Active matter has received significant attention over the last decade or so. A closely related, yet physically distinct, field is that of driven matter in which self-propulsion and collective self-organization of otherwise passive particles is achieved by external forcing of the environment. In this talk, I will discuss several topics in the nascent field of driven interfacial hydrodynamics exploring the dynamics and statistics of matter on vibrating fluid interfaces.
The first part of the talk will focus on the structural rearrangements between ground states of clusters of millimetric, hydrophobic spheres bound by capillary attractions. The structural rearrangements are driven by chaotic, supercritical Faraday waves, which in turn play the role of an active bath. In contrast to microscopic systems such as colloids, inertial effects are non-negligible in our macroscopic system, prompting the development of a Langevin model of the particle dynamics. The consequences of underdamped particle motion and long-range capillary forces on the emergent dynamics and statistics will be discussed. The second part will explore chiral objects placed on a vibrating fluid interface that are set into steady, yet reversible, rotation, with the angular speed and direction of rotation controlled by the interplay between object size and driving frequency and acceleration. Scaling laws and a simplified model of the wavefield reveal the underlying physical mechanism of rotation, while collapsing experimental measurements of the angular velocity across parameters. Informed by this discovery, we demonstrate how a floating body with both mass and geometric asymmetries can be remotely steered along 2D trajectories via modulation of the driving frequency alone. Finally (and time-permitting) we present SurferBot: a centimeter-scale, untethered, vibrating robotic device that self-propels on a fluid surface using analogous hydrodynamic mechanisms to honeybees trapped on the surface of water. Analogies with a broader class of wave-driven propulsion strategies will be discussed.
This talk will contain elements of fluid dynamics, experimental mechanics, dynamical systems, statistical physics, and simple robotics.