910 research outputs found
Optical characterization of marine phytoplankton assemblages within surface waters of the western Arctic Ocean.
An extensive data set of measurements within the Chukchi and Beaufort Seas is used to characterize the optical properties of seawater associated with different phytoplankton communities. Hierarchical cluster analysis of diagnostic pigment concentrations partitioned stations into four distinct surface phytoplankton communities based on taxonomic composition and average cell size. Concurrent optical measurements of spectral absorption and backscattering coefficients and remote-sensing reflectance were used to characterize the magnitudes and spectral shapes of seawater optical properties associated with each phytoplankton assemblage. The results demonstrate measurable differences among communities in the average spectral shapes of the phytoplankton absorption coefficient. Similar or smaller differences were also observed in the spectral shapes of nonphytoplankton absorption coefficients and the particulate backscattering coefficient. Phytoplankton on average, however, contributed only 25% or less to the total absorption coefficient of seawater. Our analyses indicate that the interplay between the magnitudes and relative contributions of all optically significant constituents generally dampens any influence of varying phytoplankton absorption spectral shapes on the total absorption coefficient, yet there is still a marked discrimination observed in the spectral shape of the ratio of the total backscattering to total absorption coefficient and remote-sensing reflectance among the phytoplankton assemblages. These spectral variations arise mainly from differences in the bio-optical environment in which specific communities were found, as opposed to differences in the spectral shapes of phytoplankton optical properties per se. These results suggest potential approaches for the development of algorithms to assess phytoplankton community composition from measurements of seawater optical properties in western Arctic waters
Non-steady-state extremely asymmetrical scattering of waves in periodic gratings
Extremely asymmetrical scattering (EAS) is a highly resonant type of Bragg
scattering with a strong resonant increase of the scattered wave amplitude
inside and outside the grating. EAS is realized when the scattered wave
propagates parallel to the grating boundaries. We present a rigorous algorithm
for the analysis of non-steady-state EAS, and investigate the relaxation of the
incident and scattered wave amplitudes to their steady-state values.
Non-steady-state EAS of bulk TE electromagnetic waves is analyzed in narrow and
wide, slanted, holographic gratings. Typical relaxation times are determined
and compared with previous rough estimations. Physical explanation of the
predicted effects is presented.Comment: 7 pages, 3 figures. This paper is freely available online at
http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-6-268 which includes
multimedia files not included in this preprint versio
Indeterminacy, Memory, and Motion in a Simple Granular Packing
We apply two theoretical and two numerical methods to the problem of a disk
placed in a groove and subjected to gravity and a torque. Methods assuming
rigid particles are indeterminate -- certain combinations of forces cannot be
calculated, but only constrained by inequalities. In methods assuming
deformable particles, these combinations of forces are determined by the
history of the packing. Thus indeterminacy in rigid particles becomes memory in
deformable ones. Furthermore, the torque needed to rotate the particle was
calculated. Two different paths to motion were identified. In the first,
contact forces change slowly, and the indeterminacy decreases continuously to
zero, and vanishes precisely at the onset of motion, and the torque needed to
rotate the disk is independent of method and packing history. In the second
way, this torque depends on method and on the history of the packing, and the
forces jump discontinuously at the onset of motion.Comment: 11 pages, 7 figures, submitted to Phys Rev
Genetic Correlations in Mutation Processes
We study the role of phylogenetic trees on correlations in mutation
processes. Generally, correlations decay exponentially with the generation
number. We find that two distinct regimes of behavior exist. For mutation rates
smaller than a critical rate, the underlying tree morphology is almost
irrelevant, while mutation rates higher than this critical rate lead to strong
tree-dependent correlations. We show analytically that identical critical
behavior underlies all multiple point correlations. This behavior generally
characterizes branching processes undergoing mutation.Comment: revtex, 8 pages, 2 fig
Force-induced desorption of a linear polymer chain adsorbed on an attractive surface
We consider a model of self-avoiding walk on a lattice with on-site repulsion
and an attraction for every vertex of the walk visited on the surface to study
force-induced desorption of a linear polymer chain adsorbed on an attractive
surface and use the exact enumeration technique for analyzing how the critical
force for desorption depends on the temperature. The curve
gives the boundary separating the adsorbed phase from the desorbed phase. Our
results show that in two dimensions where surface is a line the force
increases monotonically as temperature is lowered and becomes almost constant
at very low temperatures. In case of three-dimensions we, however, find
re-entrance, i. e. goes through a maximum as temperature is lowered.
The behaviour of the polymer chain at different values of temperature and force
is examined by calculating the probability distribution of the height from the
surface of the vertex at which external force is applied.Comment: Preprint 15 pages with 8figures and two tables. The file table-2d.ps
and table-3d.ps lists C_N(Ns,h) for given N with all possible Ns and h in two
and three dimension
Mechanical activation of vinculin binding to talin locks talin in an unfolded conformation
The force-dependent interaction between talin and vinculin plays a crucial role in the initiation and growth of focal adhesions. Here we use magnetic tweezers to characterise the mechano-sensitive compact N-terminal region of the talin rod, and show that the three helical bundles R1-R3 in this region unfold in three distinct steps consistent with the domains unfolding independently. Mechanical stretching of talin R1-R3 enhances its binding to vinculin and vinculin binding inhibits talin refolding after force is released. Mutations that stabilize R3 identify it as the initial mechano-sensing domain in talin, unfolding at ~5 pN, suggesting that 5 pN is the force threshold for vinculin binding and adhesion progression
Absorção de fosfatos mono e diamonico marcados com radiofosforo em presença e ausência de uréia pela parte aérea de mudas de abacaxizeiro (Ananas comosus (L.) Merr.)
The anisotropy of granular materials
The effect of the anisotropy on the elastoplastic response of two dimensional
packed samples of polygons is investigated here, using molecular dynamics
simulation. We show a correlation between fabric coefficients, characterizing
the anisotropy of the granular skeleton, and the anisotropy of the elastic
response. We also study the anisotropy induced by shearing on the subnetwork of
the sliding contacts. This anisotropy provides an explanation to some features
of the plastic deformation of granular media.Comment: Submitted to PR
Stretching of a polymer below the Theta point
The unfolding of a polymer below the point when pulled by an
external force is studied both in d=2 on the lattice and in off lattice.
A ground state analysis of finite length chains shows that the globule unfolds
via multiple steps, corresponding to transitions between different minima, in
both cases. In the infinite length limit, these intermediate minima have a
qualitative effect only in . The phase diagram in d=2 is determined using
transfer matrix techniques. Energy-entropy and renormalization group arguments
are given which predict a qualitatively correct phase diagram and a change of
the order of the transition from d=2 to d=3.Comment: 4 pages, 3 eps figure
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