When exposing our sample to a high RH we find a water layer would form on the surface. We want to know the maximum thickness of the water layer that your instrument can normally work. 0.2 mm, 0.5 mm or 1.0 mm?
This is something we have experienced on occasion with our in house sample analysis. So far we have not reached a limit on the water layer thickness, and certainly not in the range you mention. Provided the liquid conducts you will be sensitive to the topmost layers.
In earlier experiments of polar liquids (where we scanned across the liquid surface above the gold - aluminium interface). We found that for some liquids the interface (with 1V built-in surface work function change) was invisible, in other cases the interface was still visible but the potential changes was reduced. In these cases the liquid layer was not less than 2mm thick.
Never-the-less the liquid build up still has some practical implications:
1. If evaporation occurs this may destabilise the liquid surface level.
Use of our dedicated relative humidity chamber makes this less of an issue.
2. If you pre-treat the surface to reduce surface tension then the water may flow.
You may wish to consider making a trough or reservoir in a sample to accommodate the fluid.
In order to determine the water thickness we suggest that you first make a measurement (at gradient 300) on the bare surface and note the Z motor displacement. Then back the probe away automatically, add the salt solution and re-insert the probe towards the liquid surface. If you now record the Z displacement at gradient 300 you know the liquid layer thickness.
If you are using the SKP for these measurements a lot, it may be worth considering an upgrade to a larger diameter tip and our new in-line tip. We can accommodate any diameters (though most commonly 0.05 to 20 mm). We can also make the tip with holes in it to allows air penetration. The larger tips will result in a larger stand-off height and thus minimal disruption to the liquid surface.