21 research outputs found
Measurements of the instantaneous velocity difference and local velocity with a fiber-optic coupler
New optical arrangements with two single-mode input fibers and a fiber-optic
coupler are devised to measure the instantaneous velocity difference and local
velocity. The fibers and the coupler are polarization-preserving to guarantee a
high signal-to-noise ratio. When the two input fibers are used to collect the
scattered light with the same momentum transfer vector but from two spatially
separated regions in a flow, the obtained signals interfere when combined via
the fiber-optic coupler. The resultant light received by a photomultiplier tube
contains a cross-beat frequency proportional to the velocity difference between
the two measuring points. If the two input fibers are used to collect the
scattered light from a common scattering region but with two different momentum
transfer vectors, the resultant light then contains a self-beat frequency
proportional to the local velocity at the measuring point. The experiment shows
that both the cross-beat and self-beat signals are large and the standard laser
Doppler signal processor can be used to measure the velocity difference and
local velocity in real time. The new technique will have various applications
in the general area of fluid dynamics.Comment: Patent number: 67437 for associated information on the hardware, see
http://karman.phyast.pitt.edu/horvath
Pipe network model for scaling of dynamic interfaces in porous media
We present a numerical study on the dynamics of imbibition fronts in porous
media using a pipe network model. This model quantitatively reproduces the
anomalous scaling behavior found in imbibition experiments [Phys. Rev. E {\bf
52}, 5166 (1995)]. Using simple scaling arguments, we derive a new identity
among the scaling exponents in agreement with the experimental results.Comment: 13 pages, 3 figures, REVTeX, to appear in Phys. Rev. Let
Anomalous density dependence of static friction in sand
We measured experimentally the static friction force on the surface of
a glass rod immersed in dry sand. We observed that is extremely sensitive
to the closeness of packing of grains. A linear increase of the grain-density
yields to an exponentially increasing friction force. We also report on a novel
periodicity of during gradual pulling out of the rod. Our observations
demonstrate the central role of grain bridges and arches in the macroscopic
properties of granular packings.Comment: plain tex, 6 pages, to appear in Phys.Rev.
Proteins’ fold compactness alters disulfide-bond reducibility by 3 orders of magnitude: a comprehensive kinetic case study on the reduction of different size Trp-cage model proteins
We derived a novel approach to monitor disulfide bond
reduction in the vicinity of aromatic cluster(s) by using the near-UV
range (266–293 nm) of ECD spectra. By using combined NMR- and
ECD-spectroscopy we have determined the 3D-fold characteristics
and the associated reduction rate constants (k) of E19_SS – a highly
thermostable; disulfide bond reinforced 39-amino acid long
Exenatide mimetic – and its N-terminally truncated derivatives at
different experimental conditions. Single SS-bond reduction of the
E19_SS model (using 18-fold excess of TCEP, pH = 7, 37 °C) takes
hours, 20-30 times longer than expected, thus would not reach
completion applying the commonly used reduction protocols. We
found that structural, steric and electrostatic factors influence the
reduction rate, resulting in magnitude differences in reduction halftimes
(900 > t½ > 1 min) even for structurally similar, well-folded
derivatives of a small model-protein