Flowing Crystals: Nonequilibrium Structure of Foam

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Written by: Grant England AP225 Fall, 2011

Info

Title: Flowing Crystals: Nonequilibrium Structure of Foam

Authors: Piotr Garstecki and George M. Whitesides

@Harvard: http://gmwgroup.harvard.edu/pubs/pdf/965.pdf

Keywords

foam, lattice, metastable state

Summary

This paper describes the formation of five different types of "flowing crystals" of foam in a nonequilibrium process and attempts to describe why the different phases form by showing that it cannot be a function of interfacial energy.

Methods and Results

Foam was generated for the experiment by using a flow-focusing device as shown in the image below to focus gas into a water/surfactant mixture.

Gfoam1.jpg

Creating foam with a variety of different gas pressures produced 5 different lattices of the 31 unique structures which satisfy the Plateau rules. When calculating the interfacial energies of the observed "crystals", it was found that the lowest and highest energies of the set were both represented along with others in between--indicating that interfacial energy was not a factor in the choice of lattice structure (and, therefore, we have a nonequilibrium process). The graph on the bottom shows the relative occurrence through an experiment for different pressures of the different lattice structures.

Gfoam2.jpg

It was concluded that the formation of the different lattice structures occurred through a limit cycle where the current lattice structure served as a boundary condition for new bubbles being created and therefore propagated the same lattice structure. One cycle of this creation for the "snakeskin" pattern is shown below.

Gfoam3.jpg

It should be noted, however, that if there is a fluctuation in the system, the lattice which is being produced can be switched (which is why there is a statistical variation in the lattices observed at different pressures instead of a constant lattice-type being produced for a given pressure).

Conclusions

This model system produces metastable lattices in a nonequilibrium process, which can be used to study dynamic stability as the system can switch states upon perturbation.