Why Do Pepper Particles Stick Only Behind the Print on a Glass Shaker?

[Camille]

So we have this pepper shaker made of glass with a print on it.

One fine dinner, it ran out of pepper, so I opened the lid to fill it in and noticed a peculiar thing – small particles of pepper dust were stuck to the inside glass walls but only in places, where the print wasn’t present on the other side. Everywhere directly behing the print, the pepper particles almost didn’t stick to the glass.

(These photos also show vertical traces from a spoon, which are not relevant, I hope you’ll see the phenomenon anyway.)

I’ve been wondering about this for a while and been thinking whether it’s got to do with light, or light and humidity, or electric charge…

But as for now, I don’t have the final answer, so I’ve decided to ask the Internet.

 

[Simon Bridge]

Fine particles stick to stuff through electric charge and/or trace moisture. I'd suspect charge here ...

 

[Dale]

I wonder if sunlight charges the glass, so the print blocked the sun.

If you wipe it off does the pepper still stick? I wonder if you can find out how long it takes to get static built up. Then you could see if keeping it out of the sun changes that.

 

[Vanadium 50]

I wonder if sunlight charges the glass, so the print blocked the sun.

Or heats the glass, reducing the condensation.

 

[Dale]

Or heats the glass, reducing the condensation.

Yeah, it would be hard to control for that without a careful setup which is probably not feasible.

 

[Simon Bridge]

I don't think there is enough information to confirm the dominant mechanism ... I usually think of charging by sunlight or rubbing when looking at glass.
... among other things that is.