7. Acoustic Shaping Technology: R&D

ASI's core technology is the Acoustic Shaping technology proven by the GT team in over 120 parabolas of microgravity flight on NASA's KC-135 out of Houston in 1997 and 1998. Acoustic Shaping uses the fact that sound fields can generate mean flows, and net forces, in a fluid medium. This basic phenomenon has been known for over a century in various forms [Andrade1932]. Early observations were of dust heaps formed inside pipes and coal mine shafts. Most school children have seen demonstrations of "acoustic levitation". In acoustic levitation, the force due to a sound field generated by powerful speakers is used to lift small objects and suspend them above a surface, balancing the gravitational force. Levitation of steel spheres has been demonstrated on Earth [Whymark 1975] using powerful, cooled ultrasonic drivers, for non-contact, microwave melting operations. NASA Space Shuttle experiments demonstrated the use of Acoustic Positioning in microgravity, where individual droplets of molten material were suspended at the center of an oven, in order to develop perfectly spherical and pure spheres in microgravity. The sound field in these experiments was ultrasonic, and required cooling of the speakers. Numerous patents and papers, listed in [Wanis 1998a] have considered the enhancement of the levitation force, and the control of particle positions using passive geometric and active means.

The Georgia Tech Microgravity student flight team developed Acoustic Shaping technology to form complex shapes using large numbers of solid particles in air, in a rectangular box. Their 1997 experiments demonstrated that:

• Particles self-align to form walls in the sound field, rather than agglomerate around a single stable point as suggested by the previous single-particle experiments.
• The wall shapes correspond with high fidelity to predicted shapes from acoustic mode calculations.
• Wall shapes can be flat, or have varying radius of curvature in multiple dimensions, as predicted from sound field theory.
• Particle shapes can be heterogeneous, such as Styrofoam pieces and Rice Krispies cereal, representative of crushed solid material from lunar mines.
• Low, audible-range sound levels are adequate in microgravity to form multiple walls of either straight or curved shape.
• Harmonics could be used to form intersecting walls.
• Varying speaker location, phase and amplitude could be used to control the behavior of particles of various sizes, shapes and densities.

On June 6^th 1999, a wall made up of 1.5 mm orange spheres coated with Agarose was permanently formed after being shaped using the infamous (1 1 0) mode and hardened. The repeatability of this experiment was also successfully demonstrated on June 8^th 1999. A wall of Styrofoam, chalk, Al₂O₃ and the Agarose was formed and hardened.

The 1999 ground tests mark the very first permanent shape in our research to date. From several patent/literature search and talks to others in the field this seems to be also the first object to be manufactured using this method/approach-acoustic shaping. The repeatability of this experiment was also successfully demonstrated on June 8^th 1999. A wall of Styrofoam, chalk, Al₂O₃ and the Agarose was formed and hardened.

In our setup, a suitable grain was placed in the warm Agarose mixture, to give it strength upon formation. Knowing that our current acoustic shaper produces single particle thickness surfaces, it was forecasted that if no strengthening grain was inserted into the Agarose mixture, the formed surface would collapse after formation due to its soft gel-like structure. Therefore, 1.5 mm diameter plastic orange spheres were used as the grain in the first experiment. The second experiment had Styrofoam, Al₂O₃, and chalk powder as the grain.

Of course, this process introduced temperature gradients in the acoustic chamber. This was accounted for by first obtaining an estimate of the frequency needed to produce the (1 1 0) mode at 60^oC from the MATLAB code. The chamber was warmed up by adding a cup of water at 60^oC, then the estimate frequency was used to drive the chamber. Scanning in tens of Hertz the resonant frequency was found. The chamber was then drained out and the Agarose with the grain added. High gains were required to initially get the walls to form (dial set to 5). Once that was attained, the gain was lowered (3) to eliminate splashing and vibration of the wall (flutter). Note that the penalty was a reduction in the wall height. As the experiment was running, the frequency was lowered in one’s of Hertz to account for the speed of sound slowing to maintain resonance as the Agarose mixture cooled down as well as its surroundings. The Agarose was intentionally left to cool since that is the main method for it to harden. The Sound Pressure Level (gain) was also lowered with time in order to save the speakers. This did not affect the wall formation since by that time it had taken shape/form. The sound field was left on to act as a guide for the wall to form into a harder solid. The actual time for the wall to form and become independent of the sound field has not yet been determined but seems to be around a minute. Although this experiment was successful regardless of the temperature variations, since they were accounted for by forecasted knowledge of what’s going to happen, a simple feedback controller would be ideal for our purposes. In other experiments where positioning of a single sphere was desired, heat was a basically a dead end. But in our purposes, it does not seem to be such a hurdle. The next immediate step to demonstrate the impact of heat on our configuration is to setup the same experiment but not account for the change in speed of sound and observe the same process of the melt cooling down, and recording what happens to the wall formation process. Depending on how much the effect is, a suitable feedback control system will be designed and put to the test.

This technology is being developed into a full-scale space manufacturing capability, including

• first-principles-based prediction
• empirical simulation
• inverse design
• model-scale validation in earth-based microgravity flight experiments
• model-scale validation in space-based experiments