936 research outputs found
Transfinite order dimension
We give two different transfinite extensions of the covering dimension based on the Borst's order of certain families of boundaries of basic open sets. We compare them and we study their main properties
Structure and dynamics of colloidal depletion gels: coincidence of transitions and heterogeneity
Transitions in structural heterogeneity of colloidal depletion gels formed
through short-range attractive interactions are correlated with their dynamical
arrest. The system is a density and refractive index matched suspension of 0.20
volume fraction poly(methyl methacyrlate) colloids with the non-adsorbing
depletant polystyrene added at a size ratio of depletant to colloid of 0.043.
As the strength of the short-range attractive interaction is increased,
clusters become increasingly structurally heterogeneous, as characterized by
number-density fluctuations, and dynamically immobilized, as characterized by
the single-particle mean-squared displacement. The number of free colloids in
the suspension also progressively declines. As an immobile cluster to gel
transition is traversed, structural heterogeneity abruptly decreases.
Simultaneously, the mean single-particle dynamics saturates at a localization
length on the order of the short-range attractive potential range. Both
immobile cluster and gel regimes show dynamical heterogeneity. Non-Gaussian
distributions of single particle displacements reveal enhanced populations of
dynamical trajectories localized on two different length scales. Similar
dependencies of number density fluctuations, free particle number and dynamical
length scales on the order of the range of short-range attraction suggests a
collective structural origin of dynamic heterogeneity in colloidal gels.Comment: 14 pages, 10 figure
Diffusivity and configurational entropy maxima in short range attractive colloids
We study tagged particle diffusion at large packing fractions, for a model of
particles interacting with a generalized Lennard-Jones 2n-n potential, with
large n. The resulting short-range potential mimics interactions in colloidal
systems. In agreement with previous calculations for short-range potential, we
observe a diffusivity maximum as a function of temperature. By studying the
temperature dependence of the configurational entropy -- which we evaluate with
two different methods -- we show that a configurational entropy maximum is
observed at a temperature close to that of the diffusivity maximum. Our
findings suggest a relationbetween dynamics and number of distinct states for
short-range potentials.Comment: 4 pages, 3 figures, submited to Physical Review Lette
Theory and simulation of gelation, arrest and yielding in attracting colloids
We present some recent theory and simulation results addressing the phenomena
of colloidal gelation at both high and low volume fractions, in the presence of
short-range attractive interactions. We discuss the ability of mode-coupling
theory and its adaptations to address situations with strong heterogeneity in
density and/or dynamics. We include a discussion of the effect of attractions
on the shear-thinning and yield behaviour under flow.Comment: 17 pages, 6 figure
Characterization of Pediococcus ethanolidurans CUPV141: A b-D-glucan- and Heteropolysaccharide-Producing Bacterium
Pediococcus ethanolidurans CUPV141 is an exopolysaccharide (EPS)-producing lactic acid bacterium, first isolated from Basque Country cider (Spain). Physicochemical analysis of the EPS synthesized by the bacterium revealed that CUPV141 produces mostly a homopolysaccharide (HoPS), characterized as a 2-substituted (1,3)-beta-D-glucan, together with a small quantity of a heteropolysaccharide (HePS) composed of glucose, galactose, glucosamine, and glycerol-3-phosphate, this being the first Pediococcus strain described to produce this kind of polymer. On the contrary, an isogenic strain CUPV141NR, generated by chemical mutagenesis of CUPV141, produced the HePS as the main extracellular polysaccharide and a barely detectable amount of 2-substituted (1,3)-beta-D-glucan. This HoPS is synthesized by the transmembrane GTF glycosyltransferase (GTF), encoded by the gtf gene, which has been previously reported to be located in the pPP2 plasmid of the Pediococcus parvulus 2.6 strain. Southern blot hybridization revealed that in CUPV141 the gtf gene is located in a plasmid designated as pPE3, whose molecular mass (34.4 kbp) is different from that of pPP2 (24.5 kbp). Analysis of the influence of the EPS on the ability of the producing bacteria to adhere to the eukaryotic Caco-2 cells revealed higher affinity for the human enterocytes of CUPV141NR compared to that of CUPV141. This result indicates that, in contrast to the 2.6 strain, the presence of the HoPS does not potentiate the binding ability of P. ethanolidurans. Moreover, it supports that the phosphate-containing bacterial HePS improved the interaction between P. ethanolidurans and the eukaryotic cells.This work was supported by a grant from the Department of Environment, Territorial Planification, Agriculture and Fishing and the project no. IT 866-13 from the Basque Government and by the Spanish Ministry of Science and Innovation (AGL 2015-65010-C3-1-R)
Dynamics in Colloidal Liquids near a Crossing of Glass- and Gel-Transition Lines
Within the mode-coupling theory for ideal glass-transitions, the mean-squared
displacement and the correlation function for density fluctuations are
evaluated for a colloidal liquid of particles interacting with a square-well
potential for states near the crossing of the line for transitions to a gel
with the line for transitions to a glass. It is demonstrated how the dynamics
is ruled by the interplay of the mechanisms of arrest due to hard-core
repulsion and due to attraction-induced bond formation as well as by a nearby
higher-order glass-transition singularity. Application of the universal
relaxation laws for the slow dynamics near glass-transition singularities
explains the qualitative features of the calculated time dependence of the
mean-squared displacement, which are in accord with the findings obtained in
molecular-dynamics simulation studies by Zaccarelli et. al [Phys. Rev. E 66,
041402 (2002)]. Correlation functions found by photon-correlation spectroscopy
in a micellar system by Mallamace et. al [Phys. Rev. Lett. 84, 5431 2000)] can
be interpreted qualitatively as a crossover from gel to glass dynamics.Comment: 13 pages, 12 figure
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