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Swinburne University of Technology Australia

A Quantum Physical View of Time Crystals: OAV Air Bearing Used To Stabilize the Electrodynamic Shaker's Resonances for Experimentations on Non-Equilibrium Physics of Droplets

The Optical Sciences Centre at the Swinburne University of Technology in Melbourne, Australia aimed to better understand the behavior of liquid droplets bouncing on the surface of a fluid bath through a quantum physical lens. When droplets are subject to periodic forcing, they may begin to "walk" on the surface of the fluid, a result of each impact of the droplet with the fluid surface triggering a capillary wave, and the subsequent gradients in the fluid surface driving the planar motion of the droplets. The team characterized these droplets as "droplet time crystals" (DTC), which are periodically driven systems that exhibit a persistent oscillatory response with an integer multiple of the driving period. The main components of the experiment included the electrodynamic shaker, the OAV Air Bearing [OAVBX5050], the fluid bath, the droplet printer, and the optical imaging system used to observe and process the information from the experiments.

Using a custom optical table that rested on passive vibration isolation legs for elimination of external noise, the driving force was provided by an electrodynamic shaker bolted onto a layer cake structure, which rested on machine mounts. The shaker was connected to the OAV Air Bearing through a drive rod, and the air bearing was mounted on an aluminum plate on top of the optical table, connected to a high-purity compressed air supply. The fluid bath was leveled, and the air bearing assembly was clamped in place before the air bearing slider bar was connected to the drive rod. The design assembly can be seen in the below graphic.

Droplet Time Crystals System Set-Up Using Air Bearing

The OAV Air Bearing was a crucial component in the assembly. Indeed, the OAVBX5050 was used to reduce the transverse vibrations through stabilization of the entire system due its smooth and ultra-precise frictionless motion. Frictionless motion in the axial direction prevented adverse motion in the transverse plane. The team chose an air bearing with a large enough surface area to maintain the total payload to a minimum, reducing the shaker resonances.

The fluid bath had a fluid containing diameter of 100 mm, with mass totaling to 570 g. It was mounted on an air bearing slider bar and was precisely aligned by tilting the whole optical table using a two-axis digital level. Vibrations of the bath were measured using two piezoelectric single-axis accelerometers. Droplets were introduced onto the fluid bath surface using a droplet printer consisting of a computer-controlled two-axis linear translation stage and a piezoelectric droplet generator. The droplet generator utilized a 35 mm diameter piezoelectric buzzer disk and M6 threaded brass nozzle with 0.1-1.0 mm nozzle size. The fluid was pumped into the generator by a peristaltic pump, and the fluid level was set using a micrometer translation stage. The droplets' motion was tracked via a top view camera and a side view camera.

In the experiment, multiple subsystems were used to control the data acquisition and measurement. A computer was used to generate the driving signal for the shaker and read the accelerometer data, both at a 32 kHz sampling rate. The signals were monitored using a digital storage oscilloscope and a software feedback loop maintained a fixed driving amplitude. A droplet printer was used to deposit droplets onto the fluid surface and two microcontrollers were used to monitor temperature probes and the droplet printer. The cameras were triggered manually and the images were processed manually. However, all subsystems were integrated and controlled by a single workstation. Baseline measurements were performed to characterize the mechanical resonance properties of the shaker and study its dependence on payload. The shaker resonances were found to conform to the expectations and were in good agreement with previous studies. 

The laboratory had air conditioning system to maintain 0.5 °C temperature stability, monitored by two PT100 platinum RTD probes and a microcontroller. Calibration was done relative to each other at 21 °C ambient temperature. The fluid used for the experiments was silicone oil with density of 950 kg/m3 and viscosity of 20 cSt at 25 °C. The thermal characterization depicted that the temperature of the fluid and the air remained within the air conditioning system specifications. However, the high-speed imaging light (135 W LED) could generate turbulent air currents and affect the droplet dynamics, so it should only be turned on when necessary. These adverse effects can be eliminated by protecting the fluid bath with enclosures. Continuous measurement of fluid temperature is not necessary unless extreme precision is required.

When the fluid bath vibrated above a certain frequency-dependent amplitude, called the Faraday threshold, Faraday waves emerged on the fluid surface. The authors observed Faraday patterns with square and triangular unit cells that repeated at a lower frequency than the driving frequency of the fluid bath. The authors also introduced droplets onto the fluid surface and observed that they stably bounced in a (2,1) mode, where their center of mass undergoes vertical periodic oscillations at half the driving frequency of the fluid bath. The droplets also supported internal vibrational modes in free space. The authors fixed the driving frequency and amplitude and studied the effect of varying the droplet size on the droplet's bouncing dynamics.

The droplet printer-generator produced arbitrary two-dimensional patterns of droplets on a bath. The resulting structures were determined by droplet-droplet interactions. An example of this is a square lattice of droplets, which after 5 minutes, undergoes a transmutation to a triangle lattice. This is a non-trivial phenomenon as the system is driven far from the Faraday threshold and the preference for a triangle lattice over a square lattice cannot be explained by energy conservation or higher packing fraction alone. The observed behavior is due to complex, self-consistent interplay between wave-mediated many-body interactions between the droplets and subtle boundary effects.


Tapio Simula 2023 Phys. Scr. 98 035004

This material is based on work supported by The Optical Sciences Centre at the Swinburne University of Technology in Melbourne, Australia.

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