Unexpected Effect of Internal Degrees of Freedom on Transverse Phonons in Supercooled Liquids

We show experimentally that in a supercooled liquid composed of molecules with internal degrees of freedom the internal modes contribute to the frequency dependent shear viscosity and damping of transverse phonons, which results in an additional broadening of the transverse Brillouin lines. Earlier, only the effect of internal modes on the frequency dependent bulk viscosity and damping of longitudinal phonons was observed and explained theoretically in the limit of weak coupling of internal degrees of freedom to translational motion. A new theory is needed to describe this new effect. We also demonstrate, that the contributions of structural relaxation and internal processes to the width of the Brillouin lines can be separated by measurements under high pressure

Collective diffusion in charge-stabilized suspensions: Concentration and salt effects

The authors present a joint experimental-theoretical study of collective diffusion properties in aqueous suspensions of charge-stabilized fluorinated latex spheres. Small-angle x-ray scattering and x-ray photon correlation spectroscopy have been used to explore the concentration and ionic-strength dependence of the static and short-time dynamic properties including the hydrodynamic function H (q), the wave-number-dependent collective diffusion coefficient D (q), and the intermediate scattering function over the entire accessible range. They show that all experimental data can be quantitatively described and explained by means of a recently developed accelerated Stokesian dynamics simulation method, in combination with a modified hydrodynamic many-body theory. In particular, the behavior of H (q) for de-ionized and dense suspensions can be attributed to the influence of many-body hydrodynamics, without any need for postulating hydrodynamic screening to be present, as it was done in earlier work. Upper and lower boundaries are provided for the peak height of the hydrodynamic function and for the short-time self-diffusion coefficient over the entire range of added salt concentrations.Fil: Gapinski, J.. A. Mickiewicz University; PoloniaFil: Patkowski, A.. A. Mickiewicz University; PoloniaFil: Banchio, Adolfo Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Holmqvist, P.. Helmholtz Gemeinschaft. Forschungszentrum Jülich; AlemaniaFil: Meier, Guillermo Enrique. Helmholtz Gemeinschaft. Forschungszentrum Jülich; AlemaniaFil: Lettinga, M.P.. Helmholtz Gemeinschaft. Forschungszentrum Jülich; AlemaniaFil: Nägele, G.. Helmholtz Gemeinschaft. Forschungszentrum Jülich; Alemani

Interparticle correlations due to electrostatic interactions: A small angle x-ray and dynamic light scattering study. I. Apoferritin

The structure and dynamics of the spherical protein Apoferritin in aqueous solution are studied over a wide range of protein concentrations and ionic strengths. At high ionic strength and low protein concentration, the intermolecular forces are screened and, therefore, the proteins behave like uncharged molecules. Under these conditions, the form factor of Apoferritin was measured by means of small angle x-ray scattering ͑SAXS͒ and the hydrodynamic radius was determined by means of dynamic light scattering ͑DLS͒. The sample was found to be highly monodisperse. By decreasing the content of salt added, interactions between the Apoferritin particles were initiated. These intermolecular forces lead to a pronounced maximum in the SAXS intensity. At the same time, a slow mode appears in the relaxation time distribution, additionally to the diffusive mode. The relative amplitudes and correlation times of the diffusive and the slow mode were investigated and compared with predictions of the coupled mode theory. By assuming the slow mode to be related to the correlated motion of ordered domains, the size of these domains was derived from the slow relaxation time. From the x-ray data and the Apoferritin form factor, structure factors of ordered solutions were calculated. The shape of the structure factor peaks was studied as a function of Apoferritin and salt concentration. Finally, by using the DLS information regarding the size of correlated domains, we analyzed the degree of polyelectrolyte ordering within the paracrystalline domains in Apoferritin solutions

Fast Light-Driven Motion of Polydopamine Nanomembranes

[Image: see text] The actuation of micro- and nanostructures controlled by external stimuli remains one of the exciting challenges in nanotechnology due to the wealth of fundamental questions and potential applications in energy harvesting, robotics, sensing, biomedicine, and tunable metamaterials. Photoactuation utilizes the conversion of light into motion through reversible chemical and physical processes and enables remote and spatiotemporal control of the actuation. Here, we report a fast light-to-motion conversion in few-nanometer thick bare polydopamine (PDA) membranes stimulated by visible light. Light-induced heating of PDA leads to desorption of water molecules and contraction of membranes in less than 140 μs. Switching off the light leads to a spontaneous expansion in less than 20 ms due to heat dissipation and water adsorption. Our findings demonstrate that pristine PDA membranes are multiresponsive materials that can be harnessed as robust building blocks for soft, micro-, and nanoscale actuators stimulated by light, temperature, and moisture level

Inelastic x-ray scattering reveals the ergodic to nonergodic transition of salol, a liquid with local order

We have studied the high-frequency dynamics of salol by inelastic x-ray scattering over a wide temperature range between 50 and 450 K, across the glass transition. We find that salol efficiently realizes the mechanism of dynamical arrest described by the mode-coupling theory, as manifested by a cusp singularity in the behaviour of the non-ergodicity parameter and a $Q$ dependence of the critical non-ergodicity parameter that is in phase with the static structure factor. These results confront positively the mode-coupling theory with liquids with local order.Comment: 7 pages, 6 figure

Molecular dynamics simulation of the fragile glass former ortho-terphenyl: a flexible molecule model

We present a realistic model of the fragile glass former orthoterphenyl and the results of extensive molecular dynamics simulations in which we investigated its basic static and dynamic properties. In this model the internal molecular interactions between the three rigid phenyl rings are described by a set of force constants, including harmonic and anharmonic terms; the interactions among different molecules are described by Lennard-Jones site-site potentials. Self-diffusion properties are discussed in detail together with the temperature and momentum dependencies of the self-intermediate scattering function. The simulation data are compared with existing experimental results and with the main predictions of the Mode Coupling Theory.Comment: 20 pages and 28 postscript figure

Sucrose Monoester Micelles Size Determined by Fluorescence Correlation Spectroscopy (FCS)

One of the several uses of sucrose detergents, as well as other micelle forming detergents, is the solubilization of different membrane proteins. Accurate knowledge of the micelle properties, including size and shape, are needed to optimize the surfactant conditions for protein purification and membrane characterization. We synthesized sucrose esters having different numbers of methylene subunits on the substituent to correlate the number of methylene groups with the size of the corresponding micelles. We used Fluorescence Correlation Spectroscopy (FCS) and two photon excitation to determine the translational D of the micelles and calculate their corresponding hydrodynamic radius, Rh. As a fluorescent probe we used LAURDAN (6-dodecanoyl-2-dimethylaminonaphthalene), a dye highly fluorescent when integrated in the micelle and non-fluorescent in aqueous media. We found a linear correlation between the size of the tail and the hydrodynamic radius of the micelle for the series of detergents measured

Hydrodynamic modelling of protein conformation in solution: ELLIPS and HYDRO

The last three decades has seen some important advances in our ability to represent the conformation of proteins in solution on the basis of hydrodynamic measurements. Advances in theoretical modeling capabilities have been matched by commensurate advances in the precision of hydrodynamic measurements. We consider the advances in whole-body (simple ellipsoid-based) modeling—still useful for providing an overall idea of molecular shape, particularly for those systems where only a limited amount of data is available—and outline the ELLIPS suite of algorithms which facilitates the use of this approach. We then focus on bead modeling strategies, particularly the surface or shell–bead approaches and the HYDRO suite of algorithms. We demonstrate how these are providing great insights into complex issues such as the conformation of immunoglobulins and other multi-domain complexes