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Thixotropy

-September 30, 2013

You've surely seen these kind of anti-shoplifting tags on store merchandise. They are high-Q tank circuits which if carried between a transmitting and receiving panel at a store's doorway, they enhance 9 MHz signal propagation between those two panels and the alarm goes off. When a purchase is made, the store clerk deactivates the tag with a high energy 9 MHz energy burst that blows the capacitor to a short circuit. The resonance is gone and the tag can be carried out of the store without setting off the alarm.

 

Usually.

 

 

Unfortunately, the shorted capacitor is only shorted because of an extremely narrow tendril of aluminum having been formed between the capacitor's plates, a fragile tendril that is very easily broken. Tendril breakage can restore a tag to resonance, the so-called "Lazarus Effect", causing false alarms to occur against innocent customers.

 

I wanted to find a way to deactivate an RF merchandise tag permanently. To that end, I had an idea, but it was an idea that ultimately failed.

 

I imagined a means of moving a quantity of some kind of gel to in between the 100 pF capacitor's plates in response to the non-contact passage of a nearby object. The gel might be electrically conductive to cause a capacitor to short out or it might be just a dielectric material to shift the tank circuit's resonant frequency. If I could make that happen, there was no way that the gel would ever move back again. Tag restoration would become impossible.

 

I tried mixing iron filings into my gel and letting the passing object be a very strong magnet. It worked, but not very well.

 

 

The gel turned out to be thixotropic meaning, in my case, that it was of high viscosity when at rest while changing to a lower viscosity when in motion. This made it harder for the moving magnet to get the gel moving from its resting place than to keep it in motion once under way.

 

It struck me that the gel's behavior was analogous to the effects of static friction versus dynamic friction for a solid object sliding on a flat surface. However, that analogy turned out to be simplistic because thixotropy is a more complex topic than that.

 

I recently found a nice discussion of thixotropy on Wikipedia.

 

In any case, the effect proved too troublesome for the purpose at hand, so nothing ever came of my effort.

 

Oh well. You can't win 'em all.

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