CiteULike: dchens rotation

Translation-Rotation Paradox for Diffusion in Glass-Forming Polymers: The Role of the Temperature Dependence of the Relaxation Time Distribution

David Hall — 2007-12-06T15:38:54-00:00

Physical Review Letters, Vol. 79, No. 1. (7 July 1997), 103.

Comparisons are made of the translational and rotational diffusion of small-molecule probes in a polymer near its glass transition temperature; T g . In the rubbery state; 1.1 T g > T > T g ; translational diffusion is much less temperature dependent than rotational reorientation; in a “quenched” glass; translation and rotation have similar temperature dependencies. This is explained to be a consequence of the fact that in the rubbery state near T g the breadth of the polymer relaxation distribution is strongly temperature dependent; while in the quenched glass it is temperature invariant.

Role of orientation disorder in the formation of fragility of glassy water and glycerol-like liquids

Sergey Lishchuk — 2008-06-10T15:27:23-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 6. (2007)

The role of H bonds in the formation of the fragility and dielectric properties of highly viscous liquids is investigated. The heuristic supposition about the proportionality between the logarithm of the shear viscosity and oscillatory contributions to the mean-square displacement of a molecule is presented. Concrete calculations are carried out for the H-bond subsystem of the two-dimensional model lattice water. The conjecture on the interrelation between the phase transition in the subsystem of H bonds and the glassification point is formulated. It is shown that (i) the glassification temperature is proportional to the H-bonding energy and (ii) the fragilities of glycerol-like liquids differ from each other as a consequence of distinct interaction energies between H bonds. The existence of a close connection between the fragility parameter and dielectric permittivity is established.

Torque Detection using Brownian Fluctuations

Giovanni Volpe — 2006-11-27T18:32:15-00:00

Physical Review Letters, Vol. 97, No. 21. (2006)

We report the statistical analysis of the movement of a submicron particle confined in a harmonic potential in the presence of a torque. The absolute value of the torque can be found from the auto- and cross-correlation functions of the particle's coordinates. We experimentally prove this analysis by detecting the torque produced onto an optically trapped particle by an optical beam with orbital angular momentum.

Microrheology from Rotational Diffusion of Colloidal Particles

Efren Reyes — 2007-05-18T11:12:10-00:00

Physical Review Letters, Vol. 94, No. 10. (2005)

The microrheology of viscoelastic fluids is obtained from rotational diffusion of optically anisotropic spherical colloidal probes, measured by depolarized dynamic light scattering. The storage and loss moduli obtained from the rotational mean squared displacement is in excellent agreement with those obtained from translational diffusion and by mechanical measurements. We also show that this method is applicable to samples with strong light scattering components. This extends the capabilities of the microrheological methods based on the diffusional motion of colloidal probes.

Translations and Rotations Are Correlated in Granular Gases

NV Brilliantov — 2008-03-19T01:35:01-00:00

Physical Review Letters, Vol. 98, No. 12. (2007)

In a granular gas of rough particles the axis of rotation is shown to be correlated with the translational velocity of the particles. The average relative orientation of angular and linear velocities depends on the parameters which characterize the dissipative nature of the collision. We derive a simple theory for these correlations and validate it with numerical simulations for a wide range of coefficients of normal and tangential restitution. The limit of smooth spheres is shown to be singular: even an arbitrarily small roughness of the particles gives rise to orientational correlations.

Observation of Immobilized Water Molecules around Hydrophobic Groups

YLA Rezus — 2007-10-24T12:22:45-00:00

Physical Review Letters, Vol. 99, No. 14. (2007)

We have used femtosecond midinfrared spectroscopy to study the orientational mobility of water molecules in the hydration shells of hydrophobic groups. Our results show that hydrophobic groups are surrounded by a number of water molecules that display much slower orientational dynamics than the bulk liquid and that are therefore effectively immobilized. It turns out that each methyl group is surrounded by four immobilized water OH groups.

Shape Transition and Propulsive Force of an Elastic Rod Rotating in a Viscous Fluid

Bian Qian — 2008-03-18T00:42:13-00:00

Physical Review Letters, Vol. 100, No. 7. (2008)

The deformation of an elastic rod rotating in a viscous fluid is considered, with applications related to flagellar motility. The rod is tilted relative to the rotation axis, and experiments and theory are used to study the shape transition when driven either at constant torque or at constant speed. At low applied torque, the rod bends gently and generates small propulsive force. At a critical torque, the rotation speed increases abruptly, and the rod forms a helical shape with increased propulsive force. We find good agreement between theory and experiment. A simple physical model is presented to capture and explain the essential behavior.

Heterogeneity at the Glass Transition: Translational and Rotational Self-Diffusion

I Chang — 2007-11-27T18:50:35-00:00

J. Phys. Chem. B, Vol. 101, No. 43. (23 October 1997), pp. 8794-8801.

Abstract: Self-diffusion coefficients, D, have been measured in the glass forming liquids salol, glycerol, phenolphthaleine dimethyl ether (PDE), cresolphthaleine dimethyl ether (CDE), and -trinaphthylbenzene (TNB) in the supercooled regime. The NMR static magnetic field gradient technique was applied where D >10-14 m2 s-1 can be attained. The results are similar to previous diffusion experiments where an enhancement of translational diffusion was found in comparison with rotational diffusion and shear viscosity. Various models of spatial heterogeneity are related to a phenomenological environmental fluctuation model in view of recent diffusion and relaxation data close to the glass transition.

Connection of translational and rotational dynamical heterogeneities with the breakdown of the Stokes-Einstein and Stokes-Einstein-Debye relations in water

Marco Mazza — 2007-10-16T14:47:04-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

We study the Stokes-Einstein (SE) and the Stokes-Einstein-Debye (SED) relations, Dt=kBT/6R and Dr=kBT/8R3, where Dt and Dr are the translational and rotational diffusivity, respectively, T is the temperature, the viscosity, kB the Boltzmann constant, and R the “molecular” radius. Our results are based on molecular dynamics simulations of the extended simple point charge model of water. We find that both the SE and SED relations break down at low temperature. To explore the relationship between these breakdowns and dynamical heterogeneities (DHs), we also calculate the SE and SED relations for subsets of the 7% “fastest” and 7% “slowest” molecules. We find that the SE and SED relations break down in both subsets, and that the breakdowns occur on all scales of mobility. Thus these breakdowns appear to be generalized phenomena, in contrast with a view where only the most mobile molecules are the origin of the breakdown of the SE and SED relations, embedded in an inactive background where these relations hold. At low temperature, the SE and SED relations in both subsets of molecules are replaced with “fractional” SE and SED relations, Dt~(/T)−t and Dr~(/T)−r, where t0.84(<1) and r0.75(<1). We also find that there is a decoupling between rotational and translational motion, and that this decoupling occurs in both the fastest and slowest subsets of molecules. Further, we find that, the decoupling increases upon cooling, but that the probability of a molecule being classified as both translationally and rotationally fastest also increases. To study the effect of time scale for SE and SED breakdown and decoupling, we introduce a time-dependent version of the SE and SED relations, and a time-dependent function that measures the extent of decoupling. Our results suggest that both the decoupling and SE and SED breakdowns originate at a time scale corresponding to the end of the cage regime, when diffusion starts. This is also the time scale when the DHs are more relevant. Our work also demonstrates that selecting DHs on the basis of translational or rotational motion more strongly biases the calculation of diffusion constants than other dynamical properties such as relaxation times.