Wireless actuation with functional acoustic surfaces

Abstract

Miniaturization calls for micro-actuators that can be powered wirelessly and addressed individually. Here, we develop functional surfaces consisting of arrays of acoustically resonant micro-cavities, and we demonstrate their application as two-dimensional wireless actuators. When remotely powered by an acoustic field, the surfaces provide highly directional propulsive forces in fluids through acoustic streaming. A maximal force of ∼0.45 mN is measured on a 4 × 4 mm2 functional surface. The response of the surfaces with bubbles of different sizes is characterized experimentally. This shows a marked peak around the micro-bubbles' resonance frequency, as estimated by both an analytical model and numerical simulations. The strong frequency dependence can be exploited to address different surfaces with different acoustic frequencies, thus achieving wireless actuation with multiple degrees of freedom. The use of the functional surfaces as wireless ready-to-attach actuators is demonstrated by implementing a wireless and bidirectional miniaturized rotary motor, which is 2.6 × 2.6 × 5 mm3 in size and generates a stall torque of ∼0.5 mN·mm. The adoption of micro-structured surfaces as wireless actuators opens new possibilities in the development of miniaturized devices and tools for fluidic environments that are accessible by low intensity ultrasound fields.

Active Acoustic Surfaces Enable the Propulsion of a Wireless Robot

“…The acoustic streaming was imaged by particle imaging velocimetry (PIV) with a high‐speed camera and the recoil force was measured by a force sensor (details in Experimental Section). We find that the propulsive force increases linearly with the driving voltage of the piezoelectric transducer (up to 100 V), where it saturates reaching a maximum of about 0.45 mN (black diamonds in Figure b) with ≈6200 microbubbles on a 4 mm × 4 mm surface, consistent with the previous observations . We find that the propulsive force does not increase linearly with the number of bubbles, as for larger surfaces not enough fluid can reach the center of the array to sustain streaming, which limits the incoming water supply and decreases the overall streaming velocity.…”

“…Fabrication of the Active Surfaces : A thin film of SU‐8 photoresist containing cylindrical hole arrays was fabricated on a silicon wafer by photolithography, as reported previously . The microcavities with diameters smaller than 30 µm had a spacing equal to its diameter, and microcavities with diameters equal to or larger than 30 µm had a constant spacing of 20 µm.…”

“…Ultrasound Excitation Setup : A customized setup was used to excite the active surfaces, as reported previously . A total of 2.4 g L −1 sodium dodecyl sulfate aqueous solution was used to stabilize the interface, lower the surface tension, and increase the propulsive force .…”

“…The BASS actuators exploit bubbles in the micrometer size range to minimize the size of the cavities and so that they can be excited by ultrasound (>20 kHz). We proved that they can generate just enough force to actuate millimeter‐sized devices . However, the actuation of devices at the millimeter‐to‐centimeter scale is desirable in many fields, especially for medical applications.…”

“…Recently we reported functional interfaces that respond to ultrasound actuation and that can be applied as wireless surface actuators taking up a minimal amount of space . The active surfaces consist of 2D arrays of microbubbles, and when remotely powered by ultrasound, they provide directional propulsive forces in fluids through acoustic streaming (see Figure a,b).…”