6,256 research outputs found
A topographic mechanism for arcing of dryland vegetation bands
Banded patterns consisting of alternating bare soil and dense vegetation have
been observed in water-limited ecosystems across the globe, often appearing
along gently sloped terrain with the stripes aligned transverse to the
elevation gradient. In many cases these vegetation bands are arced, with field
observations suggesting a link between the orientation of arcing relative to
the grade and the curvature of the underlying terrain. We modify the water
transport in the Klausmeier model of water-biomass interactions, originally
posed on a uniform hillslope, to qualitatively capture the influence of terrain
curvature on the vegetation patterns. Numerical simulations of this modified
model indicate that the vegetation bands change arcing-direction from
convex-downslope when growing on top of a ridge to convex-upslope when growing
in a valley. This behavior is consistent with observations from remote sensing
data that we present here. Model simulations show further that whether bands
grow on ridges, valleys, or both depends on the precipitation level. A survey
of three banded vegetation sites, each with a different aridity level,
indicates qualitatively similar behavior.Comment: 26 pages, 13 figures, 2 table
A unified design space of synthetic stripe-forming networks
Synthetic biology is a promising tool to study the function and properties of gene regulatory networks. Gene circuits with predefined behaviours have been successfully built and modelled, but largely on a case-by-case basis. Here we go beyond individual networks and explore both computationally and synthetically the design space of possible dynamical mechanisms for 3-node stripe-forming networks. First, we computationally test every possible 3-node network for stripe formation in a morphogen gradient. We discover four different dynamical mechanisms to form a stripe and identify the minimal network of each group. Next, with the help of newly established engineering criteria we build these four networks synthetically and show that they indeed operate with four fundamentally distinct mechanisms. Finally, this close match between theory and experiment allows us to infer and subsequently build a 2-node network that represents the archetype of the explored design space
Competing Alignments of Nematic Liquid Crystals on Square Patterned Substrates
A theoretical analysis is presented of a nematic liquid crystal confined
between substrates pat- terned with squares that promote vertical and planar
alignment. Two approaches are used to eluci- date the behavior across a wide
range of length scales: Monte Carlo simulation of hard particles and
Frank-Oseen continuum theory. Both approaches predict bistable degenerate
azimuthal alignment in the bulk along the edges of the squares; the continuum
calculation additionally reveals the possi- bility of an anchoring transition
to diagonal alignment if the polar anchoring energy associated with the pattern
is sufficiently weak. Unlike the striped systems previously analyzed, the Monte
Carlo simulations suggest that there is no "bridging" transition for
sufficiently thin cells. The extent to which these geometrically patterned
systems resemble topographically patterned substrates, such as square wells, is
also discussed.Comment: 11 pages, 12 figure
Two-dimensional band structure in honeycomb metal-organic frameworks
Metal-organic frameworks (MOFs) are an important class of materials that
present intriguing opportunities in the fields of sensing, gas storage,
catalysis, and optoelectronics. Very recently, two-dimensional (2D) MOFs have
been proposed as a flexible material platform for realizing exotic quantum
phases including topological and anomalous quantum Hall insulators.
Experimentally, direct synthesis of 2D MOFs has been essentially confined to
metal substrates, where the interaction with the substrate masks the intrinsic
electronic properties of the MOF. Here, we demonstrate synthesis of 2D
honeycomb metal-organic frameworks on a weakly interacting epitaxial graphene
substrate. Using low-temperature scanning tunneling microscopy (STM) and atomic
force microscopy (AFM) complemented by density-functional theory (DFT)
calculations, we show the formation of 2D band structure in the MOF decoupled
from the substrate. These results open the experimental path towards MOF-based
designer quantum materials with complex, engineered electronic structures
Sequential establishment of stripe patterns in an expanding cell population
Periodic stripe patterns are ubiquitous in living organisms, yet the underlying developmental processes are complex and difficult to disentangle. We describe a synthetic genetic circuit that couples cell density and motility. This system enabled programmed Escherichia coli cells to form periodic stripes of high and low cell densities sequentially and autonomously. Theoretical and experimental analyses reveal that the spatial structure arises from a recurrent aggregation process at the front of the continuously expanding cell population. The number of stripes formed could be tuned by modulating the basal expression of a single gene. The results establish motility control as a simple route to establishing recurrent structures without requiring an extrinsic pacemaker.published_or_final_versio
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