  We explore the behavior of micron-scale autophoretic Janus (Au/Pt) rods,
having various Au/Pt length ratios, swimming near a wall in an imposed
background flow. We find that their ability to robustly orient and move
upstream, i.e. to rheotax, depends strongly on the Au/Pt ratio, which is easily
tunable in synthesis. Numerical simulations of swimming rods actuated by a
surface slip show a similar rheotactic tunability when varying the location of
the surface slip versus surface drag. Slip location determines whether swimmers
are Pushers (rear-actuated), Pullers (front-actuated), or in between. Our
simulations and modeling show that Pullers rheotax most robustly due to their
larger tilt angle to the wall, which makes them responsive to flow gradients.
Thus, rheotactic response infers the nature of difficult to measure flow-fields
of an active particle, establishes its dependence on swimmer type, and shows
how Janus rods can be tuned for flow responsiveness. We demonstrate the
effectiveness of a simple geometric sieve for rheotactic ability.
