Showing papers by "Peter Schurtenberger published in 2014"

Effective interactions between soft-repulsive colloids: Experiments, theory, and simulations

Abstract: We describe a combined experimental, theoretical, and simulation study of the structural correlations between cross-linked highly monodisperse and swollen Poly(N-isopropylacrylamide) microgel dispersions in the fluid phase in order to obtain the effective pair-interaction potential between the microgels. The density-dependent experimental pair distribution functions g(r)'s are deduced from real space studies using fluorescent confocal microscopy and compared with integral equation theory and molecular dynamics computer simulations. We use a model of Hertzian spheres that is capable to well reproduce the experimental pair distribution functions throughout the fluid phase, having fixed the particle size and the repulsive strength. Theoretically, a monodisperse system is considered whose properties are calculated within the Rogers-Young closure relation, while in the simulations the role of polydispersity is taken into account. We also discuss the various effects arising from the finite resolution of the microscope and from the noise coming from the fast Brownian motion of the particles at low densities, and compare the information content from data taken in 2D and 3D through a comparison with the corresponding simulations. Finally different potential shapes, recently adopted in studies of microgels, are also taken into account to assess which ones could also be used to describe the structure of the microgel fluid.

Field-induced assembly of colloidal ellipsoids into well-defined microtubules

Abstract: Current theoretical attempts to understand the reversible formation of stable microtubules and virus shells are generally based on shape-specific building blocks or monomers, where the local curvature of the resulting structure is explicitly built-in via the monomer geometry. Here we demonstrate that even simple ellipsoidal colloids can reversibly self-assemble into regular tubular structures when subjected to an alternating electric field. Supported by model calculations, we discuss the combined effects of anisotropic shape and field-induced dipolar interactions on the reversible formation of self-assembled structures. Our observations show that the formation of tubular structures through self-assembly requires much less geometrical and interaction specificity than previously thought, and advance our current understanding of the minimal requirements for self-assembly into regular virus-like structures.

Hard sphere-like glass transition in eye lens α-crystallin solutions

Abstract: We study the equilibrium liquid structure and dynamics of dilute and concentrated bovine eye lens α-crystallin solutions, using small-angle X-ray scattering, static and dynamic light scattering, viscometry, molecular dynamics simulations, and mode-coupling theory We find that a polydisperse Percus-Yevick hard-sphere liquid-structure model accurately reproduces both static light scattering data and small-angle X-ray scattering liquid structure data from α-crystallin solutions over an extended range of protein concentrations up to 290 mg/mL or 49% vol fraction and up to ca 330 mg/mL for static light scattering The measured dynamic light scattering and viscosity properties are also consistent with those of hard-sphere colloids and show power laws characteristic of an approach toward a glass transition at α-crystallin volume fractions near 58% Dynamic light scattering at a volume fraction beyond the glass transition indicates formation of an arrested state We further perform event-driven molecular dynamics simulations of polydisperse hard-sphere systems and use mode-coupling theory to compare the measured dynamic power laws with those of hard-sphere models The static and dynamic data, simulations, and analysis show that aqueous eye lens α-crystallin solutions exhibit a glass transition at high concentrations that is similar to those found in hard-sphere colloidal systems The α-crystallin glass transition could have implications for the molecular basis of presbyopia and the kinetics of molecular change during cataractogenesis

Dynamic Depolarized Light Scattering of Small Round Plasmonic Nanoparticles: When Imperfection is Only Perfect

Abstract: Although small round gold nanoparticles (Au NPs) possess only a small degree of shape anisotropy, they support localized surface plasmon resonances and exhibit intrinsic optical anisotropy. These inherent features promote depolarized light scattering, whose temporal fluctuations carry information about rotational Brownian dynamics, and thus can be used to describe the size distribution of round Au NPs. We demonstrate that this allows for a much more accurate determination of particle size and polydispersity through depolarized dynamic light scattering when compared to standard particle sizing with light scattering.

Rheology and internal dynamics of colloidal gels from the dilute to the concentrated regime

Abstract: We explore the relations between dynamical, rheological and structural properties of colloidal gels extending the range previously considered towards higher particle volume fractions . Up to the spectrum of the time-dependent mean-square displacement is shown to be fully described by models accounting for the fractal character of gels. Beyond that, the direct relations between structural and stress-bearing properties used in fractal descriptions do not hold anymore. However, by considering the concepts of passive tracer rheology we show that the macroscopic shear modulus G0 can be derived from the dynamical properties up to even higher volume fractions.

Advanced multiresponsive comploids: from design to possible applications.

Abstract: We extend the commonly used synthesis strategies for responsive microgels to the design of novel multiresponsive and multifunctional nanoparticles that combine inorganic magnetic, metallic/catalytic and thermoresponsive organic moieties. Magnetic responsiveness is implemented through the integration of silica-coated maghemite nanoparticles into fluorescently labeled crosslinked poly(N-isopropylmethacrylamide) microgels. These particles are then employed as templates for the in situ reduction of catalytically active gold nanoparticles. In order to tune the reactivity of the catalyst through a thermally controlled barrier, an additional layer of crosslinked poly(N-isopropylacrylamide) is added in the final step. We subsequently demonstrate that these particles can be employed as smart catalysts. We show that the thermoresponsive nature of the outer particle shell not only provides control over the catalytic activity, but when combined with a magnetic core allows for very efficient removal of the catalytic system through temperature-controlled reversible coagulation and subsequent magnetophoresis in an applied magnetic field gradient. We finally discuss the use of this design principle for the synthesis of complex hybrid particles for various applications that would all profit from their multiresponsive and multifunctional nature.

Direct comparison of SESANS and SAXS to measure colloidal interactions

Abstract: We present a systematic comparison of conventional small-angle X-ray scattering (SAXS) with the relatively new real-space technique, spin-echo small-angle neutron scattering (SESANS). We apply both techniques to colloidal suspensions where the particles interact as hard spheres, with electrostatic repulsions, or with polymer-induced depletion attractions. We successfully model the data sets with the appropriate colloid-colloid interaction potentials to determine the colloid radius, effective volume fraction, and surface charge, while illustrating the different representations of colloid structuring related to both techniques.