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Register a new userTwo-stage crystallization of charged colloids at low supersaturations
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We report simulations on the homogeneous liquid-fcc nucleation of charged colloids for both low and high contact energy values. As a precursor for crystal formation, we observe increased local order at the position where the crystal will form, but no correlations with the local density. Thus, the nucleation is driven by order fluctuations rather than density fluctuations. Our results also show that the transition involves two stages in both cases, first a transition liquid-bcc, followed by a bcc-hcp/fcc transition. Both transitions have to overcome free energy barriers, so that a spherical bcc-like cluster is formed first, in which the final fcc-like structure is nucleated mainly at the surface of the crystallite. This means that the bcc-fcc phase transition is a heterogeneous nucleation, even though we start from a homogeneous bulk liquid. The height of the bcc-hcp/fcc free energy barrier strongly depends on the contact energies of the colloids. For low contact energy this barrier is low, so that the bcc-hcp/fcc transition happens spontaneously. For the higher contact energy, the second barrier is too high to be crossed spontaneously by the colloidal system. However, it was possible to ratchet the system over the second barrier and to transform the bcc nuclei into the stable hcp/fcc phase. The transitions are dominated by the first liquid-bcc transition and can be described by Classical Nucleation Theory using an effective surface tension.
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Hydrophobic PMMA colloidal particles, when dispersed in oil with a relatively high dielectric constant, can become highly charged. In the presence of an interface with a conducting aqueous phase, image charge effects lead to strong binding of colloidal particles to the interface, even though the particles are wetted very little by the aqueous phase. In this paper, we study both the behavior of individual colloidal particles as they approach the interface, and the interactions between particles that are already interfacially bound. We demonstrate that using particles which are minimally wetted by the aqueous phase allows us to isolate and study those interactions which are due solely to charging of the particle surface in oil. Finally, we show that these interactions can be understood by a simple image-charge model in which the particle charge $q$ is the sole fitting parameter.
We discuss the peculiarities of the Seebeck effect in stabilized electrolytes containing the colloidal particles. Its unusual feature is the two-stage character, with the linear increase of differential thermopower as the function of colloidal particles concentration $n_{odot}$ during the first stage and dramatic drop of it at small $n_{odot}$ during the second one (steady state). We show that the properties of the initial state are governed by the thermo-diffusion flows of the mobile ions of the stabilizing electrolyte medium itself and how the colloidal particles participate in formation of the electric field in the bulk of suspension. In its turn the specifics of the steady state in thermoelectric effect we attribute to considerable displacements of the massive colloidal particles in process of their slow thermal diffusion and break down of their electroneutrality in the vicinity of electrodes
We determined bulk crystal nucleation rates in aqueous suspensions of charged spheres at low metastability. Experiments were performed in dependence on electrolyte concen-tration and for two different particle number densities. The time-dependent nucleation rate shows a pronounced initial peak, while post-solidification crystal size distributions are skewed towards larger crystallite sizes. At each concentration, the nucleation rate density initially drops exponentially with increasing salt concentration. The complete data set, however, shows an unexpected scaling of the nucleation rate densities with met-astability times the number density of particles. Parameterization of our results in terms of Classical Nucleation Theory reveals unusually low interfacial free energies of the nu-cleus surfaces and nucleation barriers well below the thermal energy. We tentatively attribute our observations to the presence of doublets introduced by the employed con-ditioning technique and acting as nucleation seeds.
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