Tuning the Upstream Swimming of Microrobots by Shape and Cargo Size

The navigation of microrobots in complex flow environments is controlled by rheotaxis, the reorientation with respect to flow gradients. Here, we demonstrate how payloads can be exploited to enhance the motion against flows. Using fully resolved hydrodynamic simulations, the mechanisms are described that allow microrobots of different shapes to reorient upstream. We find that cargo pullers are the fastest at most flow strengths but that pushers feature a nontrivial optimum as a function of the counterflow strength. Moreover, the rheotactic performance can be maximized by tuning the microrobot shape or cargo size. These results may be used to control microswimmer navigation but they also apply to rheotaxis in microbial ecology and the prevention of bacterial-contamination dynamics.