Jozua Laven

Ion-association complexes unite classical and non-classical theories for the biomimetic nucleation of calcium phosphate

Despite its importance in many industrial, geological and biological processes, the mechanism of crystallization from supersaturated solutions remains a matter of debate. Recent discoveries show that in many solution systems nanometre-sized structural units are already present before nucleation. Still little is known about the structure and role of these so-called pre-nucleation clusters. Here we present a combination of in situ investigations, which show that for the crystallization of calcium phosphate these nanometre-sized units are in fact calcium triphosphate complexes. Under conditions in which apatite forms from an amorphous calcium phosphate precursor, these complexes aggregate and take up an extra calcium ion to form amorphous calcium phosphate, which is a fractal of Ca(2)(HPO(4))(3)(2-) clusters. The calcium triphosphate complex also forms the basis of the crystal structure of octacalcium phosphate and apatite. Finally, we demonstrate how the existence of these complexes lowers the energy barrier to nucleation and unites classical and non-classical nucleation theories.

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.

Nucleation and Growth of Monodisperse Silica Nanoparticles

Although monodisperse amorphous silica nanoparticles have been widely investigated, their formation mechanism is still a topic of debate. Here, we demonstrate the formation of monodisperse nanoparticles from colloidally stabilized primary particles, which at a critical concentration undergo a concerted association process, concomitant with a morphological and structural collapse. The formed assemblies grow further by addition of primary particles onto their surface. The presented mechanism, consistent with previously reported observations, reconciles the different theories proposed to date.

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.

Current understanding of the deformation of latex particles during film formation

This article reviews models of polymer particle deformation in film formation. Dillon et al. modelled polymer particles as viscous bodies whose surface tension provides the main driving force for particle deformation. In Browns alternative model, deformation is driven by capillary pressure due to water evaporation and opposed by the elastic force of the polymer itself. Brown, however, assumes the area on which capillary and deformation pressures act to be equal and the particles to be elastic rather than visco-elastic. The first assumption is addressed by Mason and the second by Lamprecht. Brown, Mason and Lamprecht modelled the polymer particles response as the response of two non-attracting spheres pressed together. Kendall and Padget followed an alternative approach using the JKR theory thus incorporating the van der Waals attraction. The existence of a sharp transition in the film forming qualities near Tg is predicted by these theories through a sharp change in the elastic modulus. The predicted inverse proportionality for the particle radius is not observed experimentally. Experiments performed by Sperry et al. cast further doubt on the applicability of these theories. Keddie et al. propose a new approach to particle deformation. According to them transparent films can either be obtained along an ‘easy’ route, by particle deformation due to capillary forces, or along a “hard” route, by particle deformation due to other surface forces; e.g. the polymer/water interfacial tension, the polymer/air surface tension or the forces due to residual water left between the particles.

Forces operative during film formation from latex dispersions

Abstract In this paper, the different forces operative on the latex particles during film formation are examined and estimates are given of the contribution of the forces to the deformation of these particles. The forces examined are gravitational forces, Van der Waals forces, electrostatic repulsion forces due to the overlap of diffuse double layers, capillary forces due to the receding water/air interface, and capillary forces due to liquid bridges between the latex particles. The magnitude of these forces is compared to the force needed to obtain sufficient deformation, i.e. the closure of the voids between the particles. Calculations show that both capillary forces are from the same order of magnitude, 1–3 10 −7 N. The Van der Waals contribution is smaller by a factor of 20 than the contribution due to the capillary forces. However, for deformation the Van der Waals forces may be of considerable importance since the Van der Waals forces diverge for very small distances. A sound incorporation of the Van der Waals forces can be achieved by using the JKR equations. Under the assumption of constant potential, the electrostatic repulsion forces are approximately a factor of 1000 smaller than the capillary forces. The gravitational forces, 1×10 −16 N, are negligible. The force needed for successful deformation amounts to 10 −7 N, assuming that the Hertz theory is applicable in the description of polymer particle deformation. Furthermore, an equation for the capillary force due to the receding water/air interface is derived which is applicable for a wider range of degrees of deformation than is the Mason-equation. Three descriptions of the particles response to deformation are examined: (i) the Hertz theory for purely elastic spheres, (ii) the JKR-theory for purely elastic spheres in the presence of Van der Waals forces, and (iii) the Yang-theory for linear visco-elastic spheres. These descriptions are combined with both capillary forces resulting in criterions determining successful deformation.

Should the Gibbs analysis be revised

Recently, some arguments were published that cast doubt on the validity of the Gibbs adsorption isotherm. The doubt was on whether the often visible linearly declining part in the surface tension versus logarithm of concentration plot of a surfactant solution, just before the critical micelle concentration, really represents a situation of constant adsorption. Those published arguments are partly of a conceptual nature and partly based on experimental evidence. The conceptual arguments appear to be based on a misunderstanding of the theory, while the arguments based on experimental evidence stem from an inaccurate treatment of these data. Our conclusion is that none of the relevant arguments put forward are valid. The experimental evidence, if properly treated, is in line with the Gibbs theory.

Dynamics of cross linking fronts in alkyd coatings

The dynamics of the curing process of alkyd coatings is an important aspect for coating performance. The formation of cross links in an alkyd coating film has been followed in time using a microimaging nuclear magnetic resonance setup, having a spatial resolution of 5μm perpendicular to the film. During this cross-linking process a front has been observed inside the coating film. The position of this front varied with the square root of time. With the help of a simple reaction model, we have proven that this dynamics results from the fact that the curing rate is limited by the oxygen flux into the coating. This model can also explain, the differences in curing rates observed for various coatings.