In this paper, we present a qualitative analysis of the dissipative processes during the failure of the interface between a viscoelastic polymer, characterized by a weak adhesion, and a solid surface. We reassess the "viscoelastic trumpet" model [P.-G. de Gennes, C. R. Acad. Sci. Paris, 307, 1949(1988], to express the viscous energy dissipated in the bulk as a function of the rheological moduli of the material, involving the local frequencies of sollicitation during crack propagation. We deduce from this integral expression the dhesion energy for different kind of materials: (i) we show that, for a crosslinked polymer, the dissipation had been underestimated at low velocities. Indeed, the interface toughness G(V ) starts from a relatively low value, G 0 , daue to local processes near the fracture tip, and rises up to a maximum of order G 0 (µ ∞ /µ 0 ) (where µ 0 and µ ∞ stand for the elastic modulus of the material, respectively at low and high strain frequencies). This enhancement of fracture energy is due to far-field viscous dissipation in the bulk material, and begins for peel-rates V much lower than previously thought. (ii) For a polymer melt, the adhesion energy is predicted to scale as 1/V . In the second part of this paper, we compare some of these latest theoretical predictions with experimental results about the viscoelastic adhesion between a polydimethylsiloxane polymer melt and a glass surface. In particular, the expected dependence of the fracture energy versus separation rate is confirmed by the experimental data, and the observed changes in the concavity of the crack profile are in good agreement with our simple model. More generally, beyond the qulitative and simple picture used for our approach, we expect our theoretical treatment to apply for relatively weak viscoelastic adhesives, for which the crack-tip dissipative term G 0 is weakly dependent on the fracture velocity.

Abstract. -We discuss the behaviour of a thin soap film facing a frame element: the pressure in the Plateau border around the frame is lower than the film pressure, and the film thins out over a certain distance λ(t), due to the formation of a well-localized pinched region of thickness h(t) and extension w(t). We construct a hydrodynamic theory for this thinning process, assuming a constant surface tension: Marangoni effects are probably important only at late stages, where instabilitites set in. We find λ(t) ∼ t 1/4 , and for the pinch dimensions, h(t) ∼ t −1/2 and w(t) ∼ t −1/4 . These results may play a useful role for the discussion of later instabilitites leading to a global film thinning and drainage, as first discussed by K. Mysels under the name "marginal regeneration".Early experiments by K. Mysels and coworkers [1] showed that a vertical soap film, suspended on a frame, (and made with "mobile" surfactant) thins out by nucleation and growth of black, thin spots near the Plateau borders. They called this process "marginal regeneration".There are in fact (at least) two steps in marginal regeneration: a) The pressure in the Plateau border is lower than the pressure in the film. This thins out the film near the border, and leads to a "pinch". b) The pinched state must have an intrinsic instability leading to the black spots. The two steps are very different: the pinch can occur at constant surface tension-i.e. without any Marangoni effect. On the other hand, the later instabilities are probably triggered by Marangoni flows, as pointed out by a number of authors [2][3][4][5].Our aim in the present note is restricted to the first step, i.e. the description of the pinched state with its dynamics. Pinching has already been studied in connection with the elimination of dimples in the coalescence of drops [6,7] or in the drainage of thin films [8,9]. However, in these problems, the extension of the dimpled zone is prescribed and fixes one of the spatial scales involved. Our case is different: we consider a semi-infinite film (of initial thickness e 0 ) facing a straight Plateau border. At t = 0 the film begins to pinch, and is perturbed over a certain distance λ(t) increasing with time (as we shall see λ(t) ∼ t 1/4 ). Also the width w(T ) of the small pinched region decreases with time (w(t) ∼ t −1/4 ) and so does the thickness of the pinch h(t) ∼ t −1/2 . It seems important to know these scales in detail before embarking into the second step: all data on dimples suggest that instabilities can only occur after a long

In many industrial processes, pieces of the same polymer material are brought into contact at a temperature above the glass transition. Interdiffusion takes place across the interface and leads to a strengthening of the junction. Often, a cross-linker agent is also added in order to improve the global mechanical properties of the material, as in the formation of latex films from dispersed solutions of polymer particles. We studied theoretically the competition between the interdiffusion and the cross-linking reaction, and found that the control parameter tuning the balance between these two processes is R ) Qτ 0A 0 / N 3 b 3 /Ne, where Qτ0 accounts for the reactivity of the cross-linker, A 0 / is the initial concentration of sites capable of cross-linking on the polymer chains, N is the polymerization index, Ne the number of segments between entanglements, and b a distance comparable to the segment length. The case of practical interest is R , 1: the reaction locks the interfacial chains once a significant mixing has developed, resulting in films with good mechanical properties.

We present a minimal model for spatiotemporal oscillation and rheochaos in shear-thickening complex fluids at zero Reynolds number. In the model, a tendency towards inhomogeneous flows in the form of shear bands combines with a slow structural dynamics, modelled by delayed stress relaxation. Using Fourier-space numerics, we study the nonequilibrium 'phase diagram' of the fluid as a function of a steady mean (spatially averaged) stress, and of the relaxation time for structural relaxation. We find several distinct regions of periodic behavior (oscillating bands, travelling bands, and more complex oscillations) and also regions of spatiotemporal rheochaos. A low-dimensional truncation of the model retains the important physical features of the full model (including rheochaos) despite the suppression of sharply defined interfaces between shear bands. Our model maps onto the FitzHugh-Nagumo model for neural network dynamics, with an unusual form of long-range coupling.

Living cells contain a very large amount of membrane surface area, which potentially influences the direction, the kinetics, and the localization of biochemical reactions. This paper quantitatively evaluates the possibility that a lipid monolayer can adsorb actin from a nonpolymerizing solution, induce its polymerization, and form a 2D network of individual actin filaments, in conditions that forbid bulk polymerization. G- and F-actin solutions were studied beneath saturated Langmuir monolayers containing phosphatidylcholine (PC, neutral) and stearylamine (SA, a positively charged surfactant) at PC:SA = 3:1 molar ratio. Ellipsometry, tensiometry, shear elastic measurements, electron microscopy, and dark-field light microscopy were used to characterize the adsorption kinetics and the interfacial polymerization of actin. In all cases studied, actin follows a monoexponential reaction-limited adsorption with similar time constants (approximately 10(3) s). At a longer time scale the shear elasticity of the monomeric actin adsorbate increases only in the presence of lipids, to a 2D shear elastic modulus of mu approximately 30 mN/m, indicating the formation of a structure coupled to the monolayer. Electron microscopy shows the formation of a 2D network of actin filaments at the PC:SA surface, and several arguments strongly suggest that this network is indeed causing the observed elasticity. Adsorption of F-actin to PC:SA leads more quickly to a slightly more rigid interface with a modulus of mu approximately 50 mN/m.

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We study theoretically situations where competition arises between an interdiffusion process and a cross-linking chemical reaction at interfaces between pieces of the same polymer material. An example of such a situation is observable in the formation of latex films, where, in the presence of a cross-linking additive, colloidal polymer particles initially in suspension come at contact as the solvent evaporates and, optimally, coalesce into a continuous coating. We considered the low cross-link density situation in a previous paper and presented a simple control parameter that determines the final state of the interface. In the present article, with the help of simple scaling arguments, we extend our description to higher cross-link densities. We provide predictions for the strength of the interface in different favorable and unfavorable regimes and discuss how it can be optimized.

5 pagesInternational audienceWe study theoretically the polarizability of a single metallic nanoparticle immersed into an externally pumped, active gain medium able to couple to the plasmon resonance. Within the frame of a simple long-wavelength, macroscopic description, and under steady-state conditions, we show that localized plasmons can be strongly amplified, until becoming singular for a specific amount of surrounding gain; however, we find that such gain-assisted singular plasmons exhibit spectrally spread imaginary responses and are therefore intrinsically different from singular plasmons in idealized, lossless metals. More generally, we carry a systematic study of how the plasmonic response transforms under changes in the amount of gain, and show that the coupled particle and activemedium act as a self-tuned Fano resonant system. The resulting plasmons exhibit strongly distorted line shapes with unusual but interesting features. One particularly attractive situation is that of "conjugate plasmons," which, at resonance, display a strong real response in association with minimal losses. These findings could have some applications in plasmonics, nanoantennas, nanosensing, and optical metamaterials