View source for Flowing Crystals: Nonequilibrium Structure of Foam

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.

[[Image: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.

[[Image: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.

[[Image: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.