John Dolbow's KEYNOTE seminar "Surfactant-driven Fracture of Particulate Rafts: the Mechanics Behind the Pepper Experiment"

John_Dolbow_ March_17_2022.pdf (pdf 177 kB)

Abstract. When a densely-packed monolayer of hydrophobic particles is placed on a fluid surface, the particles interact through capillary bridges, leading to the formation of a particulate raft or ``praft". Densely-packed monolayers exhibit a two-dimensional elastic response, and they are capable of supporting both tension and compression. The introduction of a controlled amount of surfactant generates a surface tension gradient, producing Marangoni forces and causing the surfactant to spread, fracturing the monolayer. These systems have a common analog in an at-home science exercise (the pepper experiment) that has been used by educators to illustrate some of the fundamental concepts behind surface tension. More broadly, these systems are of interest to materials scientists and engineers because they provide an idealized setting for investigating the interplay between fluid flow and fracture. Previous studies of the surfactant-induced fracture of prafts have examined the role of viscosity and the packing fraction on the temporal evolution of the fractures. Precisely why densely-packed monolayers fracture in particular and repeatable patterns has remained an open question. This seminar will describe a continuum-based model and simulations that accounts for the interplay between the pressure exerted by a spreading surfactant and the elastic response of the praft, including the fracture toughness. This work was performed in collaboration with Christian Peco, now at Penn State University, and Yingjie Liu, now at Cubist.