6/22/2007

How Cassie and Wenzel Got Owned (or not)

Chances are good that unless you do a lot of surface chemistry, or read Langmuir extensively, that you don't know who Wenzel and Cassie are, nor are familiar with their work. I wasn't myself, so it was quite a surprise to me that someone would publish a paper in Langmuir entitled "How Wenzel and Cassie Were Wrong." To the novice surface chemist, the title certainly begs the question: who the hell are Wenzel and Cassie? What are they wrong about? And who on earth would be so hell-bent on proving them wrong as to publish a paper describing how wrong they are? Such provocation is quite unusual for Langmuir, which is probably the premier journal of surface chemistry. It's led the paper to be one of the most highly-accessed articles in the journal this year, and has led someone to publish a rebuttal this week in Langmuir's ASAP asking, "Were They Really So Wrong?"

So, who the hell are Wenzel and Cassie? They were two physicists back in the thirties and forties who worked on the physical nature of surface wettability, and published two major papers on the subject (the original Wenzel paper is not online). Sounds pretty boring to the non-surface chemist, but in reality, the wettability of surfaces combines many of chemistry's and physics' most pressing fundamental problems: what happens when two phases interact, how can we measure the energy of the interaction, and how do microscopic interactions affect macroscale properties. A major way in which the wettability of surfaces are quantified is with the "contact angle" θ. The contact angle is the angle that forms between a droplet and a surface that the droplet is on.



A droplet "wets" the surface if θ is zero, and forms a hemispherical shape at the interface if it does not wet the surface. Contact angles can be as high as 180° (ie. the droplet does not interact with the surface at all). The properties of the liquid and surface affect the contact angle: a hydrophilic surface (like clean glass) will be wetted by water, while a hydrophobic surface (like Teflon) will not. Furthermore, the topography of the surface (smooth, rough or patterned surfaces) also affects the contact angle. You measure the contact angle with an instrument called a goiniometer, which isn't much more than a lens that renders objects on a stage opaque. The free energies of the interaction between solid/liquid (γSL) solid/vapor (γSV), and vapor/liquid (γVL) relate to the contact angle θ by Young's equation: cos θ = (γSV - γSL) / γVL.

To be VERY brief, Wenzel and Cassie's contributions were to explain the relationship between energy and contact angles on rough and biphasic surfaces, by adding a few basic modifiers to Young's equation. The McCarthy group's paper with the provocative title is a response to the greatly-increased number of citations to these two fundamental papers, particularly with the increasing interest in "superhydrophobic" surfaces. McCarthy claims the major flaw in their thinking was assuming that the contact area was the determining factor in the energy and contact angles, as opposed to the line of contact. They underwent a series of experiments with various patterned and rough surfaces to show that the area of contact is completely independent of the contact angle measurement, showing that the calculated contact angles using the Cassie and Wenzel modified equations do not coincide at all with the measured contact angles. In effect, Cassie and Wenzel's assertions don't pan out, McCarthy says. Furthermore, claims McCarthy, previous experimental findings in the literature which contradict the Wenzel and Cassie models have been largely ignored.

Next time, I'll look a little deeper into McCarthy's findings, whether the provocation was worthwhile, and the latest rebuttal in Langmuir.

14 comments:

milkshake said...

How they were wrong: Wenzel and Cassie got lost in the woods. Eventually they saw a gingerbread house in the clearing. So they rung the door and asked the old lady nicely about her contact angle.

David Eaton said...

Great post! I'd add that surface thermodynamic (along with thermodynamics of phases in general) are one of those things I've had to learn post formal education. It's incredibly important, and I just didn't see much emphasis on it in school.

I'm especially interested in electrowetting these days. That literature is full of models that everyone knows are either wrong or incomplete or both.

I hadn't seen this paper. I'm looking forward to seeing the controversy play out.

Uncle Al said...

If you are brewing biointerfaces, a monolayer of radiation-grafted poly(N-vinylpyrrolidinone) on polymer will have a saline contact angle as low as 18 degrees and biologically disappear: cellular, humoral, and contact abrasion. (A singular exception is PDMS. Silicone is remarkably horrible.)

If you want a decent yield of decent devices... Uncle Al's benevolence can be leased. Incorporating a persistent fluorescent marker is no biggie.

David Eaton said...

Clearly grammar and proofreading are things I should have also given a bit of work...Sheesh.

Ashutosh said...

Next time, paraphrasing Wolfgang Pauli, there's going to be a paper with the title "Wenzel and Cassie were not even wrong"

Chemgeek said...

This brings back memories of when I worked as a chemical technician for a certain unnamed comapny and had to measure these angles for a variety of materials used in artificial heart valves and pacemakers. Wow, what memories....

Hap said...

Why the failure kitty? What did he/she do (other than get cut on)?

Excimer said...

Well, the cat is Cassie and Wenzel, and the food is a deeper understanding of surface physics? Does that work? I dunno, I felt like posting it cause it's funny and it seemed vaguely apropos

Ψ*Ψ said...

Plus, CBC always needs more kitties.

Hap said...

I like cats and putting more of them isn't a bad thing. I just didn't understand why the cat deserved to fail.

He is a cute cat, though.

Anonymous said...

I think your contact angles are wrong, because the way you've drawn it, it wets at 180 and it's dry at 0.

Excimer said...

aaah, yes, you're right. the ray parallel to the surface should be pointing toward the droplet, not away. I will fix that. Thanks!

pucho said...

Anonymous, you are wrong. At 180° the droplet avoids any contact with the surface. And at 0° the droplet is flat, thus, it wets completely the surface, and it will extend to wet the maximum surface (it will form a thin film).

Excimer said...

pucho,

anonymous was right, and i had the angles pointing the wrong way. the current pic was corrected from the original.