In an effort to create the 'starting blocks' for the 3D Nanoprinter, I found an interesting phenomenon where microspheres suspended at a water-air interface would, under some conditions, spontaneously arrange themselves into stable non-close-packed structures. In the experiments that followed, I tested the stability of the structures, the effects of water and solvent cleanliness, electrical charge of the microspheres, as well as surface tension effects. Ultimately I found that the explanation for the resulting structure is notably different from the commonly used 'surface tension attraction/electrostatic repulsion' (instead involving Stern/diffuse layer interactions similar to interatomic potentials), and that the non-close-packed lattice is indeed an energy minimizing arrangement.
The annotated CAD of the facility, followed by a photo of the complete facility.
This plastic enclosure was built for the purpose of ultrasonic deposition of microspheres. An ultrasonic piezo element is used to make a microsphere mist, and since dispersed microspheres are hazardous to breathe, this airtight container is used to keep them from reaching the experimenter. The container itself is housed inside a fume hood. Having an airtight seal also prevents the air currents around the contained from affecting the deposition process, which requires very still air. A vertically adjustable sample stage is included to transfer samples from the deposition height to the observation height without opening the container.
Some complicated circular structures.
The microspheres sometimes form orderly arrays. But more likely, they form complex structures with circles, chains, and voids, as seen in the above microscope photo. The microspheres are 1um diameter.
From left to right, a progression from the initial unordered structure to a highly ordered structure.
The microspheres are initially dry and have no order, as seen on the left of the above image. Poorly controlling parameters leads to a complex structure like the middle section of above image. By refining experimental parameters, the ordered non-close-packed structure like the one on the right is achieved in most cases. Some parameters are still not understood, so at times the ordered lattice does not appear.