  Understanding mixing and transport of passive scalars in active fluids is
important to many natural (e.g. algal blooms) and industrial (e.g. biofuel,
vaccine production) processes. Here, we study the mixing of a passive scalar
(dye) in dilute suspensions of swimming Escherichia coli in experiments using a
two-dimensional (2D) time-periodic flow and in a simple simulation. Results
show that the presence of bacteria hinders large scale transport and reduce
overall mixing rate. Stretching fields, calculated from experimentally measured
velocity fields, show that bacterial activity attenuates fluid stretching and
lowers flow chaoticity. Simulations suggest that this attenuation may be
attributed to a transient accumulation of bacteria along regions of high
stretching. Spatial power spectra and correlation functions of dye
concentration fields show that the transport of scalar variance across scales
is also hindered by bacterial activity, resulting in an increase in average
size and lifetime of structures. On the other hand, at small scales, activity
seems to enhance local mixing. One piece of evidence is that the probability
distribution of the spatial concentration gradients is nearly symmetric with a
vanishing skewness. Overall, our results show that the coupling between
activity and flow can lead to nontrivial effects on mixing and transport.
