The authors of this work, led by the Magnetic Soft Matter Group, found that when many particles are suspended in a liquid, the hydrodynamic interactions, i.e., the interactions due to the fluid motion generated by the other particles, can make it harder to force particles to move along a line. This result contrasts with previous observations, which showed that hydrodynamic interactions could help particles move faster. The reason for these different findings lies in the driving method: when particles are dragged by the fluid that surrounds them, then any force resisting the motion, for example, a periodic array of optical traps, will be enhanced by its effect on the neighboring particles, and the particles will slow down. On the other hand, when particles are moved by an external force, like that generated by a magnet, the motion of the other particles helps overcome the drag and enhances the collective velocity. Understanding this difference has profound implications in microfluidics devices and biological transport processes.
Hydrodynamic Interactions Can Induce Jamming in Flow-Driven Systems. Eric Cereceda-López, Dominik Lips, Antonio Ortiz-Ambriz, Artem Ryabov, Philipp Maass, and Pietro Tierno. Phys. Rev. Lett. 127, 214501 – Published 17 November 2021