New insights could advance microfluidics and drug delivery systems.
- New study finds obstacles can trap rolling microparticles in fluid
- Through simulations and experiments, physicists attribute the trapping effect to stagnant pockets of fluid, created by hydrodynamics
- Random motions of the molecules within the fluid then ‘kick’ the microroller into a stagnant pocket, effectively trapping it
- Size of the obstacle also controls how easy it is to trap a microroller and how long it remains trapped
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 study will be published on 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.
The microrollers in Driscoll’s lab are plastic with an iron oxide core, which gives them a weak magnetic field. By putting the microrollers in a sealed microchamber (100 millimeters by 2 millimeters by 0.1 millimeters in size), researchers can control the direction they move by manipulating a rotating magnetic field around the sample. To change the way the microrollers move, researchers simply reprogram the motion of the magnetic field to pull the microrollers in different directions.
But microfluidic devices and the human body are, of course, much more complex landscapes compared to a featureless sample chamber. So, Driscoll and her collaborators added obstacles to the system to see how microrollers could navigate the environment.
“For true-to-life applications, you aren’t just going to have this system with particles sitting in an open space,” Driscoll said. “It’s going to be a complex landscape. You might have to move the particles through winding channels. So, we wanted to first explore the simplest version of the problem: One microroller and one obstacle.”
In both computer simulations and the experimental environment, Driscoll and her team added cylindrical obstacles to the sample chamber. Sometimes the microroller sailed around the obstacle without issue, but other times it would swing around the obstacle and then get trapped behind it.
“We watched the particle stop moving past the obstacle and kind of get stuck,” Driscoll said. “We saw the same behavior in the simulations and in the experiments.”
By changing the parameters within the simulations and analyzing the data, Driscoll and her team found the hydrodynamics of the fluid inside the sample chamber created stagnant areas. In other words, the spinning microroller caused the fluid to flow in the chamber. But the flows also created pockets — including one directly behind the obstacle — where the fluid remained still and unflowing. When the particle entered the stagnant area, it stopped moving and became stuck.
But to reach the stagnant area, the particle had to perform a baffling U-turn. After moving past the obstacle, the microroller curved around it, becoming stuck to its backside. Driscoll found that random motions (called Brownian motion) of the molecules within the fluid “kicked” the microroller into the stagnant area.
“Tiny materials are subject to Brownian fluctuations,” Driscoll explained. “The fluid is not actually a continuum but is composed of individual, little molecules. Those molecules are constantly ramming into the particle at random orientations. If the particle is small enough, these collisions can move it. That’s why if you look at tiny particles under a microscope, they look like they are juggling around a little bit.”
Driscoll’s team also found that the size of the obstacle controls how long the particle will remain trapped before escaping. For example, it’s easier for Brownian fluctuations to kick the particle into the trapping region when the obstacle is smaller. By changing the obstacle size, researchers can increase the trapping time by orders of magnitude.
“Usually, Brownian fluctuations are destructive to experiments because they are a source of noise,” Driscoll said. “Here, we can leverage Brownian motion to do something useful. We can enable this hydrodynamic trapping effect.”

News
The Surprising Origin of a Deadly Hospital Infection
C. diff might not originate from external transmission but rather from within the infected patient themselves. Hospital staff dedicate significant effort to safeguard patients from infections during their hospital stay. Through practices ranging from [...]
Google AI breakthrough – huge step in finding genes that cause diseases
Google says it has made a significant step in identifying disease-causing genes, which could help spot rare genetic disorders. A new model named AlphaMissense is able to confidently classify 89 per cent of all [...]
New Study: Everyday Pleasures Can Boost Cognitive Performance
MINDWATCH study reveals cognitive peaks with everyday pleasures. Listening to music and drinking coffee are the sorts of everyday pleasures that can impact a person’s brain activity in ways that improve cognitive performance, including [...]
Moderna reveals new highly targeted COVID-19 vaccine mRNA-1283
Moderna has developed a new and improved version of its COVID-19 vaccine. The unique formulation (mRNA-1283) reduces the vaccine's content from the full-length SARS-CoV-2 spike protein to a narrowly focused encoding of just two [...]
New nanotech weapon takes aim at hard-to-treat breast cancer
Breast cancer in its various forms affects more than 250,000 Americans a year. One particularly aggressive and hard-to-treat type is triple-negative breast cancer (TNBC), which lacks specific receptors targeted by existing treatments. The rapid [...]
Scientists upcycle plastics into liquids that can store hydrogen energy
Scientists from Nanyang Technological University, Singapore (NTU Singapore) have created a process that can upcycle most plastics into chemical ingredients useful for energy storage, using light-emitting diodes (LEDs) and a commercially available catalyst, all [...]
Yale Scientists Uncover How the Immune System Can Alter Our Behavior
The mere scent of seafood can severely sicken those allergic to it — and therefore they are more likely to avoid it. Similarly, individuals who experience food poisoning from a specific dish tend to [...]
Whirlwind Tech – The Future of Energy-Efficient Spintronics Computing
Researchers in Germany and Japan have been able to increase the diffusion of magnetic whirls, so-called skyrmions, by a factor of ten. In today’s world, our lives are unimaginable without computers. Up until now, [...]
Omicron’s Silver Lining: Significantly Lower Risk of Long COVID
Omicron infections have a lower risk of long COVID than earlier variants, according to a study analyzing data from 11,000 participants. The risk of developing long COVID is significantly lower following an infection with [...]
The Hidden Mechanism Connecting Diabetes and Cancer
Researchers have discovered that insulin resistance, typically linked with type 2 diabetes, is also present in cancer patients and can accelerate the spread of the disease. In the 1920s, scientists found that the urine [...]
Scientists Unveil Urea’s Secret Role in the Origin of Life
Scientists from ETH Zurich and the University of Geneva have developed a new technique that allows them to observe chemical reactions taking place in liquids at extremely high temporal resolution. This innovation enables them to track how molecules [...]
Viagra Lowers Alzheimer’s Risk by Almost 70%, Early Study Finds
Research published recently suggests that Pfizer’s erectile dysfunction drug Viagra can decrease the risk of developing Alzheimer’s disease by up to 69 percent. The research, which was published in Nature, found that the medication has [...]
Future of Medical Imaging: Advanced AI Can Tell Your True Age by Looking at Your Chest
An AI-powered model utilizes chest X-rays to help develop biomarkers for aging. What if determining “your age” was based on your chest rather than your face? Scientists from Osaka Metropolitan University have crafted an [...]
Ultra-sensitive biosensors detect cancer in a blood test
Cancer biomarkers circulating in body fluids can be used for diagnosis and treatment monitoring. However, current detection technology lacks the required sensitivity, limiting biomarker use in clinical applications. Colorectal cancer is the second most [...]
Viruses cause 200+ diseases. This one drug may be able to treat them all.
By taking aim at a process common across many viruses, the drug could one day stop any number of known viruses — and new ones. t’s about as audacious an idea as you can [...]
Scientists Identify Potential Treatment for Rare and Devastating Lung Disease
The findings could lead to a cure for LAM. Researchers from the University of Cincinnati may have identified a potential treatment for lymphangioleiomyomatosis (LAM), a rare lung condition resembling cancer found predominantly in women of reproductive [...]