When physicists steered a tiny microparticle toward a cylindrical obstacle, they expected one of two outcomes to occur. The particle would either collide into the obstacle or sail around it. The particle, however, did neither.
The researcher team — led by Northwestern University and École Polytechnique in France — was surprised and puzzled to watch the particle curve around the obstacle and then stick to its backside. The obstacle, it seemed, had the particle effectively trapped.
After a series of simulations and experiments, the researchers unraveled the physics at play behind this strange phenomenon. Three factors caused the unexpected trapping behavior: electrostatics, hydrodynamics and erratic random movement of the surrounding molecules. The size of the obstacle also determined how long the particle remained trapped before escaping.
The new insights could be leveraged to advance microfluidic applications and drug delivery systems — both of which rely on microparticles to navigate complex, structured landscapes.
The study was published today (March 8) in the journal Science Advances.
“I didn’t expect to see trapping in this system at all,” said Northwestern’s Michelle Driscoll, who co-led the study. “But trapping adds a lot of utility to the system because now we have a way to gather up particles. Tasks like trapping, mixing and sorting are very difficult to do at such small scales. You can’t just scale down standard processes for mixing and sorting because a different kind of physics kicks in at this size limit. So, it’s important to have different ways to manipulate particles.”
Driscoll is an assistant professor of physics at Northwestern’s Weinberg College of Arts and Sciences. She co-led the study with Blaise Delmotte, a researcher at École Polytechnique.
Similar in size to bacteria, microrollers are synthetic, microscopic particles with the ability to move in a fluid environment. Driscoll and her team are particularly interested in microrollers for their ability to move freely — and quickly — in different directions and their potential to carry and deliver cargo in complex, confined environments, including within the human body.