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arXiv · 2609.37946

Emergent activity powers a path to chaos for pairs of acoustically trapped spheres

Matthew K. Gronert·Ben Cao·Ella M. King·David G. Grier
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Abstract

Pairs of spheres sharing an acoustic trap sometimes spin intermittently despite having no self-propulsion, no obvious power source, and no symmetry-breaking mechanism, switching between rocking and spinning within a single trajectory. Nonreciprocal wave scattering powers this emergent activity, and drag shapes their dynamics by coupling rotations to translations. Mismatched pairs can pass through a supercritical Hopf bifurcation into a quasiperiodic torus on which the pair's orbit reverses regularly and, farther from equilibrium, into deterministic chaos. Experiments motivate this mechanism and share its qualitative signatures; simulations reveal a full noise-free route from passivity to chaos, and suggest how analogous dynamics can emerge in other active-matter systems.

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Matthew K. Gronert, Ben Cao, Ella M. King, David G. Grier. 2026-09-29. Emergent activity powers a path to chaos for pairs of acoustically trapped spheres. https://arxiv.org/abs/2609.37946

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