How fast can you feasibly make it out of a confined space littered with obstacles? The average path length of a random walk inside an arbitrary closed domain is given by the volume-to-surface-area ratio of the space, a relationship that has been exploited in studies of light propagation in turbid media. Giacomo Frangipane and colleagues have now shown that they can use this proportionality to predict the average path length of bacteria swimming inside microstructures.
The team tracked bacteria as they moved through different microchambers containing randomly distributed pillars. The pillars provided a degree of tunable complexity that allowed the researchers to sample a range of volume-to-surface-area ratios, and ultimately confirm the general invariance property predicted for confined random walks. In doing so, they succeeded in showing that introducing more obstacles doesn’t actually increase the average residence time. Instead, it shortens it by decreasing the volume in which the bacteria can swim.