When physicists pointed a tiny microparticle at a cylindrical barrier, they expected one of two outcomes to occur. A particle either collides with an obstacle or flies around it. But the particle did neither. A research team led by Northwestern University and Ecole Polytechnique in France was surprised and confused when they saw the particle bend around the barrier and then stick to its backside. The obstacle actually seemed to retain the particle.

After a series of simulations and experiments, the researchers uncovered the physics behind this strange phenomenon. The unexpected trapping behavior was caused by three factors: electrostatic, hydrodynamic, and the unsteady random motion of the surrounding molecules. The size of the obstacle also determined how long the particle was trapped before escaping.

The new understanding can be used to develop microfluidic applications and drug delivery systems, both of which rely on microparticles to navigate complex, structured landscapes. The study will be published March 8 in the journal Science Advances.

“I never expected to see the excitement in this system,” said Michelle Driscoll of Northwestern, who led the study. “But capturing benefits the system so much because we have now found a way to collect particles. Tasks like capturing, mixing, and sorting are very difficult to perform on such a small scale. You can’t shrink the standard processes for blending and sorting, because at that size limit other physics come into play. Therefore, it is important to have different ways of manipulating particles.”

Driscoll is a professor of physics at Northwestern’s Weinberg College of Arts and Sciences. He led the study with Blaise Delmott, a researcher at the Ecole Polytechnique.

Microcylinders, similar in size to bacteria, are synthetic microscopic particles that can move in a liquid medium. Driscoll and his team are particularly interested in microcylinders because of their ability to move freely and rapidly in multiple directions, as well as their potential to transport and deliver cargo in complex confined environments, including inside the human body.

The microspools in Driscoll’s lab are plastic with an iron oxide core that gives them a weak magnetic field. By placing the microrolls in a closed microchamber (100 millimeters by 2 millimeters by 0.1 millimeters), the researchers can control the direction of their movement by manipulating the rotating magnetic field around the sample. To change the way the microrolls move, the researchers reprogrammed the movement of the magnetic field to pull the microrolls in different directions.

But microfluidic devices and the human body are of course much more complex landscapes compared to the featureless sample chamber. Driscoll and his collaborators added barriers to the system to see how the microcylinder could navigate in their environment.

“For realistic applications, you won’t have this system where particles sit in open space,” Driscoll said. “It will be a difficult landscape. You may have to pass particles through the winding channels. So we wanted to first explore the simplest version of the problem: a microcylinder and a barrier.”