Elastic sheets in shear flows, from single sheet behavior to exfoliation process: an experimental study.*

Graphene has attracted much attention of the past decade as a candidate material for applications in a variety fields such as electronics, energy generation and storage, and biomedicine. However, the fabrication of high quality graphene at industrial scale and affordable price remains a challenge. One of the most promising techniques to produce graphene from graphite is so-called liquid-phase exfoliation. In this technique, graphite is sheared in a liquid until layers of graphene detach from the bulk material.

While being poorly understood, the mechanism of exfoliation is known to rely on three physical ingredients: adhesion strength between sheets, their bending rigidity and the large scale imposed viscous shear stress. For the dynamic processes, exfoliation can be composed of different modes of deformation: peeling of two sheets, sliding between two sheets or a buckling instability, du to the compressive component of a shear flow, that leads to the formation of a blister and detach two sheets. To investigate the role of the three ingredients and the modes of deformation, here we present the first model experiment of elastic adhesive sheets in shear flows at the macroscopic scale. As a requirement one needs to understand the characteristics of viscous forces exerted on elastic sheets. To answer that question, we performed systematic studies of the dynamic of two sheets suspended in a viscous shear flow. Surprisingly, our experiments demonstrate that the critical shear rate to bend two sheets depends non-monotonically on the distance between the two sheets. The evidence of this new behavior can strongly impact the rheology a suspension composed of deformable sheets and could have many repercussions on the industrial processes of suspensions.

ERC funding (project FLEXNANOFLOW, 715475)