A team of scientists has uncovered how heavy, motorized objects climb steep
slopes — a newly discovered mechanism that also mimics how rock climbers
navigate inclines.
The findings, which appear in the journal Soft Matter, stem from a series of
experiments in which motorized objects were placed in liquid and then moved
up tilted surfaces.
“These ‘micro-swimmers’ are about 20 times heavier than the fluid they swim
in, but they were able to climb steep slopes that are almost vertical,”
explains Jun Zhang, one of the paper’s authors and a professor of physics
and mathematics at New York University’s Courant Institute of Mathematical
Sciences and NYU Shanghai.
The work enhances our understanding of “gravitaxis” — directional movement
in response to gravity. The phenomenon is a vital consideration in not only
engineering, but also in medicine and pharmaceutical development. It
explains, in part, how bacteria move through the body and provides insights
into ways to create more effective drug-delivery mechanisms.
In the Soft Matter research, the scientists created swimmers, or nanorods,
whose length is roughly 1/40th the width of a strand of human hair. These
motorized swimmers were tasked with moving up an inclined surface while
immersed in a liquid solution within a walled container. The swimmers were
composed of two types of metal — gold and rhodium as well as gold and
platinum — a makeup that gave them unbalanced densities given the varying
weights of these metals.
The swimmers’ composition, liquid environment, and juxtaposition of surfaces
enabled them to move upward, despite their significant weight.
“These motors reorient themselves upward against gravity thanks to their
density imbalance — much like a seesaw reorients itself in response to the
movement and weight of its riders,” adds Michael Shelley, a professor at the
Courant Institute and director of the Flatiron Institute’s Center for
Computational Biology. “A hydrodynamic effect amplifies this movement —
swimming next to a wall yields a bigger torque in repositioning the motors’
bodies upwards. This is important because the microscopic world is noisy —
for the motor it’s always two steps up and one step down — and the bigger
torque improves their ability to move vertically.”
In previous work, published in Physical Review Letters, Zhang, Shelley, and
their colleagues created “nano-motors” in uncovering an effective means of
movement against currents. The new research expands on these findings by
revealing how heavy objects can move up steeply inclined surfaces, offering
the promise of even more sophisticated maneuvers.
“Now that these micro-swimmers are able to climb very steep slopes against
gravity, we can look toward developing even more difficult assignments,”
observes Zhang. “Future, advanced motors will be designed to reach targeted
locations and to perform designated functions.”
Reference:
Metallic microswimmers driven up the wall by gravity by Quentin Brosseau,
Florencio Balboa Usabiaga, Enkeleida Lushi, Yang Wu, Leif Ristroph, Michael
D. Ward, Michael J. Shelley and Jun Zhangaef, 11 June 2021, Soft Matter.
DOI:
10.1039/D1SM00554E