Pattern formation in draining thin film suspensions

In collaboration between the Soft Matter Group in Leeds, and researchers at Oslo University, a striking transition was discovered between two classes of self-organizing patterns in draining thin-film suspensions.

The patterns are surprisingly common in everyday life, and immediately recognizable. This is because draining thin-film suspensions encompass two extremely familiar cases: dirt and food. From the channel patterns on a half-emptied yogurt pot, to the band patterns of dirt accreted under a car's wheel-arch, or of honey on the side of a jar, these two classes of self-forming patterns are ubiquitous. Why do they keep cropping up?

To answer that question is to gain fundamental insights into the universal principles governing the behaviour of non-equilibrium matter.

In our interdisciplinary work, we report a striking transition between two classes of self-organizing patterns in draining thin-film suspensions. Increasing the mean particle size induces the emergent pattern to switch from vertical channels to horizontal bands, marking the cross-over from colloidal to granular dynamics.

Liquid films containing suspended particles are ubiquitous throughout biology, sedimentary geology, and complex-fluids processing. The universal pattern-formation mechanisms are relevant to current research in biophysics, geology, soft-matter physics, surface phenomena, and emergent complexity.

Our description of the generic mechanisms draws on non-equilibrium thermodynamics, colloidal physics, phenomenology of granular media, lubrication theory, and two-phase flow dynamics. The work is published in the article "Pattern formation in draining thin film suspensions", M. Buchanan, D. Molenaar, S. de Villiers & R. M. L. Evans, Langmuir 23, 3732 (2007) [link], and was favourably reviewed in the the "Research Highlights" section of Nature [Nature 446, 587 (2007)] [link], and in the "Research News and Discoveries In Brief" section of New Scientist (24 March 2007) [link], as well as Forensic Magazine [link] and the Danish scientific magazine, Aktuel Naturvidenskab.