Françoise Brochard-Wyart

Capillarity and Wetting Phenomena

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Cascades of Transient Pores in Giant Vesicles: Line Tension and Transport

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Colloid & Polymer Science

Under ordinary circumstances, the membrane tension of a giant unilamellar vesicle is essentially nil. Using visible light, we stretch the vesicles, increasing the membrane tension until the membrane responds by the sudden opening of a large pore (several micrometers in size). Only a single pore is observed at a time in a given vesicle. However, a cascade of transient pores appear, up to 30-40 in succession, in the same vesicle. These pores are transient: they reseal within a few seconds as the inner liquid leaks out. The membrane tension, which is the driving force for pore opening, is relaxed with the opening of a pore and the leakage of the inner liquid; the line tension of the pores edge is then able to drive the closure of a pore. We use fluorescent membrane probes and real-time videomicroscopy to study the dynamics of the pores. These can be visualized only if the vesicles are prepared in a viscous solution to slow down the leakout of the internal liquid. From measurements of the closure velocity of the pores, we are able to infer the line tension,. We have studied the effect of the shape of inclusion molecules on. Cholesterol, which can be modeled as an inverted cone-shaped molecule, increases the line tension when incorporated into the bilayers. Conversely, addition of cone-shaped detergents reduces. The effect of some detergents can be dramatic, reducing by two orders of magnitude, and increasing pore lifetimes up to several minutes. We give some examples of transport through transient pores and present a rough measurement of the leakout velocity of the inner liquid through a pore. We discuss how our results can be extended to less viscous aqueous solutions which are more relevant for biological systems and biotechnological applications.

Bursting of sensitive polymersomes induced by curling.

AbstractCertain water soluble polymers may have a repulsive two-body interaction, but an attractiven-body interaction induced by certain “clustering” effects. In the bulk this may lead to a “

Spreading dynamics and wetting transition of cellular aggregates.

\bar \theta

Vesicles surfing on a lipid bilayer: Self-induced haptotactic motion

point” in the phase diagram. Here, with polymer brushes, we construct the theoretical density profiles, using a local mean-field approximation. The brush often shows two layers (one dense near the wall, and one dilute), but the concentrations in both layers depend on the distance to the wall. The location of the interlayer boundary can be derived from a Maxwell construction.

Inkjet formation of unilamellar lipid vesicles for cell-like encapsulation.

Polymersomes, which are stable and robust vesicles made of block copolymer amphiphiles, are good candidates for drug carriers or micro/nanoreactors. Polymer chemistry enables almost unlimited molecular design of responsive polymersomes whose degradation upon environmental changes has been used for the slow release of active species. Here, we propose a strategy to remotely trigger instantaneous polymersome bursting. We have designed asymmetric polymer vesicles, in which only one leaflet is composed of responsive polymers. In particular, this approach has been successfully achieved by using a UV-sensitive liquid-crystalline copolymer. We study experimentally and theoretically this bursting mechanism and show that it results from a spontaneous curvature of the membrane induced by the remote stimulus. The versatility of this mechanism should broaden the range of applications of polymersomes in fields such as drug delivery, cosmetics and material chemistry.

Polymer Chains Under Strong Flows: Stems and Flowers

We have visualized macroscopic transient pores in mechanically stretched giant vesicles. They can be observed only if the vesicles are prepared in a viscous solution to slow down the leak-out of the internal liquid. We study here theoretically the full dynamics of growth (driven by surface tension) and closure (driven by line tension) of these large pores. We write two coupled equations of the time evolution of the radii r(t) of the hole and R(t) of the vesicle, which both act on the release of the membrane tension. We find four periods in the life of a transient pore: (I) exponential growth of the young pore; (II) stop of the growth at a maximum radius rm; (III) slow closure limited by the leak-out; (IV) fast closure below a critical radius, when leak-out becomes negligible. Ultimately the membrane is completely resealed.Notationdmembrane thicknessEsurface stretching modulusKbHelfrich bending constantQleak-out fluxrpore radiusripore radius at nucleationrcpore radius at zero tensionrLcharacteristic radius of leak-outrmradius at maximum (II)r23pore radius at cross-over between (II) and (III)r34pore radius at cross-over between (III) and (IV)Rvesicle radiusRiinitial vesicle radiusR0vesicle radius at zero tensionVLleak-out velocityV3slow closure velocity limited by leak-out (III)V4fast closure velocity at end (IV)η2lipid viscosityηssurface viscosityη0viscosity of solutionσsurface tensionσ0surface tension before pore openingτrise time of pore growth (I)Jline tension