Collective dynamics of macroscopic photoactive matter under alternating excitation patterns

Abstract

We present experiments on the collective dynamics of macroscopic photoactive self-propelled particles subjected to spatiotemporally varying excitation. The particles move within an arena divided into two regions with different illumination intensities, creating alternating bright (more active) and dark (less active) zones. Under such conditions, the system exhibits a robust migration from the more active region toward the less active region, demonstrating a strong response to external modulation. This response depends sensitively on the frequency of the illumination pattern: at low frequencies, particles follow the changing landscape, whereas at higher frequencies, the response diminishes. We show that this behavior arises from the interplay between the imposed excitation and the intrinsic dynamics of the particle clusters that form spontaneously. To explain these features, we extend a kinetic model previously introduced in L\'evay et al. [Phys. Rev. Lett. 135, 098301 (2025)], hence revealing the most important parameters governing the transition between the responsive and unresponsive regimes.