Acoustic-radiation pressure can be used to improve performance in the dispensing of liquid drops into gases or vacuum and in dispensing gas bubbles into liquids. In a typical application involving dispensing a liquid, this is accomplished by use of a high-frequency, high-intensity acoustic transducer coupled with a conventional syringe and hollow dispensing needle (similar to a hypodermic needle). A small dose of liquid passes through the needle and forms a drop at the tip. The drop is held in place by surface tension. The acoustic transducer emits a premetered acoustic tone burst at high power. The bore of the needle conducts the acoustic waves to the drop, where acoustic-radiation pressure creates a force on the droplet. When the burst of force is sufficient to overcome surface tension, the drop separates from the tip.

These Are Examples of Schemes for coupling acoustic waves from a transducer to a liquid drop or a bubble to be dispensed from the tip of a hollow needle.

Unlike in previous approaches to dispensing, it is not necessary to rely on gravitation or on the inertia of drops to cause deployment. Usually, the sizes of drops are proportional to the sizes of needles, but by use of acoustic-radiation pressure, one can deploy drops independently of needle sizes. Because deployment by acoustic-radiation pressure is controlled electronically, it is possible to adjust the acoustic excitation to deploy or dispense drops of various liquids and various sizes with various initial velocities, on command.

In a given apparatus, acoustic waves can be coupled from a transducer in any of several schemes. Examples (see figure) include (1) using the fluid in the bore of the needle as a waveguide to conduct acoustic power to the drop at the tip of the needle; (2) using an external coaxial transducer mounted at the base of the needle and focused at a bubble at the tip of the needle; (3) using the cylindrical wall of the needle as a solid waveguide to conduct the acoustic waves from transducer to the drop at the tip; or (4) using an external coaxial transducer mounted away from the needle and focused at a bubble at the tip. The fourth-mentioned scheme is suitable for the case in which a bubble does not stick to the needle; the acoustic transducer in this scheme emits an opposing acoustic beam that pins the bubble in place until it is time to release the bubble on command. All of the foregoing schemes can be used individually or in combination.

Acoustic transducers can also be used as sensors. One can exploit this sensory capability to measure positions of drops and bubbles. By monitoring the electrical signal from a transducer, one can verify deployment of a drop or bubble, without visual monitoring of the drop or bubble.

Potential applications in which one could use acoustic-radiation pressure to enhance dispensing of drops and bubbles include the following:

  • Outer-Space Applications: Specific applications include fluid-physics, drop-physics, and droplet-combustion experiments; containerless processing; and dispensing liquids in a variety of systems in which premetered drops are needed.
  • Terrestrial Applications: The behaviors of drops and bubbles could be controlled while using fewer mechanical parts and less plumbing than are now needed for such purposes. Such control could be exploited for precise placement of paints, dyes, adhesives, liquid coating materials in general, pastes (including slurry pastes), and molten solders used in manufacturing. In many applications, this acoustic-radiation-pressure approach could eliminate the need for masks and related tooling and processing. This approach can also be followed in precise dispensing of drops and bubbles in chemical processes and in medical applications.

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-16469.

NASA Tech Briefs Magazine

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

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