  Mixing timescales and residence times in reactive multiphase flows can be
essential for product selectivity. For instance when a gas species is consumed
e.g. by a competitive consecutive reaction with moderate reaction kinetics
where reaction timescales are comparable to relevant mixing timescales. To
point out the importance of the details of the fluid flow, we analyze
experimental velocity data from a Taylor bubble wake by means of Lagrangian
methods. By adjusting the channel diameter in which the Taylor bubble rises,
and thus the rise velocity, we obtain three different wake regimes. Remarkably
the normalized residence times of passive particles advected in the wake
velocity field show a peak for intermediate rise velocities. This fact seems
unintuitive at first glance because one expects a faster removal of passive
tracers for a faster overall flow rate. However, the details of the flow
topology analyzed using Finite Time Lyapunov Exponent (FTLE) fields and
Lagrangian Coherent Structures (LCS) reveal the existence of a very coherent
vortical pattern in the bubble wake which explains the long residence times.
The increased residence times within the vortical structure and the close
bubble interface acting as a constant gas species source could enhance side
product generation of a hypothetical competitive consecutive reaction, where
the first reaction with the gas species forms the desired product and the
second the side product.
