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A Soap-Film Mobius Strip Changes Topology with a Twist Singularity

Goldstein, RE; Moffatt, HK; Pesci, AI (University of Cambridge)
Wednesday 25 July 2012, 12:10-12:30

Seminar Room 1, Newton Institute


It is well-known that a soap film spanning a looped wire can have the topology of a Möbius strip and that deformations of the wire can induce a transformation to a two-sided film, but the process by which this transformation is achieved has remained unknown. In this talk I will disucss recent experimental and theoretical work [Goldstein, Moffatt, Pesci, and Ricca, PNAS 107, 21979 (2010)] that has uncovered the dynamics of this transition. We find that this process consists of a collapse of the film toward the boundary that produces a previously unrecognized finite-time twist singularity that changes the linking number of the film's Plateau border and the centerline of the wire. We conjecture that it is a general feature of this type of transition that the singularity always occurs at the surface boundary. The change in linking number is shown to be a consequence of a viscous reconnection of the Plateau border at the moment of the singularity. High-speed imaging of the collapse dynamics of the film's throat, similar to that of the central opening of a catenoid, reveals a crossover between two power laws. Far from the singularity, it is suggested that the collapse is controlled by dissipation within the fluid film surrounding the wire, whereas closer to the transition the power law has the classical form arising from a balance between air inertia and surface tension. Analytical and numerical studies of minimal surfaces and ruled surfaces are used to gain insight into the energetics underlying the transition and the twisted geometry in the neighborhood of the singularity. A number of challenging mathematical questions arising from these observations are posed.


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