Why do droplets of condensation run at an angle on a plastic sheet?

[xtempore]

TL;DR Summary

I put some plastic sheeting over our single-glazed windows, to reduce heat loss. I was surprised to find that droplets of condensation didn't run straight down, but appeared to run at a fairly consistent angle - left or right.

I think the picture says it all! As the droplets of condensation reached a certain size they begin to run down the plastic sheet, but rather than just running straight down, they veer off to the left or right. Most of the tracks are at fairly much parallel, with some exceptions, and the pattern can go either left or right, but the angle looks to be similar.

So, obviously gravity is acting as a force, straight down, so what force is causing these droplets to run at angles? And why that reasonably consistent angle?

The plastic sheeting is meant to reduce heat loss. It's stuck to the window frame with double-sided tape, and there is a space between the plastic and the glass (basically a cheap version of double-glazing). The plastic is then stretched taut by using a hair-dryer to remove any wrinkles.

Any ideas?

 

[hmmm27]

Shrinkwrap sheets come pre-stretched in a number of directions, the normally jumbled up molecules locked to each other. Heating it up unlocks the bonds, returning the molecules to being jumbled up, again, and "shrinking" the sheet.

The water droplets are traveling down "stretch marks".

 

[DaveC426913]

Alternate theory: a very consistent wind blowing past your house from the left, then switching to from the right.

Maybe a tropical storm just passed directly over you.