Fluctuation Pressure of Biomembranes in Planar Confinement

The fluctuation pressure of a lipid-bilayer membrane is important for the stability of lamellar phases and the adhesion of membranes to surfaces. In contrast to many theoretical studies, which predict a decrease of the pressure with the cubed inverse distance between the membranes, Freund suggested very recently a linear inverse distance dependence [Proc. Natl. Acad. Sci. U.S.A. 110, 2047 (2013)]. We address this discrepancy by performing Monte Carlo simulations for a membrane model discretized on a square lattice and employ the wall theorem to evaluate the pressure for a single membrane between parallel walls. For distances that are small compared with the lattice constant, the pressure indeed depends on the inverse distance as predicted by Freund. For intermediate distances, the pressure depends on the cubed inverse distance as predicted by Helfrich [Z. Naturforsch. A 33, 305 (1978)]. Here, the crossover length between the two regimes is a molecular length scale. Finally, for distances large compared with the mean squared fluctuations of the membrane, the entire membrane acts as a soft particle and the pressure on the walls again depends linearly on the inverse distance.Comment: 4 pages, 5 figure

Wrapping of ellipsoidal nano-particles by fluid membranes

Membrane budding and wrapping of particles, such as viruses and nano-particles, play a key role in intracellular transport and have been studied for a variety of biological and soft matter systems. We study nano-particle wrapping by numerical minimization of bending, surface tension, and adhesion energies. We calculate deformation and adhesion energies as a function of membrane elastic parameters and adhesion strength to obtain wrapping diagrams. We predict unwrapped, partially-wrapped, and completely-wrapped states for prolate and oblate ellipsoids for various aspect ratios and particle sizes. In contrast to spherical particles, where partially-wrapped states exist only for finite surface tensions, partially-wrapped states for ellipsoids occur already for tensionless membranes. In addition, the partially-wrapped states are long-lived, because of an increased energy cost for wrapping of the highly-curved tips. Our results suggest a lower uptake rate of ellipsoidal particles by cells and thereby a higher virulence of tubular viruses compared with icosahedral viruses, as well as co-operative budding of ellipsoidal particles on membranes.Comment: 10 pages, 11 figure

Collective behavior of penetrable self-propelled rods in two dimensions

Collective behavior of self-propelled particles is observed on a microscale for swimmers such as sperm and bacteria as well as for protein filaments in motility assays. The properties of such systems depend both on their dimensionality and the interactions between their particles. We introduce a model for self-propelled rods in two dimensions that interact via a separation-shifted Lennard-Jones potential. Due to the finite potential barrier, the rods are able to cross. This model allows us to efficiently simulate systems of self-propelled rods that effectively move in two dimensions but can occasionally escape to the third dimension in order to pass each other. Our quasi-two-dimensional self-propelled particles describe a class of active systems that encompasses microswimmers close to a wall and filaments propelled on a substrate. Using Monte Carlo simulations, we first determine the isotropic-nematic transition for passive rods. Using Brownian dynamics simulations, we characterize cluster formation of self-propelled rods as a function of propulsion strength, noise, and energy barrier. Contrary to rods with an infinite potential barrier, an increase of the propulsion strength does not only favor alignment but also effectively decreases the potential barrier that prevents crossing of rods. We thus find a clustering window with a maximum cluster size at medium propulsion strengths.Comment: 12 pages, 13 figure

Effective Curvature Elastic Constants for Membrane-Polymer Systems

Membranes can be described using a model of mathematical surfaces where the membrane's properties are characterised by the three membrane curvature elastic constants 'spontaneous curvature', 'bending rigidity' and 'saddle-splay modulus'. Experiments show that the addition of polymers can change the properties of a membrane system considerably. One example is the polymer-boosting effect which has been discovered recently for oil-water-amphiphile mixtures. The scattering data has been described successfully by the membrane model. The effect of the polymers has been taken into account by effective membrane curvature elastic constants. The concept of effective curvature elastic constants will be introduced, and the effects of different kinds of polymer additions to membrane systems discussed in the literature will be reviewed. Using the model of freely-jointed chains for the polymers, the effects of polymers anchored to membranes will be studied for several systems by means of Monte Carlo simulations. A simulation technique is described which allows to calculate the polymer effect with high accuracy in the limit of small membrane curvatures. The effects of self-avoidance and of different polymer architectures are investigated. The self-avoidance effect for linear polymer chains is found to be small. However, the simulations show that star polymers increase the efficiency of the polymer. The effects on the bending rigidity and the spontaneous curvature per arm increase with the functionality (i.e. the number of arms) of the star, whereas the effect on the saddle-splay modulus does not depend on the functionality. Scaling arguments confirm the behaviour observed in the simulations. The properties of anchored ring polymers are studied and the effects of knots are discussed. An algorithm is presented which can be employed to calculate the effect of adsorbed polymers on the curvature elastic constants in the limit of small curvatures. For linear chains in the lamellar phase, the effect of the confined geometry is investigated, again in the limit of small membrane curvatures. The simulations show that for polymers anchored to membranes, at a small lamellar spacing the effect on the membrane curvature elastic constants changes qualitatively. While for large interlayer spacings the polymer increases the bending rigidity and decreases the saddle-splay modulus, effects of opposite sign are observed for lamellar spacings smaller than the radius of gyration of the free chain. With a model for polymers anchored to a fluctuating membrane, the polymer effect is simulated for the whole fluctuation spectrum of the membrane. We obtain a universal scaling function with a maximum at large fluctuation lengths

Diel variation in vertical distribution of an offshore ichthyoplankton community off the Oregon coast

We examined the diel ver-tical distribution, concentration, and community structure of ichthyoplank-ton from a single station 69 km off the central Oregon coast in the northeast Pacific Ocean. The 74 depth-stratified samples yielded 1571 fish larvae from 20 taxa, representing 11 families, and 128 fish eggs from 11 taxa within nine families. Dominant larval taxa were Sebastes spp. (rockfishes), Stenobra-chius leucopsarus (northern lampfish), Tarletonbeania crenularis (blue lan-ternfish), and Lyopsetta exilis (slender sole), and the dominant egg taxa were Sardinops sagax (Pacific sardine), Icichthys lockingtoni (medusafish), and Chauliodus macouni (Pacific viperfish). Larval concentrations generally increased from the surface to 50 m, then decreased with depth. Larval concentrations were higher at night than during the day, and there was evidence of larval diel vertical migration. Depth stratum was the most important factor explaining variability in larval and egg concentrations

Computational modeling of oxygen-assisted fracture in nickel-based superalloys

Nickel-based superalloys are a commonly used material in applications where high strength is required at high temperatures. A typical such example is jet engines and, in the case of polycrystalline nickel-based superalloys, components like turbine disks.Under severe loading conditions, such as cyclic loading combined with sustained dwell times at high temperatures, polycrystalline nickel-based superalloys are known to experience accelerated fatigue crack growth in oxygen-rich environments compared to vacuum. Environmentally assisted crack initiation could for example be identified as cause for turbine disk fracture in some cases of mechanical turbine failure of passenger airplanes.It has been shown by experimental work in the past that oxygen is the main deteriorating species in the case of intergranular fracture in nickel-based superalloys.In this work we develop a fully coupled chemo-mechanical cohesive zone model accounting for the interaction between oxygen transport into the grain boundaries and their stress state. The model is presented in a thermodynamic framework. Additionally, a chemo-mechanical cohesive finite element carrying both the displacement and the concentration field is suggested.The finite element formulation is complemented by a moving boundary condition for the concentration field, accounting for the increase in environment-exposed surface as edge cracks grow into the structure. Aside from handling edge cracks, the moving boundary condition yields physically meaningful results even in the case of crack initiation at interior grain boundaries.Numerical experiments are conducted on bi- and polycrystals. It is shown that the modeling framework can qualitatively reproduce experimentally observed phenomena, like the reduction of tensile strength in oxygen rich environments, the acceleration of crack growth rates upon increasing environmental oxygen concentration and saturation thereof for high environmental oxygen levels. It is also demonstrated that edge cracks can be propagated past preexisting cracks inside the polycrystal while maintaining realistic oxygen boundary conditions

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