  Transport networks, such as vasculature or river networks, provide key
functions in organisms and the environment. They usually contain loops whose
significance for the stability and robustness of the network is well
documented. However, the dynamics of their formation is usually not considered.
Such structures often grow in response to the gradient of an external field.
During evolution, extending branches compete for the available flux of the
field, which leads to effective repulsion between them and screening of the
shorter ones. Yet, in remarkably diverse processes, from unstable fluid flows
to the canal system of jellyfish, loops suddenly form near the breakthrough
when the longest branch reaches the boundary of the system. We provide a
physical explanation for this universal behavior. Using a 1D model, we explain
that the appearance of effective attractive forces results from the field drop
inside the leading finger as it approaches the outlet. Furthermore, we
numerically study the interactions between two fingers, including screening in
the system and its disappearance near the breakthrough. Finally, we perform
simulations of the temporal evolution of the fingers to show how revival and
attraction to the longest finger leads to dynamic loop formation. We compare
the simulations to the experiments and find that the dynamics of the shorter
finger are well reproduced. Our results demonstrate that reconnection is a
prevalent phenomenon in systems driven by diffusive fluxes, occurring both when
the ratio of the mobility inside the growing structure to the mobility outside
is low and near the breakthrough.
