Loss of percolation transition in the presence of simple tracer-media interactions

Random motion in disordered media is sensitive to the presence of obstacles which prevent atoms, molecules, and other particles from moving freely in space. When obstacles are static, a transition between confined motion and free diffusion occurs at a critical obstacle density: the percolation threshold. To test if this conventional wisdom continues to hold in the presence of simple tracer-media interactions from the type seen in recent experiments, we introduce the Sokoban random walk. Akin to the protagonist of an eponymous video game, the Sokoban has some ability to push away obstacles that block its path. While one expects this will allow the Sokoban to venture further away, we surprisingly find that this is not always the case. Indeed, as it moves—pushing obstacles around—the Sokoban always confines itself after traveling a characteristic distance that is set by the initial density of obstacles. Consequently, the percolation transition is lost. This finding breaks from the ruling ant in a labyrinth paradigm, vividly illustrating that even weak and localized tracer-media interactions cannot be neglected when coming to understand transport phenomena.