  Phenomenon of foam and emulsion jamming at low shear rates is explained by
considering the dynamics of thinning in the transient film, formed between the
neighboring bubbles and drops. After gradually thinning down to a critical
thickness, these films undergo instability transition and thin stepwise,
forming the so-called "black films", which are only several nanometers thick
and, thereby, lead to stronger adhesion between the dispersed particles.
Theoretical analysis shows that such film thickness instability occurs only if
the contact time between the bubbles/drops in sheared foam/emulsion is
sufficiently long, which corresponds to sufficiently low (critical) rate of
shear. Explicit expression for this critical rate is proposed and compared to
experimental data.
