I measured the contact angle of a polished ss-316 L surface (Ra =50 nm) with with 2 microlitre droplet it was 85 degree. The reported contact angle of stainless steel is usually 71 degree. I want to know whether highly polished surface can increase the contact angle or it could be error due to contamination on sample?
The CA you mention are caused be adsorbates.
The surface of stainless steel is usually an oxide surface (e.g. chromium oxide) with a high surface free energy (>> 100 mJ/m2). If you put water on it, it spreads, i.e. CA = 0°.
You can prepare a clean surface by thorough cleaning (e.g. (detergent + ultrasound) + plasma treatment; (detergent + ultrasound) + isopropanol; blazing). It will quickly regain the adsorbates.
If the surface roughness is very low (< 10 nm), it is having very low surface energy. In that case, CA should increase. However, whenever there are peaks, the surface energy at that point is very high and water can easily stick. So, the difference between 71 degrees and 85 degrees came because of lesser number of peaks I believe (low surface energy compared to 71 degrees one).
A microscopic figure of surface roughness of your stainless steel from an available microscope service (SEM, AFM, etc.) might help our understanding. We propose at least two figures, before wetting, and after. Also, an illustrative parameter is the (aging) time during wetting.
A follow up question, does CA/wettability change with different grades of stainless steel assuming same roughness and no contamination ? If yes, is there a technical table/ list of the same?
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