An acoustic-radiation-pressure phased array (ARPPA) is undergoing development at Lewis Research Center. ARPPAs are envisioned as general-purpose, nonintrusive tools for manipulating both liquids and objects suspended in liquids.

Acoustic-radiation pressure and acoustic streaming are effects created by high-intensity sound. Acoustic-radiation pressure applies forces to objects situated on acoustic paths. Acoustic streaming is a unidirectional flow that can arise because sound can exert a thrust on a liquid in which it propagates. ARPPAs would make it possible to exploit acoustic-radiation pressure and acoustic streaming (see figure) to perform such manipulation and control functions as propelling or agitating liquids, moving floating objects, controlling the shapes of liquid surfaces, or ejecting liquid drops.

The Acoustic-Phased-Array Concept, similar to the concept of phased-array antennas for electromagnetic waves, involves the exploitation of collective effects of waves emitted by elements of an array. Here, acoustic (instead of electromagnetic) beams are steered and focused by controlling the phases of excitations applied to the elements of the array.

A beam of sound can be produced by use of an array of acoustic transducer elements. Each element emits small wavelets of sound that, over a distance, overlap other wavelets. Wavelets that are in phase tend to coalesce into a single beam. If the wavelets are focused, then their amplitudes become superimposed to form a beam of high intensity. The distinguishing feature of an ARPPA is that one can electronically control the phase relationships among the elements of the array to steer the beam and adjust the size and shape of the focal region. Thus, one can adjust the position and shape of the region where acoustic-radiation pressure and acoustic streaming occur.

An interactive-computer-controlled ARPPA demonstration apparatus is under construction. This apparatus is designed to enable a user to interactively control the focus and position of an acoustic-radiation-pressure beam. The apparatus is expected to aid in the development of specific users' applications.

An ARPPA enables a user to exert some control over a liquid without intruding into its container. ARPPAs might be capable of performing the functions of such other mechanical devices as agitators, filters, probes, and manipulators. The ARPPA approach holds promise for simplifying systems by reducing the need for external plumbing and intrusive mechanisms and for such high-maintenance items as seals and bearings.

Potential uses for ARPPAs include the following:

  • Agitation of Liquids: ARPPAs could provide the agitation needed for processes that involve liquids in sealed systems. ARPPAs could be used to disperse accumulations of particles, and to form and maintain such suspensions as slurries, paints, and pastes. Furthermore, agitation of liquids by use of acoustic-radiation pressure could be used to obliterate thermal gradients or concentration gradients and thereby prevent stratification of chemicals in vessels.
  • Segregation of Gas Bubbles and Solids Suspended in Liquids: ARPPAs might be useful for segregating objects suspended in liquids in sealed systems, without using filters. Acoustic-radiation pressure could be used to force contaminant bubbles and particles into traps, where they could be rendered harmless without breaking into the system. The elimination of in-line filters would reduce probabilities of clogging and reduce the amount of maintenance needed, and could thus also be useful in reducing the risk of contamination of the environment from systems that process toxic chemicals.
  • Ejection of Liquid Drops: If droplets could be ejected from a pool of liquid without using a nozzle, then there would be no risk of clogging. Therefore, suspended particles would not hinder operation. By use of acoustic-radiation pressure with precise focus combined with tone-burst control, one could eject drops on demand, with precise control of sizes and velocities of the drops. One could use acoustic-radiation pressure in this way to dispense picoliter volumes of liquids on demand, to apply paints or other liquid coating materials without using masks, or to apply molten metals (e.g., solder in automated soldering of circuit boards).
  • Manipulation of Free Surfaces: ARPPAs could be used to control surface waves for such purposes as suppression of sloshing or of standing waves in tanks. Surfaces could be manipulated to control wetting through selective forcing of iquids into contact with solid surfaces. Solder fountains driven by acoustic-radiation pressure might be useful as means to refine the common wave-soldering method used to solder electronic-circuit boards. ARPPAs could also be used to drive surface waves on liquids for the selective application of adhesive and other coating materials.
  • Manipulation of Immersed Objects: ARPPAs could be used to manipulate such immersed objects as bubbles, drops of immiscible liquids, or partially buoyant solid objects. The focusable, steerable nature of ARPPAs could be used to move such objects though complex paths and even to oppose such forces as those associated with gravitation, fluid currents, and electromagnetic fields. ARPPAs could be employed to orient and concentrate fibers and other reinforcing constituents to be cast in composite structures. In outer space, ARPPAs could control the ingestion of gas bubbles into tanks containing liquids. ARPPAs might be useful for micromanipulation of biological tissues in liquid media. ARPPAs might even be proven suitable for nonintrusive repositioning of detached retinas in human eyes. /li>

This work was done by Richard C. Oeftering of Lewis Research Center. Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center, Commercial Technology Office, Attn: Tech Brief Patent Status, Mail Stop 7-3, 21000 Brookpark Road, Cleveland, Ohio 44135

Refer to LEW-16470.

NASA Tech Briefs Magazine

This article first appeared in the December, 1998 issue of NASA Tech Briefs Magazine.

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