Wet Cell

The wet cell used in STXM is based on a design developed for STXM III by Ulrich Neuhäusler, and modified by Michael Feser and Thorsten Schäfer. A mechanical drawing of the wet cell is located on our website

The wetcell allows one to place a fully hydrated sample (solution layer with a thickness of a few $\mu$m) as close as 350 $\mu$m away from the order sorting aperture. Thin silicon nitride windows are used as a sample support.

To use the wetcell, have the following things in hand:

• O-rings. The o-ring used has an inner diameter of 3/4'', an outer diameter of 7/8'', and a width of 1/16''.
• Screws: #0-80, 1/8 `` long, flat head.
• Silicon nitride windows currently used: 3$\times$3 mm windows, 100 nm thick, in a 9$\times$9 mm frame, wafer thickness 200 $\mu$m.
• Stainless steel shim stock, 0.001'' (25 $\mu$m) thick. Punch (e.g., using the punch set available in the machine shop) and cut a support shim as shown in Fig. 4.1.

Pictures of the wet cell pieces assembled and disassembled are show in Fig. 4.2.

Figure 4.1: Shim pieces for the wetcell.

Figure 4.2: The wetcell disassembled (left) and assembled (right).

You can then begin the steps for sample preparation:

1. Glue the shim support with nail polish to the upstream side of the upstream part of the wetcell. Make sure, that the nailpolish seals well the interface between the shim and the aluminium part of the cell properly and that there is clearance of the upper punched hole (1/8'' diameter) for pressure exchange during sample preparation.

2. Using nailpolish, glue silicon nitride windows on both the downstream side of the shim and the upstream side of the downstream part, facing their flat sides to each other. Align the windows while gluing in order to get a maximum overlap of the two windows.

3. Place a small drop of the sample you want to look at in STXM on one of the windows. Use as little as possible. If the properties of your sample make it hard to get a small drop out of an Eppendorf micropipette, you might want to use a syringe with a small diameter needle. See Fig. 4.3.

   Figure 4.3: VLM reflected light image (5$\times$) of the assembled wetcell with a drop of liquid on it.

   

4. Assemble the two parts together and start tightening the screws. The drop will start spreading out between the windows giving you a fairly thick water layer. See Fig. 4.4.

   Figure 4.4: VLM image (5 $\times$) of the wetcell hydrated, but not yet squished thin.

   

5. Continue tightening the screws and watch the layer. At some point, the center of the two windows will start to be pulled together by capillary forces. This shows in interference colors visible with the naked eye. You have to try out, if the region, where you have a thin layer gets thicker by continuing to tighten the screws or by losening the screws. What also might happen is, that you enclose too much solution between the windows, that can't escape as the outside of the wafers already touch. This shows in the windows flexing out towards you. Losen the screws again and give it another try. If you get something like shown below, you are in the right thickness range due to the appearance of interference colors. The sample shown in Fig. 4.5 is a suspension of clay/polymer aggregates with a clay size fraction of 200 nm to 2 $\mu$m. The particles are clamped between the silicon nitride windows and appear as bumps in the window surface. This suggests a layer thickness of around 1 $\mu$m, in agreement with x-ray transmission observed in STXM.

   Figure 4.5: VLM image (5$\times$ at left, 40$\times$ at right) of a clay/polymer aggregate in a 1 $\mu$m thick water layer. Photo by U. Neuhäusler.

   

   

6. When you are happy with your layer thickness, you might want to put a small drop of water thru the 1/8'' hole into the space inside the wet cell and then tape the hole with scotch tape. The wet cell is now sealed and your sample might stay hydrated for up to a day.