The formation of particle clusters near an interfacial meniscus

Abstract

A study of the behaviour of particles in dilute liquid suspensions in the proximity of a gas-liquid interface has been carried out. The experiments were performed in a thin channel, the liquid was allowed to evaporate so that a meniscus between the suspension and the vapour was formed. As this receded into the liquid, particles were concentrated in a close-packed band adjacent to the meniscus. As the number of particles in this band increased, the meniscus became unstable and bifurcated. This resulted in the particles from the band being depositied in complex patterns, the development of which was shown to be a strong function of the particle size. Where the meniscus bifurcated, a thin filament of suspension was formed. When the particles in the original suspension were small, mean diameter d ⩽ 10 μm, the liquid subsequently evaporated from the filament to produce a final deposit of particles as ordered chains. In experiments with larger partices d ⩾ 13 μm, the filaments ruptured leave “islands”. As the liquid evaporated from these, particles reassembled under the action of capillary forces to form compact discs. Using spherical particles, precipitates of copper oxalate or glass ballotini, the particles in the discs assumed a structure close to that of the lowest configurational entropy. Various parameters were studied in relation to the above-described particle ordering phenomenon. Particle distribution, concentration and morphology, surface free energy, surface tension and the presence of various types of surface-active agents were all studied. In systems where the deposit formed ordered chains, successive bifurcations of the meniscus produced branched structures, the patterns of which were shown to be reproducible. These patterns resembled river drainage nets and other similar phenomena. The analysis of the patterns showed them to be fractal structrues with a fraction dimension of 1.31.