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 (γ_SL) solid/vapor (γ_SV), and vapor/liquid (γ_VL) relate to the contact angle θ by Young's equation: cos θ = (γ_SV - γ_SL) / γ_VL.

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.