  Collective cell motility plays central roles in various biological phenomena
such as inflammatory response, wound healing, cancer metastasis and
embryogenesis. These are biological demonstrations of the unjamming transition.
However, contradictory to the typical density-driven jamming processes in
particulate assemblies, cellular systems often get unjammed in highly packed,
sometimes overcrowding tissue environments. In this work, we report that
overcrowding can unjam gap-free monolayers through increasing isotropic
compression. The transition boundary is determined by the isotropic compression
and the cell-cell adhesion. We explicitly construct the free energy landscape
for the T1 topological transition during monolayer rearrangement, and find that
it evolves from single-barrier shape to double-barrier shape upon completion of
the unjamming process. Our analyses reveal that the overcrowding and adhesion
induced unjamming transition reflects the mechanical yielding of the highly
deformable monolayer, which differs from those caused by loosing up a packed
particulate assembly.
