A team of physicists observed an unusual behavior of a microparticle as it interacted with a rotating cylindrical obstacle immersed in a liquid. The observation came from the press service of Northwestern University. In the experiments, Michel Driscoll and his colleagues introduced a microparticle toward a microcylinder that was spinning within a fluid. The scientists anticipated two possible outcomes: the particle would collide with the obstacle or travel around it. Instead, the particle neither hit nor simply went past. It behaved in a way that surprised the researchers: it circumnavigated the obstacle and then adhered to the obstacle’s backside, as if the surface appeared to trap it, holding the particle in place for a period of time.
Following a combination of computer simulations and laboratory trials, the team traced the physics behind this counterintuitive motion. Three factors emerged as the drivers of the effect: electrostatic interactions, the influence of fluid flow, and random thermal motion of the surrounding molecules. The size of the obstacle played a crucial role in how long the particle remained captured before it could escape.
The team found that aspects of fluid dynamics created stagnant regions within the experimental chamber. The spinning microcylinder induced internal liquid movement, but certain pockets formed behind the obstacle where the fluid nearly stopped. When the particle encountered one of these quiescent zones, it halted and became stuck. To reach a region of mobility, the particle had to execute a sharp reorientation around the obstacle. It then moved to the far side, effectively clinging to the rear surface. The researchers also observed that random kicks from the Brownian motion of liquid molecules could push the particle into the capture zone, a process that depended on the barrier’s size. With a smaller obstacle, these fluctuations more readily drove the particle into the trapped state, and by adjusting the barrier size, the team could extend the retention time by a noticeable margin.
This work provides a framework for retaining microparticles during experiments, offering a potential advantage for researchers who need to study particles over extended periods without continuous external manipulation. The findings underline how subtle interactions between electrostatics, hydrodynamics, and thermal motion can produce stable, counterintuitive particle behavior in confined liquid systems, especially when microcylindrical obstacles are present. The insights gained could influence experimental design across nanoscale and microscale studies, where precise control over particle position is essential. The researchers emphasize that the method has practical potential for advancing measurements and observations in a range of liquid-based experiments, where maintaining particle position under dynamic conditions can be challenging.