The cell.

We have investigated the free diffusion process that takes place when two miscible fluids are brought in contact, the mixing between adjoining regions being kept as little as possible before a measurement sequence. The liquid sample we used was an aqueous solution of glycerol, with a weight fraction of 0.3. It was diffused into pure water. The cell was filled with the two liquids, with the denser solution in the lower part to avoid convective instability. The two horizontal layers are initially separated by a fairly sharp meniscus. As SNFS is an image forming technique, at least for big objects, we were able to thoroughly check the sample for spurious disturbances at the interface before starting collecting data. As soon as the two liquids came into contact, diffusion takes place, and the nonequilibrium fluctuations arise.

The main difficulty is to fill the cell, keeping the interface between the two liquids as regular as possible. We used a Flowing Junction Cylindrical Cell (FJCC), a prototype developed for the study of nonequilibrium fluctuations in microgravity [26]. From Eq. (9.2) we see that the roll off depends on the intensity of $g$, the gravitational acceleration. As $g$ decreases, gravity acts at increasingly shorter wavevectors, and the divergence of fluctuations at small $q$ becomes more evident. From Eq. (9.3) we see that the intensity of the power spectrum for small wave vectors increases linearily in $1/g$. This divergence of the intensity of fluctuations on $g$ will be studied in an experiment performed on the Intarnational Space Station. A drawing of the prototype cell is shown in Fig. 9.1; a picture can be seen in Fig. 9.2.

Figure 9.1: The Flowing Junction Cylindrical Cell, developed for measurements of non equilibrium fluctuations in free diffusion experiments in microgravity. The gray parts are made of perspex. The glass windows are blue in the drawing. The two liquids are injected through the holes: water in hole A and a solution of water and glycerol in hole C. They fill, respectively, the upper and the lower part of the cell. The two liquids come into contact in the middle of the cell, and the solution they form flows through the slit D and is extracted from the hole C. The porous rings, red in the drawing, make the flow more regular.

Figure 9.2: A picture of the Flowing Junction Cylindrical Cell.

Two pipes feed the cell with the two liquids, in the present study water and a solution of water and glycerol, with a small pressure. The liquids enter in two ring-shaped chambers, from which they flow in the cylindrical cell passing throug porous elements. The flow is quite symmetric, due to the presence of the porous rings. The two liquids fill the cell, water on the top and glycerol on the bottom; they come into contact in the middle of the cell, and are pushed out the cell through a circular slit. The outgoing liquid is collected in a third chamber, passes through another porous ring, and is collected by a pipe.

The FJCC can be filled also in microgravity, since it is based on the flow of liquids. However, gravity greatly simplifies this task: since the denser fluid is in the lower part of the cell, big fluctuations, created by macroscopic motions, relax due to buoyancy, while small fluctuations disappear quickly due to diffusion.

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