"Water-free" computer model for fluid bilayer membranes

We use a simple and efficient computer model to investigate the physical properties of bilayer membranes. The amphiphilic molecules are modeled as short rigid trimers with finite range pair interactions between them. The pair potentials have been designed to mimic the hydrophobic interactions, and to allow the simulation of the membranes without the embedding solvent as if the membrane is in vacuum. We find that upon decreasing the area density of the molecules the membrane undergoes a solid-fluid phase transition, where in the fluid phase the molecules can diffuse within the membrane plane. The surface tension and the bending modulus of the fluid membranes are extracted from the analysis of the spectrum of thermal undulations. At low area densities we observe the formation of pores in the membrane through which molecules can diffuse from one layer to the other. The appearance of the pores is explained using a simple model relating it to the area dependence of the free energy.Comment: 12 pages, 8 figures, to appear in J. Chem. Phy

Entropic Elasticity at the Sol-Gel Transition

The sol-gel transition is studied in two purely entropic models consisting of hard spheres in continuous three-dimensional space, with a fraction $p$ of nearest neighbor spheres tethered by inextensible bonds. When all the tethers are present ($p=1$) the two systems have connectivities of simple cubic and face-centered cubic lattices. For all $p$ above the percolation threshold $p_c$, the elasticity has a cubic symmetry characterized by two distinct shear moduli. When $p$ approaches $p_c$, both shear moduli decay as $(p-p_c)^f$, where $f\simeq 2$ for each type of the connectivity. This result is similar to the behavior of the conductivity in random resistor networks, and is consistent with many experimental studies of gel elasticity. The difference between the shear moduli that measures the deviation from isotropy decays as $(p-p_c)^h$, with $h\simeq 4$.Comment: 12 pages, 3 eps figures, RevTe