Shape dependant properties of Catalytic micro-swimmers

This thesis explores catalyst driven micro-swimmers which propel via phoretic mechanisms. This is achieved with a Janus structure of active and inert sides, which when exposed to chemical fuel causes asymmetric forces across the swimmer, leading to propulsion. Catalytic micro-swimmers have proposed applications such as microfluidic transport, cargo delivery, micro-stirring, and environmental remediation. These active swimmers, whilst popular in the literature, have yet to be optimised for their intended practical applications. The randomised motion at long time scales is a problem for transport applications and synthesis methods are complicated and produce a low yield of swimmers. This thesis focusses on platinum coated insulator swimmers, which when exposed to hydrogen peroxide propel via a mechanism, which is proposed to be self-electrophoretic, in the literature. The bulk of the thesis explores the relationship between the geometry of the starting colloid, and the resulting motion and swimmer dynamics. For ellipsoidal swimmers, this was found to have a significant impact on the swimmer’s angular velocity and interactions with the fluid and bounding surfaces. Whilst investigating the motion of ellipsoidal colloids, a self-imposed flow alignment effect was observed and thoroughly investigated. The findings revealed a strong preference for motion across the flow field rather than along the streamlines. Bowl and Dumpling shaped colloids were also investigated to see if they would self-shadow and induce enough asymmetry in the catalyst cap to produce driven rotations. In certain colloid orientations, before coating, this was successful producing much higher angular velocities than spherical swimmers whilst other orientations produced negligible spin. In an effort to further investigate how geometry impacts the motion of active swimmers, the symmetry of the catalytic patch was intentionally broken by glancing angle deposition to induce beneficial rotations. This had previously been achieved with close packed spherical swimmers, but the work here aims to reduce the steps required to achieve this in the hopes of improving scalability of synthesis. The data shows that introducing even a slight glancing angle during coating provides enough shadowing of well-spaced ellipsoidal colloids to produce swimmers with angular velocities 16x the magnitude of ellipsoidal colloids coated at a direct angle.