Regular Article

Probing the local response of a two-dimensional liquid foam^★,★★

¹ Aalto University, Department of Applied Physics, PO Box 11100, 00076 Aalto, Finland
² Université de Lyon, ENSL, UCBL, CNRS, Laboratoire de Physique, Lyon, France
³ Lavrentyev Institute of Hydrodynamics, Novosibirsk, Russia

^a e-mail: leevi.viitanen@aalto.fi

Received: 19 June 2018
Received in final form: 17 December 2018
Published online: 13 February 2019

Abstract

Aqueous foams are viscoelastic yield stress fluids. Due to their complex rheology, foam flow around an obstacle embedded in a 2D Hele-Shaw cell has been widely studied. Typically, in such geometry in the moving flow reference frame the flow field of viscoelastic fluids exhibit a quadrupolar structure characterized by a negative wake. Here, we introduce a measuring geometry, new in this context, whereby instead of flowing the foam around the obstacle, we move the obstacle as an intruder inside the foam. The proposed setup makes it possible to independently control the driving velocity and the liquid foam properties, such as the gas fraction and polydispersity. We show that the liquid foam velocity field around the intruder is similar to the one observed in viscoelastic fluids, in particular the emergence of a negative wake, e.g. a velocity overshoot downstream side of the obstacle. However, surprisingly, the intensity of this velocity overshoot decreases with the number of intruder passes, probably related to the evolution of the local disordered structure of the liquid foam.