Metallic Microswimmers Driven up the Wall by Gravity

TL;DR

This study demonstrates that tail-heavy bimetallic nanorods can autonomously orient and swim upwards along a wall against gravity, driven by buoyancy and hydrodynamic interactions, mimicking biological gravitaxis.

Contribution

It reveals the mechanisms behind gravitactic behavior in synthetic nanomotors, combining experimental and theoretical analysis to understand their upward swimming along walls.

Findings

Tail-heavy nanorods preferentially swim up walls against gravity.

Two mechanisms identified: buoyancy torque and hydrodynamic drag asymmetry.

Nanorods exhibit natural gravitactic orientation and movement.

Abstract

As a natural and functional behavior, various microorganisms exhibit gravitaxis by orienting and swimming upwards against gravity. Swimming autophoretic nanomotors described herein, comprising bimetallic nanorods, preferentially orient upwards and swim up along a wall, when tail-heavy (i.e. when the density of one of the metals is larger than the other). Through experiment and theory, two mechanisms were identified that contribute to this gravitactic behavior. First, a buoyancy or gravitational torque acts on these rods to align them upwards. Second, hydrodynamic interactions of the rod with the inclined wall induce a fore-aft drag asymmetry on the rods that reinforces their orientation bias and promotes their upward motion.