  A tenuous monolayer of hydrophobic particles at the air-water interface often
forms a scum or raft. When such a monolayer is disturbed by the localized
introduction of a surfactant droplet, a radially divergent surfactant shock
front emanates from the surfactant origin and packs the particles into a
jammed, compact, annular band with a packing fraction that saturates at a peak
packing fraction $\phi^*$. As the resulting two-dimensional, disordered elastic
band grows with time and is driven radially outwards by the surfactant, it
fractures to form periodic triangular cracks with robust geometrical features.
We find the number of cracks $N$ and the compaction band radius $R^*$ at
fracture onset vary monotonically with the initial packing fraction
($\phi_{init}$). However, its width $W^*$ is constant for all $\phi_{init}$. A
simple geometric theory that treats the compaction band as an elastic annulus,
and accounts for mass conservation allows us to deduce that $N \simeq 2\pi
R^*/W^* \simeq 4\pi \phi_{RCP}/\phi_{init}$, a result we verify both
experimentally and numerically. We show the essential ingredients for this
phenomenon are an initially low enough particulate packing fraction that allows
surfactant driven advection to cause passive jamming and eventual fracture of
the hydrophobic particulate interface.
