- Attractive forces between the intimate particles of matter (atoms
or molecules) are at the heart of the conception of cohesion of matter.
This is why matter stick together : gases condense to liquids,and
liquids freeze to solids. Granular jets falling out of a funnel shaped
container, subjected to small vertical vibrations, under the action of
gravity display a strikingly liquid-like appearance. Such jets start out
spatially uniform and break up into clusters farther downstream as may
happen for ordinary liquids under the action of surface tension forces.
This is surprising since attractive forces between grains (at the heart
of surface tension or capillary forces in liquids) are much weaker than
other mechanical forces at play (gravity, friction, inelasticity).
From an analysis of the long wavelength variations of the jet
radius (induced by the vertical vibration), it turns out that these
modes are unstable and produce a long wavelength break up of the jet
which is reminiscent of a "Rayleigh-Plateau" capillary instability
(that leads to drop formation for liquids). This instability and the
break up of the jet can be inhibited when the effect of the surrounding
medium (air) is reduced by enclosing the jet in an evacuated chamber,
showing that the effective surface tension measured is the result of a
strong interaction with the surrounding air.
