Dewetting of Thin Viscoelastic Polymer Films on Slippery Substrates

Dewetting of thin polystyrene films deposited onto silicone wafers at temperatures close to the glass transition exhibits unusual dynamics and front morphologies. Here, we present a new theoretical approach of these phenomena taking into account both the viscoelastic properties of the film and the non-zero velocity of the film at the interface with the substrate (due to slippage). We then show how these two ingredients lead to : (a) A very asymmetric shape of the rim as the film dewetts, (b) A decrease of the dewetting velocity with time like $t^{-{1/2}}$ for times shorter than the reptation time (for larger times, the dewetting velocity reaches a constant value). Very recent experiments by Damman, Baudelet and Reiter [Phys. Rev. Lett. {\bf 91}, 216101 (2003)] present, however, a much faster decrease of the dewetting velocity. We then show how this striking result can be explained by the presence of residual stresses in the film.Comment: Submitted to PR

Polymer chains in confined spaces and flow-injection problems: some remarks

We revisit the classical problem of the behavior of an isolated linear polymer chain in confined spaces, introducing the distinction between two different confinement regimes (the {\it weak} and the {\it strong} confinement regimes, respectively). We then discuss some recent experimental findings concerning the partitioning of individual polymers into protein pores. We also generalize our study to the case of branched polymers, and study the flow-injection properties of such objects into nanoscopic pores, for which the strong confinement regime plays an important role.Comment: Submitted June 200

Capillary-Gravity Waves Generated by a Sudden Object Motion

We study theoretically the capillary-gravity waves created at the water-air interface by a small object during a sudden accelerated or decelerated rectilinear motion. We analyze the wave resistance corresponding to the transient wave pattern and show that it is nonzero even if the involved velocity (the final one in the accelerated case, the initial one in the decelerated case) is smaller than the minimum phase velocity $c_{min}=23 \mathrm{cm s^{-1}}$. These results might be important for a better understanding of the propulsion of water-walking insects where accelerated and decelerated motions frequently occur.Comment: Submitted to Physics of Fluid

A Scaling Theory of the Competition between Interdiffusion and Cross-Linking at Polymer Interfaces

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 (A. Aradian, E. Raphael, P.-G. de Gennes, Macromolecules 33, 9444 (2000)), 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.Comment: 19 pages, 5 figures. To appear in Macromolecule

A microscopic picture of erosion and sedimentation processes in dense granular flows

Gravity-driven flows of granular matter are involved in a wide variety of situations, ranging from industrial processes to geophysical phenomena, such as avalanches or landslides. These flows are characterized by the coexistence of solid and fluid phases, whose stability is directly related to the erosion and sedimentation occurring at the solid-fluid interface. To describe these mechanisms, we build a microscopic model involving friction, geometry, and a nonlocal cooperativity emerging from the propagation of collisions. This new picture enables us to obtain a detailed description of the exchanges between the fluid and solid phases. The model predicts a phase diagram including erosion, sedimentation, and stationary-flow regimes, in quantitative agreement with experiments and discrete-element-method simulations