  The motion of cells in tissues is an ubiquitous phenomenon. In particular, in
monolayered cell colonies in vitro, pronounced collective behavior with
swirl-like motion has been observed deep within a cell colony, while at the
same time, the colony remains cohesive, with not a single cell escaping at the
edge. Thus, the colony displays liquid-like properties inside, in coexistence
with a cell-free "vacuum" outside. How can adhesion be strong enough to keep
cells together, while at the same time not jam the system in a glassy state?
What kind of minimal model can describe such a behavior? Which other signatures
of activity arise from the internal fluidity? We propose a novel active
Brownian particle model with attraction, in which the interaction potential has
a broad minimum to give particles enough wiggling space to be collectively in
the fluid state. We demonstrate that for moderate propulsion, this model can
generate the fluid-vacuum coexistence described above. In addition, the
combination of the fluid nature of the colony with cohesion leads to preferred
orientation of the cell polarity, pointing outward, at the edge, which in turn
gives rise to a tensile stress in the colony -- as observed experimentally for
epithelial sheets. For stronger propulsion, collective detachment of cell
clusters is predicted. Further addition of an alignment preference of cell
polarity and velocity direction results in enhanced coordinated, swirl-like
motion, increased tensile stress and cell-cluster detachment.
