Correlations In Suspensions Research Articles

Correlations in suspensions confined between viscoelastic surfaces: Noncontact microrheology.

We study theoretically the velocity cross-correlations of a viscous fluid confined in a slit between two viscoelastic media. We analyze the effect of these correlations on the motions of particles suspended in the fluid. The compliance of the confining boundaries gives rise to a long-ranged pair correlation, decaying only as 1/r with the interparticle distance r. We show how this long-ranged effect may be used to extract the viscoelastic properties of the confining media without embedding tracer particles in them. We discuss the remarkable robustness of such a potential technique with respect to details of the confinement, and its expected statistical advantages over standard two-point microrheology.

Temperature-dependent dynamic correlations in suspensions of magnetic nanoparticles in a broad range of concentrations: a combined experimental and theoretical study.

The interweave of competing individual relaxations influenced by the presence of temperature and concentration dependent correlations is an intrinsic feature of superparamagnetic nanoparticle suspensions. This unique combination gives rise to multiple applications of such suspensions in medicine, nanotechnology and microfluidics. Here, using theory and experiment, we investigate dynamic magnetic susceptibility in a broad range of temperatures and frequencies. Our approach allows, for the first time to our knowledge, to separate clearly the effects of superparamagnetic particle polydispersity and interparticle magnetic interactions on the dynamic spectra of these systems. In this way, we not only provide a theoretical model that can predict well the dynamic response of magnetic nanoparticles systems, but also deepen the understanding of the dynamic nanoparticle self-assembly, opening new perspectives in tuning and controlling the magnetic behaviour of such systems in AC fields.

Correlations and fluctuations of stress and velocity in suspensions of swimming microorganisms

Active systems, which are driven out of equilibrium, can produce long range correlations and large fluctuations that are not restricted by the fluctuation-dissipation theorem. We consider here the fluctuations and correlations in suspensions of swimming microorganisms that interact hydrodynamically. Modeling the organisms as force dipoles in Stokes flow and considering run-and-tumble and rotational diffusion models of their orientational dynamics allow derivation of closed form results for the stress fluctuations in the long-wave limit. Both of these models lead to Lorentzian distributions, in agreement with some experimental data. These fluctuations are not restricted by the fluctuation-dissipation theorem, as is explicitly verified by comparing the fluctuations with the viscosity of the suspension. In addition to the stress fluctuations in the suspension, we examine correlations between the organisms. Because of the hydrodynamic interactions, the velocities of two organisms are correlated even if the positions and orientations are uncorrelated. We develop a theory of the velocity correlations in this limit and compare with the results of computer simulations. We also formally include orientational correlations in the theory; and comparing with simulations, we are able to show that these are important even in the dilute limit and are responsible in large part for the velocity correlations. While the orientation correlations cannot as yet be predicted from this theory, by inserting the results from simulations into the theory it is possible to properly determine the form of the swimmer velocity correlations. These correlations of orientations are also the key to understanding the spatial correlations of the fluid velocity. Through simulations we show that the orientational correlations decay as r−2 with distance—inserting this dependence into the theory leads to a logarithmic dependence of the velocity fluctuations on the size of the system.

Diffusion and Spatial Correlations in Suspensions of Swimming Particles

Populations of swimming micro-organisms produce fluid motions that lead to dramatically enhanced diffusion of tracer particles. Using simulations of suspensions of swimming particles in a periodic domain, we capture this effect and show that it depends qualitatively on the mode of swimming: swimmers "pushed" from behind by their flagella show greater enhancement than swimmers that are "pulled" from the front. The difference is manifested by an increase, that only occurs for pushers, of the diffusivity of passive tracers and the velocity correlation length with the size of the periodic domain. A physical argument supported by a mean field theory sheds light on the origin of these effects.

Metalloporphyrin Rods in Hydrocarbons: Highly Shear Responsive Suspensions

Suspensions of rods made up of a Zn(II) complex of a long-chain triester of meso-tetrakis[p-carboxy]phenyl porphyrin (ZnP3) in saturated hydrocarbons have been characterized by rheological measurements. The anisotropy of isointensity contour plots of small-angle neutron scattering patterns characterize the orientation correlations in suspensions at rest and under shear. The highly thixotropic ZnP3 suspensions in cyclohexane exhibit weak yield stresses and are very shear sensitive due to fragile interactions between rod ends. Rheological and scattering data indicate that the relaxation of sheared suspensions can be described as a transition from a nematic-like distribution of rods to a random three-dimensional distribution. The related kinetics is a slow first-order process corresponding to the progressive disinclination of the rigid rods forming the segmented chains.

Dynamical correlations in suspensions of charged rodlike macromolecules.

Dynamical correlations of charged rodlike colloidal particles interacting via a Yukawa segment-segment potential are investigated using extensive Brownian dynamics computer simulations. The model and the parameters used in the simulations are particularly designed for aqueous suspensions of tobacco-mosaic viruses. Over a broad range of rod concentrations we calculate the translational long-time self-diffusion coefficient and study the orientational motion in the disordered phase. It is found that the relaxation of the orientational correlation has three different regimes: a diffusive short-time motion, another diffusive motion for intermediate times, and a nondiffusive long-time relaxation. Also dynamical correlations in the nematic phase and near the isotropic-nematic transition are calculated. The results are compared with experimental data. \textcopyright{} 1996 The American Physical Society.