  Dynamics of the pendant drop coalescing with a sessile drop to form a single
daughter droplet is known to form a bridge. The bridge evolution begins with a
point contact between the two drops leading to a liquid neck of size comparable
to the diameter of the drops. To probe this phenomenon in polymeric fluids, we
quantify the neck radius growth during coalescence using high speed imaging. In
the current study, we unveil the existence of three regimes on basis of
concentration ratio $c/c^*$ namely, inertio-elastic $c/c^*<c_e/c^*$,
viscoelastic $c_e/c^*<c/c^*<20$ and elasticity dominated regimes $c/c^*>20$.
Our results suggest that the neck radius growth with time (t) obeys a power-law
behaviour $t^b$, such that the coefficient $b$ has a steady value in
inertio-elastic and viscoelastic regimes, with a monotonic decrease in
elasticity dominated regime. Based on this dependence of $b$ on concentration
ratios, we propose a new measurement technique Rheocoalescence to obtain the
relaxation time of the these fluids. We also show a deviation from universality
proposed in literature for the elasticity dominated regime.
