Displays have revolutionized the way we work and learn, and thus, the development of display technologies is of paramount importance. The possibility of a free-space display in which 3D graphics can be viewed from 360 without obstructions is an active area of research-holograms or lightfield displays can realize such a display, but they suffer from clipping and a limited field of view. Here, we use a phased array of ultrasonic emitters to realize a volumetric acoustophoretic display in which a millimetric particle is held in midair using acoustic radiation forces and moved rapidly along a 3D path. Synchronously, a light source illuminates the particle with the target color at each 3D position. We show that it is possible to render simple figures in real time (10 frames per second) as well as raster images at a lower frame rate. Additionally, we explore the dynamics of a fast-moving particle inside a phased-array levitator and identify potential sources of degradation in image quality. The dynamics are nonlinear and lead to distortion in the displayed images, and this distortion increases with drawing speed. The created acoustophoretic display shows promise as a future form of display technology.

We describe and experimentally explore a nonlinear stiffness model of the trapping of a solid particle in a single-axis acoustic levitator. In contrast to the commonly employed linear stiffness assumption, our nonlinear model accurately predicts the response of the system. Our nonlinear model approximates the acoustic field in the vicinity of the trap as a one-dimensional sinusoid and solves the resulting dynamics using numerical continuation. In particular, we predict a softening of stiffness with amplitude as well as period-doubling bifurcations, even for small excitation amplitudes of %2% of the wavelength. These nonlinear dynamic features are observed experimentally in a single-axis levitator operating at 40 kHz and trapping millimetre-scale expanded polystyrene spheres. Excellent agreement between the observed and predicted behaviour is obtained suggesting that this relatively simple model captures the relevant physical phenomena. This new model enables the dynamic instabilities of trapped particles to be accurately predicted, thereby benefiting contactless transportation and manipulation applications.

Advances in acoustophoresis have allowed the recent development of a free-space volumetric display called an Acoustophoretic Volumetric Display (AVD) that can render 3D graphics observable without obstructions. The current generation of AVD can render simple vector graphics in real-time, but larger and complex graphics require 10 seconds or more to render. Here we present a generalized model of an AVD and use this to understand its performance limits; in particular, we answer the question of how large a display (1:1 raster screen) can be created? We show that AVD performance is affected by the size and properties of the particle, along with the viscosity of the surrounding fluid. Optimal performance is achieved when the viscous drag force and inertial force are minimized relative to the acoustic radiation force. Our model suggests that, for expanded polystyrene particles (ρ p = 19 kg m −3 ), a screen size of up to 117 mm by 117 mm can be rendered at 10 Hz with an acoustic frequency of 20 kHz, and an acoustic pressure amplitude of 25 kPa.

Acoustic holograms are the keystone of modern acoustics. They encode three-dimensional acoustic fields in two dimensions, and their quality determines the performance of acoustic systems. Optimisation methods that control only the phase of an acoustic wave are considered inferior to methods that control both the amplitude and phase of the wave. In this paper, we present Diff-PAT, an acoustic hologram optimisation platform with automatic differentiation. We show that in the most fundamental case of optimizing the output amplitude to match the target amplitude; our method with only phase modulation achieves better performance than conventional algorithm with both amplitude and phase modulation. The performance of Diff-PAT was evaluated by randomly generating 1000 sets of up to 32 control points for single-sided arrays and single-axis arrays. This optimisation platform for acoustic hologram can be used in a wide range of applications of PATs without introducing any changes to existing systems that control the PATs. In addition, we applied Diff-PAT to a phase plate and achieved an increase of > 8 dB in the peak noise-to-signal ratio of the acoustic hologram.

Phased array transducers (PATs) are used in many applications, from airborne ultrasonic tactile displays to acoustic levitation. Acoustic holograms play a significant role in determining the performance of these applications. Many PATs and optimizers have been developed; however, only the following have been demonstrated in the literature: “phase” and “phase and amplitude” control of transducers and “phase” and “amplitude” only control at target points. Thus, most of the combinations of transducer state and target acoustic field conditions are yet to be explored. Here, we explore such combinations using Diff-PAT, one of the latest acoustic hologram optimizers. Diff-PAT is based on automatic differentiation and stochastic gradient descent. This optimizer achieves higher accuracy than conventional optimizers. We formulated multiple loss functions and wave propagators to enable each combination of the operation mode and quantitatively assessed the performance of each combination. The developed optimizers will offer new opportunities in the field and could allow further simplifications in PAT applications.