Colloidal stability for concentrated zirconia aqueous suspensions

This work started as part of an investigation into the mechanisms by which fine zirconia aqueous dispersions can be processed for ceramic materials engineering. Aqueous dispersions of TZ3Y fine zirconia particles obtained by dispersion of dry powder in acidic solutions (pH 3) have been subjected to compression through osmotic experiments. The results show a behavior that is unusual when compared with the classical behavior of colloidal dispersions. Indeed, the 50 nm particles are well dispersed and protected from aggregation by electrical double layers, with a high zeta potential (60–80 mV). Yet, during osmotic compression, the dispersion goes from a liquid state to a gel state at a rather low volume fraction, φ=0.2, whereas the liquid–solid transition for repelling particles is expected to occur only at φ=0.5. This early transition to a state in which the dispersion does not flow may be a severe drawback in some uses of these dispersions, and thus it is important to understand its causes. A possible cause of this early aggregation is the presence of a population of very small particles, which are seen in osmotic stress experiments and in light scattering. We propose that aggregation could result from the compression of this population, through either of the following mechanisms: (a) An increase in pressure causes the small particles to aggregate with each other and with the larger ones or (b) An increase in pressure induces a depletion flocculation phenomenon, in which the large particles are pushed together by the smaller ones

Thermal expansion within a chain of magnetic colloidal particles

We study the thermal expansion of chains formed by self-assembly of magnetic colloidal particles in a magnetic field. Using video-microscopy, complete positional data of all the particles of the chains is obtained. By changing the ionic strength of the solution and the applied magnetic field, the interaction potential can be tuned. We analyze the thermal expansion of the chain using a simple model of a one dimensional anharmonic crystal of finite size.Comment: 5 pages and 3 figure

The Sol-Gel Process Simulated by Cluster-Cluster Aggregation

The pair-correlation function $g(r,t)$ and its Fourier transform, the structure factor $S(q,t)$, are computed during the gelation process of identical spherical particles using the diffusion-limited cluster-cluster aggregation model in a box. This numerical analysis shows that the time evolution of the characteristic cluster size $\xi$ exhibits a crossover close to the gel time $t_g$ which depends on the volumic fraction $c$. In this model $t_g$ tends to infinity when the box size $L$ tends to infinity. For systems of finite size, it is shown numerically that, when $t<t_g$, the wave vector $q_m$, at which $S(q,t)$ has a maximum, decreases as $S(q_m,t)^{-1/D}$, where $D$ is an apparent fractal dimension of clusters, as measured from the slo pe of $S(q,t)$ . The time evolution of the mean number of particles per cluster $\bar {n}$ is also investigated. Our numerical results are in qualitative agreement with small angle scattering experiments in several systems.Comment: RevTex, 13 pages + 9 postscript figures appended using "uufiles". To appear in J. of Non-Cryst. Solid

A Model for the Elasticity of Compressed Emulsions

We present a new model to describe the unusual elastic properties of compressed emulsions. The response of a single droplet under compression is investigated numerically for different Wigner-Seitz cells. The response is softer than harmonic, and depends on the coordination number of the droplet. Using these results, we propose a new effective inter-droplet potential which is used to determine the elastic response of a monodisperse collection of disordered droplets as a function of volume fraction. Our results are in excellent agreement with recent experiments. This suggests that anharmonicity, together with disorder, are responsible for the quasi-linear increase of $G$ and $\Pi$ observed at $\varphi_c$.Comment: RevTeX with psfig-included figures and a galley macr

Fractionation of polydisperse systems: multi-phase coexistence

The width of the distribution of species in a polydisperse system is employed in a small-variable expansion, to obtain a well-controlled and compact scheme by which to calculate phase equilibria in multi-phase systems. General and universal relations are derived, which determine the partitioning of the fluid components among the phases. The analysis applies to mixtures of arbitrarily many slightly-polydisperse components. An explicit solution is approximated for hard spheres.Comment: 4 pages, 1 figur

Rotational Diffusion in a Chain of Particles

We study the coupled rotational diffusion in a two-particle chain on the basis of a Smoluchowski equation and calculate time-correlation functions that are measurable in an experiment. This might be used to explore hydrodynamic interactions in the limit where lubrication theory is valid.Comment: 7 pages, 2 figures, to be published in J. Phys.: Condens. Matte

Elongation of confined ferrofluid droplets under applied fields

Ferrofluids are strongly paramagnetic liquids. We study the behavior of ferrofluid droplets confined between two parallel plates with a weak applied field parallel to the plates. The droplets elongate under the applied field to reduce their demagnetizing energy and reach an equilibrium shape where the magnetic forces balance against the surface tension. This elongation varies logarithmically with aspect ratio of droplet thickness to its original radius, in contrast to the behavior of unconfined droplets. Experimental studies of a ferrofluid/water/surfactant emulsion confirm this prediction.Comment: 12 pages, 7 figures. Submitted to Phys. Rev.

Dilatancy, Jamming, and the Physics of Granulation

Granulation is a process whereby a dense colloidal suspension is converted into pasty granules (surrounded by air) by application of shear. Central to the stability of the granules is the capillary force arising from the interfacial tension between solvent and air. This force appears capable of maintaining a solvent granule in a jammed solid state, under conditions where the same amount of solvent and colloid could also exist as a flowable droplet. We argue that in the early stages of granulation the physics of dilatancy, which requires that a powder expand on shearing, is converted by capillary forces into the physics of arrest. Using a schematic model of colloidal arrest under stress, we speculate upon various jamming and granulation scenarios. Some preliminary experimental results on aspects of granulation in hard-sphere colloidal suspensions are also reported.Comment: Original article intended for J Phys Cond Mat special issue on Granular Materials (M Nicodemi, Ed.

Control of silver-polymer aggregation mechanism by primary particle spatial correlations in dynamic fractal-like geometry

Silver nanocrystals have been prepared by reacting silver nitrate with ascorbic acid in aqueous solution containing a low concentration of a commercial polynaphtalene sulphonate polymer (Daxad 19). Various crystalline morphologies have been obtained simply by tuning the reaction temperature. We have investigated the nanoparticle formation mechanism at three different temperatures by in situ and time resolved Small Angle X ray Scattering measurements. By modeling the scattering intensity with interacting spherical particles in a fractal-like polymer-Ag matrix, we found signatures of nucleation, growth and assembly of primary particles of about 15-20 nm. We observed how the time evolution of both spatial correlations between primary particles and the dynamic fractal geometry of the polymer-Ag matrix could influence and determine both the aggregation mechanism and the morphology of forming nanostructures in solution