1The resistance to fracture of reversible biopolymer hydrogels is an important control factor of the cutting/slicing and eating characteristics of food gels 1 . It is also critical for their utilization in tissue engineering, for which mechanical protection of encapsulated components is needed 2,3 . Its dependence on loading rate 4 and, recently, on the density and strength of cross-links 3 has been investigated.But no attention was paid so far to solvent nor to environment effects. Here we report a systematic study of crack dynamics in gels of gelatin in water/glycerol mixtures. We show on this model system that: (i) increasing solvent viscosity slows down cracks; (ii) soaking with solvent increases markedly gel fragility; (iii) tuning the viscosity of the (miscible) environmental liquid affects crack propagation via diffusive invasion of the crack tip vicinity. The results point toward the fact that fracture occurs by viscoplastic chain pull-out. This mechanism, as well as the related phenomenology, should be common to all reversibly cross-linked (physical) gels.Gelatin gels are constituted of denatured (coil) collagen chains, held together by crosslinks made of segments of three-stranded helices stabilized by hydrogen bonds 5 . This network, swollen by the aqueous solvent, which controls its (undrained) bulk modulus, is responsible for the finite shear modulus µ, of order a few kPa. Hence, hydrogels can be considered incompressible. One estimates average mesh sizes ξ ∼ (kT /µ) 1/3 of order 10 nm, i.e. coil segments involving a few 100 units (residues) 6 . Moreover, in the presence of pressure gradients, the solvent diffuses through the network. This poroelastic behaviour 7,8 controls e.g. slow solvent draining in or out of the gel under applied stresses.They are thermoreversible, i.e., in contrast with chemical, covalently cross-linked gels, their network "melts" close above room temperature. This behavior, assignable to their small cross-link binding energy, leads to the well studied 5 slow aging (strengthening) of µ, and to their noticeable creep under moderate stresses 9 . When stretched at constant strain rate, gelatin gels ultimately fail at a strain ∼ 1 which, though rather poorly reproducible, is clearly rate-dependent 4 . In order to get insight into the nature of the dissipative processes at play, one needs to investigate the propagation of cracks independently from their (stochastic) nucleation 10 . Here we study the fracture energy G(V ) needed to propagate a crack at constant velocity V in notched long thin plates (see Fig. 1) of gels differing by the glycerol content of their aqueous solvent.

International audienceThe rate dependence of fracture has been studied in a series of physically associating triblock copolymer gels that have a well-defined molecular structure. Compressive experiments were performed to develop a strain energy function that accurately captures the strain hardening behavior of these materials. This same strain energy function was utilized in a finite element model of the crack tip stresses, which become highly anisotropic at stress values below the failure strength of the gels. The rate dependence of the energy release rate, G, is independent of the gel concentration when G is normalized by the small strain Young's modulus, E. The gels exhibit a transition from rough, slow crack propagation to smooth, fast crack propagation for a well-defined value of the characteristic length, G/E

The recent theory of compressive sensing leverages upon the structure of signals to acquire them with much fewer measurements than was previously thought necessary, and certainly well below the traditional Nyquist-Shannon sampling rate. However, most implementations developed to take advantage of this framework revolve around controlling the measurements with carefully engineered material or acquisition sequences. Instead, we use the natural randomness of wave propagation through multiply scattering media as an optimal and instantaneous compressive imaging mechanism. Waves reflected from an object are detected after propagation through a well-characterized complex medium. Each local measurement thus contains global information about the object, yielding a purely analog compressive sensing method. We experimentally demonstrate the effectiveness of the proposed approach for optical imaging by using a 300-micrometer thick layer of white paint as the compressive imaging device. Scattering media are thus promising candidates for designing efficient and compact compressive imagers.