  The behaviour of a viscous drop squeezed between two horizontal planes is
treated by both theory and experiment. When the squeezing force F is constant
and surface tension is neglected, the theory predicts ultimate growth of the
radius a~ t^{1/8}, in excellent agreement with our experiment. Surface tension
at the drop boundary is included in the analysis, although negligibly small in
the squeezing experiments. The circular evolution is found to be stable under
small perturbations. If the force is reversed (F < 0), so that the plates are
pulled or levered apart, the boundary of the drop is subject to a fingering
instability. The effect of a trapped air bubble at the centre of the drop is
then considered. The annular evolution of the drop under constant squeezing is
still found to follow a `one-eighth' power law, but this is unstable, the
instability originating at the boundary of the air bubble. If the plates are
drawn apart, the evolution is still subject to the fingering instability driven
from the outer boundary of the annulus. This instability is realised
experimentally by levering the plates apart at one corner: fingering develops
at the outer boundary and spreads rapidly to the interior as the levering is
slowly increased. At a later stage, small cavitation bubbles appear in the very
low pressure region far from the point of leverage. This exotic behaviour is
discussed in the light of the foregoing theoretical analysis.
