12 research outputs found
Fluctuations of the Casimir-like force between two membrane inclusions
Although Casimir forces are inseparable from their fluctuations, little is
known about these fluctuations in soft matter systems. We use the membrane
stress tensor to study the fluctuations of the membrane-mediated Casimir-like
force. This method enables us to recover the Casimir force between two
inclusions and to calculate its variance. We show that the Casimir force is
dominated by its fluctuations. Furthermore, when the distance d between the
inclusions is decreased from infinity, the variance of the Casimir force
decreases as -1/d^2. This distance dependence shares a common physical origin
with the Casimir force itself.Comment: 5 pages, 3 figure
Knots in Charged Polymers
The interplay of topological constraints and Coulomb interactions in static
and dynamic properties of charged polymers is investigated by numerical
simulations and scaling arguments. In the absence of screening, the long-range
interaction localizes irreducible topological constraints into tight molecular
knots, while composite constraints are factored and separated. Even when the
forces are screened, tight knots may survive as local (or even global)
equilibria, as long as the overall rigidity of the polymer is dominated by the
Coulomb interactions. As entanglements involving tight knots are not easy to
eliminate, their presence greatly influences the relaxation times of the
system. In particular, we find that tight knots in open polymers are removed by
diffusion along the chain, rather than by opening up. The knot diffusion
coefficient actually decreases with its charge density, and for highly charged
polymers the knot's position appears frozen.Comment: Revtex4, 9 pages, 9 eps figure
Equilibrium shapes of flat knots
We study the equilibrium shapes of prime and composite knots confined to two
dimensions. Using rigorous scaling arguments we show that, due to self-avoiding
effects, the topological details of prime knots are localised on a small
portion of the larger ring polymer. Within this region, the original knot
configuration can assume a hierarchy of contracted shapes, the dominating one
given by just one small loop. This hierarchy is investigated in detail for the
flat trefoil knot, and corroborated by Monte Carlo simulations.Comment: 4 pages, 3 figure
Tightness of slip-linked polymer chains
We study the interplay between entropy and topological constraints for a
polymer chain in which sliding rings (slip-links) enforce pair contacts between
monomers. These slip-links divide a closed ring polymer into a number of
sub-loops which can exchange length between each other. In the ideal chain
limit, we find the joint probability density function for the sizes of segments
within such a slip-linked polymer chain (paraknot). A particular segment is
tight (small in size) or loose (of the order of the overall size of the
paraknot) depending on both the number of slip-links it incorporates and its
competition with other segments. When self-avoiding interactions are included,
scaling arguments can be used to predict the statistics of segment sizes for
certain paraknot configurations.Comment: 10 pages, 6 figures, REVTeX
Calcium-ion-controlled nanoparticle-induced tubulation in supported flat phospholipid vesicles
Biological nanotubes, often referred to as tunneling nanotubes, fulfill important functions within the cell, e.g. by supplying cell components, conducting signals and transporting virus particles and bacteria. Many functions are still insufficiently understood, which has placed these nanostructures in the focus of recent investigation. We report here on our observations of transient tubulation in nanoparticle-containing, supported flat giant unilamellar vesicles (FGUVs). The encapsulation of nanoparticles in FGUVs in conjunction with low (1-4 mM) Ca(2+) in the ambient buffer solution resulted in transient tube formation. Tubes extended from the FGUV up to a length of several hundred micrometres and exhibited, on some occasions, vesicle encapsulation. The findings represent an interesting confirmation of several reported theoretical and practical models of tube formation in biological or biomimetic systems
Fractal avalanche ruptures in biological membranes
Bilayer membranes envelope cells as well as organelles, and constitute the most ubiquitous biological material found in all branches of the phylogenetic tree. Cell membrane rupture is an important biological process, and substantial rupture rates are found in skeletal and cardiac muscle cells under a mechanical load(1). Rupture can also be induced by processes such as cell death(2), and active cell membrane repair mechanisms are essential to preserve cell integrity(3). Pore formation in cell membranes is also at the heart of many biomedical applications such as in drug, gene and short interfering RNA delivery(4). Membrane rupture dynamics has been studied in bilayer vesicles under tensile stress(5-8), which consistently produce circular pores(5,6). We observed very different rupture mechanics in bilayer membranes spreading on solid supports: in one instance fingering instabilities were seen resulting in floral-like pores and in another, the rupture proceeded in a series of rapid avalanches causing fractal membrane fragmentation. The intermittent character of rupture evolution and the broad distribution in avalanche sizes is consistent with crackling-noise dynamics(9). Such noisy dynamics appear in fracture of solid disordered materials(10), in dislocation avalanches in plastic deformations(11) and domain wall magnetization avalanches(12). We also observed similar fractal rupture mechanics in spreading cell membranes
Fractal avalanche ruptures in biological membranes
Bilayer membranes envelope cells as well as organelles, and constitute the most ubiquitous biological material found in all branches of the phylogenetic tree. Cell membrane rupture is an important biological process, and substantial rupture rates are found in skeletal and cardiac muscle cells under a mechanical load(1). Rupture can also be induced by processes such as cell death(2), and active cell membrane repair mechanisms are essential to preserve cell integrity(3). Pore formation in cell membranes is also at the heart of many biomedical applications such as in drug, gene and short interfering RNA delivery(4). Membrane rupture dynamics has been studied in bilayer vesicles under tensile stress(5-8), which consistently produce circular pores(5,6). We observed very different rupture mechanics in bilayer membranes spreading on solid supports: in one instance fingering instabilities were seen resulting in floral-like pores and in another, the rupture proceeded in a series of rapid avalanches causing fractal membrane fragmentation. The intermittent character of rupture evolution and the broad distribution in avalanche sizes is consistent with crackling-noise dynamics(9). Such noisy dynamics appear in fracture of solid disordered materials(10), in dislocation avalanches in plastic deformations(11) and domain wall magnetization avalanches(12). We also observed similar fractal rupture mechanics in spreading cell membranes