la8b04084_si_005.avi (1.77 MB)
ac/dc Magnetic Fields for Enhanced Translation of Colloidal Microwheels
media
posted on 2019-02-06, 00:00 authored by Dante Disharoon, Keith B. Neeves, David W. M. MarrMicroscale
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 Fg 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.