Active intruder motion in a two-dimensional granular medium

We studied an active intruder traversing a two-dimensional granular medium. First, we conducted experiments to analyze the motion of the active intruder within a bed of soft particles distributed throughout a horizontal container. Our observations revealed that the intruder preferentially moved along regions characterized by higher disorder. As a consequence of its motion, and as the intruder repeatedly traversed specific regions, the structure in the vicinity of the path, but also in more distant regions, gradually became more ordered. Additionally, the motion of the intruder tended to be localized in closed paths, with this effect becoming more pronounced at higher particle concentrations. Secondly, we analyzed the behavior of the active intruder within a slightly tilted container and under vibration and shearing conditions, which served to fluidize the granular bed. Our findings indicated that the trajectory of the intruder tended to intersect regions of the granular medium with lower structural order. The overall order and fluidization degrees were found to be the primary influences on the intruder's penetration length. We used the sixth-bond-orientational order parameter to characterize the local structural aspects of the medium and the mean squared displacement to describe the dynamics of both the intruder and the medium. We conducted an experimental analysis of the ordering of the medium and motion of the intruder using network contact forces in a similar system based on soft gelatin particles. The results of this experimental analysis are consistent with the general results, showing that media with an ordered configuration exhibit more resistance than disordered media.