by impenetrable solids
It's about 1/16 of an inch thick and measures about 18 inches by 3 feet. One side (the inside) is flat and the outside is bumpy. Actually it's made of fairly sharp points with depressions between. The object of this design is to disperse light without absorbing it so as to achieve the good effects of translucency without wasting lumens on unwanted light absorption.
Well, it doesn't absorb light but it sure absorbs water ... and ... holds it.
But not like a sponge. Plastic doesn't absorb water very well. It just "rolls" off. The flat side acts like this as you would expect. But the bumpy side holds an astonishing full cup of water (my guestimate from wicking it off onto a cloth rag and wringing it out).
That might seem unimpressive to you at first but compared to the flat side which retains maybe twenty drops it is an immense increase for such a slight variation. And ... it still retains this moisture in the vertical position, i.e. gravity will not drain it. It sticks like glue. You must either wait for evaporation or wick it off to a more "wetable" material.
The cause for this
There is a certain size water package that is pulled loose (separated) under the Earth's gravitational pull ... the "drop". Recall the approximate size of a drop and this is near to the amount that the little depression holds in the plastic surface. That's the secret. If it held two drops, gravity would undoubtedly pull it down. It would drip dry for the most part just like the flat side.
I believe this is not an intentional design by the manufacturer. It's just a fortuitous coincidence, that is, an interesting one. I see no use for this except perhaps in air conditioning but that's a very advanced field and unlikely to be improved.
A conjecture ...
I believe that the amount of water held by this plastic light cover design represents a maximum water retention per unit surface area in earth gravity for any impenetrable solid whatsoever. It's that impressive.
If you know of any thing which will hold more water per unit area strictly by VanDerWaal's forces, I would be very interested to hear about it. Maybe there's a use for it in something ... irrigation? ... cooling a roof? ... I don't know ;o)