Complex Flows and Soft Matter Group

Capillary Surfers

Over the last several decades, there has been significant interest in understanding the physics of so-called “wet” active matter systems, in which constituents consume energy in order to move through a fluid medium. Such systems are ubiquitous in biology and span a large range of length scales: microscopic organisms like bacteria interact through relatively low-speed flows dominated by viscous effects, while schools of fish and flocks of birds generate faster flows dominated by inertial effects. This research program concerns a newly-discovered active matter system that bridges the gap between these scales, as it operates in an intermediate regime in which both viscous and inertial effects are relevant. Specifically, the “capillary surfers" are millimetric objects that self-propel while floating at the interface of a vibrating fluid bath. Experiments conducted in the Harris Lab (Brown University) demonstrated that surfer pairs may lock into a variety of bound states, and larger collectives of surfers self-organize into coherent flocking states. We have developed theoretical models for these surfers that allow us to physically interpret the experimental results. Our models are computationally tractable and readily generalizable to larger collectives of surfers, thus providing a platform for studying active matter systems in which both inertial and viscous effects are relevant.

Recent Publications



A.U. Oza, G. Pucci, I. Ho, D.M. Harris, Theoretical modeling of capillary surfer interactions on a vibrating fluid bath, Phys. Rev. Fluids, 8, 114001, (2023)





We present and analyze a theoretical model for the dynamics and interactions of “capillary surfers,” which are millimetric objects that self-propel while floating at the interface of a vibrating fluid bath. In our companion paper [I. Ho et al., Phys. Rev. Fluids 8, L112001 (2023)], we reported the results of an experimental investigation of the surfer system, which showed that surfer pairs may lock into one of seven bound states, and that larger collectives of surfers self-organize into coherent flocking states. Our theoretical model for the surfers' positional and orientational dynamics approximates a surfer as a pair of vertically oscillating point sources of weakly viscous gravity-capillary waves. We derive an analytical solution for the associated interfacial deformation and thus the hydrodynamic force exerted by one surfer on another. Our model recovers the bound states found in experiments and exhibits good agreement with experimental data. Moreover, we conduct a linear stability analysis of bound state solutions and compute numerically the associated eigenvalues. We find that the spacings of the bound states are quantized on the capillary wavelength, with stable branches of equilibria separated by unstable ones. Generally, our work shows that self-propelling objects coupled by capillary waves constitute a promising platform for studying active matter systems in which both inertial and viscous effects are relevant.

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I. Ho, G. Pucci, A.U. Oza, D.M. Harris, Capillary surfers: Wave-driven particles at a vibrating fluid interface, Phys. Rev. Fluids, 8, L112001, (2023)





We present an experimental study of capillary surfers, a new fluid-mediated active system that bridges the gap between dissipation- and inertia-dominated regimes. Surfers are wave-driven particles that self-propel and interact on a fluid interface via an extended field of surface waves. A surfer's speed and interaction with its environment can be tuned broadly through the particle, fluid, and vibration parameters. The wave nature of interactions among surfers allows for multistability of interaction modes and promises a number of novel collective behaviors.

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