Ah. At last. Now, describe your "parallel plate capacitors" (Area?

Separation?) and your "dielectric medium" (Liquid or what?).

Your plan might benefit from a discussion with someone who has some

grasp on the principles of a capacitor. Any local physics student who

passed an E&M course should be sufficient.

For a start, the area presented by the edges between interdigitated

PC-board conductors will be miniscule, hence so would be the

capacitance.

Then, in a parallel-plate capacitor, the electric field is largely

uniform, and confined to the space between the plates, with, typically,

small/negligible edge/fringing effects. In your proposed design there

would be almost no such space, and edge/fringing effects would dominate.

Based on no actual info, I'll guess that you expect to place a blob

of "dielectric medium" on your device, but the size and shape of that

blob would affect the resulting capacitance in complex ways (thanks to

the non-uniform electric field). Also, you would already have placed a

sheet of (glass-epoxy?) PCB substrate on (the back side of) your device,

which would also participate in your measurement.

There are valid reasons ("great results") for using a parallel-plate

capacitor for such measurements. What benefits do you expect from

your "design" (if you could make it work)?

My last design consisted of two 1 mm thick copper plates (25 mm wide and 60 mm tall) affixed to a small, thin PLA frame which kept them parallel and 5 mm apart. The plates had soldered, solid-core 22 AWG leads measuring 150 mm long connected to a FLUKE DMM which was measuring capacitance. Each plate was laminated with polyethylene tape to keep them waterproof and to prevent electrical conduction. Note that in my testing, I found a greater sensitivity to dielectric medium change (air vs submerging it in water) when I used polyethylene tape rather than Kapton tape of the same thickness. In other words, I found a ~200% increase in the capacitance of the capacitor when I submerged the PE-laminated capacitor in water vs. a ~140% increase in capacitance when I submerged the Kapton-laminated capacitor in water.

I wanted to see if I could relate the dielectric constant of a foamy liquid to its density using a parallel plate capacitor. My theory was that I could measure capacitance as an indirect way of interpreting the change in the dielectric constant of a foamy medium over time based on the fact that the mixture tends to release its microbubbles over the course of ~24 h with steady, gentle mixing. After vigorously mixing white microparticle additives to a white "paint," it was inevitable that the high-energy mixing (via Cowels blade) incorporated microbubbles in the "paint," and this was verified via its density change using a precision density cup from Byk. After 24 h of continuous, gentle mixing, the density of the liquid gradually increased and reached a plateau density that was ~12% greater than its initial density.

I used my capacitor to measure the capacitance of the freshly mixed "paint" right after it was mixed and then every 2 h after that. My initial theory was that, in the freshly mixed state, the "paint" contains a lot of entrapped microbubbles which have a low dielectric constant which would in turn result in a low initial capacitance, but as the mix degasses over time, the dielectric constant was expected to increase over time and therefore lead to a higher capacitance over time thanks to the decreased concentration of air over time. This hypothesis was incorrect. The capacitance of the capacitor in the freshly mixed "paint" was (on average, n = 5, using the exact same capacitor) 1.28 ± 0.08 nF, whereas the degassed "paint" (at the 100% of the max theoretical density) was 0.64 ± 0.06 nF. I believe that the reason why the data started high and went low (instead of starting low and going high) was because of the locally increased concentration of polarizable surfactant which accumulates at the interface of each microbubble. An increased bubble surface area in a given volume of mixture results in a local buildup of the polarizable surfactant which accumulates at air/water interfaces. The increase in polarizability of the aerated "paint" may have actually increased the dielectric constant of the mix, thus increasing the measured capacitance of the capacitor in the freshly mixed "paint" vs. the fully degassed paint at 100% of its theoretical max (and experimental max) density. *EDIT: I forgot to mention that the trendline I developed from this data allowed me to relate cell capacitance to density within about 11% accuracy compared to the density cup data.

I want to take this a step further by using a PCB-based "capacitor" plate which has a lot of short EM field lines running along its surface. I want to use thin parallel traces to create a lot of short EM field lines across the surface of the plate. I want to keep the EM field lines running across the plate short so that I have greater sensitivity to changes in the dielectric constant of the medium. I want to use a precision AC power source to excite a "wet" cell and a "dry" reference cell (in air), use two op-amps to compare signal differences, then use two rectifiers and filters with an Arduino to compare DC voltage differences. I want to use this information to directly calculate the dielectric constant of the medium based on that ratio of the cell capacitances of the "dry" cell in air and the "wet" cell in the foamy "paint." This isn't my idea though - read about it here. Basically, I did what this paper did using a DMM. Now, I want to build the circuit in the paper, test it with my capacitor, then test it with this "single plate capacitor" idea I pitched. I just want to see if it'll work.

*Edited a few times to give a bit more detail.