1,335 research outputs found
Brownian Dynamics of a Sphere Between Parallel Walls
We describe direct imaging measurements of a colloidal sphere's diffusion
between two parallel surfaces. The dynamics of this deceptively simple
hydrodynamically coupled system have proved difficult to analyze. Comparison
with approximate formulations of a confined sphere's hydrodynamic mobility
reveals good agreement with both a leading-order superposition approximation as
well as a more general all-images stokeslet analysis.Comment: 4 pages, 3 figures, REVTeX with PostScript figure
Multidimensional optical fractionation with holographic verification
The trajectories of colloidal particles driven through a periodic potential
energy landscape can become kinetically locked in to directions dictated by the
landscape's symmetries. When the landscape is realized with forces exerted by a
structured light field, the path a given particle follows has been predicted to
depend exquisitely sensitively on such properties as the particle's size and
refractive index These predictions, however, have not been tested
experimentally. Here, we describe measurements of colloidal silica spheres'
transport through arrays of holographic optical traps that use holographic
video microscopy to track individual spheres' motions in three dimensions and
simultaneously to measure each sphere's radius and refractive index with
part-per-thousand resolution. These measurements confirm previously untested
predictions for the threshold of kinetically locked-in transport, and
demonstrate the ability of optical fractionation to sort colloidal spheres with
part-per-thousand resolution on multiple characteristics simultaneously.Comment: 4 pages, 2 figures. Accepted for publication in Physical Review
Letter
Giant Colloidal Diffusivity on Corrugated Optical Vortices
A single colloidal sphere circulating around a periodically modulated optical
vortex trap can enter a dynamical state in which it intermittently alternates
between freely running around the ring-like optical vortex and becoming trapped
in local potential energy minima. Velocity fluctuations in this randomly
switching state still are characterized by a linear Einstein-like diffusion
law, but with an effective diffusion coefficient that is enhanced by more than
two orders of magnitude.Comment: 4 pages, 4 figure
Colloidal Electrostatic Interactions Near a Conducting Surface
Charge-stabilized colloidal spheres dispersed in deionized water are supposed
to repel each other. Instead, artifact-corrected video microscopy measurements
reveal an anomalous long-ranged like-charge attraction in the interparticle
pair potential when the spheres are confined to a layer by even a single
charged glass surface. These attractions can be masked by electrostatic
repulsions at low ionic strengths. Coating the bounding surfaces with a
conducting gold layer suppresses the attraction. These observations suggest a
possible mechanism for confinement-induced attractions.Comment: 4 pages, 2 figure
Colloidal transport through optical tweezer arrays
Viscously damped particles driven past an evenly spaced array of potential
energy wells or barriers may become kinetically locked in to the array, or else
may escape from the array. The transition between locked-in and free-running
states has been predicted to depend sensitively on the ratio between the
particles' size and the separation between wells. This prediction is confirmed
by measurements on monodisperse colloidal spheres driven through arrays of
holographic optical traps.Comment: 4 pages, 4 figure
Colloidal hydrodynamic coupling in concentric optical vortices
Optical vortex traps created from helical modes of light can drive
fluid-borne colloidal particles in circular trajectories. Concentric
circulating rings of particles formed by coaxial optical vortices form a
microscopic Couette cell, in which the amount of hydrodynamic drag experienced
by the spheres depends on the relative sense of the rings' circulation.
Tracking the particles' motions makes possible measurements of the hydrodynamic
coupling between the circular particle trains and addresses recently proposed
hydrodynamic instabilities for collective colloidal motions on optical
vortices.Comment: 7 pages, 2 figures, submitted to Europhysics Letter
The Size of the Narrow-Line Emitting Region in the Seyfert 1 Galaxy NGC 5548 from Emission-Line Variability
The narrow [O III] 4959, 5007 emission-line fluxes in the spectrum of the
well-studied Seyfert 1 galaxy NGC 5548 are shown to vary with time. From this
we show that the narrow line-emitting region has a radius of only 1-3 pc and is
denser (n ~ 10^5 cm^{-3}) than previously supposed. The [O III] line width is
consistent with virial motions at this radius given previous determinations of
the black hole mass.Since the [O III] emission-line flux is usually assumed to
be constant and is therefore used to calibrate spectroscopic monitoring data,
the variability has ramifications for the long-term secular variations of
continuum and emission-line fluxes, though it has no effect on shorter-term
reverberation studies. We present corrected optical continuum and broad Hbeta
emission-line light curves for the period 1988 to 2008.Comment: 11 pages, 5 figures, 6 tables. Accepted for publication in Ap
Hydrodynamic Coupling of Two Brownian Spheres to a Planar Surface
We describe direct imaging measurements of the collective and relative
diffusion of two colloidal spheres near a flat plate. The bounding surface
modifies the spheres' dynamics, even at separations of tens of radii. This
behavior is captured by a stokeslet analysis of fluid flow driven by the
spheres' and wall's no-slip boundary conditions. In particular, this analysis
reveals surprising asymmetry in the normal modes for pair diffusion near a flat
surface.Comment: 4 pages, 4 figure
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