Fluid and Particle Preparation
Most liquids contain dissolved gases which can lead to the undesired generation of bubbles and corrosion of the metallic parts when the related containers are kept in space over a long period of time. Due to the surface-tension effects enabled accordingly (see the scientific description page , gas bubbles can interfere significantly with experiments which aim to study other aspects or phenomena. In a similar way, corrosion can cause uncontrolled variations in the composition of the considered fluids. Consequently, the fluid to be used in space experiments must be carefully prepared to ensure that they are free of dissolved gases, especially oxygen.
Although they are made of inert materials (such as glass), fine solid particles can also lead to undesired effects if they are not properly prepared or treated. Due to unavoidable electrostatic effects they can undergo undesired aggregation phenomena when dispersed in a liquid or stick to the walls of the fluid container.
The activities that Particle Vibration (T-PAOLA) science team had to implement to properly take into account all these aspects are illustrated in the following.
The Particles
The science team had to:
- Select spherical particles and ensure their size was in a specific (narrow) range.
- Remove the particles not satisfying the above conditions (non-spherical shapes, broken particles or too small or large particles).
- Treat the particles to prevent them from developing undesired hydrophobic and electrostatic behaviours (potentially hindering their mobility when dispersed in a fluid).
- “Light” particles – silver coated hollow glass microspheres with a density of 0.14 g/cm3 and diameter of 75-90 µm.
- “Standard Heavy” particles – silver coated hollow glass microspheres with a density of 1.32 g/cm3 and diameter of 75-90 µm.
- “Small Heavy” particles – silver coated solid microspheres with a density of 2.24 g/cm3 and diameter of 53-63 µm.
- “Extra Heavy” particles – silver coated solid glass microspheres with a density of 2.7 g/cm and diameter of 75-90 µm.
In order to prevent them from developing undesired aggregation effects of electrostatic nature (particles sticking to solid surfaces or forming agglomerates), a small percentage of surfactant has been added to the host liquid.
Particle Injection Procedure Definition and Optimisation
Given the fragile nature of some particles (such as the “light” ones described above), the tendency of these to be broken during their insertion in the fluid containers, and the intrinsic difficulties related to the need to inject them into the containers through a very small orifice (1 mm only), the research team had to develop, test and optimise a specific procedure to do so. The main steps of the final procedure (obtained after a number of failed attempts) are illustrated in the following:
500-5000±50 particles are taken from polypropylene centrifuge tubes using a micro spatula (Fig. a) and tipped carefully on to the end of a thin tool of aluminum or carbon ensuring a single layer formation of particles on the surface (Fig.b). Once counted, the tip of the tool is placed near the opening of the 1 mm port and tapped lightly to allow the particles to fall in, without necessitating any sweeping motion with rods or other tools, which could lead to breakages (Fig. c). A wide mouth aluminum micro-funnel with a 1 mm inner diameter tip is positioned over the filling port to facilitate this process. Since even through the aid of the funnel, some particles can remain on the innermost rim of the filling port (Fig. e), light tapping can be used to tip these particles into the cell (in general, as demonstrated by our tests, any remaining particles are sucked into the cell following application of vacuum).
Fluid Preparation
The fluids were selected, treated, and stored in the following manner:
- Selected on the basis of numerical simulations.
- Treated to remove any dissolved gases (through a freeze-pump-thaw technique)
- Stored in gas-tight equipment before being injected in the fluid containers (cell arrays).
Ethanol has been used as the main fluid for the Particle Vibration (T-PAOLA) experiments. This has been selected due to its desirable viscosity, thermal conductivity and specific heat which define the well known Prandtl number. The relatively low viscosity of this fluid has facilitated cell filling (as the fluid had to flow through a 1 mm hole).
As ethanol is known to absorb gas, a relevant degassing procedure has been applied to remove corrosive gases such as oxygen prior to injecting it and the particles into the containers.
Oxygen is typically present in liquids at ambient temperature. This presence is extremely harmful as it can generate corrosion when the liquid comes in contact with metallic surfaces or parts. Oxygen also needs to be removed prior to the freeze-pump-thaw degassing procedure to avoid oxygen condensing in the system which is highly flammable. For this reason, nitrogen gas has initially been injected into the liquid (to replace oxygen with an inert gas).
The setup consisted of bubbling nitrogen gas into a Schlenk flask filled at half capacity with ethanol.
When we store liquids over a long time, the ambient gases can dissolve into the liquid. Those dissolved gases need to be removed to prevent the formation of bubbles in the fluid container during the experiments. Indeed, if bubbles occur they can change the physics of the experiments leading to the onset of surface-tension-driven flows.
Following purging with nitrogen gas, the ethanol liquid is degassed in a two-step process:
1. Vapour Degassing
Ethanol is added to a Schlenk flask and the vapour is vacuumed for 10 minutes as an initial degassing step. A magnetic stirrer keeps the fluid moving and a cold trap is used to protect the vacuum pump.
2. Freeze-Pump-Thaw
The Schlenk flask is transferred over a Dewar of liquid nitrogen and flash frozen. Liquid nitrogen is necessary as the freezing point of ethanol is -114 °C! The space above the frozen liquid is then vacuumed before the liquid is thawed at room temperature. This allows gases trapped in the liquid to rise to the top. The process is repeated 3 times to ensure full gas removal.
Without any particular protection, the fluids will again absorb gases even after the nitrogen treatment and the degassing procedure. However, the fluids should not be in contact with any more gases. All equipment needs to be gas-tight.
To achieve this condition, the degassed fluids were stored in 10mL gas-tight syringes which were then sealed tightly until use.
Fluid Containers Filling
The fluid containers for the experiments were finally filled with the required particles and degassed liquid by means of a dedicated procedure specifically conceived to prevent air from dissolving into the pre-treated (degassed) liquid.