Actin modulates shape and mechanics of tubular membranes.

The actin cytoskeleton shapes cells and also organizes internal membranous compartments. In particular, it interacts with membranes for intracellular transport of material in mammalian cells, yeast, or plant cells. Tubular membrane intermediates, pulled along microtubule tracks, are formed during this process and destabilize into vesicles. While the role of actin in tubule destabilization through scission is suggested, literature also provides examples of actin-mediated stabilization of membranous structures. To directly address this apparent contradiction, we mimic the geometry of tubular intermediates with preformed membrane tubes. The growth of an actin sleeve at the tube surface is monitored spatiotemporally. Depending on network cohesiveness, actin is able to entirely stabilize or locally maintain membrane tubes under pulling. On a single tube, thicker portions correlate with the presence of actin. These structures relax over several minutes and may provide enough time and curvature geometries for other proteins to act on tube stability.

Dewetting of cellular monolayers.

We investigate the physical principles of cellular layer stability. We show that cohesive cellular layers deposited on non-adhesive substrates are metastable and "dewet" by nucleation and growth of dry patches. The dewetting process can be induced either chemically by a non-adhesive surface treatment or, unlike simple liquids, physically by a decrease in the substrate rigidity. We thus unveil two mechanisms by which the integrity of cellular layers can be compromised. We interpret the opening dynamics by an analogy with the dewetting of viscous films. This analogy can be exploited to estimate parameters characterizing the mechanical response of a cellular layer.

Naive model for stick-slip processes.

This paper is a tutorial description of stick-slip in soft materials (rubber beads, gels) where inertial effects are negligible. A typical example is a rubber sphere, pressed against a glass surface (JKR contact). The sphere is driven from the top at a prescribed velocity U ( approximately 100 microm/s). At moderate U (and with suitable surface treatment) a periodic stick-slip regime is often observed. We present a simple picture of the stick-slip cycle, describing the growth of a slip zone from the rear end of the sample, and the resulting plot of force vs. time. All our estimates are restricted to scaling laws.

Forced detachment of immersed elastic rubber beads.

We investigate the strength of adhesion and the dynamics of detachment of elastic beads (Young's modulus E approximately 1 MPa) adhering to a horizontal solid surface in a viscous liquid. The beads are initially compressed on the surface. Their unbinding is imposed by fast vertical stretching (above a certain threshold value). The decrease in the contact radius is monitored by interferential microscopy. We find that the dynamics of detachment involves three steps: (i) fast elastic decompression, (ii) slow adhesive detachment, and (iii) catastrophic rupture. They can be interpreted by a transfer of the Johnson Kendall Roberts (JKR) energy toward viscous losses in the liquid wedge, near the rubber/solid/liquid (R/S/L) contact line.

Tether extrusion from red blood cells: integral proteins unbinding from cytoskeleton.

We investigate the mechanical strength of adhesion and the dynamics of detachment of the membrane from the cytoskeleton of red blood cells (RBCs). Using hydrodynamical flows, we extract membrane tethers from RBCs locally attached to the tip of a microneedle. We monitor their extrusion and retraction dynamics versus flow velocity (i.e., extrusion force) over successive extrusion-retraction cycles. Membrane tether extrusion is carried out on healthy RBCs and ATP-depleted or -inhibited RBCs. For healthy RBCs, extrusion is slow, constant in velocity, and reproducible through several extrusion-retraction cycles. For ATP-depleted or -inhibited cells, extrusion dynamics exhibit an aging phenomenon through extrusion-retraction cycles: because the extruded membrane is not able to retract properly onto the cell body, each subsequent extrusion exhibits a loss of resistance to tether growth over the tether length extruded at the previous cycle. In contrast, the additionally extruded tether length follows healthy dynamics. The extrusion velocity L depends on the extrusion force f according to a nonlinear fashion. We interpret this result with a model that includes the dynamical feature of membrane-cytoskeleton association. Tether extrusion leads to a radial membrane flow from the cell body toward the tether. In a distal permeation regime, the flow passes through the integral proteins bound to the cytoskeleton without affecting their binding dynamics. In a proximal sliding regime, where membrane radial velocity is higher, integral proteins can be torn out, leading to the sliding of the membrane over the cytoskeleton. Extrusion dynamics are governed by the more dissipative permeation regime: this leads to an increase of the membrane tension and a narrowing of the tether, which explains the power law behavior of L(f). Our main result is that ATP is necessary for the extruded membrane to retract onto the cell body. Under ATP depletion or inhibition conditions, the aging of the RBC after extrusion is interpreted as a perturbation of membrane-cytoskeleton linkage dynamics.

Hydrodynamic narrowing of tubes extruded from cells.

We discuss the pulling force f required to extrude a lipid tube from a living cell as a function of the extrusion velocity L. The main feature is membrane friction on the cytoskeleton. As recently observed for neutrophils, the tether force exhibits a "shear thinning" response over a large range of pulling velocities, which was previously interpreted by assuming viscoelastic flows of the sliding membrane. Here, we propose an alternative explanation based on purely Newtonian flow: The diameter of the tether decreases concomitantly with the increase of the membrane tension in the lipid tube. The pulling force is found to vary as L(1/3), which is consistent with reported experimental data for various types of cells.

Cascade of shocks in inertial liquid-liquid dewetting.

We study the inertial dewetting of water films (A) (thickness e) deposited on highly hydrophobic liquid substrates (B). On these ideal surfaces, thin films can be made which dewet at large velocities obeying under those conditions the Culick law for the bursting of soap films. The rim collecting the water film can become coupled to the surface waves characterized by a surface tension gamma(B) upstream of the rim (coated substrate) and gamma = gamma(B) downstream, where the water film has dried. Upon decreasing the thickness, we observe a sequence of two hydraulic shocks during the dewetting inducing gravity waves behind the rim, and capillary waves ahead.

Triplon modes of puddles.

Free fluctuations of the contact line of large drops ("puddles") of wavelength lambda > kappa(-1), the capillary length, cannot be seen on a solid substrate because even a small but finite hysteresis is enough to block these slow modes. We show here that vertical vibrations of the substrate (at frequency omegaE, acceleration Lambda) above a threshold amplitude Lambda(c) release the line and excite contour oscillations (triplons). We observe harmonic modes and parametric excitations at omegaE/2. We construct the phase diagram (Lambda, omegaE) of these subharmonic modes and we study their growth dynamics: they slow down near the threshold of the contour instability.

Semiflexible polymers confined in soft tubes.

We discuss various conformations for a polymer (of persistent length l(p)) confined into a deformable tube (the wall being a lipid bilayer with a certain surface tension sigma and curvature energy K). Our study assumes that there is no adsorption of the chain on the wall. Three states are compared: (a) an unperturbed tube, plus a confined chain, (b) a tube swollen in all the region surrounding the chain (similar to a snake eating a sausage), (c) a globule, a roughly spherical coil surrounded by a strongly deformed tube. We construct a (qualitative) phase diagram for these systems with two variables: the surface tension sigma and the degree of polymerization N. Our main conclusion is that "globules" usually win over "snakes".

Wetting fibers with liposomes.

Giant unilamellar vesicles (GUVs) are deposited on glass microfibers. The vesicles adopt the classical "onduloidal" shape of liquid droplets on fibers. They spread by two simultaneous mechanisms: envelopment and emission of a precursor film. This film spreads faster than on a uniform plane surface and eventually stops, signaling the presence of defects on the rod. This fast spreading tenses the vesicles; transient pores open on the GUVs and the internal liquid leaks out. This process leads to a new technique for fiber coating.

Membrane tensiometer for heavy giant vesicles.

One key parameter of giant-vesicles adhesion is their membrane tension, sigma. A theoretically simple but delicate way to impose (and measure) it is to use micropipette manipulation techniques. But usually, the vesicles are free and their tension is unknown, until an adhesion patch grows. Sigma can be deduced from the detailed profile of the membrane close to the substrate, but this method is limited to very low tensions. We present here a rather simple way to estimate the membrane tension of heavy vesicles, which sediment close to a surface, by observing by RIM the size of the flat region of the vesicle. As an application, we follow the slow flattening of vesicles, when the surrounding sugar solution is evaporating, and their light-induced tensioning.

Adhesion between giant vesicles and supported bilayers decorated with chelated E-cadherin fragments.

Here, we present a study of adhesion between cadherin fragments using giant unilamellar vesicles and supported bilayers. These objects are partially made of nickel chelating lipids and are subsequently decorated with proteins bearing a 6His tag. Initially, we observed their fixation and correct orientation by using a fluorescent protein, the green fluorescent protein (GFP)-6His. The adhesive behavior of E-cadherin functionalized giant vesicles and supported bilayers was studied as a function of the calcium concentration and of the protein functionality by reflection interference microscopy. We show that such a system retains specific cadherin-mediated adhesion and could be used to study the statics and dynamics of adhesive plaques as well as to gain insight into the fundamental mechanisms of cellular adhesion at the mesoscopic scale.

Vibrated sessile drops: transition between pinned and mobile contact line oscillations.

We study the effects of vertical vibrations on non-wetting large water sessile drops flattened by gravity. The solid substrate is characterized by a finite contact angle hysteresis (10-15 degrees). By varying the frequency and the amplitude of the vertical displacement, we observe two types of oscillations. At low amplitude, the contact line remains pinned and the drop presents eigen modes at different resonance frequencies. At higher amplitude, the contact line moves: it remains circular but its radius oscillates at the excitation frequency. The transition between these two regimes arises when the variations of contact angle exceed the contact angle hysteresis. We interpret different features of these oscillations, such as the decrease of the resonance frequencies at larger vibration amplitudes. The hysteresis acts as "solid" friction on the contour oscillations, and gives rise to a stick-slip regime at intermediate amplitude.

Line thermodynamics: adsorption at a membrane edge.

We report a novel experimental study of line thermodynamics. Our system consists of detergent molecules adsorbing at the edges of freestanding lipid bilayers. Adsorption reduces the line tension T of the membrane edges. Measuring T as a function of the bulk detergent concentration C, we obtain a line adsorption isotherm. Using an extension of Gibbs's surface thermodynamics to lines, we estimate the "line excess density" of adsorbants and the energy of adsorption per site.

Formation of adhesive contacts: spreading versus dewetting.

A soft bead (radius Rb) is pressed with a force F against a hydrophobic glass plate through a water drop ("wet" JKR set-up). We observe with a fast camera the growth of the contact zone bridging the rubber bead to the glass. Depending on the approach velocity V, two regimes are observed: i) at large V a liquid film is squeezed at the interface and dewets by nucleation and growth of a dry contact; ii) at low velocities, the bead remains nearly spherical. As it comes into contact, the rubber bead spreads on the glass with a characteristic time (in the range of one millisecond) tau approximately eta Rb2/F, where eta is the liquid viscosity. The laws of spreading are interpreted by a balance of global mechanical and viscous forces.

Adhesion induced by mobile binders: dynamics.

We consider a vesicle bilayer loaded with molecules that can bind (upon contact) with a solid surface, following the classical model of Bell, Dembo, and Bongrand. We are interested in situations where the contact area varies with time: we assume that binders can then migrate via diffusion. The resulting dissipation and lag create a retarded force on the contact line, which could be significant in squeezing or rolling experiments. However, there are two cases where we expect the lag force to be ineffective: (i) separation by shrinking of an adhesive patch (where the Evans "tear out" process turns out to be less costly) and (ii) spontaneous growth of a patch from a point contact. In this last case, the lag force is weak, and we give detailed predictions for the growth laws.

Wetting transitions at soft, sliding interfaces.

We observe (by optical interferometry) the contact of a rubber cap squeezing a nonwetting liquid against a plate moving at velocity U. At low velocities, the contact is dry. It becomes partially wet above a threshold velocity V(c1), with two symmetrical dry patches on the rear part. Above a second velocity V(c2), the contact is totally wet. This regime U>V(c2) corresponds to the hydroplaning of a car (decelerating on a wet road). We interpret the transitions at V(c1), V(c2) in terms of a competition between (a) liquid invasion induced by shear (b) spontaneous dewetting of the liquid (between nonwettable surfaces).

Dynamics of transient pores in stretched vesicles.

We image macroscopic transient pores in mechanically stretched giant vesicles. Holes open above a critical radius r(c1), grow up to a radius r(c2), and close. We interpret the upper limit r(c2) by a relaxation of the membrane tension as the holes expand. The closing of the holes is caused by a further relaxation of the surface tension when the internal liquid leaks out. A dynamic model fits our data for the growth and closure of pores.

The life and death of "Bare" viscous bubbles

Air bubbles collect and explode at the surface of many viscous liquids, as observed with polymer foams, in glass furnaces, and during volcanic eruptions. The liquid film separating the bubble from bulk air can have a long lifetime (if it is viscous) even if it is not protected by a surfactant. These "bare" films display unusual dynamic behaviors in drainage and rupture. Two different model systems were studied: a polymer melt (silicone oil) and a molten (borosilicate) glass of comparable viscosity. Although the two systems differ greatly in their relaxation time, they are described by the same set of laws, which can be understood from a relatively simple hydrodynamic model.

Nucleation Radius and Growth of a Liquid Meniscus

We consider a horizontal solid plate P placed above the free surface of a liquid L separated by a layer of air of thickness e ( approximately 0.1 mm). With suitable P /L pairs this layer of air is metastable for thicknesses e below a certain limit e c ( approximately 1 mm). We have found a way of setting up bridges connecting the liquid surface with the plate in a controlled way (axisymmetric meniscus of horizontal radius R ). The meniscus grows if R is above a certain threshold R c (e ). If R < R c the meniscus shrinks to zero. Our method allows precise measurements of R c (e ): We were able to do this using silicone oils and two types of plates P (with different contact angles). Our results are in good agreement with classical calculations by G. I. Taylor and E. Michael (J. Fluid Mech. 58, 625 (1973)). Furthermore, When R > R c (e ), we find that R grows linearly with time t and that dR dt ~ e -0.7 1 - e e c 2 .