Abstract:
In the past, a great deal of attention has been drawn to thermal driven denaturation processes. In recent years, however, the discovery of stress-induced denaturation, observed at the one-molecule level, has revealed new insights into the complex phenomena involved in the thermo-mechanics of DNA function. Understanding the effect of local pressure variations in DNA stability is thus an appealing topic. Such processes as cellular stress, dehydration, and changes in the ionic strength of the medium could explain local pressure changes that will affect the molecular mechanics of DNA and hence its stability. In this work, a theory that accounts for hysteresis in pressure-driven DNA denaturation is proposed. We here combine an irreversible thermodynamic approach with an equation of state based on the Poisson-Boltzmann cell model. The latter one provides a good description of the osmotic pressure over a wide range of DNA concentrations. The resulting theoretical framework predicts, in general, the process of denaturation and, in particular, hysteresis curves for a DNA sequence in terms of system parameters such as salt concentration, density of DNA molecules and temperature in addition to structural and configurational states of DNA. Furthermore, this formalism can be naturally extended to more complex situations, for example, in cases where the host medium is made up of asymmetric salts or in the description of the (helical-like) charge distribution along the DNA molecule. Moreover, since this study incorporates the effect of pressure through a thermodynamic analysis, much of what is known from temperature-driven experiments will shed light on the pressure-induced melting issue.

Abstract:
The so-called extended law of corresponding states, as proposed by Noro and Frenkel [J. Chem. Phys. 113, 2941 (2000)], involves a mapping of the phase behaviors of systems with short-range attractive interactions. While it has already extensively been applied to various model potentials, here we test its applicability to protein solutions with their complex interactions. We successfully map their experimentally determined metastable gas--liquid binodals, as available in the literature, to the binodals of short-range square-well fluids, as determined by previous as well as new Monte Carlo simulations. This is achieved by representing the binodals as a function of the temperature scaled with the critical temperature (or as a function of the reduced second virial coefficient) and the concentration scaled by the cube of an effective particle diameter, where the scalings take into account the attractive and repulsive contributions to the interaction potential, respectively. The scaled binodals of the protein solutions coincide with simulation data of the adhesive hard-sphere fluid. Furthermore, once the repulsive contributions are taken into account by the effective particle diameter, the temperature dependence of the reduced second virial coefficients follows a master curve that corresponds to a linear temperature dependence of the depth of the square-well potential. We moreover demonstrate that, based on this approach and cloud-point measurements only, second virial coefficients can be estimated, which we show to agree with values determined by light scattering or by DLVO-based calculations.

Abstract:
One major goal in condensed matter physics is identifying the physical mechanisms that lead to arrested states of matter, especially gels and glasses. The complex nature and microscopic details of each particular system are relevant. However, from both scientific and technological viewpoints, a general, consistent and unified definition is of paramount importance. Through Monte Carlo computer simulations of states identified in experiments, we demonstrate that adhesive hard-sphere dispersions are the result of rigidity percolation with average number of bonds, $$, equals to 2.4. This corresponds to an established mechanism leading to phase transitions in network-forming materials. Our findings connect the concept of critical gel formation in colloidal suspensions with short-range attractive interactions to the universal concept of rigidity percolation. Furthermore, the bond, angular and local distributions along the gelation line are explicitly studied in order to determine the topology of the structure of the critical gel state.

Abstract:
Brownian motion of free particles on curved surfaces is studied by means of the Langevin equation written in Riemann normal coordinates. In the diffusive regime we find the same physical behavior as the one described by the diffusion equation on curved manifolds [J. Stat. Mech. (2010) P08006]. Therefore, we use the latter in order to analytically investigate the whole diffusive dynamics in compact geometries, namely, the circle and the sphere. Our findings are corroborated by means of Brownian dynamics computer simulations based on a heuristic adaptation of the Ermak-McCammon algorithm to the Langevin equation along the curves, as well as on the standard algorithm, but for particles subjected to an external harmonic potential, deep and narrow, that possesses a "Mexican hat" shape, whose minima define the desired surface. The short-time diffusive dynamics is found to occur on the tangential plane. Besides, at long times and compact geometries, the mean-square displacement moves towards a saturation value given only by the geometrical properties of the surface.

Abstract:
We report on the short-time dynamics in colloidal mixtures made up of monomers and dimers highly confined between two glass-plates. At low concentrations, the experimental measurements of colloidal motion agree well with the solution of the Navier-Stokes equation at low Reynolds numbers, which takes into account the increase of the drag force on each particle due to wall-particle hydrodynamic forces. We find that the ratio of the short-time diffusion coefficients of the monomer and that of the center of mass of the dimer remains independent of both the total packing fraction and the dimer molar fraction up to concentrations near to the crystallization transition. The same physical scenario is observed for the ratio between the parallel and perpendicular components of the short-time diffusion coefficients of the dimer. This dynamical behavior is corroborated by means of Molecular Dynamics computer simulations that explicitly include the particle-particle hydrodynamic forces induced by the solvent. Thus, our results point out toward that the effects of the particle-particle hydrodynamic interactions on the diffusion coefficients are identical and, thus, factorable in both species.

Abstract:
a model to study the structural and thermodynamic properties of hard-colloidal particles in contact with fluctuating membranes by means of monte carlo simulation is proposed. to test the accuracy of our model, we compare the density profile of a system composed of non-interacting point-like particles with the analytical expression derived by bickel et al. [phys. rev. e 70 (2004) 051404]. this model is applicable to colloids with finite size and it can easily be extended to binary systems or systems with long-range (coulomb-like) interactions.

Abstract:
A model to calculate the magnetisation of deoxyhemoglobin of human blood by means of Brownian dynamics simulations is presented. We consider a system made up of dipolar magnetic spheres, which can interact but do not overlap. Particles are exposed to external magnetic fields to compute the magnetisation curve, which exhibit a Langevin-like behaviour. The magnetic susceptibility of the erythrocytes and completely deoxygenated whole blood are Xp = 1.61 x 10-6(SI) and XWB = -4.46 x 10-6(SI), respectively, which are in good agreement to experimental data and theoretical calculations. Moreover, we also compute the paramagnetic component of the susceptibility of erythrocytes that in our simulations normal blood from beta thalassemia major samples could be differentiated.

Abstract:
Se propone un modelo que utiliza simulaci′on de Monte Carlo para estudiar las propiedades estructurales y termodin′amicas de esferas duras coloidales en contacto con membranas fluctuantes. Para verificar la precisi ′on de nuestro modelo, comparamos el perfil de densidad de un sistema compuesto por part′ culas puntuales no interactuantes con la expresi′on anal′ tica derivada por Bickel et al. [Phys. Rev. E 70 (2004) 051404]. Este modelo es aplicable al caso de coloides con tama no finito y puede ser f′acilmente extendido a sistemas binarios o sistemas con interacciones de largo alcance (tipo Coulomb).

Abstract:
Combining cell and Jellium model mean-field approaches, Monte Carlo together with integral equation techniques, and finally more demanding many-colloid mean-field computations, we investigate the thermodynamic behavior, pressure and compressibility of highly charged colloidal dispersions, and at a more microscopic level, the force distribution acting on the colloids. The Kirkwood-Buff identity provides a useful probe to challenge the self-consistency of an approximate effective screened Coulomb (Yukawa) potential between colloids. Two effective parameter models are put to the test: cell against renormalized Jellium models.

Abstract:
objective 17 cases of dengue were reported from the town of el playón (7°28'15"-n/73°12'00"-w/altitude 510 masl) during epidemiological study weeks 5-6 and 12-13/2010. the santander department's health secretariat sought outbreak investigation and community knowledge-attitudes-practice (kap) assessment on 16-4-2010 concerning a dengue fever outbreak in the town after an epidemiological surveillance committee had met; this was to strengthen local action regarding aedes aegypti vector promotion-prevention-control. methods this was a descriptive outbreak investigation study using an active search for cases having clinical manifestations of dengue according to protocol definitions, an entomological survey and a kap survey of 47 people selected by simple-random-sampling. data was systematised and analysed using epi-info (3.5.1) with measures of central tendency-male ratio, in-house aegypti (ia), water-storage (wi) and breatau (bi) indexes. results 4,774 clinical records were examined. there were 67 records compatible with dengue, 24 cases were notified (75 % town-25 % department), 15 (63 %) being women and 9 (38 %) men. 20 (83 %) cases occurred in the urban area and 4 (17 %) in rural areas. attack rate/cumulated incidence: initial=0.3 % (17/6 303), final=0.4 % (24/6 303), male ratio=1.5. 47 houses and 142 water-storage items proved positive: low water-stores=8.7 %(4/46), rubber tyres=8.3 %(1/12), others=4.4 %(2/45), plants=2.9 %(1/34). ia=5 %, wi=6%, bi=17%, taxonomy confirmed=15 %(7/47). 47 kap, 36 (77 %) women, 11 (23 %) men; knowledge on disease, symptoms and signs 44 (94 %), eliminating larvae deposits 32(68 %), asked about dengue 3(6 %), informed about responsibilities related to control 25(53 %). conclusions: the local community knows about risk factors and is aware of its responsibility regarding vector control. community participation should be promoted using the ecosystem as a strategy for mosquito and dengue transmission control.