Controlling speed of bacterial active droplets in a nematic environment

Water-based droplets loaded with swimming bacteria Bacillus subtilis propel themselves unidirectionally when placed in a thermotropic nematic. We investigate the effect of the viscosity of the droplet medium on the propulsion speed of the droplets. The speed of droplets first increases and then decreases as the viscosity of the medium is increased by adding nontoxic disodium cromoglycate (DSCG) and polymer carboxymethyl cellulose (CMC). In the case of DSCG, the maximum velocity is observed at 13wt% concentration of the additive. On the other hand, individual bacteria show the fastest speed in a more dilute medium with 8wt% of DSCG. The results imply that the increase in the droplet's media viscosity causes a better momentum transfer at the droplet-nematic interface, which speeds up the droplets.