  Multicellular aggregates are known to exhibit liquid-like properties. The
fusion process of two cell aggregates is commonly studied as the coalescence of
two viscous drops. However, tissues are complex materials and can exhibit
viscoelastic behaviour. It is known that elastic effects can prevent the
complete fusion of two drops, a phenomenon known as arrested coalescence. Here
we report the presence of this phenomenon in stem cell aggregates and provide a
theoretical framework which agrees with the experiments. In addition,
agent-based simulations show that cell protrusion activity controls a
solid-to-fluid phase transition, revealing that arrested coalescence can be
found in the vicinity of an unjamming transition. By analysing the dynamics of
the fusion process and combining it with nanoindentation measurements, we
obtain the effective viscosity, shear modulus and surface tension of the
aggregates. More generally, our work provides a simple, fast and inexpensive
method to characterize the mechanical properties of viscoelastic materials.
