Schematic diagram showing particles moving through porous material. Credit: Haichao WuResearchers at the University of Colorado Boulder discovered that tiny, self-propelled particles called "nanoswimmers," can escape mazes up to 20 times faster than passive particles. This opens up the possibility of their being used in everything, from industrial clean-ups and medication delivery.These tiny, synthetic nanorobots can escape from maze-like environments with remarkable effectiveness, according to the Proceedings of the National Academy of Sciences. These nanoswimmers could be used in the future to remove contaminated soil, improve water filtering, or deliver drugs to specific areas within the body like dense tissues."This is an entirely new phenomenon that points towards a wide range of potential applications," said Daniel Schwartz (senior author) and Glenn L. Murphy Endowed professor of chemical and biological engineering.The theoretical physics community first became aware of these nanoswimmers around 20 years ago. Schwartz said that people imagined many real-world applications. Unfortunately, these practical applications are not yet realized. This is partly because it has been difficult to model and observe their movements in the right environments.These tiny, spherical nanoswimmers are also known as Janus particles, named after a Roman god with two heads. They are made of silica or polymer and have different chemical properties. Chemical reactions can occur in one hemisphere, but not in the other. This creates a chemical environment that allows the particle to absorb energy from the surrounding environment and transform it into directional motion, also known as self-propulsion."In biology and living organisms cell propulsion is the predominant mechanism that causes motion to occur. Yet, engineered applications rarely use it." Schwartz said that our research suggests that self-propulsion is possible.Brownian particles are passive particles that move around randomly (a type of motion called Brownian motion). These particles were named for Robert Brown, a 19th-century scientist who studied the random motions of pollen grains suspended within water.These passive Brownian particles were converted into Janus particles (nanoswimmers), by the researchers. They then made the self-propelled nanoswimmers move through a maze of porous media. Then, they compared their performance to passive Brownian particles to see how efficient and effective they were at finding escape routes.Even researchers were shocked by the results.This image is representative of scanning electron microscopy images of inverse-opals, the porous material used in this research. The close-packed cavities are indicated by large circular patterns, while the connections between adjacent cavities are indicated by small elliptical shapes. Each cavity was connected to its neighboring cavities by twelve holes. Credit: Haichao WuJanus particles were 20 times more efficient at escaping mazea cavities than Brownian particles. This was because they moved along cavity walls looking for holes. This allowed them to quickly find exits. They also seemed to have self-propulsion, which gave them the energy they needed to get through the exit holes in the maze.We know there are many applications for nanorobots in very restricted environments. But we don't know much about how they move or what their advantages are over traditional Brownian particles. Haichao Wu (lead author and graduate student in chemical- and biological engineering), said that the paper was meant to compare these two. "And we discovered that nanoswimmers can use a completely different method to search these maze environments."These particles are extremely small at 250 nanometers, which is about the same width as a human hair (160 Nanometers), but they are still smaller than the head a pin (1-2 Millimeters). The work can be scaled. These particles can navigate and penetrate spaces as small as human tissue, carrying cargo and drugs. They also have the ability to permeate soil underground and beaches of sand to remove pollutants.Nanoswimmers in a swarmNext, we will be studying how nanoswimmers interact in groups and in combination with passive particle systems."Nanoswimmers have been known to exhibit emergent behavior in open environments. This mimics the swarming motions of flocks or schools of fish. Schwartz said that this is what has prompted a lot of research into them.This goal will be difficult because it is hard to see and understand the 3D movements of tiny particles deep within complex interconnected materials.Wu overcame the problem by using a refractive index fluid in the porous media. This is a liquid that alters how light travels through a medium. The maze was made virtually invisible and allowed for the observation of 3D particle movement using the technique double-helix spread function microscopy.Wu was able to track the three-dimensional trajectory of the particles and create visual representations. This is a significant improvement over traditional 2D modeling of nanoparticles. This breakthrough would have made it possible to better understand individual and group behavior.Wu stated, "This paper is only the first step. It provides a model system as well as an imaging platform that allows us to answer these questions." "The next step will be to test this model on a larger number of nanoswimmers to see how they interact in a restricted environment."Explore more Particles allow you to take a closer look at the rotational dynamicsAdditional information: Haichao Wu and colleagues, Mechanisms of transport improvement for self-propelled microswimmers in porous matrix, Proceedings of the National Academy of Sciences (2021). Information from the Journal: Proceedings of the National Academy of Sciences Haichao Wu and colleagues, Mechanisms of transport improvement for self-propelled microswimmers within a porous matrix. (2021). DOI: 10.1073/pnas.2101807118
