PhD Project: Brittle-to-Ductile Yielding Transition in Colloidal Gels and Glasses
see link: 2024PhD_advertisement_BTD
Understanding the response of soft materials under stress is of paramount importance, both fundamentally and in applications. Very often, the mechanical response of the material is dictated by its interactions with surfaces. To overcome this issue and probe the intrinsic properties of materials, we will produce unique spherical beads of gel and glasses of millimetric size. The gels and glasses are based on colloidal particles. The gels consist in a homogeneous porous stress-bearing network structure and the glasses consist in a dense packing of the particles. Our objective is to rationalize the fate of the beads under a mechanical compression, and especially understand how they yield. We want to investigate the complex interplay between the flow of liquid through the pores (the poroelasticity), the plasticity and fracture of the structure. We expect to unveil the physical mechanisms that control whether a bead of gel or glass yields in a brittle or ductile manner.
We will use a multiscale approach that combines mechanical measurements under compression, image analysis, and time- and space-resolved light scattering. We will explore several strategies to produce unique beads of colloidal gels and glasses with tunable toughness and brittleness. We will build an optical set-up to measure with a space and time resolution the microscopic rearrangements of the colloids in the bead during compression. We will measure concomitantly the mechanical response of the bead to the compressive stress and image the overall behavior of the bead.
The work will be performed within the Soft Matter Team (https://www.softmatter-l2c.fr/) of the Laboratoire Charles Coulomb in Montpellier, France, under the supervision of Laurence Ramos and Luca Cipelletti, in close collaboration with Christian Ligoure.
Expected starting time: September 2024
(gauche) Schéma de l’expérience de frottement entre le gel et une sphere en silice : la vitesse V et la force normale Fn sont imposées et on mesure la force de frottement Ft (droite) Cette mesure macroscopique permet d’accéder aux caractéristiques physiques des mécanismes moléculaires à l’origine du frottement : Les chaines polymères du gel s’adsorbe sur la silice, elles sont ensuite étirées lors du glissement de la silice par rapport au gel, puis se désorbent quand la force f de rappel élastique devient trop grande. Ce processus dissipatif est à l’origine du frottement, qui dépend de l’énergie d’adsorption moléculaire et de l’élasticité des chaines. L’expérience permet donc une mesure des interactions moléculaires.