  We consider a monolayer of particles floating at a horizontal liquid-gas
interface -- a particle raft. Upon compressing the monolayer in a Langmuir
trough, the particles at first pack but ultimately the monolayer buckles out of
the plane. We measure the stress profile within the raft at the onset of
buckling and show for the first time that such systems exhibit a Janssen
effect: the stress decays exponentially away from the compressing barriers over
a length scale that depends on the width of the trough. We find quantitative
agreement between the rate of decay and the simple theory presented by Janssen
and others. This demonstrates that floating particle rafts have a granular, as
well as elastic, character, which is neglected by current models. Finally, we
suggest that our experimental setup may be suitable for exploring granular
effects in two dimensions without the complications of gravity and basal
friction.
