Dynamics of Surfactant-Driven Fracture of Particle Rafts

Abstract

We investigate the dynamic fracture of a close-packed monolayer of particles, or particle raft, floating at a liquid-gas interface induced by the localised addition of surfactant. Unusually for a two-dimensional solid, our experiments show that the speed of crack propagation here is not affected by the elastic properties of the raft. Instead it is controlled by the rate at which surfactant is advected to the crack tip by means of the induced Marangoni flows. Further, the velocity of propagation is not constant in time and the length of the crack scales as t 3/4 . More broadly, this surfactant induced rupture of interfacial rafts suggests ways to manipulate them for applications.The curious behavior of particulate interfaces that separate liquids is a source of interesting questions at the intersection of hydrodynamics and elasticity. For instance, liquid drops coated with a fine hydrophobic powder become non-wetting [1], forming an artificial analog of a much older solution stumbled upon by insects [2]. Similarly, the addition of particles to the surface of liquid drops prior to coalescence stabilises the coalesced drops to the common pinch-off instability and can lead to reversible morphological instabilities such as buckling when subject to pressure Recent experiments [7,8] show that a densely packed monolayer of particles at an interface (a 'particle raft') has many of the characteristics of a two-dimensional linear elastic solid in certain regimes. For example, under compressive loading, a particle raft statically buckles out of the plane demonstrating that collectively its constituent particles possess a non-zero shear modulus, with γ the surface tension coefficient of the pure liquid-gas interface and d the particle diameter. This ability to sustain finite shear stresses arises from a combination of capillary forces and the short range steric constraints due to particle-particle contact, and is seen in a variety of similar systems such as armored bubbles, drying colloidal drops and suspensions [5,9,10,11], although understanding the kinetics of onset of this elastic behavior constitutes work in progress. Going beyond the linear elastic behavior, the ability to sustain finite shear stresses suggests that these rafts should also be able to sustain fractures which relieve stresses primarily in one direction [8]. This fracture can be observed in the particulate scum that forms on the surface of a cup of black tea, which is then fractured by the addition of milk. A similar phenomenon is observed in pond scum when fractured by the ripples induced by a pebble. Here we use an interfacial particle raft as a vehicle to study the cracks induced by the addition of surfactant and provide a model for

Instabilities in Droplets Spreading on Gels

“…In addition, prior studies of viscoelastic substrates have focused on substrate breakup or fracture when subjected to stresses [10,11,12,13]. Here, we observe a failure of the gel driven by surface tension.…”

“…Intriguingly, recent work on the surfactantinduced fracture of a particle raft [12], saw a multi-armed morphology similar to the starbursts observed here and t 3/4 spreading behavior, which the authors associated with liquid-on-liquid behavior.…”

Controlling Pickering Emulsion Destabilisation: A Route to Fabricating New Materials by Phase Inversion

“…Vella et al [35] observed that adding surfactant could disrupt particle layers attached to planar water surfaces. They examined densely packed monolayers of polymeric particles ( r p = 50 μm) on the surfaces of water-glycerol solutions.…”

“…They used a needle to inject a drop of non-ionic surfactant (polyoxyethylene sorbitan monoleate) into the layer. Providing the particles were not jammed together, Vella et al [35] observed a crack form, where the needle touched the particles, and propagate through the monolayer. They argued that localised reduction of the surface tension caused tensile stress in the particle layer, forcing the particles to rearrange [35].…”

Tearing of thin sheets: Cracks interacting through an elastic ridge

“…This process occurs mainly at the atomic scale near the crack tip, where the energy focuses, but also at much larger scale for particle rafts [21]. Nevertheless, macroscopic parameters, like work of fracture γ or fracture toughness K, can be defined (and measured) to describe the progression of cracks when the material properties are uniform without necessarily resorting to microscopic analysis [1,22,23].…”