We study the influence of hydrodynamic interactions on the self-diffusion of super-paramagnetic colloidal particles suspended in water. The colloids interact via repulsive dipolar forces due to an applied magnetic field, and their motion is effectively confined to two dimensions. By comparing experimental data with the results from extensive computer simulations, where the water flow fields are treated at the Rotne-Prager level, we show that the diffusivity is enhanced at all times due to hydrodynamic interactions. The enhancement effect becomes stronger with increasing particle density, but is almost unaffected by the temperature.

No abstract

We investigate aging in glassy systems based on a simple model, where a point in configuration space performs thermally activated jumps between the minima of a random energy landscape. The model allows us to show explicitly a subaging behavior and multiple scaling regimes for the correlation function. Both the exponents characterizing the scaling of the different relaxation times with the waiting time and those characterizing the asymptotic decay of the scaling functions are obtained analytically by invoking a "partial equilibrium" concept.

Aging dynamics in glassy systems is investigated by considering the hopping motion in a rugged energy landscape whose deep minima are characterized by an exponential density of states ρ(E) = T −1 g exp(E/Tg), −∞ < E ≤ 0. In particular we explore the behavior of a generic two-time correlation function Π(tw + t, tw) below the glass transition temperature Tg when both the observation time t and the waiting time tw become large. We show the occurrence of ordinary scaling behavior, Π(tw + t, tw) ∼ F1(t/t µ 1 w ), where µ1 = 1 (normal aging) or µ1 < 1 (subaging), and the possible simultaneous occurrence of generalized scaling behavior, tw ) with µ2 < µ1 (subaging). Which situation occurs depends on the form of the effective transition rates between the low lying states. Employing a "Partial Equilibrium Concept", the exponents µ1,2 and the asymptotic form of the scaling functions are obtained both by simple scaling arguments and by analytical calculations. The predicted scaling properties compare well with Monte-Carlo simulations in dimensions d = 1 − 1000 and it is argued that a mean-field type treatment of the hopping motion fails to describe the aging dynamics in any dimension. Implications for more general situations involving different forms of transition rates and the occurrence of many scaling regimes in the t-tw plane are pointed out.

We study theoretically and experimentally static and diffusional properties of a strongly asymmetric binary mixture of super-paramagnetic colloids confined to an air-water interface. The colloids interact via repulsive dipolar forces, induced by a magnetic field applied perpendicular to the planar interface. Brownian dynamics (BD) computer simulations are employed to analyse the microstructure and the tracer-diffusion of both components. The profound interaction asymmetry leads to unusual features in the partial pair distribution functions, and to significantly enhanced tracer-diffusion. We find good agreement between our experimental data and the computer simulation results.