Peter R. Lang

Direct measurements of polymer-induced forces

Colloid–polymer mixtures are found in dispersions that are an important part of peoples everyday lives. The dynamics and phase stability of colloid–polymer mixtures depend on the interactions that are present in these systems. Therefore, knowledge of interactions is of basic interest. Depending on their adsorption affinity polymers added to the colloidal suspension can cause steric stabilization or flocculation due to depletion or adsorption (bridging). This paper reviews theoretical and experimental work performed on polymer-induced interactions in colloidal suspensions. Theoretically, polymers have mainly been treated as ideal flexible chains or even generalized as non-interacting (phantom) spheres. Many relevant experiments, however, have been performed with polymer chains, which are polydisperse and/or charged and/or self-interacting. These cases are challenging for theoreticians: a limited amount of work performed on these systems is also discussed here. We particularly concentrate in this review on the direct experimental measurement of polymer-induced interactions. A brief description of techniques which enable these measurements is given and their strengths and weaknesses are discussed.

Colloidal dynamics near a wall studied by evanescent wave light scattering: Experimental and theoretical improvements and methodological limitations

The dynamics of colloidal spheres near to a wall is studied with an evanescent wave scattering setup that allows for an independent variation of the components of the scattering wave vector normal and parallel to the wall. The correlation functions obtained with this novel instrumentation are interpreted on the basis of an expression for their short time behavior that includes hydrodynamic interactions between the colloidal spheres and the wall. The combination of the evanescent wave scattering setup and the exact expression for the short time behavior of correlation functions allows for an unambiguous measurement of the particle mobility parallel and normal to the wall by means of light scattering. It is possible to measure the viscous wall drag effect on the dynamics of particles with radii as small as 27 nm, where, however, the method reaches its limits due to the low scattering intensities of such small particles.

Local moments of 3d, 4d, and 5d atoms at Cu and Ag (001) surfaces

Abstract We present ag-initio calculations for the electronic structure of 3d, 4d and 5d transition-metal impurities at the (001) surface of Cu and Ag and determine the surface enhancement of the local moments. For 3d impurities we find a sizable enhancement of the local moments, being most important for V and Cr. Large local moments are obtained for 4d and 5d impurities which are in general non-magnetic in the bulk. Some of the adatoms (Nb, Mo, Tc, W, Re) on Ag (001) have “giant” magnetic moments between 3 and 4 μ B .

Flow Dichroism as a Reliable Method to Measure the Hydrodynamic Aspect Ratio of Gold Nanoparticles

Particle shape plays an important role in controlling the optical, magnetic, and mechanical properties of nanoparticle suspensions as well as nanocomposites. However, characterizing the size, shape, and the associated polydispersity of nanoparticles is not straightforward. Electron microscopy provides an accurate measurement of the geometric properties, but sample preparation can be laborious, and to obtain statistically relevant data many particles need to be analyzed separately. Moreover, when the particles are suspended in a fluid, it is important to measure their hydrodynamic properties, as they determine aspects such as diffusion and the rheological behavior of suspensions. Methods that evaluate the dynamics of nanoparticles such as light scattering and rheo-optical methods accurately provide these hydrodynamic properties, but do necessitate a sufficient optical response. In the present work, three different methods for characterizing nonspherical gold nanoparticles are critically compared, especially taking into account the complex optical response of these particles. The different methods are evaluated in terms of their versatility to asses size, shape, and polydispersity. Among these, the rheo-optical technique is shown to be the most reliable method to obtain hydrodynamic aspect ratio and polydispersity for nonspherical gold nanoparticles for two reasons. First, the use of the evolution of the orientation angle makes effects of polydispersity less important. Second, the use of an external flow field gives a mathematically more robust relation between particle motion and aspect ratio, especially for particles with relatively small aspect ratios.

One-particle correlation function in evanescent wave dynamic light scattering

In order to interpret measured intensity autocorrelation functions obtained in evanescent wave scattering, their initial decay rates have been analyzed recently [P. Holmqvist, J. K. G. Dhont, and P. R. Lang, Phys. Rev. E 74, 021402 (2006); B. Cichocki, E. Wajnryb, J. Blawzdziewicz, J. K. G. Dhont, and P. R. Lang, J. Chem. Phys. 132, 074704 (2010); J. W. Swan and J. F. Brady, ibid. 135, 014701 (2011)]. A theoretical analysis of the longer time dependence of evanescent wave autocorrelation functions, beyond the initial decay, is still lacking. In this paper we present such an analysis for very dilute suspensions of spherical colloids. We present simulation results, a comparison to cumulant expansions, and experiments. An efficient simulation method is developed which takes advantage of the particular mathematical structure of the time-evolution equation of the probability density function of the position coordinate of the colloidal sphere. The computer simulation results are compared with analytic, first and second order cumulant expansions. The only available analytical result for the full time dependence of evanescent wave autocorrelation functions [K. H. Lan, N. Ostrowsky, and D. Sornette, Phys. Rev. Lett. 57, 17 (1986)], that neglects hydrodynamic interactions between the colloidal spheres and the wall, is shown to be quite inaccurate. Experimental results are presented and compared to the simulations and cumulant expansions.

Surface induced ordering effects in soft condensed matter systems

The symmetry break of the particle interaction field at surfaces causes a variety of interfacial effects including the variation of the relative stability of different phases of a given compound. In many materials surface melting is observed, i.e. a surface near a portion of the system is in a less ordered state than the bulk. Over the last two decades, surface ordering, i.e. the stabilization of the higher ordered phase by the surface, has been observed for an increasing number of soft condensed matter systems. For melts of low molar mass compounds, surface freezing has been reported so far only for chain molecules, while in systems of colloidal length scale, surface ordering has been observed for spherical particles as well. In this contribution we will review experimental studies, some theoretical approaches as well as computer simulations of surface ordering, both in the field of molecular melts and of colloidal suspensions.

Translational and rotational near-wall diffusion of spherical colloids studied by evanescent wave scattering

In this article we extend recent experimental developments [Rogers et al., Phys. Rev. Lett., 2012, 109, 098305] by providing a suitable theoretical framework for the derivation of exact expressions for the first cumulant (initial decay rate) of the correlation function measured in Evanescent Wave Dynamic Light Scattering (EWDLS) experiments. We focus on a dilute suspension of optically anisotropic spherical Brownian particles diffusing near a planar hard wall. In such a system, translational and rotational diffusion are hindered by hydrodynamic interactions with the boundary which reflects the flow incident upon it, affecting the motion of colloids. The validity of the approximation by the first cumulant for moderate times is assessed by juxtaposition to Brownian dynamics simulations, and compared with experimental results. The presented method for the analysis of experimental data allows the determination of penetration-depth-averaged rotational diffusion coefficients of spherical colloids at low density.

Ab Initio Calculation of Quantum Well States in Cu/Co (100)

A first-principles calculation of spin-polarized quantum well states within Cu/Co has been performed using a KKR-Greens function method. Results for up to 25 ML Cu and several ML Co are presented, which are in very good agreement with experiments. In addition an important new effect is found arising from the resonant interaction of QW states with size-quantized d-states of the Co layers. This effect leads to branch jumps in the dispersion of the QW states and to a strong reduction of their confinement.

The intensity correlation function in evanescent wave scattering

As a first step toward the interpretation of dynamic light scattering with evanescent illumination from suspensions of interacting spheres, in order to probe their near wall dynamics, we develop a theory for the initial slope of the intensity autocorrelation function. An expression for the first cumulant is derived that is valid for arbitrary concentrations, which generalizes a well-known expression for the short-time, wave-vector dependent collective diffusion coefficient in bulk to the case where a wall is present. Explicit expressions and numerical results for the various contributions to the initial slope are obtained within a leading order virial expansion. The dependence of the initial slope on the components of the wave vector parallel and perpendicular to the wall, as well as the dependence on the evanescent-light penetration depth are discussed. For the hydrodynamic interactions between colloids and between the wall, which are essential for a correct description of the near-interface dynamics, we include both far-field and lubrication contributions. Lubrication contributions are essential to capture the dynamics as probed in experiments with small penetration depths. Simulations have been performed to verify the theory and to estimate the extent of the concentration range where the virial expansion is valid. The computer algorithm developed for this purpose will also be of future importance for the interpretation of experiments and to develop an understanding of near-interface dynamics, at high colloid concentrations.

Depletion interaction mediated by polydisperse rods

The interaction between a colloidal hard sphere of radius R and a wall or between two spheres in a dilute suspension of infinitely thin rods of length L is calculated numerically. The method allows the study of depletion potentials for any value of LR and, consequently, the influence of rod length polydispersity can be investigated. It was observed that both the depth and the range of the potential increase drastically if the relative standard deviation sigma of the length distribution is larger than 0.25, while the potential is virtually indistinguishable from that caused by monodisperse rods, if sigma < or similar to 0.1.