Theory and practice: about "dielectric breakdowns"


#1

Dear DMF-developer,
I’m trying to find some answers for my theory-part about DMF and breakdowns of
my thesis. I would like to talk to somebody who got more in-depth experience and
knowledge with the mechanics of dielectric breakdowns and their origins. If
anyone could spare some time to answer some specific questions or have a good
book or paper (or YouTube video), you could do me a big favor.
My goal is to understand and describe the following cases in a satisfying way:
1st case:
Assume we have a material, its properties seem to be marvelous, let’s call it
CEP (cyanoethylpullulan ;). The relative permittivity is about 20, It got a
breakdown-strength of 80 volt per micrometer. I made a CEP layer of 10µm
thickness, a 100nm Teflon coating, and I moved just a normal water droplet with
around 60 Volt. In theory this layer thickness should survive 800 V, but after
some minutes or after an hour, the breakdown occurs.
Why…?
2nd case:
there is an interesting paper about simulation of breakdowns (and CA
saturation) in DMF A. I. Drygiannakis, A. G. Papathanasiou and A. G.
Boudouvis, 2009, On the Connection between Dielectric Breakdown Strength,
Trapping of Charge, and Contact Angle Saturation in Electrowetting
,
Langmuir, 25, 147-152.
In their calculation, they show, that a voltage of
64V results in an strong E-field above the local breakdown-strength near the
droplet. The dielectric material was SiO2 with a EBstrength of 1000V/µm.
Let’s build a device in our mind: we use 5µm CEP, that would result in a
driving voltage around 50V in a 200µm gap and with 2mm x 2mm electrodes. But
now we want to apply a 100-200nm layer of SiO2 on top (don’t care if possible,
or not now). The driving voltage should still be around 50-55V with this
additional layer.
BUT does the idea work: to protect a high-permittivity layer with a
high-breakdown-strength layer?
Or will the CEP layer induce a strong
field, which exceed - again - over the local breakdown strength of the SiO2
layer.

3rd case:

@wheeler labs: Are your Parylen-chips 100% stable over hours or do they
break in the end, too? If they are stable, WHY
are they stable? :wink:
I mean, how does the surface of the parylen near
the droplet survive the higher local fields at the edges (assuming the model of
the paper from case 2 fits here)?

Have a nice weekend,

Sebastian