  The effect of surfactants on the tail and film dynamics of elongated gas
bubbles propagating through circular capillary tubes is investigated by means
of an extensive three-dimensional numerical study using a hybrid
front-tracking/level-set method. The focus is on the visco-inertial regime,
which occurs when the Reynolds number of the flow is much larger than unity.
Under these conditions, `clean' bubbles exhibit interface undulations in the
proximity of the tail \cite{Magnini_prf_2017}, with an amplitude that increases
with the Reynolds number. We perform a systematic analysis of the impact of a
wide range of surfactant properties, including elasticity, bulk surfactant
concentration, solubility, and diffusivity, on the bubble and flow dynamics in
the presence of inertial effects. The results show that the introduction of
surfactants is effective in suppressing the tail undulations as they tend to
accumulate near the bubble tail. Here, large Marangoni stresses are generated,
which lead to a local `rigidification' of the bubble. This effect becomes more
pronounced for larger surfactant elasticities and adsorption depths. At reduced
surfactant solubility, a thicker rigid film region forms at the bubble rear,
where a Couette film flow is established, while undulations still appear at the
trailing edge of the downstream `clean' film region. In such conditions, the
bubble length becomes an influential parameter, with short bubbles becoming
completely rigid.
