt/wt) ethylene glycol/glycerol mixture. The suspension had two stress components, the Brownian stress σ B and the hydrodynamic stress σ H After start-up of flow at various shear rates $$\dot \gamma $$ , the viscosity growth function η+ (t, $$\dot \gamma $$ ) was measured with time t until it reached the steady state. The viscosity decay function η− (t, $$\dot \gamma $$ ) was measured after cessation of flow from the steady as well as transient states. At low $$\dot \gamma $$ where the steady state viscosity η ( $$\dot \gamma $$ ) exhibited the shear-thinning, the η− (t, $$\dot \gamma $$ ) and η+ (t, $$\dot \gamma $$ ) data were quantitatively described with a BKZ constitutive equation utilizing data for nonlinear relaxation moduli G (t, γ). This result enabled us to attribute the thinning behavior to the decrease of the Brownian contribution η B = σ B / $$\dot \gamma $$ (considered in the BKZ equation through damping of G (t, γ)). On the other hand, at high $$\dot \gamma $$ where η ( $$\dot \gamma $$ ) exhibited the thickening, the BKZ prediction largely deviated from the η+ (t, $$\dot \gamma $$ ) and η+ (t, $$\dot \gamma $$ ) data, the latter obtained after cessation of steady flow. This result suggested that the thickening was due to an enhancement of the hydrodynamic contribution η H = σ H / $$\dot \gamma $$ (not considered in the BKZ equation). However, when the flow was stopped at the transient state and only a small strain (<0.2) was applied, η H was hardly enhanced and the η− (t, $$\dot \gamma $$ ) data agreed with the BKZ prediction. Correspondingly, the onset of thickening of η+ (t, $$\dot \gamma $$ ) was characterized with a $$\dot \gamma $$ -insensitive strain (≌ 0.2). On the basis of these results, the enhancement of η H (thickening mechanism) was related to dynamic clustering of the particles that took place only when the strain applied through the fast flow was larger than a characteristic strain necessary for close approach/collision of the particles.

Nonlinear rheological behavior of a concentrated spherical silica suspension

Linear and nonlinear viscoelastic properties were examined for a 50 wt% suspension of spherical silica particles (with radius of 40 nm) in a viscous medium, 2.27/1 (wt/wt) ethylene glycol/glycerol mixture. The effective volume fraction of the particles evaluated from zero-shear viscosities of the suspension and medium was 0.53. At a quiescent state the particles had a liquid-like, isotropic spatial distribution in the medium. Dynamic moduli G* obtained for small oscillatory strain (in the linear viscoelastic regime) exhibited a relaxation process that reflected the equilibrium Brownian motion of those particles. In the stress relaxation experiments, the linear relaxation modulus G(t) was obtained for small step strain γ(≤0.2) while the nonlinear relaxation modulus G(t, γ) characterizing strong stress damping behavior was obtained for large γ(>0.2). G(t, γ) obeyed the time-strain separability at long time scales, and the damping function h(γ) (−G(t, γ)/G(t)) was determined. Steady flow measurements revealed shear-thinning of the steady state viscosity η(γ) for small shear rates γ( τ−1). Corresponding changes were observed also for the viscosity growth and decay functions on start up and cessation of flow, η + (t, γ) and η− (t, γ). In the shear-thinning regime, the γ and τ dependence of η+(t,γ) and η−(t,γ) as well as the γ dependence of η(γ) were well described by a BKZ-type constitutive equation using the G(t) and h(γ) data. On the other hand, this equation completely failed in describing the behavior in the shear-thickening regime. These applicabilities of the BKZ equation were utilized to discuss the shearthinning and shear-thickening mechanisms in relation to shear effects on the structure (spatial distribution) and motion of the suspended particles.

オパール状物質の合成

The Synthesis of Opalin Materials Takajiro Shimohira, Tsutomu Hiroshima: National Institute for Researches in Inorganic Materials, Sakura-mura, Niiharigun, Ibaraki Opalin materials which show some of the characteristics of natural precious opal have been synthesized. The process of making the materials was as follows;1) Monodispersed suspensions of spherical silica particles of colloidal size were produced by means of hydrolysis of ethyl silicate.2) An ordered close-packed array of hydrated silica particles has been achieved by sedimentation over a period of several months.3) Sintering and impregnation method were used to harden the arrays of stacked spheres.