Contactless deformation of fluid interfaces by acoustic radiation pressure

Abstract

Reversible and programmable shaping of surfaces promises wide-ranging applications in tunable optics and acoustic metasurfaces. Based on acoustic radiation pressure, contactless and real-time deformation of fluid interface can be achieved. This paper presents an experimental and numerical study to characterize the spatiotemporal properties of the deformation induced by acoustic radiation pressure. Using localized ultrasonic excitation, we report the possibility of on-demand tailoring of the induced protrusion at water-air interface in space and time, depending on the shape of the input pressure field. The experimental method used to measure the deformation of the water surface in space and time shows close agreement with simulations. We demonstrate that acoustic radiation pressure allows shaping protrusion at fluid interfaces, which could be changed into a various set of spatiotemporal distributions, considering simple parameters of the ultrasonic excitation. This paves the way for novel approach to design programmable space and time-dependent gratings at fluid interfaces.

Figure 1

Space–time tracking of the surface elevation. A confocal displacement sensor CL-P070 is used to measure the time evolution of the water–air surface displacement induced by the ARP. The confocal laser measurement is synchronized with an ultrasonic excitation generated by an amplifier delivering a 1 MHz electrical burst to an immersed transducer. Triggering is performed through an electrical pulse from a function generator remotely driven by a computer.

Figure 2

3D characterization of the interface deformation induced by a transient acoustic excitation. Three phases are highlighted: rise (a,d,g), maximum (b,e,h),and decrease (c,f,i). (a–c) Pictures of the deformation shot with a reflex camera for $p_{i0}=3.5\text{MPa}$ and $\tau =50\text{ms}$. (d–f) Experiment results of a 2D scan for $p_{i0}=2.1\text{MPa}$ and $\tau =50\text{ms}$. (g–i) 3D view of a simulation results for $p_{i0}=2.1\text{MPa}$ and $\tau =50\text{ms}$. The time evolution of the measured and simulated surface deformation are available in Supplementary Video 1a,b.

Figure 3

Time dependences and spatiotemporal shapes of the deformation for different input pressure fields. Experiments (solid lines) and FEM simulations (dashed lines) of the time evolution of the point located on the transducer axis are compared for (a) a spherical and (b) a planar transducer. (c) 2D shape of the interface deformation resulting from a spherical transducer at its maximum height (t = 8 ms), during the decrease (t = 16 ms) and for its minimum height (t = 45 ms). (d) 2D shape of the interface deformation resulting from a planar transducer at its maximum height (t = 13 ms), during the decrease (t = 38 ms) and at its minimum height (t = 95 ms). Insets in (c,d) are the linear input pressure fields from hydrophone measurements for each transducer.

Figure 4

Influence of the time sequence of the excitation. (a) The time evolution of the surface displacement at r = 0 (solid blue line) induced by a periodic burst excitation (solid red line). The 1 MHz 30 period sine burst (highlighted with a black frame) is repeated at a frequency of 100 Hz during 200 ms, (b) shows an extended view of one burst. (c) 2D shape of the resulting interface deformation at $t=58\text{ms}$ and (d) Cross-sectional profile of the interface deformation along $x$ at $y=0$.