Properties of mixed colloidal crystals

Abstract

IntroductionColloidal crystals are the subject of intense research for several reasons. First, owing to their speci®c length and time scales their properties are readily accessible by various forms of light scattering and microscopy. Second, a number of interesting questions of fundamental interest in condensed matter physics can be addressed in a well-de®ned model systems. This concerns both equilibrium properties (like diusive dynamics, structure or phase behaviour), response to external ®elds (like electrokinetic properties or sedimentation) and situations far from equilibrium (like the glass transition, shear-melting or crystallisation kinetics). Third, however, important applications may also result from a better understanding of the generic properties. Optical-band-gap materials are one of these.Recently focus has shifted from single-component suspensions to mixtures. The most prominent examples deal with hard-sphere or hard-sphere-polymer mixtures of large size dierence, where phase separation is observed owing to attractive terms in the potential of the mean force [1,2]. Much less is known about charged mixtures. Only a few reports exist on the phase behaviour [3,4,5], and fewer on even other properties, like shear rigidity [6], diusion [7,8] or crystallisation kinetics.We report on a mixture of two species of polystyrene latex spheres in deionised aqueous suspension. Their size ratio is C 0.85. Such a small value should not induce an entropically driven instability. Nevertheless, the phase behaviour is not immediately obvious. The formation of alloy structures of glasses or of precipitats can be expected. Alloys may either be compositionally ordered (e.g. of NaCl structure) or disordered (e.g. Cu/Au). We conducted a number of dierent experiments to comprehensively characterise the resulting solids. Besides the static structure factor, S(q), we also measured the conductivity, r, and the static shear modulus, G. The former is very sensitive to the formation of precipitates; the latter has been shown to yield valuable information about the local crystal structure [9]. Finally, we also report some of the ®rst dynamic light scattering measurements on polycrystalline samples. Their interpretation can be qualitative only, since a rigorous scattering theory of such nonergodic and inhomogeneous materials is still demanded. The combination of our results shows that at low-to-intermediate particle concentrations randomly substituted alloys of body-centered-cubic (bcc) structure are formed.Abstract We prepared colloidal crystals from aqueous suspensions of spherical, charged polystyrene spheres under deionised conditions. Using a home-built multipurpose light scattering apparatus we measured the static structure factor, the static shear modulus and the intermediate scattering function. In addition we also monitored also the conductivity as a function of the composition and the particle number density. For the mixture investigated the data are well described assuming the formation of randomly substituted body-centere...

Crystallization kinetics of colloidal model suspensions: recent achievements and new perspectives

“…Effective charges determined from conductivity experiments in the fluid or crystalline phase are numerically very close to theoretically calculated renormalized charges [108,109,110,111]. Effective charges including many body interactions are accessible from measurements of the elasticity in the crystalline phase [93,94] or the static structure factor, S(q), of the fluid phase [112]. The effective charge, Z eff , obtained from an interpretation of elasticity measurements in terms of an effective HCY potential has been combined with the results from computer simulations to accurately predict the phase behaviour of CS systems [113].…”

“…Due to the long ranged nature of the CS repulsion, surface roughness or softness as well as short ranged van der Waals attractions can in most cases be neglected. The crucial parameter for the interaction strength, rather than , is the particle number density, n. Adjustment of n and salt concentration, c, is readily facilitated employing a closed conditioning circuit [90,91,92] and controlled in situ by combining static light scattering with conductivity and/or elasticity measurements [93,94]. For most polymer latex particles the number of charge generating surface groups, N, is fixed by synthesis and readily accessible by titration experiments [95].…”

Melting and freezing lines for a mixture of charged colloidal spheres with spindle-type phase diagram

“…The effective charge from elasticity Z eff G , gives a measure of the interaction strength. Also this quantity was observed to be independent of n in the crystalline phase [49].…”

“…The effective elasticity charge reflects the influence of triplet and higher order interactions [44, ]. In contrast to the larger effective charge derived from conductivity (describing the number of uncondensed counter-ions), this kind of effective charge gives a very accurate description of the phase behavior of single component systems [11]. In addition, we carefully checked that the chosen interaction energy, Eqn.…”

Phase behavior of a de-ionized binary mixture of charged spheres in the presence of gravity

“…Measurements of the shear modulus employed the non-invasive optical technique of torsional resonance detection [53]. In short, the sample is set into low amplitude torsional oscillation about its vertical axis.…”