Dynamic interactions between fast microscale rotors

(Figure Presented) Trimetallic catalytic microrotors were fabricated by electrodeposition of cylindrical Au-Ru rods in the pores of anodic alumina membranes, dissolution of the template membrane, and then sequential vapor deposition of Cr, SiO₂, Cr, Au, and Pt on one side of each rod. This design provides two force vectors for the catalytic motor, including one perpendicular to the rod axis. The rods rotated rapidly (∼180 rpm) in 15% aqueous H₂O₂ solution with minimal orbital or translational movement. The rotation was rapid enough to observe qualitatively different interactions between pairs of co- and counter-rotating rods. Counter-rotating rods were able to approach each other closely and underwent frequent tip-to-tip collisions. Co-rotating rods could approach each other only to a distance of ∼0.9 μm. This difference is rationalized on the basis of shear forces generated by the catalytically driven rotation of the rods.