Research: Particle Rafts

When a large number of particles float in a close packed array at an interface, they change the area of liquid air interface available and so alter the surface tension of that interface. For very dense packings, this can increase the surface tension coefficient by an order of magnitude. In addition, the presence of solid particles constrains the fluid to such an extent that we can accumulate large anisotropic stresses. If we then compress this "particle raft" then it is observed to buckle in a time independent manner (shown in figs. 1 (a) and (b)) leading to a well defined wrinkling pattern with a wavelength. The value of this wavelength can be determined from classical elasticity theory and hence allows us to probe the physical properties of the raft, such as its Young's modulus.

As well as this buckling instability, particle rafts have other, solid-like, properties. For example, in tension they will fracture very easily. A graphic illustration of this is shown in fig. 1 (c) where a drop of surfactant has been added to the raft. The addition of surfactant leads to a surface tension gradient, which is sufficient to open a crack in the solid that propagates in a stick-slip manner.

Figure 1: (a) A buckled particle raft for spherical particles with diameter 0.297 mm. (b) A buckled particle raft with irregular shaped particles of diameter around 0.02 mm (c) Almost the final state of a cracked particle raft consisting of particles of diameter around 0.15 mm. The pin here (of diameter 1 mm) only generates a crack if it is coated with some form of surfactant. The sharpness of the pin does not cause the cracking, since a clean pin will only pierce the solid. The scale bars in (a) and (b) represent 5 mm.

In many ways, these particle rafts are similar to the bubble rafts introduced by Bragg and Nye. However, they are much stiffer than these bubble rafts and are more stable (particles don't pop!). They are also distinct from bubble rafts because of the buckling instability which does not occur in bubble rafts, since it is easier for bubbles to form a foam. We also think that these particle rafts are related to a number of other experiments that have been performed in recent years. For example, liquid marbles are essentially drops of liquid encased in a spherical particle raft - this prevents them from wetting the substrate on which they are placed so that they can even roll on solids! In a similar way, the Weitz Group have done a lot of work on the encapsulation of drugs in thin elastic shells consisting of small particles.

References:

L. Bragg and J. F. Nye, "A dynamical model of crystal structure", Proc. R. Soc. Lond. A 190, 474 (1947).
P. Aussillous and D. Quere, "Liquid marbles", Nature 411, 924 (2001).
D. Vella, P. Aussillous and L. Mahadevan, "Elasticity of interfacial particle rafts", Europhys. Lett. 68 (2004), 212. arXiv:cond-mat/0406723

Abstract:

We study the collective behaviour of a close packed monolayer of non-Brownian particles at a fluid-liquid interface. Such a particle raft forms a two-dimensional elastic solid and can support anisotropic stresses and strains, e.g. it buckles in uniaxial compression and cracks in tension. We characterise this solid in terms of a Young's modulus and Poisson ratio derived from simple theoretical considerations and show the validity of these estimates by using an experimental buckling assay to deduce the Young's modulus.

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