839 research outputs found
Maximum or Minimum Differentiation? An Empirical Investigation into the Location of Firms
We empirically test some implications from location theory using the location of Los Angeles area gasoline stations in physical space and in the space of product attributes. We consider the effect of demand patterns, entry costs, and several proxies for competition -- the total number of stations, the proportion of independent stations, and the proportion of same-brand stations in a market -- on the tendency for a gasoline station to be physically located more or less closely to its competitors. Using an estimation procedure that controls for spatial correlation and controlling for market characteristics as well as non- spatial product attributes, we find that firms locate their stations in an attempt to spatially differentiate their product as general market competition increases. In other words, the incentive to differentiate in order to soften price competition dominates the incentive to cluster locations to attract consumers from rivals. We also find that spatial differentiation increases as stations become more differentiated in other station characteristics.product differentiation, spatial theory, location theory, retail gasoline
Global cross-over dynamics of single semiflexible polymers
We present a mean-field dynamical theory for single semiflexible polymers
which can precisely capture, without fitting parameters, recent fluorescence
correlation spectroscopy results on single monomer kinetics of DNA strands in
solution. Our approach works globally, covering three decades of strand length
and five decades of time: it includes the complex cross-overs occurring between
stiffness-dominated and flexible bending modes, along with larger-scale
rotational and center-of-mass motion. The accuracy of the theory stems in part
from long-range hydrodynamic coupling between the monomers, which makes a
mean-field description more realistic. Its validity extends even to short,
stiff fragments, where we also test the theory through Brownian hydrodynamics
simulations.Comment: 6 pages, 5 figures; updated with minor changes to reflect published
versio
Interfaces and Grain Boundaries of Lamellar Phases
Interfaces between lamellar and disordered phases, and grain boundaries
within lamellar phases, are investigated employing a simple Landau free energy
functional. The former are examined using analytic, approximate methods in the
weak segregation limit, leading to density profiles which can extend over many
wavelengths of the lamellar phase. The latter are studied numerically and
exactly. We find a change from smooth chevron configurations typical of small
tilt angles to distorted omega configurations at large tilt angles in agreement
with experiment.Comment: 9 pages, 6 figures 9 pages, 6 figure
Discrete elastic model for stretching-induced flagellar polymorphs
Force-induced reversible transformations between coiled and normal polymorphs
of bacterial flagella have been observed in recent optical-tweezer experiment.
We introduce a discrete elastic rod model with two competing helical states
governed by a fluctuating spin-like variable that represents the underlying
conformational states of flagellin monomers. Using hybrid Brownian dynamics
Monte-Carlo simulations, we show that a helix undergoes shape transitions
dominated by domain wall nucleation and motion in response to externally
applied uniaxial tension. A scaling argument for the critical force is
presented in good agreement with experimental and simulation results.
Stretching rate-dependent elasticity including a buckling instability are
found, also consistent with the experiment
Counterions at Charged Cylinders: Criticality and universality beyond mean-field
The counterion-condensation transition at charged cylinders is studied using
Monte-Carlo simulation methods. Employing logarithmically rescaled radial
coordinates, large system sizes are tractable and the critical behavior is
determined by a combined finite-size and finite-ion-number analysis. Critical
counterion localization exponents are introduced and found to be in accord with
mean-field theory both in 2 and 3 dimensions. In 3D the heat capacity shows a
universal jump at the transition, while in 2D, it consists of discrete peaks
where single counterions successively condense.Comment: 4 pages, 3 figures; submitted to Phys. Rev. Lett. (2005
Water-like hierarchy of anomalies in a continuous spherical shouldered potential
We investigate by molecular dynamics simulations a continuous isotropic
core-softened potential with attractive well in three dimensions, introduced by
Franzese [cond-mat/0703681, to appear on Journal of Molecular Liquids], that
displays liquid-liquid coexistence with a critical point and water-like density
anomaly. Here we find diffusion and structural anomalies. These anomalies occur
with the same hierarchy that characterizes water. Yet our analysis shows
differences with respect to the water case. Therefore, many of the anomalous
features of water could be present in isotropic systems with soft-core
attractive potentials, such as colloids or liquid metals, consistent with
recent experiments showing polyamorphism in metallic glasses.Comment: 27 pages, 9 figures. to appear in J. Chem. Phy
Nonlinear fractional waves at elastic interfaces
We derive the nonlinear fractional surface wave equation that governs compression waves at an elastic interface that is coupled to a viscous bulk medium. The fractional character of the differential equation comes from the fact that the effective thickness of the bulk layer that is coupled to the interface is frequency dependent. The nonlinearity arises from the nonlinear dependence of the interface compressibility on the local compression, which is obtained from experimental measurements and reflects a phase transition at the interface. Numerical solutions of our nonlinear fractional theory reproduce several experimental key features of surface waves in phospholipid monolayers at the air-water interface without freely adjustable fitting parameters. In particular, the propagation distance of the surface wave abruptly increases at a threshold excitation amplitude. The wave velocity is found to be of the order of 40 cm/s in both experiments and theory and slightly increases as a function of the excitation amplitude. Nonlinear acoustic switching effects in membranes are thus shown to arise purely based on intrinsic membrane properties, namely, the presence of compressibility nonlinearities that accompany phase transitions at the interface
Thermodynamic and dynamic anomalies for a three dimensional isotropic core-softened potential
Using molecular dynamics simulations and integral equations (Rogers-Young,
Percus-Yevick and hypernetted chain closures) we investigate the thermodynamic
of particles interacting with continuous core-softened intermolecular
potential. Dynamic properties are also analyzed by the simulations. We show
that, for a chosen shape of the potential, the density, at constant pressure,
has a maximum for a certain temperature. The line of temperatures of maximum
density (TMD) was determined in the pressure-temperature phase diagram.
Similarly the diffusion constant at a constant temperature, , has a maximum
at a density and a minimum at a density .
In the pressure-temperature phase-diagram the line of extrema in diffusivity is
outside of TMD line. Although in this interparticle potential lacks
directionality, this is the same behavior observed in SPC/E water.Comment: 16 page
Electrostatic colloid-membrane complexation
We investigate numerically and on the scaling level the adsorption of a
charged colloid on an oppositely charged flexible membrane. We show that the
long ranged character of the electrostatic interaction leads to a wrapping
reentrance of the complex as the salt concentration is varied. The membrane
wrapping depends on the size of the colloid and on the salt concentration and
only for intermediate salt concentration and colloid sizes we find full
wrapping. From the scaling model we derive simple relations for the phase
boundaries between the different states of the complex, which agree well with
the numerical minimization of the free energy.Comment: 7 page, 11 figure
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