  The collective behaviour of confluent cell sheets is strongly influenced both
by polar forces, arising through cytoskeletal propulsion and by active
inter-cellular forces, which are mediated by interactions across cell-cell
junctions. We use a phase-field model to explore the interplay between these
two contributions and compare the dynamics of a cell sheet when the polarity of
the cells aligns to (i) their main axis of elongation, (ii) their velocity, and
(iii) when the polarity direction executes a persistent random walk.In all
three cases, we observe a sharp transition from a jammed state (where cell
rearrangements are strongly suppressed) to a liquid state (where the cells can
move freely relative to each other) when either the polar or the inter-cellular
forces are increased. In addition, for case (ii) only, we observe an additional
dynamical state, flocking (solid or liquid), where the majority of the cells
move in the same direction. The flocking state is seen for strong polar forces,
but is destroyed as the strength of the inter-cellular activity is increased.
