DATE : Vendredi 6 octobre 2017 – 11h00
Salle 050 Bat 11B
Intervenant : Angelo Pommella, Laboratoire Charles Coulomb, Université de Montpellier
Structural changes and rupture phenomena are ubiquitous in soft matter systems under stress, beingoften related and central to determine the final properties of medical, food, and personal care products. Indeed, some bio-applications deal with soft objects (e.g. vesicles) under flow while othersrequire specific supports (bio-gels) in static stressed conditions. In both cases, the dynamics and the rupture strongly depend on the time-dependent evolution of the internal structure. In this talk, I will present three studies aiming to elucidate some key mechanisms of the dynamics and rupture of
vesicles and bio-gels under different stress conditions. In the first part of the talk, I will focus on multilamellar vesicles. Giant vesicles (typical diameter 10 – 100 microns) are usually generated by the folding of a lamellar phase made of amphiphilic molecules and can be either unilamellar (made of a single bilayer) or multilamellar (made of a stack of bilayers). Their collective behaviour has been studied extensively but experimental results on the flow behaviour of individual multilamellar ve- sicles are lacking. We used a simple shear flow to investigate the dynamic behaviour of multilamellar
vesicles. We compared the results with the well-known dynamics of unilamellar vesicles and emulsion droplets finding surprisingly analogies with both systems. We explained these analogies analysing the evolution of the vesicle microstructure under flow. In the second part of the talk I will present a study on unilamellar vesicles. While the flow-induced dynamics upon small deformation of giant vesicles is well-known, the behaviour upon large deformation, such that unilamellar vesicles break, has rarely been studied for the case of flow-induced deformation. We investigated how break-up of unilamellar vesicles is affected by the mechanical properties of vesicle membranes under flow. We
used an acoustic microstreaming flow, a complex flow generated by an ultrasound-driven bubble, to probe the effect of the excess area and the area stretching modulus. We demonstrated that selective break-up based only on the mechanical properties of different vesicle populations can be achieved opening the way to vesicle sorting mechanism under flow. In the last part of the talk, I will show an investigation on the failure of agarose gels. So far, many studies approached the failure of soft systems from a macroscopic point of view while, very few focussed on the microscopic structure and, usually, only after failure takes place. We investigated the time-evolution of the dynamics of the gel microstructure when a constant stress is imposed to the gel leading to the material failure. We used a novel experimental setup combining rheology and dynamic light scattering. The results show the presence of a heterogeneous dynamic activity in the gel microstructure predicting the imminent failure. These dynamic events are not present in the rheological macroscopic behaviour.