10,945 research outputs found
Complex oscillatory yielding of model hard sphere glasses
The yielding behaviour of hard sphere glasses under large amplitude
oscillatory shear has been studied by probing the interplay of Brownian motion
and shear-induced diffusion at varying oscillation frequencies. Stress,
structure and dynamics are followed by experimental rheology and Browian
Dynamics simulations. Brownian motion assisted cage escape dominates at low
frequencies while escape through shear-induced collisions at high ones, both
related with a yielding peak in\ . At intermediate
frequencies a novel, for HS glasses, double peak in is
revealed reflecting both mechanisms. At high frequencies and strain amplitudes
a persistent structural anisotropy causes a stress drop within the cycle after
strain reversal, while higher stress harmonics are minimized at certain strain
amplitudes indicating an apparent harmonic response.Comment: 4 figures placed at the end with following order: Figure 1, figure 3,
figure 4 and figure
Stokesian Dynamics simulation of Brownian suspensions
The non-equilibrium behaviour of concentrated colloidal dispersions is studied by Stokesian Dynamics, a general molecular-dynamics-like technique for simulating particles suspended in a viscous fluid. The simulations are of a suspension of monodisperse Brownian hard spheres in simple shear flow as a function of the PĂŠclet number, Pe, which measures the relative importance of shear and Brownian forces. Three clearly defined regions of behaviour are revealed. There is first a Brownian-motion-dominated regime (Pe ⤠1) where departures from equilibrium in structure and diffusion are small, but the suspension viscosity shear thins dramatically. When the Brownian and hydrodynamic forces balance (Pe â 10), the dispersion forms a new âphaseâ with the particles aligned in âstringsâ along the flow direction and the strings are arranged hexagonally. This flow-induced ordering persists over a range of Pe and, while the structure and diffusivity now vary considerably, the rheology remains unchanged. Finally, there is a hydrodynamically dominated regime (Pe > 200) with a dramatic change in the long-time self-diffusivity and the rheology. Here, as the PĂŠclet number increases the suspension shear thickens owing to the formation of large clusters. The simulation results are shown to agree well with experiment
Yielding of Hard-Sphere Glasses during Start-Up Shear
Concentrated hard-sphere suspensions and glasses are investigated with rheometry, confocal microscopy, and Brownian dynamics simulations during start-up shear, providing a link between microstructure, dynamics, and rheology. The microstructural anisotropy is manifested in the extension axis where the maximum of the pair-distribution function exhibits a minimum at the stress overshoot. The interplay between Brownian relaxation and shear advection as well as the available free volume determine the structural anisotropy and the magnitude of the stress overshoot. Shear-induced cage deformation induces local constriction, reducing in-cage diffusion. Finally, a superdiffusive response at the steady state, with a minimum of the time-dependent effective diffusivity, reflects a continuous cage breakup and reformation
Anisotropic Diffusion Limited Aggregation
Using stochastic conformal mappings we study the effects of anisotropic
perturbations on diffusion limited aggregation (DLA) in two dimensions. The
harmonic measure of the growth probability for DLA can be conformally mapped
onto a constant measure on a unit circle. Here we map preferred directions
for growth of angular width to a distribution on the unit circle which
is a periodic function with peaks in such that the width
of each peak scales as , where defines the
``strength'' of anisotropy along any of the chosen directions. The two
parameters map out a parameter space of perturbations that allows a
continuous transition from DLA (for or ) to needle-like fingers
as . We show that at fixed the effective fractal dimension of
the clusters obtained from mass-radius scaling decreases with
increasing from to a value bounded from below by
. Scaling arguments suggest a specific form for the dependence
of the fractal dimension on for large , form which compares
favorably with numerical results.Comment: 6 pages, 4 figures, submitted to Phys. Rev.
The New Eclipsing Cataclysmic Variable SDSS 154453+2553
The cataclysmic variable SDSS154453+2553 was recently identified in the Sloan
Digital Sky Survey. We obtained spectra and photometry at the MDM Observatory,
which revealed an eclipse with a 6.03 hour period. The H{\alpha} emission line
exhibits a strong rotational disturbance during eclipse, indicating that it
arises in an accretion disk. A contribution from an M-type companion is also
observed. Time-series photometry during eclipse gives an ephemeris of
2454878.0062(15) + 0.251282(2)E. We present spectroscopy through the orbit and
eclipse photometry. Our analysis of the secondary star indicates a distance of
800 {\pm} 180 pc.Comment: 6 pages, 3 figures, Accepted for publication in PAS
Stress development, relaxation, and memory in colloidal dispersions: Transient nonlinear microrheology
The motion of a single Brownian particle in a complex fluid can reveal material behavior both at and away from equilibrium. In active microrheology, a probe particle is driven by an external force through a complex medium and its motion studied in order to infer properties of the embedding material. Most work in microrheology has focused on steady behavior and established the relationship between the motion of the probe, the microstructure, and the effective microviscosity of the medium. Transient behavior in the near-equilibrium, linear-response regime has also been studied via its connection to low-amplitude oscillatory probe forcing and the complex modulus; at very weak forcing, the microstructural response that drives viscosity is indistinguishable from equilibrium fluctuations. But important information about the basic physical aspects of structural development and relaxation in a medium is captured by startup and cessation of the imposed deformation in the nonlinear regime, where the structure is driven far from equilibrium. Here, we study theoretically and by dynamic simulation the transient behavior of a colloidal dispersion undergoing nonlinear microrheological forcing. The strength with which the probe is forced, Fext, compared to thermal forces, kT/b, governs the dynamics and defines a PĂŠclet number, Pe = F^ext/(kT/b), where kT is the thermal energy and b is the colloidal bath particle size. For large Pe, a boundary layer (in which unsteady advection balances diffusion) forms at particle contact on the time scale of the flow, a/U, where a is the probe size and U its speed, whereas the wake forms over O(Pe) diffusive time steps. Similarly, relaxation following cessation occurs over several time scales corresponding to distinct physical processes. For very short times, the time scale for relaxation is set by a boundary layer of thickness δ âź (a+b)/Pe, and so Ď âź Î´^2/D_r, where Dr is the relative diffusivity between the probe of size a and a bath particle. Nearly all stress relaxation occurs during this time. At longer times, the Brownian diffusion of the bath particles acts to close the wake on a time scale set by how long it takes a bath particle to diffuse laterally across it, Ď âź (a+b)^2/D_r. Although the majority of the microstructural relaxation occurs during this wake-healing process, it does so with little change in the stress. Also during relaxation, the probe travels backward in the suspension; this recovered strain is proportional to the free energy stored in the compressed particle configuration, an indicator that the stress is proportional to the free energy density stored entropically in the microstructure. Theoretical results are compared with Brownian dynamics simulation where it is found that the dilute theory captures the correct behavior even for concentrated suspensions. Two modes of forcing are studied: Constant force and constant velocity. Results are compared to analogous macrorheology results for suspensions undergoing simple shear flow
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