---
title: Can't Get Rid of Drops? Surface and Hydrophobicity Are Keys
description: Learn how hydrophobicity and surface roughness impact moisture adhesion on glass surfaces.
---

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# [Can't Get Rid of Drops? Surface and Hydrophobicity Are Keys](https://www.silcotek.com/blog/bid/104637/can-t-get-rid-of-drops-surface-and-hydrophobicity-are-keys)

 Written by [Marty Higgins](https://www.silcotek.com/blog/author/marty-higgins) | January 24 2014

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:

DI water on treated surfaces:  **Rough**

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DI water on treated surfaces:  **Smooth**

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

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. 

Source:  Microstruct superhydrophobic.png. In *Wikipedia*. Retrieved August 23, 2012, from [http://en.wikipedia.org/wiki/File:Microstruct_superhydrophobic.png](http://en.wikipedia.org/wiki/File:Microstruct_superhydrophobic.png)

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.

Conclusion:

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!](https://www.silcotek.com/hs-fs/file-470738655-wmv/flash/bouncing_drop_1.wmv)

[Learn more](https://www.silcotek.com/hydrophobicity-moisture-resistant-solutions/) about hydrophobicity and how to manage effects of moisture.

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[View full post](https://www.silcotek.com/blog/bid/104637/can-t-get-rid-of-drops-surface-and-hydrophobicity-are-keys)

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