  Flexible rings and rectangle structures floating at the surface of water are
prone to deflect under the action of surface pressure induced by the addition
of surfactant molecules on the bath. While the frames of rectangles bend inward
or outward for any surface pressure difference, circles are only deformed by
compression beyond a critical buckling load. However, compressed frames also
undergo a secondary buckling instability leading to a rhoboidal shape.
Following the pioneering works of \cite{Hu} and \cite{Zell}, we describe both
experimentally and theoretically the different elasto-capillary deflection and
buckling modes as a function of the material parameters. In particular we show
how this original fluid structure interaction may be used to probe the
adsorption of surfactant molecules at liquid interfaces.
