We investigate the delayed rupture of biopolymer gels under a constant shear load by simultaneous dynamic light scattering and rheology measurements. We unveil the crucial role of normal stresses built up during gelation: all samples that eventually fracture self-weaken during the gelation process, as revealed by a partial relaxation of the normal stress concomitant to a burst of microscopic plastic rearrangements. Upon applying a shear stress, weakened gels exhibit in the creep regime distinctive signatures in their microscopic dynamics, which anticipate macroscopic fracture by up to thousands of seconds. The dynamics in fracturing gels are faster than those of non-fracturing gels and exhibit large spatio-temporal fluctuations. A spatially localized region with significant plasticity eventually nucleates, expands progressively, and finally invades the whole sample triggering macroscopic failure.

We exploit the deformation of drops spinning in a denser background fluid to investigate the effective interfacial tension (EIT) between miscible molecular fluids. We find that, for sufficiently low interfacial tension, spinning drops develop dumbbell shapes, with two large heads connected by a thinner central body. We show that this shape depends not only on the density and viscosity contrast between the drop and background fluids, but also on the fluid molecular structure, and hence on the stresses developing at their interface due to a different molecular interaction. We systematically investigate the dynamics of dumbbell-shaped drops of water-glycerol mixtures spinning in a pure glycerol reservoir. By developing a model for the deformation based on the balance of the shear stress opposing the deformation, the imposed normal stress on the drop, and an effective interfacial tension, we exploit the time evolution of the drop shape to measure the EIT. Our results showthat the EIT in water-glycerol systems is orders of magnitude lower than that reported in previous experimental measurements, and in excellent agreement with values calculated via the phase field model proposed by Truzzolillo et al. [Phys. Rev. X 6, 041057 (2016)].

Taylor dispersion analysis (TDA) is a powerful sizing technique very well suited for (macro)molecules between angstroms and submicrons (typically up to 200 nm). However, new detection modes are required for non-UV-absorbing (macro)molecules such as most of the polysaccharides, including starches. In this work, two different detection modes were compared, backscattering interferometry (BSI) and UV-photooxidation detection (UV-POD). TDA-BSI measures the relative change of the refractive index between eluent and sample (water as eluent in this work), whereas TDA-UV-POD detects the UV-absorbing photo-oxidized products of polysaccharides/starches in strong alkaline media (130 mM NaOH or 1 M KOH). TDA-UV-POD detection was evaluated for linearity and sensitivity at two wavelengths, 214 and 266 nm. The mass-average hydrodynamic radius (Rh) obtained by TDA-BSI and TDA-UV-POD was found to be in excellent agreement, whereas higher average Rh values were obtained by batch dynamic light scattering (DLS) under the same conditions, because of the higher sensitivity of DLS to large-size solutes and aggregates. The hydrodynamic radius distributions obtained by TDA and DLS are intrinsically different but both techniques were found to be complementary, providing useful information on sample dispersity. Owing to the absence of the stationary phase, low sample consumption with straightforward sample preparation (no filtration), and no calibration, TDA is anticipated to become a method of choice for the size-based characterization of polysaccharides, including starches.

The mechanical properties of soft matter are of great importance in countless applications, in addition of being an active field of academic research. Given the relative ease with which soft materials can be deformed, their non-linear behavior is of particular relevance. Large loads eventually result in material failure. In this Perspective article, we discuss recent work aiming at detecting precursors of failure by scrutinizing the microscopic structure and dynamics of soft systems under various conditions of loading. In particular, we show that the microscopic dynamics is a powerful indicator of the ultimate fate of soft materials, capable of unveiling precursors of failure up to thousands of seconds before any macroscopic sign of weakening.

Cells and tissues have the remarkable ability to actively generate the forces required to change theirshape. This active mechanical behavior is largely mediated by the actin cytoskeleton, a crosslinkednetwork of actin filaments that is contracted by myosin motors. Experiments and active gel theorieshave established that the length scale over which gel contraction occurs is governed by a balancebetween molecular motor activity and crosslink density. By contrast, the dynamics that govern thecontractile activity of the cytoskeleton remain poorly understood. Here we investigate the microscopicdynamics of reconstituted actin–myosin networks using simultaneous real-space video microscopy andFourier-space dynamic light scattering. Light scattering reveals different regimes of microscopicdynamics as a function of sample age. We uncover two dynamical precursors that precede macroscopicgel contraction. One is characterized by a progressive acceleration of stress-induced rearrangements,while the other consists of sudden, heterogeneous rearrangements. Intriguingly, our findings suggest aqualitative analogy between self-driven rupture and collapse of active gels and the delayed rupture ofpassive gels observed in earlier studies of colloidal gels under external loads.

We present a new light scattering setup coupled to a commercial rheometer operated in the plate-plate geometry. The apparatus allows the microscopic dynamics to be measured, discriminating between the contribution due to the affine deformation and additional mechanisms, such as plasticity. Light backscattered by the sample is collected using an imaging optical layout, thereby allowing the average flow velocity and the microscopic dynamics to be probed with both spatial and temporal resolution. We successfully test the setup by measuring the Brownian diffusion and flow velocity of diluted colloidal suspensions, both at rest and under shear. The potentiality of the apparatus are explored in the startup shear of a biogel. For small shear deformations, $\gamma \le 2\%$, the rheological response of the gel is linear. However, striking deviations from affine flow are seen from the very onset of deformation, due to temporally and spatially heterogeneous rearrangements bearing intriguing similarities with a stick-slip process.