arXiv subjects

Stuart J. Thomson

Publications and source records attributed to Stuart J. Thomson.

7 recordsLinked to original sources

arXiv2023

Non-equilibrium thermodynamics in driven macroscopic self-assembly

Equilibrium statistical mechanics provides a robust framework for characterizing phase transitions in systems whose microsopic dynamics are time-reversible. Efforts to develop and validate theoretical frameworks for time-irreversible, non-equilibrium systems are constrained by experimental data that capture only partial measurements of the system dynamics. We herein overcome this limitation using a tunable macroscopic platform for non-equilibrium physics: millimetric spheres bound by capillary attractions at the fluid interface and driven out of equilibrium by a field of supercritical Faraday waves. The external driving induces correlated fluctuations in the particle trajectories, which in turn excite structural rearrangements between distinct metastable cluster topologies. By tracking all microstate transitions experimentally, we directly measure a non-zero entropy production rate reflecting broken detailed balance and quantifying the system's departure from equilibrium. The measured stochastic dynamics are in quantitative agreement with a many-body active Ornstein-Uhlenbeck model, thus establishing a bridge to a wider class of athermal, self-propelled systems at the microscale. These results invite parallel studies of non-equilibrium self-assembly kinetics using active colloids or passive particles immersed in bacterial baths whose dynamics and irreversibility are likewise governed by correlated active forces and tunable inter-particle interactions.

cond-mat.soft↗

arXiv2022

Bidirectional Wave-Propelled Capillary Spinners

When a solid body floats at the interface of a vibrating liquid bath, the relative motion between the object and interface generates outwardly propagating surface waves. It has recently been demonstrated that millimetric objects with fore-aft mass asymmetry generate an associated asymmetric wavefield and consequently self-propel in unidirectional motion. Harnessing this wave-powered mechanism of propulsion, we here demonstrate that chiral objects placed on a vibrating fluid interface are set into steady, yet reversible, rotation, with the angular speed and direction of rotation controlled by the interplay between object geometry and driving parameters. 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. Leveraging the control over the chiral object's direction of rotation, we then demonstrate that a floating body with an asymmetric mass distribution and chirality can be remotely steered along two-dimensional trajectories via modulation of the driving frequency alone. This accessible and tunable macroscopic system serves as a potential platform for future explorations of chiral active and driven matter, and demonstrates a mechanism by which wave-mediated fluid forces can be manipulated for directed propulsion.

physics.flu-dyn↗

arXiv2022

SurferBot: A wave-propelled aquatic vibrobot

Nature has evolved a vast array of strategies for propulsion at the air-fluid interface. Inspired by a survival mechanism initiated by the honeybee (Apis mellifera) trapped on the surface of water, we here present the SurferBot: a centimeter-scale vibrating robotic device that self-propels on a fluid surface using analogous hydrodynamic mechanisms as the stricken honeybee. This low-cost and easily assembled device is capable of rectilinear motion thanks to forces arising from a wave-generated, unbalanced momentum flux, achieving speeds on the order of centimeters per second. Owing to the dimensions of the SurferBot and amplitude of the capillary wave field, we find that the magnitude of the propulsive force is similar to that of the honeybee. In addition to a detailed description of the fluid mechanics underpinning the SurferBot propulsion, other modes of SurferBot locomotion are discussed. More broadly, we propose that the SurferBot can be used to explore fundamental aspects of active and driven particles at fluid interfaces, as well as in robotics and fluid mechanics pedagogy.

physics.flu-dyn↗

arXiv2021

Taylor dispersion of elongated rods

Particles transported in fluid flows, such as cells, polymers, or nanorods, are rarely spherical. In this study, we numerically and theoretically investigate the dispersion of an initially localized patch of passive elongated Brownian particles constrained to one degree of rotational freedom in a two-dimensional Poiseuille flow, demonstrating that elongated particles exhibit an enhanced longitudinal dispersion. In a shear flow, the rods translate due to advection and diffusion and rotate due to rotational diffusion and their classical Jeffery's orbit. The magnitude of the enhanced dispersion depends on the particle's aspect ratio and the relative importance of its shear-induced rotational advection and rotational diffusivity. When rotational diffusion dominates, we recover the classical Taylor dispersion result for the longitudinal spreading rate using an orientationally averaged translational diffusivity for the rods. However, in the high-shear limit, the rods tend to align with the flow and ultimately disperse more due to their anisotropic diffusivities. Results from our Monte Carlo simulations of the particle dispersion are captured remarkably well by a simple theory inspired by Taylor's original work. For long times and large Peclet numbers, an effective one-dimensional transport equation is derived with integral expressions for the particles' longitudinal transport speed and dispersion coefficient. The enhanced dispersion coefficient can be collapsed along a single curve for particles of high aspect ratio, representing a simple correction factor that extends Taylor's original prediction to elongated particles.

physics.flu-dyn↗

arXiv2020

A discrete complex Ginzburg-Landau equation for a hydrodynamic active lattice

A discrete and periodic complex Ginzburg-Landau equation, coupled to a discrete mean equation, is systematically derived from a driven and dissipative oscillator model, close to the onset of a supercritical Hopf bifurcation. The oscillator model is inspired by recent experiments exploring active vibrations of quasi-one-dimensional lattices of self-propelled millimetric droplets bouncing on a vertically vibrating fluid bath. Our systematic derivation provides a direct link between the constitutive properties of the lattice system and the coefficients of the resultant amplitude equations, paving the way to compare the emergent nonlinear dynamics---namely discrete bright and dark solitons, breathers, and traveling waves---against experiments. Further, the amplitude equations allow us to rationalize the successive bifurcations leading to these distinct dynamical states. The framework presented herein is expected to be applicable to a wider class of oscillators characterized by the presence of a dynamic coupling potential between particles. More broadly, our results point to deeper connections between nonlinear oscillators and the physics of active and driven matter.

nlin.PS↗

arXiv2020

Collective vibrations of confined levitating droplets

We report a new type of fluid-based driven dissipative oscillator system consisting of a lattice of millimetric fluid droplets bouncing on a vertically vibrating liquid bath and bound within an annular ring. We characterize the system behavior as it is energized through a progressive increase in the bath's vibrational acceleration. Depending on the number of drops, the onset of motion of the lattice may take the form of either out-of-phase oscillations or a striking solitary wave-like instability. Theoretical modeling demonstrates that these behaviors may be attributed to different bifurcations at the onset of instability. The results presented here demonstrate the potential and utility of the walking droplet system as a platform for investigating wave-mediated, inertial, non-equilibrium particle dynamics at the macroscale.

cond-mat.soft↗

arXiv2020

Collective vibrations of a hydrodynamic active lattice

Recent experiments show that quasi-one-dimensional lattices of self-propelled droplets exhibit collective instabilities in the form of out-of-phase oscillations and solitary-like waves. This hydrodynamic lattice is driven by the external forcing of a vertically vibrating fluid bath, which invokes a field of subcritical Faraday waves on the bath surface, mediating the spatio-temporal droplet coupling.By modelling the droplet lattice as a memory-endowed system with spatially nonlocal coupling, we herein rationalise the form and onset of instability in this new class of dynamical oscillator. We identify the memory-driven instability of the lattice as a function of the number of droplets, and determine equispaced lattice configurations precluded by geometrical constraints. Each memory-driven instability is then classified as either a super- or sub-critical Hopf bifurcation \emph{via} a systematic weakly nonlinear analysis, rationalising experimental observations. We further discover a previously unreported symmetry-breaking instability, manifest as an oscillatory-rotary motion of the lattice. Numerical simulations support our findings and prompt further investigations of this nonlinear dynamical system.

physics.flu-dyn↗