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Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2025, Tutors: Demian Levis, Juan Pablo Carrillo-Mora

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Hexbug robots are self-propelled particles that can behave as both chiral and nonchiral active Brownian particles. We experimentally determined key parameters: self-propulsion speed, chirality, rotational diffusion and self-alignment length, for seven robots by studying their free trajectories and angular dynamics under translational forcing. To explore collective behavior, we assembled active trimers by linking three robots in a linear chain with rods allowing free rotation.
From these experiments, we developed a trimer model and ran extensive numerical simulations to analyze its dynamics. Our results reveal two dynamical regimes depending on the central robot’s chirality: one resembling active Brownian motion and another similar to chiral run-and-tumble dynamics. This behavior emerges from the interplay of two time scales related to chirality and self-alignment. By analyzing center-of-mass trajectories, we studied the mean squared displacement to measure the effective translational diffusion coefficient of the trimer. It decreases with chirality in the first regime but increases in the second, where tumbling events emerge. A minimum occurs where the two time scales balance, indicating a smooth crossover