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arXiv:1806.03759 · Chaos An Interdisciplinary Journal of Nonlinear Science

Hong–Ou–Mandel-like two-droplet correlations

Rahil N. Valani, Anja C. Slim, Tapio Simula

Published Sep 1, 2018• 24 citations
arXivHTMLPDF

Abstract

We present a numerical study of two-droplet pair correlations for in-phase droplets walking on a vibrating bath. Two such walkers are launched toward a common point of intersection. As they approach, their carrier waves may overlap and the droplets have a non-zero probability of forming a two-droplet bound state. The likelihood of such pairing is quantified by measuring the probability of finding the droplets in a bound state at late times. Three generic types of two-droplet correlations are observed: promenading, orbiting, and chasing pair of walkers. For certain parameters, the droplets may become correlated for certain initial path differences and remain uncorrelated for others, while in other cases, the droplets may never produce droplet pairs. These observations pave the way for further studies of strongly correlated many-droplet behaviors in the hydrodynamical quantum analogs of bouncing and walking droplets.

DOI
10.1063/1.5032114

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We present a numerical study of two-droplet pair correlations for in-phase droplets walking on a vibrating bath. Two such walkers are launched toward a common point of intersection. As they approach, their carrier waves may overlap and the droplets have a non-zero probability of forming a two-droplet bound state. The likelihood of such pairing is quantified by measuring the probability of finding the droplets in a bound state at late times. Three generic types of two-droplet correlations are observed: promenading, orbiting, and chasing pair of walkers. For certain parameters, the droplets may become correlated for certain initial path differences and remain uncorrelated for others, while in other cases, the droplets may never produce droplet pairs. These observations pave the way for further studies of strongly correlated many-droplet behaviors in the hydrodynamical quantum analogs of bouncing and walking droplets.
A droplet bouncing on the surface of a vibrating liquid bath can move horizontally guided by the wave it produces on impacting the bath. The wave itself is modified by the environment, and thus, the interactions of the moving droplet with the surroundings are mediated through the wave. This forms an example of a pilot-wave system. Taking the Oza–Rosales–Bush description for walking droplets as a theoretical pilot-wave model, we investigate the dynamics of two interacting identical, in-phase bouncing droplets theoretically and numerically. A remarkably rich range of behaviors is encountered as a function of the two system parameters, the ratio of inertia to drag, κ, and the ratio of wave forcing to drag, β. The droplets typically travel together in a tightly bound pair, although they unbind when the wave forcing is large and inertia is small or inertia is moderately large and wave forcing is moderately small. Bound pairs can exhibit a range of trajectories depending on parameter values, including straight lines, sub-diffusive random walks, and closed loops. The droplets themselves may maintain their relative positions, oscillate toward and away from one another, or interchange positions regularly or chaotically as they travel. We explore these regimes and others and the bifurcations between them through analytic and numerical linear stability analyses and through fully nonlinear numerical simulation.
It was recently discovered that millimeter-sized droplets bouncing on the surface of an oscillating bath of the same fluid can couple with the surface waves it produces and begin walking across the fluid bath. These walkers have been shown to behave similarly to quantum particles; a few examples include single-particle diffraction, tunneling, and quantized orbits. Such behavior occurs because the drop and surface waves depend on each other to exist, making this the first and only known macroscopic pilot-wave system. In this paper, the quantized orbits between two identical drops are explored. By sending a perturbation to a pair of orbiting walkers, the orbit can be disrupted and transition to a new orbit. The numerical results of such transitions are analyzed and discussed.
Pilot-wave dynamics of walking droplets is a field in hydrodynamics that has been studied extensively for its interesting features previously thought to be found only in quantum mechanics. The walking droplets in combination with their pilot-wave form a spatially delocalized entity called a walker, wherefrom quantum-like features arise. \n\nThe behavior and dynamics of the droplet have been described theoretically, where the walker moves in a two-dimensional (plane) bath. We start the theoretical development of an analogous one-dimensional version of this theory that describes the walker's behavior in an one-dimensional (long and narrow) bath. We set up the model basis for the equation of motion of the vertically bouncing droplet by looking at a quasi-static droplet in our one-dimensional system. \n\nOur goal is to develop a theory that eventually describes and rationalizes the dynamics and statistical behavior of a walker in our one-dimensional setting. We predict that the simplification to an one-dimensional setting will make the theory behind the behavior of the walker system more understandable. Hence, the theory, which is the basis for explaining the emerging quantum-like features, will eventually shed some light on what is and what is not fundamental to quantum mechanics.\n\nSpecifically, the surface- and potential energy of a quasi-static droplet are calculated and expressed in an harmonic decomposition of the droplet's shape. The expansion coefficients that arise in the harmonic decomposition are calculated by performing an averaging method on the potential energy and minimizing the total energy of a droplet resting on a rigid surface. The expansion coefficients are shown to accurately describe the droplet's shape for a range of Bond numbers (a dimensionless number representing the relative magnitude of gravity to surface tension per unit area), where the range depends on the contact area that the droplet makes with the underlying surface. These results can be used to further develop the one-dimensional walker theory.
Bouncing walking droplets possess fascinating properties due to their peculiar wave-particle interaction leading to unexpected quantumlike behaviors. We propose a study consisting in droplets walking along annular cavities. We show that, in this geometry, they spontaneously form a string of synchronized bouncing droplets that share a common coherent wave propelling the group at a speed faster than single walkers. The formation of this coherent wave and the collective droplet behaviors are captured by a model. Those are at the opposite of the ones found in two-dimensional geometries. Our results shed light on walking dynamics.
Bouncing fluid droplets can walk on the surface of a vibrating bath forming a wave-particle association. Walking droplets have many quantum-like features. Research efforts are continuously exploring quantum analogues and respective limitations. Here, we demonstrate that two oscillating particles (millimetric droplets) confined to separate potential wells exhibit correlated dynamical features, even when separated by a large distance. A key feature is the underlying wave mediated dynamics. The particles' phase space dynamics is given by the system as a whole and cannot be described independently. Numerical phase space histograms display statistical coherence; the particles' intricate distributions in phase space are statistically indistinguishable. However, removing one particle changes the phase space picture completely, which is reminiscent of entanglement. The model here presented also relates to nonlinearly coupled oscillators where synchronization can break out spontaneously. The present oscillator-coupling is dynamic and can change intensity through the underlying wave field as opposed to, for example, the Kuramoto model where the coupling is pre-defined. There are some regimes where we observe phase-locking or, more generally, regimes where the oscillators are statistically indistinguishable in phase-space, where numerical histograms display their (mutual) most likely amplitude and phase.
Couder et al. [1] discovered that the behavior of droplets bouncing on a vibrated bath mimics a variety of quantum phenomena, through their coupling with a Faraday wave. Here, we introduce a continuous model inspired from this experiment, which encapsulates its essential features into a system of three simple differential equations. We show, numerically and analytically, the existence of self-propelled walkers in this framework. Finally, making use of the model’s familiar formulation, we suggest that a particle coupled with a propagating wave, instead of a standing one, behaves much like a bouncing droplet.