The purpose of the Spun Microgravity Liquid Experiment (SMiLE) is to investigate the development of droplets produced when a liquid exits a circular nozzle at a constant flow rate in a microgravity environment. SMiLE consists of a liquid injection assembly and a viewing chamber assembly which includes image recording capability. The experiment provides precise control of the flow velocity (Reynolds and Weber numbers) via a syringe pump driven by a stepper motor, and records the surface profiles of the produced drops via a high resolution stereoscopic camera setup. Fluid temperature and system state properties are monitored and logged. A large number of experiments (N > 1,000) are required to provide statistically relevant results on drop formation dynamics.
To avoid the need for a large fluid and waste reservoir, this experiment incorporates a novel centrifugal separation and recovery system to reuse the fluid. The experiment has been designed to conform to a standard 1.5U payload slot (150 x 100 x 100 mm), weigh less than 1.5 kg and consume less than a 2W peak supplied power. The experiment contains only a 20 mL reservoir of water, and will be capable of creating flows with Reynolds numbers ranging from 54 to 785. The system is powered through a setup of twelve super-capacitors which recharge via a 2W power supply, and is controlled by a compact microcontroller. With an estimated operational cycle length of 100 seconds, the experiment works through three main operational modes – injection and visual recording, centrifugal spin and liquid gathering, and liquid return. Captured surface profiles are processed on-board to provide drop diameter and time of formation as well as a transient record of position.
Jet breakup has been explored since the early 19th century. From such studies and further experimental work it is understood there are three distinct regimes of liquid jet breakup in normal Earth conditions.
The characterisation of jet breakup is important for use in drop on demand applications, including inkjet printing, genome printing, and pharmaceuticals. For consistent successful operation, these applications require controlled, consistent drops to be formed.
The breakup of a liquid jet occurs due to instabilities that form on the surface the jet and grow in magnitude until they cause necking and then capillary collapse.
Instabilities are characterised as convective or absolute. Convective instabilities travel from the point of initiation in one direction, and are what allow a long cylindrical jet to start forming droplets some distance away from the nozzle. Absolute instabilities travel simultaneously upstream and downstream, leading to the formation of drops at or very close to the nozzle. Lord Rayleigh’s initial analysis of a theoretical jet identified that the fastest growing convective perturbation had a wavelength of 9.016(r0), where r0 is the initial radius of the jet.
Jet breakdown is sensitive to a wide variety of factors including gravity and jet velocity, as well as fluid parameters like density, viscosity and surface tension. Surface tension is one of the major driving forces promoting drop formation, causing the cylindrical jet to form into a sphere, the most energy efficient shape. Thus, higher surface tension promotes the development of necking and capillary collapse. With increasing velocity, the known regimes are periodic dripping, chaotic dripping and liquid jetting.
Typically, jet breakup is described by the following parameters:
· Reynolds number (Re) – compares inertial forces to viscous forces
· Weber number (We) – compares inertial forces to surface tension
· Bond number (Bo) – compares gravitational forces to surface tension
While jet breakup in normal earth conditions has been studied quite rigorously, less experimental work has been completed in reduced gravity conditions due to the difficulty in achieving such conditions. Interestingly, three jet breakup regimes have also been observed in environments where gravity no longer dominates minor forces: jetting, chaotic dripping, and quasi-steady, where the drop does not detach from the nozzle (see above). While initial theoretical analyses on jet breakup did not include gravity, only recently has the chaotic dripping regime in reduced gravity been experimentally proven to exist.
The most commonly utilised methods are drop towers or parabolic flights, though new opportunities for experiments on board the ISS are being made available through such platforms as NanoRacks.
Drop tower testing (results shown above) has pointed to the existence of a chaotic dripping regime in reduced gravity conditions. To further study this flow regime, a new experiment is being designed for use on board the International Space Station (ISS), the Spun Microgravity Liquid Experiment - SMiLE.
The Spun Microgravity Liquid Experiment (SMiLE) that will fly on the International Space Station (ISS) operates in a different manner to earlier versions designed for use in terrestrial drop towers. Once placed in the NanoRack on board the ISS, SMiLE (see below, left) has four distinct operational modes designed circulate fluid through the system:
1. Injection and Recording: A linear actuator stepper motor pumps water from the reservoir into the viewing chamber. The outlet solenoid valve is set to open state. Droplet formation will be recorded by two Raspberry Pi cameras.
2. Spin up and Recording: Once the reservoir is depleted and the stepper motor has ceased operation, and the centrifuge motor will start. Centrifugal force is expected to cause the droplets to move to the sloped outer wall of the viewing chamber, and from there be funnelled to the return line and on to the reservoir. The solenoid valves will be closed in this initial spin up, and the cameras will still be recording.
3. Drainage and Return: At this point, the cameras are turned off, and the stepper motor is run in reverse. This creates a back pressure on the collected water and draws the water to the reservoir by suction. The inlet solenoid valve is set to open state. A bubble sensor monitors the fluid line leaving the viewing chamber to ensure that no air bubbles are drawn into the reservoir.
4. Charging: To power the experiment, super-capacitors are required. In between the high power consumption stages, all of the motors and lights are turned off to allow the super-capacitors to recharge.
A high level electrical schematic of the SMiLE control system is shown above right. Super-capacitors provide power to the internal components via a slip-ring. The internal components are driven by a Raspberry Pi computer module, contained on a custom printed circuit board.