169 research outputs found
Confinement of knotted polymers in a slit
We investigate the effect of knot type on the properties of a ring polymer
confined to a slit. For relatively wide slits, the more complex the knot, the
more the force exerted by the polymer on the walls is decreased compared to an
unknotted polymer of the same length. For more narrow slits the opposite is
true. The crossover between these two regimes is, to first order, at smaller
slit width for more complex knots. However, knot topology can affect these
trends in subtle ways. Besides the force exerted by the polymers, we also study
other quantities such as the monomer-density distribution across the slit and
the anisotropic radius of gyration.Comment: 9 pages, 6 figures, submitted for publicatio
Effects of awareness that food intake is being measured by a universal eating monitor on the consumption of a pasta lunch and a cookie snack in healthy female volunteers
To date, there have been no studies that have explicitly examined the effect of awareness on the consumption of food from a Universal Eating Monitor (UEM - hidden balance interfaced to a computer which covertly records eating behaviour). We tested whether awareness of a UEM affected consumption of a pasta lunch and a cookie snack. 39 female participants were randomly assigned to either an aware or unaware condition. After being informed of the presence of the UEM (aware) or not being told about its presence (unaware), participants consumed ad-libitum a pasta lunch from the UEM followed by a cookie snack. Awareness of the UEM did not significantly affect the amount of pasta or cookies eaten. However, awareness significantly reduced the rate of cookie consumption. These results suggest that awareness of being monitored by the UEM has no effect on the consumption of a pasta meal, but does influence the consumption of a cookie snack in the absence of hunger. Hence, energy dense snack foods consumed after a meal may be more susceptible to awareness of monitoring than staple food items
Bacteria solve the problem of crowding by moving slowly
Bacteria commonly live attached to surfaces in dense collectives containing billions of cells1. While it is known that motility allows these groups to expand en masse into new territory2,3,4,5, how bacteria collectively move across surfaces under such tightly packed conditions remains poorly understood. Here we combine experiments, cell tracking and individual-based modelling to study the pathogen Pseudomonas aeruginosa as it collectively migrates across surfaces using grappling-hook-like pili3,6,7. We show that the fast-moving cells of a hyperpilated mutant are overtaken and outcompeted by the slower-moving wild type at high cell densities. Using theory developed to study liquid crystals8,9,10,11,12,13, we demonstrate that this effect is mediated by the physics of topological defects, points where cells with different orientations meet one another. Our analyses reveal that when defects with topological charge +1/2 collide with one another, the fast-moving mutant cells rotate to point vertically and become trapped. By moving more slowly, wild-type cells avoid this trapping mechanism and generate collective behaviour that results in faster migration. In this way, the physics of liquid crystals explains how slow bacteria can outcompete faster cells in the race for new territory
Equilibrium and dynamical properties of the ANNNI chain at the multiphase point
We study the equilibrium and dynamical properties of the ANNNI (axial
next-nearest-neighbor Ising) chain at the multiphase point. An interesting
property of the system is the macroscopic degeneracy of the ground state
leading to finite zero-temperature entropy. In our equilibrium study we
consider the effect of softening the spins. We show that the degeneracy of the
ground state is lifted and there is a qualitative change in the low temperature
behaviour of the system with a well defined low temperature peak of the
specific heat that carries the thermodynamic ``weight'' of the ground state
entropy. In our study of the dynamical properties, the stochastic Kawasaki
dynamics is considered. The Fokker-Planck operator for the process corresponds
to a quantum spin Hamiltonian similar to the Heisenberg ferromagnet but with
constraints on allowed states. This leads to a number of differences in its
properties which are obtained through exact numerical diagonalization,
simulations and by obtaining various analytic bounds.Comment: 9 pages, RevTex, 6 figures (To appear in Phys. Rev. E
Thermodynamic Curvature of the BTZ Black Hole
Some thermodynamic properties of the Ba\~nados-Teitelboim-Zanelli (BTZ) black
hole are studied to get the effective dimension of its corresponding
statistical model. For this purpose, we make use of the geometrical approach to
the thermodynamics: Considering the black hole as a thermodynamic system with
two thermodynamic variables (the mass and the angular momemtum ), we
obtain two-dimensional Riemannian thermodynamic geometry described by positive
definite Ruppeiner metric. From the thermodynamic curvature we find that the
extremal limit is the critical point. The effective spatial dimension of the
statistical system corresponding to the near-extremal BTZ black holes is one.
Far from the extremal point, the effective dimension becomes less than one,
which leads to one possible speculation on the underlying structure for the
corresponding statistical model.Comment: 19 pages, LaTeX with revtex macro, 4 figures in eps file
Simple Fluids with Complex Phase Behavior
We find that a system of particles interacting through a simple isotropic
potential with a softened core is able to exhibit a rich phase behavior
including: a liquid-liquid phase transition in the supercooled phase, as has
been suggested for water; a gas-liquid-liquid triple point; a freezing line
with anomalous reentrant behavior. The essential ingredient leading to these
features resides in that the potential investigated gives origin to two
effective core radii.Comment: 7 pages including 3 eps figures + 1 jpeg figur
Surface critical exponents at a uniaxial Lifshitz point
Using Monte Carlo techniques, the surface critical behaviour of
three-dimensional semi-infinite ANNNI models with different surface
orientations with respect to the axis of competing interactions is
investigated. Special attention is thereby paid to the surface criticality at
the bulk uniaxial Lifshitz point encountered in this model. The presented Monte
Carlo results show that the mean-field description of semi-infinite ANNNI
models is qualitatively correct. Lifshitz point surface critical exponents at
the ordinary transition are found to depend on the surface orientation. At the
special transition point, however, no clear dependency of the critical
exponents on the surface orientation is revealed. The values of the surface
critical exponents presented in this study are the first estimates available
beyond mean-field theory.Comment: 10 pages, 7 figures include
Phase separating binary fluids under oscillatory shear
We apply lattice Boltzmann methods to study the segregation of binary fluid
mixtures under oscillatory shear flow in two dimensions. The algorithm allows
to simulate systems whose dynamics is described by the Navier-Stokes and the
convection-diffusion equations. The interplay between several time scales
produces a rich and complex phenomenology. We investigate the effects of
different oscillation frequencies and viscosities on the morphology of the
phase separating domains. We find that at high frequencies the evolution is
almost isotropic with growth exponents 2/3 and 1/3 in the inertial (low
viscosity) and diffusive (high viscosity) regimes, respectively. When the
period of the applied shear flow becomes of the same order of the relaxation
time of the shear velocity profile, anisotropic effects are clearly
observable. In correspondence with non-linear patterns for the velocity
profiles, we find configurations where lamellar order close to the walls
coexists with isotropic domains in the middle of the system. For particular
values of frequency and viscosity it can also happen that the convective
effects induced by the oscillations cause an interruption or a slowing of the
segregation process, as found in some experiments. Finally, at very low
frequencies, the morphology of domains is characterized by lamellar order
everywhere in the system resembling what happens in the case with steady shear.Comment: 1 table and 12 figures in .gif forma
Electron-induced neutron knockout from He
The differential cross section for electron-induced neutron knockout in the reaction 4He(e,e′n)3He has been measured for the first time with a statistical accuracy of 11%. The experiment was performed in quasielastic kinematics at a momentum transfer of 300 MeV/c and in the missing-momentum range of 25–70 MeV/c. The comparison of the data with theoretical calculations shows an impressive increase of the cross section resulting from final state interaction effects. Specifically , the p−n charge-exchange process dominates the cross section in this kinematical regime. (APS
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