Navigating disordered landscapes with touch: Lessons from blackworms

We will discuss the dynamics of {it Lumbriculus variegatus}, also known as the blackworm, as they navigate a series of chambers connected by narrow passages where steric interactions with confining walls lead to significant barriers for transport. By observing these long slender worms inside transparent hard-walled chambers, we visualize and track the entire shape of their body as they collide with the walls, locate passages, and repeatedly pass back and forth between the chambers. We show that the worm does not explore the chambers ergodically but remains close to the boundary as its head repeatedly collide with the wall, while its tail and center of mass stay away, as the worm follows the boundary. In a regime where the passage width is greater than the worm width, the frequency with which the worm's head locates the passage is found to increase rapidly before essentially becoming constant with increasing width. We discuss this crossover in terms of the time scale and distance over which the head loses contact with the boundary. We further show the impact of the body strokes on the penetration depth of the worm down the passage, and in successfully traversing the passage between the chambers. The implication of the measured traversal rates on the transport properties in a randomly connected set of chambers representing a quasi-2D porous medium will be discussed.