Competition of time and spatial scales in polymer glassy dynamics: Rejuvenation and confinements effects

We use molecular-dynamics (MD) simulations and an original lateral contact experiment to explore the influence of mechanical history on polymer mechanical behavior and segmental mobility. Two typical glassy polymers are considered: bulk acrylate (experiments) and atactic polystyrene (aPS) in a bulk and in thin films (simulations). Stress-strain behavior has been investigated both experimentally for sheared, 50 µm thick, acrylate films and by MD simulations of an aPS in a bulk for two different strain rates in a closed extension–recompression loops. Cyclic shear strains applied in the plastic regime were found experimentally to induce a progressive transition of the mechanical response of the polymer glass toward a steady state which is characterized by a strong reduction of the apparent – non linear – shear modulus. The dynamics of the polymer glass in this yielded state was subsequently analyzed from a measurement of the time dependent linear viscoelastic properties at various imposed frequencies. Immediately after the cyclic plastic deformation, mechanical "rejuvenation" of the polymer is evidenced by a drop in the storage modulus and an increase in the loss modulus, as compared to the initial values recorded before plastic deformation. A progressive recovery of the viscoelastic properties is also measured as a function of time as a result of the enhanced aging rate of the system. This experimentally observed mechanical rejuvenation of polymer has been for the first time connected to the drastic increase in the simulated segmental mobility. A simulated distribution of relaxation times shows a shift to shorter times of the a and ß relaxation processes which is consistent with the observed experimental changes in the viscoelastic modulus after rejuvenation. Finally, we present our first findings on the thickness- and substrate-dependence of the simulated glass transition temperature for thin aPS films. We observe the decrease of the glass transition temperature with film thickness, but for extremely thin (less than 2 nm) films

Slip dynamics at a patterned rubber/glass interface during stick-slip motions

We report on an experimental study of heterogeneous slip instabilities generated during stick-slip motions at a contact interface between a smooth rubber substrate and a patterned glass lens. Using a sol-gel process, the glass lens is patterned with a lattice of parallel ridges (wavelength, 1.6 μm, amplitude 0.35 μm). Friction experiments using this patterned surface result in the systematic occurrence of stick-slip motions over three orders of magnitude in the imposed driving velocity while stable friction is achieved with a smooth surface. Using a contact imaging method, real-time displacement fields are measured at the surface of the rubber substrate. Stick-slip motions are found to involve the localized propagation of transverse interface shear cracks whose velocity is observed to be remarkably independent on the driving velocity

Variable response to electric shark deterrents in bull sharks, Carcharhinus leucas

International audienceAlthough relatively rare, human-shark interactions and sharks bites are increasing globally, which has led to the development of various mitigation measures. Electric shark deterrents (ESDs) have, so far, been the most efective personal deterrents, but have only been scientifcally tested on one of the species most frequently responsible for shark bites, i.e. white shark (Carcharodon carcharias). We tested the efectiveness of fve ESDs (E-Shark Force, NoShark, Rpela v2, Freedom+Surf, Freedom+Surf—Shortboard) on bull sharks, Carcharhinus leucas, over a period of 21 days in September 2019, in New Caledonia. Standardised bait was attached 30 cm below an experimental board that had an active ESD for up to 15 min, or until a bull shark touched the bait or the board. We compared the numbers of baits taken, numbers of passes and reactions around the board, as well as the distance between the sharks and the board among ESDs and against a control board with bait and no active ESD. The Freedom+Surf was the most efective ESD, reducing the amounts of baits taken by 42.3%, while the Rpela v2 and Freedom+Surf—Shortboard also signifcantly reduced the number of baits taken by 16.5% and 16.2% respectively. Mean distance between sharks and the bait was not afected by the ESDs, but the number of approaches and the proportion of reactions were both signifcantly higher when the Freedom+Surf was active compared to other ESDs. The efectiveness of all ESDs decreased over time, with the likelihood of the bait being taken increasing and the number of approaches and distance between sharks and the bait decreasing. Our fndings show that the ability of ESDs to deter bull shark varies between products, with the Freedom+Surf resulting in the most behavioural changes, followed by the Rpela v2 and Freedom+Surf—Shortboard. However, none of the products tested completely stopped sharks from taking the bait

Normal contact and friction of rubber with model randomly rough surfaces

International audienceWe report on normal contact and friction measurements of model multicontact interfaces formed between smooth surfaces and substrates textured with a statistical distribution of spherical micro-asperities. Contacts are either formed between a rigid textured lens and a smooth rubber, or a flat textured rubber and a smooth rigid lens. Measurements of the real area of contact A versus normal load P are performed by imaging the light transmitted at the microcontacts. For both interfaces, A(P) is found to be sub-linear with a power law behavior. Comparison with two multi-asperity contact models, which extend the Greenwood–Williamson (J. Greenwood and J. Williamson, Proc. Royal Soc. London Ser. A, 295, 300 (1966)) model by taking into account the elastic interaction between asperities at different length scales, is performed, and allows their validation for the first time. We find that long range elastic interactions arising from the curvature of the nominal surfaces are the main source of the non-linearity of A(P). At a shorter range, and except for very low pressures, the pressure dependence of both density and area of microcontacts remains well described by Greenwood–Williamson's model, which neglects any interaction between asperities. In addition, in steady sliding, friction measurements reveal that the mean shear stress at the scale of the asperities is systematically larger than that found for a macroscopic contact between a smooth lens and a rubber. This suggests that frictional stresses measured at macroscopic length scales may not be simply transposed to microscopic multicontact interfaces

Variable response to electric shark deterrents in bull sharks, Carcharhinus leucas

Although relatively rare, human-shark interactions and sharks bites are increasing globally, which has led to the development of various mitigation measures. Electric shark deterrents (ESDs) have, so far, been the most effective personal deterrents, but have only been scientifically tested on one of the species most frequently responsible for shark bites, i.e. white shark (Carcharodon carcharias). We tested the effectiveness of five ESDs (E-Shark Force, NoShark, Rpela v2, Freedom+Surf, Freedom+Surf-Shortboard) on bull sharks, Carcharhinus leucas, over a period of 21 days in September 2019, in New Caledonia. Standardised bait was attached 30 cm below an experimental board that had an active ESD for up to 15 min, or until a bull shark touched the bait or the board. We compared the numbers of baits taken, numbers of passes and reactions around the board, as well as the distance between the sharks and the board among ESDs and against a control board with bait and no active ESD. The Freedom+Surf was the most effective ESD, reducing the amounts of baits taken by 42.3%, while the Rpela v2 and Freedom+Surf-Shortboard also significantly reduced the number of baits taken by 16.5% and 16.2% respectively. Mean distance between sharks and the bait was not affected by the ESDs, but the number of approaches and the proportion of reactions were both significantly higher when the Freedom+Surf was active compared to other ESDs. The effectiveness of all ESDs decreased over time, with the likelihood of the bait being taken increasing and the number of approaches and distance between sharks and the bait decreasing. Our findings show that the ability of ESDs to deter bull shark varies between products, with the Freedom+Surf resulting in the most behavioural changes, followed by the Rpela v2 and Freedom+Surf-Shortboard. However, none of the products tested completely stopped sharks from taking the bait

Slip dynamics at a patterned rubber/glass interface during stick-slip motions

International audienceWe report on an experimental study of heterogeneous slip instabilities generated during stick- slip motions at a contact interface between a smooth rubber substrate and a patterned glass lens. Using a sol-gel process, the glass lens is patterned with a lattice of parallel ridges (wave length, 1.6 μm, amplitude 0.35μm). Friction experiments using this patterned surface result in the systematic occurrence of stick-slip motions over three orders of magnitude in the imposed driving velocity while stable friction is achieved with a smooth surface. Using a contact imaging method, real-time displacement fields are measured at the surface of the rubber substrate. Stick-slip motions are found to involve the localized propagation of transverse interface shear cracks whose velocity is observed to be remarkably independent on the driving velocity