The soap froth provides an idealised example of a cellular structure, which
evolves or coarsens over laboratory time-scales and for which topological
measures appear to provide an intuitive characterisation. However, froth is
an intrinsically non-equilibrium system, and topological measures are not
universally applicable to such processes. Recently, persistence has been pr
oposed as a more general probe of non-equilibrium dynamics, where the froth
is viewed as a two-phase system through construction of a virtual phase. W
e use a direct simulation method to investigate persistence fur random 2-D
(Voronoi) and hexagonal froths of size up to 2500 bubbles. We find that sim
ulation results are qualitatively similar to those of experiment, with the
normalised average area, (A*(t)/A(0)), of persistent regions within a bubbl
e at time t approaching an equilibrium Value for a range of volume (or samp
ling) fraction values, phi, for the Voronoi froth. The case for the hexagon
al is less clear, since exclusion of the defect (or defects) from the virtu
al phase leads to rapid decline in the average area of persistent bubbles.
Simulation times required are very long, however, and evolution is slow for
long-term survivors. Consequently, persistent behaviour is not demonstrate
d satisfactorily for the fraction of survivors, N*(t)/N(t), in a random sys
tem of this size, although for the hexagonal with one or more seeded defect
s, there is some indication that decay depends on phi, for some colouring p
atterns. However, limiting slope values are probably not established. (C) 2
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