Just like traffic engineers design the flow of traffic, scientists at the Department of Energy’s (DOE) Argonne National Laboratory have developed a way to control the motion of swimming bacteria using 3-D-printed microscopic pillars. Manipulating the bacteria could aid in the development of new materials with unusual properties for many applications, including microscopic transport, biomedicine and even microrobotics.
When suspended in drops of water, the bacterial “swimmers” will shoot around chaotically, together forming tiny, unstable tornados that drift, decay and annihilate each other. The scientists discovered that when they introduce an array of microscopic towers into the turbulent bacterial bath, the swimmers self-organize into a coherent structure of vortices around the towers.
“You can think of this like chaotic motion of cars in an open lot,” said Argonne’s Andrey Sokolov, a scientist on the study. “When you introduce tiny objects into the system, like traffic cones, the chaotic motion self-organizes into ordered motion.”
“When you introduce tiny objects into the system, like traffic cones, the chaotic motion self-organizes into ordered motion.” — Andrey Sokolov, Argonne scientist
The “traffic cones” in this experiment are vertical pillars 10 times thinner than the thickness of a hair. The scientists produce the towers, made of photopolymer, using a state-of-the-art 3-D printing process called multiphoton lithography within Argonne’s Materials Science division.
When the scientists immerse the towers in the bacterial suspension, the swimmers form a lattice of vortices whose directions of rotation alternate like the colors on a checkerboard. This pattern, which scientists call antiferromagnetic order, is most famously displayed by the spinning electrons inside some magnetic materials.
“Usually, we hear the words ‘vortex’ and ‘antiferromagnetic’ in the context of magnetic systems and superconductors,” said Sokolov, “but we observed a remarkably similar structure in a very different system — a biological system.”
In addition, the scientists discovered that the direction of the individual vortices can be further controlled with the introduction of chiral towers, or small pillars with slats on the sides. The bacteria will swim clockwise or counterclockwise depending on the orientation of the slats in the towers around them. Using this technique, the scientists can manipulate the vortices and design a wide range of patterns on demand.
The effect that these towers have on the bacterial “hive mind” is still somewhat mysterious to the scientists. The towers take up only around five percent of the area in the system, but they have a profound effect on bacterial motion. Through the experiments, the scientists determined that the effect is controlled by the spacing of the towers.
“The patterns are strongest when the pillar spacing is on the same scale as the size of the typical vortex when the bacteria are unconstrained,” said Sokolov.
The study is part of a larger investigation of active matter, or matter that continuously consumes energy and uses it for mechanical motion. Countless examples of active matter show up in everyday life, including flocks of birds, swarms of bees or even rice on a shaking pan. Scientists are trying to control these systems to have them accomplish particular tasks, such as transporting a microscopic object over a material like a rock star surfing a crowd.
The bacterial suspension in this study is active matter because the bacteria constantly convert chemical energy from nutrients into mechanical energy to swim. “The outcome of our research is a new way to manipulate active matter on a microscopic level,” said Sokolov.
The bacteria used in the study, Bacillus subtilis, are commonly found in soil and the intestines of animals. The scientists suspect that other swimmers, like E. Coli or artificial swimmers, would behave in a similar way.
A paper on the study, titled “Engineering bacterial vortex lattice via direct laser lithography,” was published in Nature Communications on Oct. 26, 2018.
The research was funded by the DOE Office of Science, Basic Energy Sciences Program.
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