Physicists have discovered an unusual behavior of a microparticle when rolling a cylindrical obstacle. This was reported by the press service of the American Northwestern University.
Michel Driscoll and colleagues sent a microparticle to a cylindrical obstacle (microcylinder) rotating in a liquid medium during the experiments. They expected one of two outcomes: the particle must either hit an obstacle or get around it. But the particle did neither. The scientists were surprised to see how the particle got around the obstacle and then “sticked” to its backside. The obstacle seemed to contain the particle effectively.
After a series of simulations and experiments, the researchers uncovered the physics behind this strange phenomenon. The unexpected behavior was caused by three factors: electrostatic, hydrodynamic, and disordered random motion of surrounding molecules. The size of the obstacle also determined how long the particle was trapped before escaping.
It turned out that due to the peculiarities of hydrodynamics, stagnant zones appeared inside the sample chamber. In other words, the rotating microcylinder caused the liquid to flow inside the chamber. But the flows formed pockets—including pockets just behind an obstacle—where the fluid remained stationary and did not flow. When the particle hit the stagnant zone, it stopped and got stuck. However, to reach the rest zone, the particle had to make a sharp turn. Reaching the obstacle, the particle went around it, sticking to the back. Driscoll found that the Brownian motion of molecules in a liquid “ejects” the particle into a stationary region. It also turns out that the size of the barrier determines how long the particle will be trapped before escaping. For example, it is easier for Brownian fluctuations to push a particle into the capture region when the barrier is smaller. By varying the size of the barrier, the researchers could increase the retention time by an order of magnitude.
The authors hope their method for retaining microparticles during scientific experiments will be in demand.