Thank you Prosper! I found this paper and was hoping for something more simple. Maybe even something one could measure in the field. Thanks for your fast response!

Hi Rene,

If you want to measure in the field you don't want to carry instruments. We have the same sometimes in industrial practice. What I've used in the past are the test kits from AccuDyne ( https://www.accudynetest.com/dyne_fluids.html). These are either liquids which you apply with a swab stick, or pens. The fact whether the test liquid will contract into droplets, or remain as a line, like you applied it, will tell you if the sample surface is more hydrophobic or hydrophilic. We have used this successfully to investigate polypropylene car bumpers for pollution, prior to painting. You would need to find out which range of surface tensions you need and will discriminate enough for your work by looking at the contact angle on a few samples.

Hope this helps, kind regards, Leo

That's a great answer and more than helpful! Thanks a lot, Leo!

You're welcome !! Good luck with your research !!