Fluctuation-dissipation theorem in an aging colloidal glass

We provide a direct experimental test of the Stokes-Einstein relation as a special case of the fluctuation-dissipation theorem (FDT) in an aging colloidal glass. The use of combined active and passive microrheology allows us to independently measure both the correlation and response functions in this non-equilibrium situation. Contrary to previous reports, we find no deviations from the FDT over several decades in frequency (1 Hz-10 kHz) and for all aging times. In addition, we find two distinct viscoelastic contributions in the aging glass, including a nearly elastic response at low frequencies that grows during aging. This is the clearest change in material properties of the system with aging.Comment: 5 pages,4 figure

Short-time inertial response of viscoelastic fluids measured with Brownian motion and with active probes

We have directly observed short-time stress propagation in viscoelastic fluids using two optically trapped particles and a fast interferometric particle-tracking technique. We have done this both by recording correlations in the thermal motion of the particles and by measuring the response of one particle to the actively oscillated second particle. Both methods detect the vortex-like flow patterns associated with stress propagation in fluids. This inertial vortex flow propagates diffusively for simple liquids, while for viscoelastic solutions the pattern spreads super-diffusively, dependent on the shear modulus of the medium

MOLECULAR MECHANICS OF CYTOSKELETAL COMPONENTS

Summary Semiflexible polymers are of great biological importance in determining the mechanical properties of cells. We have used laser interferometry to passively trap and detect the motions of pairs of micron sized silica spheres in solutions of semiflexible polymer. The single-and dual-bead frequency dependent complex shear moduli were extracted from the auto-and cross-correlation response of the beads respectively, with the response being derived from position fluctuation data using dispersion relations from linear response theory

High-bandwidth one- and two-particle microrheology in solutions of wormlike micelles

We have developed a large-bandwidth two-particle microrheology technique to measure loss and storage moduli of viscoelastic materials from 0.1 Hz to about 100 kHz using laser trapping and interferometry. We found that quantitative agreement between one- and two-particle microrheology exist in entangled solutions of wormlike micelles chosen as a simple model viscoelastic system. These results validate both experimental method and data interpretation. The consistent results also prove that in a simple system, where the solution length scales are much smaller than the micron probe size, one-particle microrheology can accurately measure bulk viscoelastic parameter

Twin optical traps for two-particle cross-correlation measurements: Eliminating cross-talk

The correlated motions of two micron-sized particles reflect the (micro-) rheological properties of a fluid and can be conveniently detected using two optical traps in combination with interferometric displacement detection. When the correlations become small, cross-talk between the two beams becomes important. We have used dual optical traps created by either two orthogonally polarized laser beams derived from one laser source, or by two independent lasers of different wavelengths for microrheology experiments. High numerical aperture lenses (objective and condenser) in the optical path can introduce depolarization, and polarizing beam splitters are not perfect, both of which can lead to optical cross-talk. We have characterized the cross-talk in our setup and demonstrate that the use of two independent laser eliminates cross-talk entirely. © 2008 American Institute of Physics