ac/dc Magnetic Fields for Enhanced Translation of Colloidal Microwheels Dante Disharoon Keith B. Neeves David W. M. Marr 10.1021/acs.langmuir.8b04084.s005 https://acs.figshare.com/articles/media/ac_dc_Magnetic_Fields_for_Enhanced_Translation_of_Colloidal_Microwheels/7732859 Microscale devices must overcome fluid reversibility to propel themselves in environments where viscous forces dominate. One approach, used by colloidal microwheels (μwheels) consisting of superparamagnetic particles assembled and powered by rotating ac magnetic fields, is to employ a nearby surface to provide friction. Here, we used total internal reflection microscopy to show that individual 8.3 μm particles roll inefficiently with significant slip because of a particle–surface fluid gap of 20–80 nm. We determined that both gap width and slip increase with the increasing particle rotation rate when the load force is provided by gravity alone, thus providing an upper bound on translational velocity. By imposing an additional load force with a dc magnetic field gradient superimposed on the ac field, we were able to decrease the gap width and thereby enhance translation velocities. For example, an additional load force of 0.2 <i>F</i><sub>g</sub> provided by a dc field gradient increased the translational velocity from 40 to 80 μm/s for a 40 Hz rotation rate. The translation velocity increases with the decreasing gap width whether the gap is varied by dc field gradient-induced load forces or by reducing the Debye length with salt. These results present a strategy to accelerate surface-enabled rolling of microscale particles and open the possibility of high-speed μwheel rolling independent of the gravitational field. 2019-02-06 00:00:00 translational velocity load force particle rotation rate dc field gradient 40 Hz rotation rate 0.2 F g ac gap width translation velocity increases Colloidal Microwheels Microscale devices 8.3 μ m particles roll inefficiently dc field gradient-induced load forces