Can't Get Rid of Drops? Surface and Hydrophobicity Are Keys

January 24 2014 Process Analytical

Dursan vials

by Dr. David Smith

Surface hydrophobicity & smoothness have a very large effect on contact angle values on glass and metal surfaces.  Below is a comparison of contact angles with chemically identical surfaces, but with rough or smooth topographies on 316 stainless steel coupons:

Rough surface comparision

DI water on treated surfaces:  Rough

Smooth surface comparison

DI water on treated surfaces:  Smooth

So with glass vials, we are obviously dealing with a smooth glass surface. 

untreated vs. Dursan

A smooth surface has one strike against it because smooth surfaces produce lower contact angles, but there is more.  A “rough” surface may show an artificially high contact angle.  However; the rough surface may effect the “mobility” of that droplet and may or may not allow droplet movement in a desired or predictable way (i.e. like a ball bearing may move around on a tilted surface). 

There are two types of interactions between a liquid drop and the rough surface it is in contact with. 

Droplet comparision

Source:  Microstruct superhydrophobic.png. In Wikipedia. Retrieved August 23, 2012, from

The Cassie-Baxter state requires not just surface roughness, but a heterogeneous surface roughness.  Since there is essentially zero roughness on a glass surface, the conformal Dursan deposition (i.e. it does not create roughness to the surface) will not allow a classic Cassie-Baxter state.  Once the contact angle on a smooth surface approaches approx. 120°, the angle is extreme enough that one will start to see the ball-bearing effect. Also, higher volume drops of contact angles > 90° have enough mass to fall away or roll off a surface.  Lower volume drops that do not have enough mass, and are therefore exhibiting Wenzel, non-mobility behavior.


In a Wenzel state, the drop may have a high contact angle, but there is wetting of the drop down through the surface roughness.  This wetting acts as an anchor and prevents mobility of the drop.  In a Cassie-Baxter state, there is no wetting of the roughness structure, and the air gap throughout that substructure translates to no “anchor”, and frees the drop to move around (like a ball bearing).


See a drop bounce like a ball bearing!

Learn more about hydrophobicity and how to manage effects of moisture.