CiteULike: dchen's charge

Diffusion-Driven Pattern Formation in Ionic Chemical Solutions

Zsanett Virányi — 2008-05-07T23:25:52-00:00

Phys. Rev. Lett. 100, 088301 (2008)

The driving force in diffusion-driven pattern formation is the difference in the diffusional flux of the key species, which in the case of ionic systems builds up a local electric field at the concentration gradients. The arising additional migrational flux not only decreases but also enhances the instability of the base state, depending on the charge distribution among the components. The opposite charges on the slower diffusing autocatalyst and its reacting counterpart favor pattern formation and shift the onset of instability to a smaller difference in the diffusion coefficients. The same charges, in addition to having the opposite effect, may even lead to the complete stabilization of planar reaction fronts unstable in the neutral system.

Density Dependent Interactions and Structure of Charged Colloidal Dispersions in the Weak Screening Regime

Rojas Ochoa — 2008-05-05T20:12:24-00:00

Physical Review Letters, Vol. 100, No. 17. (2008)

We determine the structure of charge-stabilized colloidal suspensions at low ionic strength over an extended range of particle volume fractions using a combination of light and small angle neutron scattering experiments. The variation of the structure factor with concentration is analyzed within a one-component model of a colloidal suspension. We show that the observed structural behavior corresponds to a nonmonotonic density dependence of the colloid effective charge and the mean interparticle interaction energy. Our findings are corroborated by similar observations from primitive model computer simulations of salt-free colloidal suspensions.

Dipolar Correlations and the Dielectric Permittivity of Water

Manu Sharma — 2008-05-04T15:35:50-00:00

Physical Review Letters, Vol. 98, No. 24. (2007)

The static dielectric properties of liquid and solid water are investigated within linear response theory in the context of ab initio molecular dynamics. Using maximally localized Wannier functions to treat the macroscopic polarization we formulate a first-principles, parameter-free, generalization of Kirkwood's phenomenological theory. Our calculated static permittivity is in good agreement with experiment. Two effects of the hydrogen bonds, i.e., a significant increase of the average local moment and a local alignment of the molecular dipoles, contribute in almost equal measure to the unusually large dielectric constant of water.

A comparative study on the phase behaviour of highly charged colloidal spheres in a confining wedge geometry

Ana Fontecha — 2007-09-27T16:53:09-00:00

Journal of Physics: Condensed Matter, Vol. 17, No. 31. (2005), pp. S2779-S2786.

We studied the structures formed in aqueous dispersions of charged colloidal spheres under a constant low salt concentration of c = 6 × 10[?]6 mol l[?]1. Particles of diameter 2a = 1000 nm were confined to a low angle wedge geometry with plate separation 0<S<50 µm and observed with video microscopy. Irrespective of the initial particle density n we reproducibly observe the particles to migrate to the narrow wedge side on the timescale of a few days. Thereby an interface between a crystalline structure and a near particle free region is formed, which propagates slowly until the dilute region is exhausted of particles. While the origin of this separation is still unclear, the final extension of the crystalline region is stable on the timescale of months. Within the crystal phase we observe a characteristic sequence of structures with increasing plate separation similar to that seen in previous experiments on hard sphere-like systems but here with non-touching particles. Moreover, we find that mechanical equilibrium is a necessary prerequisite for observing the full richness of different phases. A detailed comparison to recent theoretical calculations for bilayers was performed and semi-quantitative agreement with the predictions observed.

Noncentral Forces in Crystals of Charged Colloids

D Reinke — 2008-03-19T17:11:08-00:00

Physical Review Letters, Vol. 98, No. 3. (2007)

The elastic properties of fcc crystals consisting of charge stabilized colloidal particles are determined from real space imaging experiments using confocal microscopy. The normal modes and the force constants of the crystal are obtained from the fluctuations of the particles around their lattice sites using the equipartition theorem. We show that the Cauchy relation is not fulfilled and that only noncentral many-body forces can account for the elastic properties.

Colloidal Interactions and Transport in Nematic Liquid Crystals

SA Tatarkova — 2007-04-22T14:04:11-00:00

Physical Review Letters, Vol. 98, No. 15. (2007)

We describe a new nematic liquid-crystal colloid system which is characterized by both charge stabilization of the particles and an interaction force. We estimate the effective charge of the particles by electrophoretic measurements and find that in such systems the director anchoring energy W is very low and the particles have little director distortion around them. The interaction force is created by producing a radial distribution of the nematic order parameter around a locally isotropic region created by ir laser heating. We theoretically describe this as being due to the induced flexoelectric polarization, the quadrupolar symmetry of which provides the required long-range force acting on charged particles.

Disappearance of the Gas-Liquid Phase Transition for Highly Charged Colloids

AP Hynninen — 2007-05-08T12:03:35-00:00

Physical Review Letters, Vol. 98, No. 19. (2007)

We calculate the full phase diagram of spherical charged colloidal particles using Monte Carlo free energy calculations. The system is described using the primitive model, consisting of explicit hard-sphere colloids and point counterions in a uniform dielectric continuum. We show that the gas-liquid critical point becomes metastable with respect to a gas-solid phase separation at colloid charges Q20 times the counterion charge. Approximate free energy calculations with only one and four particles in the fluid and solid phases, respectively, are used to determine the critical line for highly charged colloids up to Q=2000. We propose the scaling law Tc*" align="middle">~Q1/2 for this critical temperature.

Charged Particles on Surfaces: Coexistence of Dilute Phases and Periodic Structures at Interfaces

Sharon Loverde — 2008-03-18T22:19:38-00:00

Physical Review Letters, Vol. 98, No. 23. (2007)

We consider a mixture of two immiscible oppositely charged molecules strongly adsorbed to an interface, with a neutral nonselective molecular background. We determine the coexistence between a high density ionic periodic phase and a dilute isotropic ionic phase. We use a strong segregation approach for the periodic phase and determine the one-loop free energy for the dilute phase. Lamellar and hexagonal patterns are calculated for different charge stoichiometries of the mixture. Molecular dynamics simulations exhibit the predicted phase behavior. The periodic length scale of the solid phase is found to scale as /(lB3/2), where is the effective charge density, lB is the Bjerrum length, and is the cohesive energy.