This work reports a study of in-line bubble interactions in non-Newtonian fluids by varying the injection period between bubbles. The experimental data reveal that the bubble rise velocity depends upon the injection period, obviously due to in-line interactions. The original rheological simulation, based on the close relationship between the rise of a chain of bubbles and consecutive shear deformations, proves for the first time that in-line interactions are mainly governed by a dynamical competition between the creation and relaxation of stresses in fluids. This interaction mechanism is clearly corroborated by a newly developed visualisation method based on birefringence. With the decrease of the injection period, such interactions become strong enough and coalescence occurs. The spectral analysis of the bubble passage at different heights in the column demonstrates that the interactions are no longer linear accompanied by successive bifurcations leading to chaotic coalescence.