December 2011

Sometimes you have to lower your standards. This morning I headed off into the woods behind our office on a quest for a lotus leaf and a rose petal. I came back with skunk cabbage and a fuzzy New Jersey weed. I knew from the start that I'd be lucky to find one of them. The reality is that there is very little growing in the woods of New Jersey in the wintertime.  The skunk cabbage, well, it stunk, so we got rid of it and focused on the fuzzy weed.

A Fuzzy New Jersey Weed

My mission was to do some hands-on lab work in an effort to illustrate the "petal effect" and contrast that with the "lotus effect". (More on this can be found here: http://en.wikipedia.org/wiki/Wetting#.22Petal_effect.22_vs._.22lotus_effect.22.5B16.5D) So my mission was fifty percent successful since I was able to identify a real-world example of something that exhibits a Cassie impregnated wetting regime (aka the "petal effect"). The water contact angle on the fuzzy weed leaf averaged 110.2°. And - like the rose petal - the drop did not roll off even with a tilt angle of 90°. This behavior is the result of a hierarchy of micro and nanoscopic structures on the leaf. The leaf's hairy surface provides a roughness (for more on roughness, see our Sept 2010 Newsletter) which promotes a hydrophobic surface while the nanoscopic hairs lock into the drop to deter wetting and dewetting. While the exact geometry varies from that of the rose petal, the result is the same: a hydrophobic surface with high adhesive force.

This contrasts with the "lotus effect" which is also characterized by a hydrophobic behavior but with very low adhesive force. With the lotus leaf, the water drop will roll off with a very low tilt angle and thus has a low contact angle hysteresis. This can be explained by the drop which sits entirely on top of the nanostructures in a true Cassie state - with no hairs in the drop. There is very little surface area in contact with the drop and thus very low adhesive force.

The fuzzy weed, on the other hand, exhibited a 13.3° contact angle hysteresis with an advancing contact angle of 116.6° and a receding contact angle of 103.3° at a tilt of 90°. And like the rose petal, the drop stays stuck even when the leaf is upside down.

I should point out that the fuzzy weed does not enjoy the self-cleaning attributes of the lotus leaf. While the water drops easily roll off the lotus leaf taking with them dust and dirt, the drops on the fuzzy weed stubbornly stay in place despite the wind and motion.

So our quest continues. Maybe next month we will go exploring for a lotus leaf...or perhaps a leftover poinsettia will work.
 

Both ramé-hart DROPimage Standard and DROPimage Advanced software programs include the Acid-Base tool for determining surface energy. This utility allows the user to take contact angle measurements on a given surface using three different liquids, one apolar and two polar. One possible combination might include methylene iodide for the apolar liquid and water and glycerol for the polar pair. In our experiment today, we will use those three liquids. van Oss, et al, proposed that a more accurate picture could be captured by separating the surface energy into three components: the dispersive component and the polar component which, in turn,  is broken down by the acidic and basic interactions.

Since the Acid-Base theory uses three components, it's more robust than standard two-component theories (such as Owens/Wendt or Fowkes). However, the Acid-Base theory is particularly sensitive to surface active elements such as sulfur or oxygen. Even minute quantities of sulfur, for example, can greatly decrease the surface tension of the liquids.

The above video shows how we measure the contact angle on a treated steel surface using water, glycerol, and methylene iodide. We then calculate the surface energy using the Acid-Base tool in DROPimage. You can also access this video at: http://youtu.be/VlbI7bUBuIk
 

From time to time when measuring contact angle on hydrophobic surfaces, DROPimage will report an error due to the white hole in the center of the sessile drop. This occurs as a result of the drop acting as a lens. This is an easy problem to overcome. Simply use the right line option and then place the yellow right line to the right of the white hole and the green left line to the left of the white hole and then measure contact angle like usual and you will not have an error. The video below shows the sequence for using this feature while measuring water contact angle on a piece of treated steel.

Click on the above video to watch or go to: http://www.youtube.com/watch?v=5w0wtzTwpeo&feature=colike
 

Finally and most importantly, from all of us at ramé-hart instrument co., we wish you and yours a Happy Holiday and Prosperous New Year in 2012. And thank you for helping make 2011 a successful year as we commemorate fifty years of continuous operation.