Active matter transport in porous mediums*

Many organisms including bacteria, nematodes, and vertebrates, spanning in length from microns to meters, can be found in porous fluid-saturated mediums ranging from tissue to granular beds. Depending on their relative size, such active matter may move entirely within the pore space or rearrange the material locally, depending on the topology and strength of the medium. There is considerable interest in understanding the strategies employed in achieving transport, towards applications in robotics, oil recovery, logic design in microfluidic devices, and high-throughput motility-based sorters. We shall discuss a series of studies with Lumbriculus variegatus, also known as California blackworms, as they move through model porous medium in the form of transparent index matched granular hydrogels, or laser-cut mazes consisting of chambers connected by narrow passages. These slender limbless worms perform undulatory and peristaltic strokes and use their head to actively probe their surroundings. We will show how the worm employs the different strokes to move through the medium, which changes character with increasing overburden pressure.  The importance of boundary following and body strokes in determining how active matter escapes from enclosed spaces will be highlighted in the limit where the interaction with the solid component of the medium is steric. We will discuss active polymer models and the degree to which they can capture the trapping and diffusion statistics of the worm. Finally, we will address questions on the influence of activity as it relates to the entanglement of these filamentary worms and clogging while they move through narrow passages.