Biomedical subjects

Thomas Palberg

Publications and source records attributed to Thomas Palberg.

7 recordsLinked to original sources

PubMed2006

Effective charges along the melting line of colloidal crystals.

The shear modulus G of charged colloidal crystals was measured at several constant particle densities n and varying salt concentrations c up to the melting salt concentration cM using torsional resonance spectroscopy. Far from the phase boundary the samples are polycrystalline and the shear modulus stays roughly constant as a function of c. Upon approaching the melting transition an increasing amount of wall based crystal material is formed surrounding a shrinking polycrystalline core and G drops nearly linearly. When the transition is complete G again stays constant. The morphologic transitions may be scaled upon a single master curve. For the polycrystalline morphology, the elastic data are evaluated in terms of a pairwise additive screened Coulomb interaction yielding a particle effective charge Z(G)*. Under de-ionized conditions Z(0,G)* is independent of n and significantly lower than expected from charge renormalization theory. With increasing salt concentration Z(G)* increases. The increase becomes more pronounced at larger n. By extrapolation we further obtain the melting line effective elasticity charge Z(M,G)*. Z(M,G)* shows a steplike increase with increasing nM and cM to values consistent with charge renormalization theory. Interestingly, the increase coincides semi-quantitatively with the one expected from the universal melting line for charged spheres, thus facilitating a consistent description of phase behavior and elasticity over an extended range of the phase diagram.

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PubMed2006

Electrophoretic flow behaviour and mobility of colloidal fluids and crystals.

We report on measurements of the electrophoretic mobility mu of charged colloidal spheres in the deionized state, where the suspensions show fluid or crystalline order. In the fluid state, parabolic flow profiles are observed due to electro-osmotic solvent flow. In the crystalline state, complex flow profiles occur due to additional crystal cohesion. The mobility mu then may inferred from the flow velocity averaged over the complete cell cross section as performed in our home built super-heterodyne Doppler velocimeter. For two particle species of 68 and 122 nm diameter we measured mu as a function of particle concentration. Starting from a plateau value at low concentration, mu decreases approximately logarithmically with increased concentration. Interestingly, the decrease of mu is not affected by the phase transition, indicating that electro-kinetic properties may be viewed as single particle properties even in the case of structure formation. Moving further along this line of thought, we show that this behaviour may indeed be captured near quantitatively by an ad hoc combination of charge renormalization calculations and the Standard Electro-kinetic Model originally designed for isolated, non-interacting particles. From this coincidence, we may conclude that (i) counter-ion condensation behind the hydrodynamic slip plane is the dominant effect of finite particle concentrations and leads to a decrease of the effective particle charge and (ii) an increased colloid concentration can be understood in terms of an elevated total ion concentration and that (iii) it can thus be reduced to a modification of the screening parameter, such that the problem can be mapped onto a single particle theory for an infinite electrolyte.

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PubMed2005

Microscopic investigations of homogeneous nucleation in charged sphere suspensions.

We studied the homogeneous nucleation kinetics of an aqueous suspension of charged colloidal spheres under de-ionized conditions. Samples of equilibrium crystalline structure were shear molten and the metastable melt left to solidify after cessation of shear. At low particle number densities n, corresponding to low metastability of the melt, nucleation was monitored directly via video microscopy. We determined the nucleation rates gamma(t) by counting the number of newly appearing crystals in the observation volume per unit time. Using a suitable discrete adaptation of Avrami's [J. Chem. Phys. 7, 1003 (1939); ibid.8, 212 (1940); ibid.9, 177 (1941)] model for solidification via homogeneous nucleation and subsequent growth, we calculate the remaining free volume VF(t) to obtain the rate densities J(t) = gamma(t)/VF(t). We observe J(t) to rise steeply, display a plateau at a maximum rate density Jmax, and to decrease again. With increased n the plateau duration shrinks while Jmax increases. At low to moderate number densities fully solidified samples were analyzed by microscopy to obtain the grain-size distribution and the average crystallite size angle brackets(L). Under the assumption of stationarity, we obtained the nucleation rate density J(Avr), which increased strongly with increasing n. Interestingly, J(Avr) agrees quantitatively to Jmax and to J(Avr) as obtained previously from scattering data taken on the same sample at large n. Thus, by combination of different methods, reliable nucleation rate densities are now available over roughly one order of magnitude in n and eight orders of magnitude in J.

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PubMed2005

Drude-type conductivity of charged sphere colloidal crystals: density and temperature dependence.

We report on extensive measurements in the low-frequency limit of the ac conductivity of colloidal fluids and crystals formed from charged colloidal spheres suspended in de-ionized water. Temperature was varied in a range of 5 degrees C < Theta < 35 degrees C and the particle number density n between 0.2 and 25 microm(-3) for the larger, respectively, 2.75 and 210 microm(-3) for the smaller of two investigated species. At fixed Theta the conductivity increased linearly with increasing n without any significant change at the fluid-solid phase boundary. At fixed n it increased with increasing Theta and the increase was more pronounced for larger n. Lacking a rigorous electrohydrodynamic treatment for counterion-dominated systems we describe our data with a simple model relating to Drude's theory of metal conductivity. The key parameter is an effectively transported particle charge or valence Z(*). All temperature dependencies other than that of Z(*) were taken from literature. Within experimental resolution Z(*) was found to be independent of n irrespective of the suspension structure. Interestingly, Z(*) decreases with temperature in near quantitative agreement with numerical calculations.

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PubMed2005

Crystallization in charged two-component suspensions.

We report on the crystallization of colloidal crystals comprising of charged particles with different size ratio dispersed in thoroughly deionized water. Single components were characterized carefully and their nucleation behavior was investigated before the preparation of mixtures. Mixtures investigated at constant particle number densities showed body centred cubic structure, conductivity, and shear moduli comply with the assumption of a randomly substituted crystal. Most importantly, for the first time we obtain the dependence of the nucleation rate densities in dependence on the composition and (for one fixed composition) the particle number density. The process of nucleation in random substitutional crystals is found to be similar to the one-component case.

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PubMed2003

Sheared colloidal crystals in confined geometry: a real space study on stationary structures under shear.

We constructed an optical plate-plate shear cell suitable for the study of aqueous suspensions of charged colloidal spheres under low electrolyte concentrations (10(-6) M). The variable gap height was adjusted to 30 microm corresponding to 15-30 interparticle distances. The concentration of 300 microm diameter polystyrene spheres was chosen around 1 microm(-1) where previous studies had revealed the equilibrium structure to be fluid or body centred cubic. Under shear, layer structures of hexagonal symmetry form, often coexistent with a fluid phase. We used an adapted high resolution video microscope to perform a detailed study of the structure. The central ray of light was prevented from entering the objective and the images are constructed from higher order scattered light. The cover glass correction was adjusted in such a way that several layers of particles were visible simultaneously. The effective depth of sharpness was thus increased to several microm. From the 2D video frames we constructed time averaged 2D particle correlation diagrams (PCD) which correspond to the distribution functions for the projections of particle positions into the x y-plane. They allow us to discriminate different layers and to analyse their structure and their relative position. Layer structures showed approximately hexagonal symmetry bat with an elongation in the flow direction. They were always observed to be wall based. Melting in the z-direction occurred before in-plane melting and with a different mechanism. The resulting salt concentration-shear rate phase diagram is discussed in comparison with results from computer simulation.

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PubMed2001

A Multipurpose Instrument To Measure the Vitreous Properties of Charged Colloidal Solids.

We present a new high-precision light-scattering setup to study the properties of colloidal solids. It combines static light scattering, dynamic light scattering, and torsional resonance spectroscopy with a flexible and reliable preparational procedure. All three experiments can be performed quasi-simultaneously on the same mechanically undisturbed sample. Thus, unequivocal identification of glassy behavior in a comprehensive characterization dependent on interaction parameters becomes possible. A detailed description of the mechanics is given. We thoroughly tested the new apparatus on dilute colloidal samples and against commercial reference instruments. We performed a first systematic series of experiments on near index matched charged particles, varying the packing volume fraction between 0.006 and 0.11. The results indicate a new route into the glassy state different from that observed in hard-sphere systems. The range of applications, accuracy, limitations, and possible extensions of the technique are discussed. Copyright 2001 Academic Press.

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