H. Versmold

Liquid‐like ordered colloidal suspensions: The influence of the particle concentration

The density of dependence of the static structure factor S(Q) of liquid‐like ordered colloidal suspensions is investigated by light scattering techniques. We found that variation of the particle number density even by a factor of 8 produced only a modest change of the height of the first peak of S(Q). In order to obtain a consistent theoretical description of S(Q) at various particle number densities, the effective charge Z*p of the particles and the Debye screening parameter κ* were calculated on the basis of the Poisson–Boltzmann–Cell (PBC) model and the Poisson–Boltzmann–Jellium (PBJ) model, respectively. The resulting Z*p and κ* values were used to calculate S(Q) via the rescaled mean spherical approximation (RMSA). It is found that a consistent description of the density dependence of S(Q) is possible with the PBC model. By contrast, the PBJ model predicts too weak a liquid‐like order than observed experimentally.

THE GLASS TRANSITION OF CHARGED POLYMER COLLOIDS

Dynamic and static light scattering is applied to concentrated suspensions of a new species of charged colloidal suspensions with a low refractive index. The polymer colloids which were obtained by polymerizing perfluorobutylacrylate could be index matched in a water/glycerol mixture. At volume fractions Φ greater than 0.22 a glass transition can be observed, which is manifested by the nondecaying components of the intermediate scattering functions. The intermediate scattering functions for a supercooled liquid sample and the predictions of the mode‐coupling theory for the β relaxation are compared over a large Q range. By using an exponent parameter λ=0.76 a good agreement between theory and experiment can be obtained.

Structure and dynamics of polymer colloid suspensions from dynamic light scattering and Brownian dynamics simulation

A dilute suspension of ordered polymer colloid particles was investigated by static and dynamic light scattering. The obtained results are compared with a Brownian dynamic (BD) simulation of a model system consisting of hard spheres interacting with an exponentially screened Coulomb potential. The long time properties of the self‐ and collective correlation functions are investigated in detail. The obtained mean squared displacement is best described by a stretched exponential model and the collective long time diffusion from the BD simulation can only be approximately described by a kinetic theory based on a hard sphere system.

Computer simulation study of the Rayleigh and Raman spectra of fluid N2

Computer simulations of the Rayleigh and Raman spectral time‐correlation functions (TCFs) are reported for N2 at 323 K and densities ranging from 0.48 to 2.5 times the critical value. The results are compared with experiment. Particular attention is paid to the density dependence of the spectral intensity due to the correlated permanent polarizability and to the interaction‐induced contributions to the polarizability. The partial cancellations that occur between various two‐, three‐, and four‐body terms in the cross and collision induced (CI) parts of the spectral TCFs are evaluated. It is shown that these terms are significant in both the Raman and the Rayleigh spectra, but cancellation greatly reduces their net contribution at all densities studied. The weak but uncancelled TCFs that are associated with orientational correlations of the molecular polarizability anisotropies are shown to be a significant part of the high density Rayleigh TCFs. It is argued that the long‐range nature of this TCF means tha...

Microscopic investigations of the single particle dynamics in colloidal crystals

In this paper we investigated the single particle dynamics in a colloidal crystal by means of video microscopy and digital imaging methods. To obtain the self term of the van Hove function Gs(x,τ) with a reasonable good statistical accuracy we analyzed up to two million particle displacements per time channel τ of Gs(x,τ), which corresponds to the analysis of 3000 video frames, each with approximately 1000 particles. It was found that the Gaussian approximation almost perfectly accounts for the experimental results, so that the time evolution of Gs(x,τ) can solely be described by the mean square displacement 〈x2(τ)〉. The measured mean square displacements were compared with the model of a Brownian particle bound in a harmonic potential [Physica A 157, 705 (1989)]. The experimental curves approach the plateau value 〈x2(τ→∞)〉 of the mean square displacement much slower than is predicted by the model. From the plateau value of the mean square displacement the Lindemann parameter was determined for various sa...

NEUTRON DIFFRACTION FROM SHEAR ORDERED COLLOIDAL DISPERSIONS

Recently, it has been pointed out that the structure of shear ordered colloidal dispersions can adequately be accounted for by the intensity distribution I(l) along Bragg rods [Phys. Rev. Lett. 75, 763 (1995)]. Information concerning packing of the shear induced layers and on their stacking order is contained in I(l). In this paper I(l) is determined by measuring the small angle neutron scattering distribution as a function of the sample orientation. For the investigated charge stabilized system with particle diameter σ=143 nm and a distance of nearest neighbors in the layers a=237 nm, a structure close to random close‐packed hexagonal layers is found. As compared with close‐packed systems the distance between the layers c is elongated so that c≊a. Further, it is shown that the application of shear leads to a more uniform distribution of I(l) along the rods, which demonstrates that the loss of correlation between the hexagonal layers under sheared conditions can also be determined by small angle neutron s...

Small angle neutron scattering experiments from colloidal dispersions at rest and under sheared conditions

Neutron diffraction from charge stabilized shear ordered colloidal dispersions at rest and under sheared conditions are presented. A newly designed shear cell is used to generate a linear shear profile. Hexagonal scattering patterns were observed both at rest and under sheared conditions. The stacking probability A is determined by measuring the intensity dependence of the Bragg spots as a function of the angle between the incoming neutron beam and the sample cell. The shear experiments are discussed in terms of a continuous distortion [W. Loose and B. J. Ackerson, J. Chem. Phys. 101, 7211 (1994)] at small shear rates, and shear melting at higher shear rates.

Concentrated colloidal dispersions: On the relation of rheology with small angle x-ray and neutron scattering

Although rheology as well as neutron and x-ray scattering studies of ordered dispersions have been performed no coherent picture is discussable. In this paper we report our synchrotron x-ray and neutron scattering data of concentrated, charge stabilized, monodisperse polymer colloids and compare them with rheological data. We found that the scattering data can be understood in terms of viscoelastic flow. When ordered by flow-, solid- and liquid-like behavior can be separated with two independent experiments: (a) The solid-like character can be seen at perpendicular incidence as Bragg reflections, (b) the liquid-like character becomes apparent by sample rotation as the intensity distribution along Bragg rods.

Tracer particle diffusion in crystal-like ordered colloidal suspensions

Dynamic light scattering experiments are reported on crystal-like ordered colloidal suspensions composed of two types of electrostatically interacting particles having similar size and effective particle charge. The particles differ only in their refractive indices. We obtain the incoherent (self) intermediate scattering function of these mixtures by using only a trace concentration of one of the species and matching the refractive index of the suspending liquid medium to that of the other. Localization of the tracer particles about randomly located lattice sites of the colloidal crystal is manifested in the nonergodicity of the incoherent intermediate scattering functions. The latter, and the particle mean-squared displacements obtained from them, plateau to finite values at long times. In analogy to neutron scattering these plateau values of the incoherent intermediate scattering functions may be called the elastic incoherent structure factor.

Fusion of colloidal crystals induced by monovalent and asymmetric salt: an osmotic pressure and ultra-small-angle X-ray scattering study

Abstract Electrostatically stabilized diluted suspensions of negatively-charged poly(p-bromostyrene) particles have been studied by ultra small angle X-ray scattering (USAXS) and simultaneous osmotic pressure measurements. We investigated the structure and the interparticle interactions as a function of the crystal stiffness controlled by the salinity. By adding monovalent salt (NaCl) or multivalent +1/−5 salt (polyd(T)5), we reduced electrostatic repulsions and finally induced fusion of the colloidal crystal. The osmotic pressure drops and the crystal melts when the salt concentration in the reservoir is higher than 2.10−4 M for NaCl. In the case of a multivalent co-ion, 3.10−3 M polyd(T)5 is enough to reduce the osmotic pressure to zero. Thus, an experimental determination of the pressure versus distance relation allows the detection of less than micromolar amounts of multicharged co-ions. Surprisingly, the Poisson–Boltzmann cell model remains valid even when only a few screening co-ions are present in each unit cell of the colloidal crystal. By calculating the mean square displacement of a particle inside the cage made of nearest neighbours from the eccentric Poisson–Boltzmann Cell model, we have verified that the Lindemann criterion for the crystal–liquid transition is satisfied when the crystal melts due to the addition of monovalent salt.