Combined effects of aspect ratio and catalytic coating location on active-rod dynamics.

Abstract

HYPOTHESIS: A central goal in micro/nanomotor research is not only to make particles move, but to control how they move. The motion of anisotropic active particles depends on both particle aspect ratio and catalytic coating location. Their combined influence is therefore expected to govern active-rod transport and orientational dynamics. EXPERIMENTS: We used lithographically defined Pt-coated SU-8 rods with three aspect ratios, AR = 2.2, 2.8, and 3.8, and two coating locations: Pt localized near one rod end and Pt deposited along the lateral sidewall. Combining trajectory-level analysis with simplified COMSOL-based phoretic-slip simulations, we investigated how aspect ratio and coating location influence propulsion direction, translational transport, and orientational dynamics. FINDINGS: End-coated rods exhibited a pronounced intermediate-aspect-ratio optimum, with AR = 2.8 showing the largest effective diffusion coefficient, axial velocity, and accumulated in-plane orientational change. In contrast, sidewall-coated rods displayed predominantly lateral propulsion and broadly comparable ensemble mean square displacement across aspect ratios, while loop size and turning behavior varied with rod length. These results identify aspect ratio and catalytic coating location as coupled structural parameters that regulate near-wall active-rod motion by redistributing the relative contributions of translational, looping, and mixed trajectories. This framework links particle geometry and surface activity distribution to trajectory-level transport descriptors in catalytic colloidal rods.

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