arXiv subjects

J. -B. Fournier

Publications and source records attributed to J. -B. Fournier.

16 recordsLinked to original sources

arXiv2010

On the surface tension of fluctuating quasi-spherical vesicles

We calculate the stress tensor for a quasi-spherical vesicle and we thermally average it in order to obtain the actual, mechanical, surface tension $τ$ of the vesicle. Both closed and poked vesicles are considered. We recover our results for $τ$ by differentiating the free-energy with respect to the proper projected area. We show that $τ$ may become negative well before the transition to oblate shapes and that it may reach quite large negative values in the case of small vesicles. This implies that spherical vesicles may have an inner pressure lower than the outer one.

cond-mat.soft↗

arXiv2006

Determination of the interactions in confined macroscopic Wigner islands: theory and experiments

Macroscopic Wigner islands present an interesting complementary approach to explore the properties of two-dimensional confined particles systems. In this work, we characterize theoretically and experimentally the interaction between their basic components, viz., conducting spheres lying on the bottom electrode of a plane condenser. We show that the interaction energy can be approximately described by a decaying exponential as well as by a modified Bessel function of the second kind. In particular, this implies that the interactions in this system, whose characteristics are easily controllable, are the same as those between vortices in type-II superconductors.

cond-mat.mes-hall↗

arXiv2003

Dynamin recruitment by clathrin coats: a physical step?

Recent structural findings have shown that dynamin, a cytosol protein playing a key-role in clathrin-mediated endocytosis, inserts partly within the lipid bilayer and tends to self-assemble around lipid tubules. Taking into account these observations, we make the hypothesis that individual membrane inserted dynamins imprint a local cylindrical curvature to the membrane. This imprint may give rise to long-range mechanical forces mediated by the elasticity of the membrane. Calculating the resulting many-body interaction between a collection of inserted dynamins and a membrane bud, we find a regime in which the dynamins are elastically recruited by the bud to form a collar around its neck, which is reminiscent of the actual process preempting vesicle scission. This physical mechanism might therefore be implied in the recruitment of dynamins by clathrin coats.

cond-mat.soft↗

arXiv2002

Wormlike chain or tense string? A question of resolution

It is shown that a wormlike chain, i.e., a filament with a fixed contour-length S and a bending elasticity kappa, attached to a frame of length L, can be described--at low resolutions--by the same type of elastic free-energy as a tense string. The corresponding tension is calculated as a function of temperature, L, kappa and S.

cond-mat.soft↗

arXiv2002

Exact calculations of the paranematic interaction energy for colloidal dispersions in the isotropic phase of a nematogenic material

In a recent paper [Phys. Rev. E 61, 2831 (2000)], Borstnik, Stark and Zumer have studied the stability of a colloidal dispersion of micron-sized spherical particles in the isotropic phase of a nematogenic material. Close to the nematic transition, the attraction due to a surface-induced paranematic order can yield flocculation. Their calculation of the nematic-mediated interaction was based on an ansatz for the order-parameter profile. We compare it with an exact numerical calculation, showing that their results are qualitatively correct. Besides, we point out that in the considered regime, the exact interaction is extremely well approximated by a simple analytical formula which is asymptotically exact.

cond-mat.soft↗

arXiv2001

Anisotropic Capillary Interactions and Jamming of Colloidal Particles Trapped at a Liquid-Fluid Interface

We determine the capillary attraction and equilibrium configurations of particles trapped at a liquid-fluid interface due to the pinning of their contact-line. We calculate analytically the asymptotic interaction energy between two particles and, numerically, the multi-body energy landscape for up to four contacting particles. Our results are consistent with recent experiments. We show that a system composed of a large number of such particles behaves as a jammed system.

cond-mat.soft↗

arXiv2001

Analytical characterization of adhering vesicles

We characterize vesicle adhesion onto homogeneous substrates by means of a perturbative expansion around the infinite adhesion limit, where curvature elasticity effects are absent. At first order in curvature elasticity, we determine analytically various global physical quantities associated with adhering vesicles: height, adhesion radius, etc. Our results are valid for adhesion energies above a certain threshold, that we determine numerically. We discuss the haptotactic force acting on a vesicle in the limit of weak adhesion gradients. We also propose novel methods for measuring adhesion energies and we suggest a possible way of determining the size of suboptical vesicles using controlled adhesion gradients.

cond-mat.soft↗

arXiv2001

Effective Area-Elasticity and Tension of Micro-manipulated Membranes

We evaluate the effective Hamiltonian governing, at the optically resolved scale, the elastic properties of micro-manipulated membranes. We identify floppy, entropic-tense and stretched-tense regimes, representing different behaviors of the effective area-elasticity of the membrane. The corresponding effective tension depends on the microscopic parameters (total area, bending rigidity) and on the optically visible area, which is controlled by the imposed external constraints. We successfully compare our predictions with recent data on micropipette experiments.

cond-mat.soft↗

arXiv2001

Nematic-Wetted Colloids in the Isotropic Phase: Pairwise Interaction, Biaxiality and Defects

We calculate the interaction between two spherical colloidal particles embedded in the isotropic phase of a nematogenic liquid. The surface of the particles induces wetting nematic coronas that mediate an elastic interaction. In the weak wetting regime, we obtain exact results for the interaction energy and the texture, showing that defects and biaxiality arise, although they are not topologically required. We evidence rich behaviors, including the possibility of reversible colloidal aggregation and dispersion. Complex anisotropic self-assembled phases might be formed in dense suspensions.

cond-mat.soft↗

arXiv2001

Casimir Torques between Anisotropic Boundaries in Nematic Liquid Crystals

Fluctuation-induced interactions between anisotropic objects immersed in a nematic liquid crystal are shown to depend on the relative orientation of these objects. The resulting long-range ``Casimir'' torques are explicitely calculated for a simple geometry where elastic effects are absent. Our study generalizes previous discussions restricted to the case of isotropic walls, and leads to new proposals for experimental tests of Casimir forces and torques in nematics.

cond-mat.soft↗

arXiv1999

Long-range Casimir interactions between impurities in nematic liquid crystals and the collapse of polymer chains in such solvents

The elastic interactions between objects embedded in a nematic liquid crystal are usually caused by the average distorsion-rather than by the fluctuations-of the nematic orientational field. We argue that for sufficiently small particles, the nematic-mediated interaction originates purely from the fluctuations of the nematic director. This Casimir interaction decays as d^(-6), d being the distance between the particles, and it dominates van der Waals interactions close to the isotropic-to-nematic transition. Considering the nematic as a polymer solvent, we show that the onset of this Casimir interaction at the isotropic-to-nematic transition can discontinuously induce the collapse of a flexible polymer chain from the swollen state to the globular state, without crossing the Theta-point.

cond-mat.soft↗

arXiv1999

N-body Study of Anisotropic Membrane Inclusions: Membrane Mediated Interactions and Ordered Aggregation

We study the collective behavior of inclusions inducing local anisotropic curvatures in a flexible fluid membrane. The N-body interaction energy for general anisotropic inclusions is calculated explicitly, including multi-body interactions. Long-range attractive interactions between inclusions are found to be sufficiently strong to induce aggregation. Monte Carlo simulations show a transition from compact clusters to aggregation on lines or circles. These results might be relevant to proteins in biological membranes or colloidal particles bound to surfactant membranes.

cond-mat.soft↗

arXiv1999

Coarse-grained surface energies and temperature-induced anchoring transitions in nematic liquid crystals

We introduce a coarse-grained description of the surface energy of a nematic liquid crystal. The thermal fluctuations of the nematic director close to the surface renormalize at macroscopic scales the bare surface potential in a temperature-dependent way. The angular dependence of the renormalized potential is dramatically smoothed, thus explaining the success of the Rapini-Papoular form. Close to the isotropic phase, the anchoring energy is strongly suppressed and the change of its shape allows for anchoring transitions. Our theory describes quantitatively the temperature dependence of the anchoring energy and the temperature-induced anchoring transitions reported in the literature.

cond-mat.soft↗

arXiv1998

Microscopic membrane elasticity and interactions among membrane inclusions: Interplay between the shape, dilation, tilt and tilt-difference modes

A phenomenological Landau elasticity for the shape, dilation, and lipid-tilt of bilayer membranes is developed. The shape mode couples with the sum of the monolayers' tilt, while the dilation mode couples with the difference of the monolayers' tilts. Interactions among membrane inclusions within regular arrays are discussed. Inclusions modifying the membrane thickness and/or inducing a tilt-difference due to their convex or concave shape yield a dilation-induced attraction and a tilt-difference-induced repulsion. The resulting interaction can stabilize 2D crystal phases, with the possible coexistence of different lattice spacings when the dilation-tilt-difference coupling is large. Inclusions favoring crystals are those with either a long-convex or a short-concave hydrophobic core. Inclusions inducing a local membrane curvature due to their conical shape repel one another. At short inclusions separations, a tilt comparable with the inclusion's cone angle develops: it relaxes the membrane curvature and reduces the repulsion. At large separations the tilt vanishes, whatever the value of the shape-tilt coupling.

cond-mat.soft↗

arXiv1998

Coupling between membrane tilt-difference and dilation: a new ``ripple'' instability and multiple crystalline inclusions phases

A continuum Landau theory for the micro-elasticity of membranes is discussed, which incorporates a coupling between the bilayer thickness variation and the difference in the two monolayers' tilts. This coupling stabilizes a new phase with a rippled micro-structure. Interactions among membrane inclusions combine a dilation-induced attraction and a tilt-difference-induced repulsion that yield 2D crystal phases, with possible coexistence of different lattice spacings for large couplings. Inclusions favoring crystals are those with either a long-convex or a short-concave hydrophobic core.

cond-mat.soft↗

arXiv1998

Bi-defects of Nematic Surfactant Bilayers

We consider the effects of the coupling between the orientational order of the two monolayers in flat nematic bilayers. We show that the presence of a topological defect on one bilayer generates a nontrivial orientational texture on both monolayers. Therefore, one cannot consider isolated defects on one monolayer, but rather associated pairs of defects on either monolayer, which we call bi-defects. Bi-defects generally produce walls, such that the textures of the two monolayers are identical outside the walls, and different in their interior. We suggest some experimental conditions in which these structures could be observed.

cond-mat.soft↗