4. Acoustic Shaping
4.1 Prior Results

In the preceding sections we showed that significant forces could be generated in unsteady fields, especially standing wave fields, due to interaction between the field and solid (or liquid) particles. Comparison of optical, microwave and acoustic forces shows that significant accelerations, much higher than the disturbances from “g-jitter” in orbit, can be achieved using all three kinds of waves. In the following section, we show flight validation of the idea of using such forces in a resonant field, to form prescribed shapes of walls. Acoustic shaping in reduced-gravity experiments is used, since an opportunity provided by NASA on their Reduced-Gravity Flight Laboratory made this feasible. These results were obtained before the present NIAC grant; however, they are reproduced here to emphasize that constructing predictable shapes is valid and practical.

 

Figure 4.1 Rectangular chamber geometry used for reduced-gravity acoustic shaping experiments [23-35]

Wanis et al [23] used a rectangular plexiglass box with speakers mounted on two sides across a corner as shown in Fig. 4.1. Only the speaker in the end face was used. With solid particles placed inside the box, the setup flown in reduced gravity, and the speaker driven at a natural frequency of the box, the particles migrate rapidly and stand along the nodal planes of the box (Figure 4.2.)

Figure 4.2 Single-particle thick walls of irregular ellipsoidal grains, forming parallel to the nodal surfaces in reduced-gravity flight experiments

This is a crucial demonstration: in the ultrasonics and optics experiments performed elsewhere, the primary interest was in holding one particle close to a pre-selected point of minimum potential. Here it is seen that when a multitude of particles are placed in a resonant potential field, they migrate, not to the single point of least potential, but to fill entire surfaces. These are thus self-forming walls - videotapes of the flight tests show that the particles "jostle" each other and fill up vacant spaces in the. The clarity of the video frame shown above confirms our observation that the walls are stable, and irregular-shaped particles stay fixed in position, with no rotation or vibration.

 

The environment where the above results were obtained is shown in Fig. 4.3. The frequencies and gains were set by the experimenter with the cap, (Andres Sercovich) while their values were read out by his teammate (Ron Sostaric). There was no feedback control or correction for differences in air temperature between the ground experiment and the flight test. The best results (stable walls) were obtained when the input intensity was quite low - so that secondary effects such as acoustic streaming were not strong.

Figure 4.3 Experiment on board the NASA KC-135 Reduced-gravity flight laboratory where the image in Figure 4.2 was obtained

 

 


Abstract 
Intro
Theory
Near-term: Acoustic
Mid-Term: L2 Habitat
Space Economy 
Far-Term: Radio-Wave Construction
Comments
Issues
Conclusions
Acknowledgements
References

 

Various other results have been obtained on such wall formations - these are reported in [23-25]. The mode shapes even for a rectangular container can be quite complex, and the measured locations of the walls are parallel to or coincident with the nodal surfaces. Figure 4.4shows comparison between predicted and measured wall locations, with the measurements being made in ground tests where particles arranged themselves along the floor of the chamber. 

Figure 4.4 Measured locations of Styrofoam particles along the floor of a rectangular resonator operated at its 110 mode in a ground experiment.

 

 


 
 
In Figure 4.5, we see that the walls do not touch the walls of the resonator, but extend close to the walls. Also, the formation of large walls along the nodal surfaces does not alter the sound field enough to shift the resonance to another frequency. Figure 4.6 is one of several flight test results showing the formation of walls with hollow aluminum oxide spheres (white) mixed with hollow aluminum spheres (shiny metal).

Figure 4.5 Walls of Styrofoam particles form in an acoustic resonator operated at its 110 mode. Reduced-gravity flight test, April 1997.

 

 

Figure 4.6 (below, left) Walls of hollow aluminum oxide spheres and hollow aluminum spheres. Flight test, acoustic field. The walls are usually 1 particle thick. The small aluminum particles are seen to occupy the space between the larger aluminum oxide particles.

One issue that has come up in previous Acoustic Positioning experiments in Space is the ability to hold particles still as the temperature field around them alters. Wang et al [20] note that interaction between the streaming flow and the thermal boundary layer of a heated particle would reduce or even reverse the trapping force in the standing wave field, and hence cause the trapped particle to drift away, as it cools following melting in an oven. This effect occurs in the Mie scattering regime, where the interaction between the incident and scattered radiation has a strong directionality. When audible-frequency sound and millimeter-sized particles are used, the interaction is in the Rayleigh scattering regime, and such problems are much less significant. Figure 4.7 is a preliminary attempt to demonstrate that a wall formed of molten particles can be cooled without the trapping force being destroyed. The material in this case consisted of millimeter-sized balls which were heated to the melting point, then placed in a resonant acoustic field and allowed to cool, with the frequency continuously changed to accommodate changes in speed of sound. This experiment was performed in the lab in1-G, so the wall is not very high. The particles remained in a vertical wall until the material solidified, forming the first solid object built in an acoustic field.

 


Abstract 
Intro
Theory
Near-term: Acoustic
Mid-Term: L2 Habitat
Space Economy 
Far-Term: Radio-Wave Construction
Comments
Issues
Conclusions
Acknowledgements
References

 

 

Figure 4.7 T-shaped object solidified from molten millimeter-sized balls of Agarose, with the driver frequency continuously varied to maintain resonance during cooling. Ground experiment in 1G

The above experiment demonstrates that liquids will form into walls along the nodal surfaces in a resonant acoustic field in 1-G. Thus the nodal regions are clearly regions of low static pressure, in addition to being the trap region for small particles. This interesting discovery was investigated in forming walls of water in 1-G. It was found that the walls would shatter at the top, with a spray of droplets escaping, causing a fountain effect as more water was pushed up into the wall from below. When a powder was suspended on the water surface, the sheets remained much more stable, and steady walls of water with suspended powder were formed, again with very small thickness. We have not measured the thickness, but it appears to be of the order of a millimeter. Examples are seen in Figure 4.8.

 

 

 

 

 

Figure 4.8 Curved (quarter-spherical) walls of water with suspended powder, formed in an acoustic resonator. Ground test, 1-G.
 
 
 
 
 
 

 


Abstract 
Intro
Theory
Near-term: Acoustic
Mid-Term: L2 Habitat
Space Economy 
Far-Term: Radio-Wave Construction
Comments
Issues
Conclusions
Acknowledgements
References

 

 

4.2 Implications of the Acoustic Shaping Results

From the above experiments, it is clear that acoustic fields can be used with solids, liquids and phase change, to form solidified objects with thin walls, in reduced gravity. The height of liquid walls can be much greater, with a different choice of acoustic medium, static pressure, and g-level, offering the potential for various manufacturing processes in lunar or artificial / variable- gravity stations. An example of such a process is conceptualized below in Fig. 4.10: A solid object is built in several parts in an acoustic field, with processes involving solar-powered heating, cooling and robotic assembly. Objects up to about 3 meters in size can be built with conventional technology using appropriate gases, provided that large chambers can be provided in reduced-gravity environments.

Figure 4.10: Formation of a part of a cylindrical object using acoustic shaping, with color and texture changes indicating temperature variations and change from granular material to solid-walled part.

 

4.3 Development of Shape-Design Software for Tailored Force Fields

By combining several modes, with variable amplitudes, various interesting shapes can be built. To visualize these and permit experimentation, a software package was developed, using Matlab. Currently a more user-friendly, stand-alone version of this, suitable for use by non-technical personnel, is being developed. Results from our experiments with this software are shown below in Figure 4.11. The software can be modified to simulate other types of radiation such as microwaves or radio waves quite readily.


2 2 0
1 1 0
1 1 0 + 0 2 0
2 3 0 + 1 0 0

 

FIGURE 4.11. Simulation results on useful wall shapes combining low-order acoustic modes in a rectangular chamber.

4.4  Concluding Remarks on the Findings in Section 4.
In this section we have presented the idea of tailoring unsteady potential fields to form solid particles of arbitrary shapes into walls of prescribed geometry. We have considered applications and theoretical results from optics and acoustics to show that the force generation mechanisms bear similarity. Choosing a material with a wide range of forms, but generally uniform density (silicon dioxide as glass and regolith) we have performed a direct comparison of the particle accelerations achievable using optical, microwave and acoustic standing wave fields. Experiments using acoustic shaping confirm that particles of arbitrary shape migrate rapidly to nodal surfaces whose shapes correspond to simple predictions. Generalizing using the similarities between different wave fields, we conclude that electromagnetic shaping could become a viable option to consider for automatically forming useful objects in Space, when high-power, long-wave resonators are developed.

There are, no doubt, complexities in building refined parts using acoustics. There is a rich body of literature on the theory relating to ultrasonic positioning. Liquid manipulation using acoustics is also a topic of strong research interest [27]. The issue of driving multiple resonances is dealt with in [28]. However, the experimental results shown here are proof that the basic concept is feasible and worthy of further exploitation. The major breakthrough in our work is the confirmation that tailored force fields of various types of radiation can be used, with essentially similar and predictable characteristics. This permits excellent simplifications in our view of what appeared at first sight to be an intractable problem.

While the formation of parts by acoustic shaping is feasible today, acoustic shaping requires containers with gas atmospheres. The formation of walls and useful shapes from microwave and other electromagnetic radiation can be performed in vacuum, but require substantial power sources, and will become realistic when there is a movement towards the construction of large power sources such as solar power satellites or power beamed from the Moon. For any large-scale construction in Space, massive resources from extraterrestrial locations (Moon or asteroids) are essential.  Large-scale extraction of extraterrestrial resources requires an economic framework with long-term payoffs.  In following sections we discuss an architecture for building the first massive human habitat at the Earth-Moon L-2 or L-5 regions, as part of a coherent plan for a Space-based economy. The project is conceived as a synergy between concepts for lunar-based solar power plants, lunar robotic mining and metal processing,  robotic fabrication plants, and a set of  electromagnetic launchers as a lunar equatorial  space launch systems.  Railcar-sized rectangular containers filled with 2m thickness of lunar regolith (dominantly SiO2) are launched off the lunar surface. They are captured in space by “shepherd” space tugs which guide them to the axis of the cylinder, and positioned using an electromagnetic grid onto the outer cable grid of the cylinder, before being robotically fixed to each other.  A quasi-steady magnetic positioning and assembly technique is used in this process.
 
 
 
Abstract 
Intro
Theory
Near-term: Acoustic
Mid-Term: L2 Habitat
Space Economy 
Far-Term: Radio-Wave Construction
Comments
Issues
Conclusions
Acknowledgements
References