On the free volume in nuclear multifragmentation

In many statistical multifragmentation models the volume available to the $N$ nonoverlapping fragments forming a given partition is a basic ingredient serving to the simplification of the density of states formula. One therefore needs accurate techniques for calculating this quantity. While the direct Monte-Carlo procedure consisting of randomly generating the fragments into the freeze-out volume and counting the events with no overlapped fragments is numerically affordable only for partitions with small $N$, the present paper proposes a Metropolis - type simulation which allows accurate evaluations of the free volume even for cases with large $N$. This procedure is used for calculating the available volume for various situations. Though globally this quantity has an exponential dependence on $N$, variations of orders of magnitude for partitions with the same $N$ may be identified. A parametrization based on the virial approximation adjusted with a calibration function, describing very well the variations of the free volume for different partitions having the same $N$ is proposed. This parametrization was successfully tested within the microcanonical multifragmentation model from [Al. H. Raduta and Ad. R. Raduta, Phys. Rev. C {\bf 55}, 1344 (1997); {\it ibid.}, {\bf 56}, 2059 (1997)]. Finally, it is proven that parametrizations of the free volume solely dependent on $N$ are rather inadequate for multifragmentation studies producing important deviations from the exact results.Comment: 20 pages, 9 figures, Nucl. Phys. A (in press

Free-volume kinetic models of granular matter

We show that the main dynamical features of granular media can be understood by means of simple models of fragile-glass forming liquid provided that gravity alone is taken into account. In such lattice-gas models of cohesionless and frictionless particles, the compaction and segregation phenomena appear as purely non-equilibrium effects unrelated to the Boltzmann-Gibbs measure which in this case is trivial. They provide a natural framework in which slow relaxation phenomena in granular and glassy systems can be explained in terms of a common microscopic mechanism given by a free-volume kinetic constraint.Comment: 4 pages, 6 figure

An investigation of microstructural characteristics of contact-lens polymers

The free volume and gas permeability in several contact lens specimens were measured as part of a Space Commercialization Program. Free volume was measured using positron lifetime spectroscopy, while permeability for O2, N2, CO2 gases was measured using mass spectrometry and polarography. Permeability for all gases increases with the mean free volume cell size in the test samples. As might be expected, the specimens with the highest free volume fraction also exhibit the lowest Rockwell Hardness Number. An interesting corollary is the finding that the presence of fluorine atoms in the lens chemical structure inhibits filling up of their free volume cells. This is expected to allow the lenses to breathe freely while in actual use

Model for the free-volume distributions of equilibrium fluids

We introduce and test via molecular simulation a simple model for predicting the manner in which interparticle interactions and thermodynamic conditions impact the single-particle free-volume distributions of equilibrium fluids. The model suggests a scaling relationship for the density-dependent behavior of the hard-sphere system. It also predicts how the second virial coefficients of fluids with short-range attractions affect their free-volume distributions.Comment: 7 pages, 5 figure

Impact of Cholesterol on Voids in Phospholipid Membranes

Free volume pockets or voids are important to many biological processes in cell membranes. Free volume fluctuations are a prerequisite for diffusion of lipids and other macromolecules in lipid bilayers. Permeation of small solutes across a membrane, as well as diffusion of solutes in the membrane interior are further examples of phenomena where voids and their properties play a central role. Cholesterol has been suggested to change the structure and function of membranes by altering their free volume properties. We study the effect of cholesterol on the properties of voids in dipalmitoylphosphatidylcholine (DPPC) bilayers by means of atomistic molecular dynamics simulations. We find that an increasing cholesterol concentration reduces the total amount of free volume in a bilayer. The effect of cholesterol on individual voids is most prominent in the region where the steroid ring structures of cholesterol molecules are located. Here a growing cholesterol content reduces the number of voids, completely removing voids of the size of a cholesterol molecule. The voids also become more elongated. The broad orientational distribution of voids observed in pure DPPC is, with a 30% molar concentration of cholesterol, replaced by a distribution where orientation along the bilayer normal is favored. Our results suggest that instead of being uniformly distributed to the whole bilayer, these effects are localized to the close vicinity of cholesterol molecules