1,774 research outputs found
Nonconservative forcing and diffusion in refractive optical traps
Refractive optical trapping forces can be nonconservative in the vicinity of
a stable equilibrium point even in the absence of radiation pressure. We
discuss how nonconservative 3D force fields, in the vicinity of an equilibrium
point, reduce to circular forcing in a plane; a simple model of such forcing is
the refractive trapping of a sphere by a four rays. We discuss in general the
diffusion of an anisotropically trapped, circularly forced particle and obtain
its spectrum of motion. Equipartition of potential energy holds even though the
nonconservative flow does not follow equipotentials of the trap. We find that
the dissipated nonconservative power is proportional to temperature, providing
a mechanism for a runaway heating instability in traps.Comment: To appear in the Journal of the Optical Society of America
Determining Microscopic Viscoelasticity in Flexible and Semiflexible Polymer Networks from Thermal Fluctuations
We have developed a new technique to measure viscoelasticity in soft
materials such as polymer solutions, by monitoring thermal fluctuations of
embedded probe particles using laser interferometry in a microscope.
Interferometry allows us to obtain power spectra of fluctuating beads from 0.1
Hz to 20 kHz, and with sub-nanometer spatial resolution. Using linear response
theory, we determined the frequency-dependent loss and storage shear moduli up
to frequencies on the order of a kHz. Our technique measures local values of
the viscoelastic response, without actively straining the system, and is
especially suited to soft biopolymer networks. We studied semiflexible F-actin
solutions and, as a control, flexible polyacrylamide (PAAm) gels, the latter
close to their gelation threshold. With small particles, we could probe the
transition from macroscopic viscoelasticity to more complex microscopic
dynamics. In the macroscopic limit we find shear moduli at 0.1 Hz of G'=0.11
+/- 0.03 Pa and 0.17 +/- 0.07 Pa for 1 and 2 mg/ml actin solutions, close to
the onset of the elastic plateau, and scaling behavior consistent with G(omega)
as omega^(3/4) at higher frequencies. For polyacrylamide we measured plateau
moduli of 2.0, 24, 100 and 280 Pa for crosslinked gels of 2, 2.5, 3 and 5%
concentration (weight/volume) respectively, in agreement to within a factor of
two with values obtained from conventional rheology. We also found evidence for
scaling of G(omega) as \omega^(1/2), consistent with the predictions of the
Rouse model for flexible polymers.Comment: 16 pages, with 15 PostScript figures (to be published in
Macromolecules
Experimental study of the Fluctuation-Dissipation-Relation during an aging process
The validity of fluctuation dissipation relations in an aging system is
studied in a colloidal glass during the transition from a fluid-like to a
solid-like state. The evolution of the rheological and electrical properties is
analyzed in the range . It is found that at the beginning of the
transition the fluctuation dissipation relation is strongly violated in
electrical measurements. The amplitude and the persistence time of this
violation are decreasing functions of frequency. At the lowest frequencies of
the measuring range it persists for times which are about 5% of the time needed
to form the colloidal glass. This phenomenology is quite close to the recent
theoretical predictions done for the violation of the fluctuation dissipation
relation in glassy systems. In contrast in the rheological measurements no
violation of the fluctuation dissipation relation is observed. The reasons of
this large difference between the electrical and rheological measurements are
discussed.Comment: to be published on physica
Endothelial Progenitors Exist within the Kidney and Lung Mesenchyme
The renal endothelium has been debated as arising from resident hemangioblast precursors that transdifferentiate from the nephrogenic mesenchyme (vasculogenesis) and/or from invading vessels (angiogenesis). While the Foxd1-positive renal cortical stroma has been shown to differentiate into cells that support the vasculature in the kidney (including vascular smooth muscle and pericytes) it has not been considered as a source of endothelial cell progenitors. In addition, it is unclear if Foxd1-positive mesenchymal cells in other organs such as the lung have the potential to form endothelium. This study examines the potential for Foxd1-positive cells of the kidney and lung to give rise to endothelial progenitors. We utilized immunofluorescence (IF) and fluorescence-activated cell sorting (FACS) to co-label Foxd1-expressing cells (including permanently lineage-tagged cells) with endothelial markers in embryonic and postnatal mice. We also cultured FACsorted Foxd1-positive cells, performed in vitro endothelial cell tubulogenesis assays and examined for endocytosis of acetylated low-density lipoprotein (Ac-LDL), a functional assay for endothelial cells. Immunofluorescence and FACS revealed that a subset of Foxd1-positive cells from kidney and lung co-expressed endothelial cell markers throughout embryogenesis. In vitro, cultured embryonic Foxd1-positive cells were able to differentiate into tubular networks that expressed endothelial cell markers and were able to endocytose Ac-LDL. IF and FACS in both the kidney and lung revealed that lineage-tagged Foxd1-positive cells gave rise to a significant portion of the endothelium in postnatal mice. In the kidney, the stromal-derived cells gave rise to a portion of the peritubular capillary endothelium, but not of the glomerular or large vessel endothelium. These findings reveal the heterogeneity of endothelial cell lineages; moreover, Foxd1-positive mesenchymal cells of the developing kidney and lung are a source of endothelial progenitors that are likely critical to patterning the vasculature. © 2013 Sims-Lucas et al
Elementary simulation of tethered Brownian motion
We describe a simple numerical simulation, suitable for an undergraduate
project (or graduate problem set), of the Brownian motion of a particle in a
Hooke-law potential well. Understanding this physical situation is a practical
necessity in many experimental contexts, for instance in single molecule
biophysics; and its simulation helps the student to appreciate the dynamical
character of thermal equilibrium. We show that the simulation succeeds in
capturing behavior seen in experimental data on tethered particle motion.Comment: Submitted to American Journal of Physic
Locating the source of projectile fluid droplets
The ill-posed projectile problem of finding the source height from spattered
droplets of viscous fluid is a longstanding obstacle to accident reconstruction
and crime scene analysis. It is widely known how to infer the impact angle of
droplets on a surface from the elongation of their impact profiles. However,
the lack of velocity information makes finding the height of the origin from
the impact position and angle of individual drops not possible. From aggregate
statistics of the spatter and basic equations of projectile motion, we
introduce a reciprocal correlation plot that is effective when the polar launch
angle is concentrated in a narrow range. The vertical coordinate depends on the
orientation of the spattered surface, and equals the tangent of the impact
angle for a level surface. When the horizontal plot coordinate is twice the
reciprocal of the impact distance, we can infer the source height as the slope
of the data points in the reciprocal correlation plot. If the distribution of
launch angles is not narrow, failure of the method is evident in the lack of
linear correlation. We perform a number of experimental trials, as well as
numerical calculations and show that the height estimate is insensitive to
aerodynamic drag. Besides its possible relevance for crime investigation,
reciprocal-plot analysis of spatter may find application to volcanism and other
topics and is most immediately applicable for undergraduate science and
engineering students in the context of crime-scene analysis.Comment: To appear in the American Journal of Physics (ms 23338). Improved
readability and organization in this versio
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