2,379 research outputs found
Colloidal dynamics in polymer solutions: Optical two-point microrheology measurements
We present an extension of the two-point optical microrheology technique
introduced by Crocker \textit{et al.} [Phys. Rev. Lett. \textbf{85}, 888
(2000)] to high frequencies. The correlated fluctuations of two probe spheres
held by a pair of optical tweezers within a viscoelastic medium are determined
using optical interferometry. A theoretical model is developed to yield the
frequency-dependent one- and two-particle response functions from the
correlated motion. We demonstrate the validity of this method by determining
the one- and two-particle correlations in a semi-dilute solution of polystyrene
in decalin. We find that the ratio of the one- and two-particle response
functions is anomalous which we interpret as evidence for a slip boundary
condition caused by depletion of polymer from the surface of the particle
Non-additivity of pair interactions in charged colloids
It is general wisdom that the pair potential of charged colloids in a liquid
may be closely approximated by a Yukawa interaction, as predicted by the
classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. We experimentally
determine the effective forces in a binary mixture of like-charged particles,
of species 1 and 2, with blinking optical tweezers. The measured forces are
consistent with a Yukawa pair potential but the (12) cross-interaction is not
equal to the geometric mean of the (11) and (22) like-interactions, as expected
from DLVO. The deviation is a function of the electrostatic screening length
and the size ratio, with the cross-interaction measured being consistently
weaker than DLVO predictions. The corresponding non-additivity parameter is
negative and grows in magnitude with increased size asymmetry.Comment: Final versio
Measurement of Effective Temperatures in an Aging Colloidal Glass
We study the thermal fluctuations of an optically confined probe particle,
suspended in an aging colloidal suspension, as the suspension transforms from a
viscous liquid into an elastic glass. The micron-sized bead forms a harmonic
oscillator. By monitoring the equal-time fluctuations of the tracer, at two
different laser powers, we determine the temperature of the oscillator,
. In the ergodic liquid the temperatures of the oscillator and
its environment are equal while, in contrast, in a nonequilibrium glassy phase
we find that substantially exceeds the bath temperature.Comment: 4 pages (minor changes, accepted Phys. Rev. Lett.
Conversion Efficiencies of Heteronuclear Feshbach Molecules
We study the conversion efficiency of heteronuclear Feshbach molecules in
population imbalanced atomic gases formed by ramping the magnetic field
adiabatically. We extend the recent work [J. E. Williams et al., New J. Phys.,
8, 150 (2006)] on the theory of Feshbach molecule formations to various
combinations of quantum statistics of each atomic component. A simple
calculation for a harmonically trapped ideal gas is in good agreement with the
recent experiment [S. B. Papp and C. E. Wieman, Phys. Rev. Lett., 97, 180404
(2006)] without any fitting parameters. We also give the conversion efficiency
as an explicit function of initial peak phase space density of the majority
species for population imbalanced gases. In the low-density region where
Bose-Einstein condensation does not appear, the conversion efficiency is a
monotonic function of the initial peak phase space density, but independent of
statistics of a minority component. The quantum statistics of majority atoms
has a significant effect on the conversion efficiency. In addition,
Bose-Einstein condensation of an atomic component is the key element
determining the maximum conversion efficiency.Comment: 46 pages, 32 figure
Phase separation dynamics in colloid-polymer mixtures: the effect of interaction range
Colloid-polymer mixtures may undergo either fluid-fluid phase separation or
gelation. This depends on the depth of the quench (polymer concentration) and
polymer-colloid size ratio. We present a real-space study of dynamics in phase
separating colloid-polymer mixtures with medium- to long-range attractions
(polymer-colloid size ratio q_R=0.45-0.89, with the aim of understanding the
mechanism of gelation as the range of the attraction is changed. In contrast to
previous studies of short-range attractive systems, where gelation occurs
shortly after crossing the equilibrium phase boundary, we find a substantial
region of fluid-fluid phase separation. On deeper quenches the system undergoes
a continuous crossover to gel formation. We identify two regimes, `classical'
phase separation, where single particle relaxation is faster than the dynamics
of phase separation, and `viscoelastic' phase separation, where demixing is
slowed down appreciably due to slow dynamics in the colloid-rich phase.
Particles at the surface of the strands of the network exhibit significantly
greater mobility than those buried inside the gel strand which presents a
method for coarsening.Comment: 8 page
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