A Star is Born: Distinct Marangoni Particulate Patterns Emerging in Confined Spaces

A gradient in surface tension can induce the Marangoni effect. To visualize it, tracker particles (such as pepper flakes) can be distributed over the surface and local introduction of surfactants will result in a particle-free region due to the Marangoni effect. Typically, this region is perfectly circular as a result of rotational symmetry in stresses. However, if the number of tracker particles per surface area is too large, the assumption of negligible interaction breaks down. In this case, the particulate layer can fracture [1]. The densification can be a shock driven jamming and subsequent fracture [2], but it can also be induced alone by the reservoir’s finite size. It is due to the surfactants clearing an area for particles that in turn increases particle density in the rest of the reservoir. We investigated this alternative route to fracture. In particular, we studied the (dynamic) transition from circle to star shape as a function of both initial particle density and added soap volume.

[1] D. Vella, H. Y. Kim, P. Aussillous, and L. Mahadevan, Dynamics of Surfactant-Driven Fracture of Particle Rafts, Phys. Rev. Lett. 96, 1 (2006).
[2] M. M. Bandi, T. Tallinen, and L. Mahadevan, Shock-Driven Jamming and Periodic Fracture of Particulate Rafts, Epl 96, (2011).