After a preliminary design review, the focus for this past week shifted to logistics. This includes creating a detailed schedule of work, milestones, and materials needed. Thus far, structural requirements have been figured and a material list for the necessary goods is complete. In addition to the microfluidic devices, propellant tanks must be assembled to feed the valves. The goal for this past week was simply to source and assemble these small tanks for mounting and use on the flight rig. A teflon inner tank hold the propellant while a harder plastic outer shell provides structure and mounting points. A central wick acts as the primary flow path for the propellant to the valves. These assembled are shown in the figure below.
Ordering of parts begins this week. Unfortunately, a slight delay in new valve fab was incurred due to an issue with a tool for depositing oxides on silicon wafers. This has since been fixed and we are back on track for pushing flight component fabrication! Once the new components are ordered, the flight rig can start being assembled.
With all the fab work under way, it’s time to take a step back and ensure all the big-picture elements are in place for the actual zero-g flight. This includes fleshing out the details of operation for each parabola during flight, addressing safety concerns, sourcing materials and cataloging them, planning data collection, finalizing component layout, and looking at the criteria for a successful flight.
80/20 aluminum frame rails and plexiglass will be ordered to form the base structure for the experiment. A mid-level baseplate will serve as a mounting point for all hardware to be tested as well as the power supply and data collection devices. Once on hand, these materials will be easy to assemble and strong enough to handle the 9G load requirements set out for the flight.
Designing the initial flight experiment has begun! This means fitting as much good data gathering equipment, microfluidic devices, and necessary structure into the prescribed plane’s footprint. Thus far, a simple 80x20 aluminum box frame will support a baseplate that can be sized to accommodate the needed components. These components will include: microfluidic devices in a hermetically sealed container with accompanying fluid reservoirs, microcontroller, 100 Volt power supply, switch board, and camera. Final layout will be determined by the dimensions of the power supply and camera setup.
Following the final fabrication of the last flow controller design, it was decided that the extremely dense packed layout suitable for a space mission was not necessary to test the underlying conceptual operation. Therefore, a new flow controller layout has been set to a larger 2cmx2cm square chip size. This will allow for thicker features that are more robust during fabrication and flight. Fabrication of these new flight-oriented test articles has begun. To further improve the photolithographic feature resolution and etch-resistance, an oxide mask will be implemented. This is achieved by thermally growing silicon-dioxide on a bare silicon wafer at approximately 1000 degrees celsius. The images below show the darkened wafers with a recent oxide growth completed in the glowing furnace tube. These wafers will be the base for the next fabrication steps.
Final photolithographic and etch steps were carried out. The primary lesson learned had to do with the energy dispersion of the laser in the photoresist. It would seem the fluid channel walls, while captured in photoresist, were not wide enough to withstand the long duration etch to expose the capillaries. The next step will be to re-design the channels and fluid controlling feature geometries to make a more robust test article and ensure a successful fabrication with the available process equipment.
Initial materials were identified for the flight unit as well. These will include a plexiglass enclosure, non-pressurized fluid tank, power supply, micro-controller, and a structural interface for the various microfluidic design iterations to be controlled by the micro-controller. Code for the microcontroller and CAD for the enclosure and experiment structure are now underway. Lastly, with the ability to control liquid in zero-g, there must be some artistic ways to demonstrate these devices. More to come on that. In the meantime, how to best capture the device operation is being sorted out.
With a strong backing of nanofabrication trials and fluid modeling out of the way, construction of the various flow controller test articles is under way!
All fabrication begins with pure silicon wafers. Thus far, oxidation has been deposited and thin layers of photo-activated polymers were deposited and patterned to allow for a plasma etch to cut the desired fluid-controlling features and geometries into the underlying silicon.
Next up, the final photolithographic and etching steps will be completed prior to dicing the wafer into individual flow controllers and valves for testing. Once the best candidates are found, they will be incorporated into a experimental apparatus to be flow on the zero-g flight!
The booming market of smaller, cheaper, and lighter CubeSats has created the need for equally small propulsion systems. Electric propulsion is a very efficient option for most large spacecraft, but small spacecraft typically lack the power needed to operate them. MIT’s Space Propulsion Lab has developed the ion Electrospray Propulsion System to address this operational regime to make CubeSats viable platforms for numerous missions that traditional chemical propulsion cannot enable. To accompany these lightweight and low-power propulsion systems, propellant control must also be low mass and require minimal power. Recent developments in microfabrication techniques have shown that electrostatic fields can effectively initiate the flow of propellant via wicking materials and capillary tubes with no need for heavy mechanical valves and pressure vessels that require more power. These devices are being built
for testing in a zero-gravity environment to conclusively show their operational capabilities for future spacecraft.