What does it mean in Wenzel-model, that this theory is for chemically homogeneous substrate? If I modified the surfaces by plasma, so I have of course some chemical changes on the surface, does it mean I can´t use in this case Wenzel-theory? And what does that mean that Cassie´s model is for chemically heterogeneous surfaces? I only understand this, if I put on my surface a (water) drop, then drops are suspended atop asperities, and leave air (or vapor) between them. So I have here heterogeneous interface (e.g. solid-air). I read a lot of papers about this 2 theories, but this - about homogeneous and heterogeneous surfaces - I really don´t understand.
Yangping is right IF we use Oxygen plasma. If we use Ar there will be only roughness, or SF6 there will be fluorine roots created on the surface.
As for your question. Wettability is influenced both from chemistry and structures on the surface. If you have a flat surface for example that is hydrophilic, if the chemistry i the same and you create roughness (with the same material) then it will be superHydrophilic if roughness is high enough. If the initial surface is hydrophobic, then with roughness it becomes superhydrophobic.
In the case of Oxygen plasma you will have -OH, -COOH etc on the surface meaning that apart from roughness you increase hydrophilic behavior and you make your material more hydrophilic.
This is what happens generally in the case of plasma. You change both topography and surface chemistry.
It is quite simple. Wenzel's model (or state) applies in the case where wetting is on a suface without any air as an intermediate layer. Caussie - Baxter model is applied when air is trapped in between the structures (or roughness) and water.
By heterogeneous they mean wetting actually not the surface itself.
I hope I helped. Ask me if you didn't understand what I wrote.
Thank you, this part I understand, but in some papers they talking about chemically homogeneous or heterogeneous, for me is "chemically" something with material/substrate surface itself or not? maybe I missunderstood this. But which state do I have, when my surfaces before treatment were hydrophobic and after treatment hydrophilic, the roughness is increased? maybe I can only describe this by chemically modification e.g. functional groups, polar groups. In this case I can´t describe by wenzel or cassie stat or? Thank´s a lot!
plasma modified surfaces will creat -COOH, -OH bond on the surface, these bond will change the wettability of the surface.
And what about the roughness? Does it has an influence on wettability? Or only chemically modification influence wettability of my surfaces. I have both chemically changes as well as roughness changes.
Yangping is right IF we use Oxygen plasma. If we use Ar there will be only roughness, or SF6 there will be fluorine roots created on the surface.
As for your question. Wettability is influenced both from chemistry and structures on the surface. If you have a flat surface for example that is hydrophilic, if the chemistry i the same and you create roughness (with the same material) then it will be superHydrophilic if roughness is high enough. If the initial surface is hydrophobic, then with roughness it becomes superhydrophobic.
In the case of Oxygen plasma you will have -OH, -COOH etc on the surface meaning that apart from roughness you increase hydrophilic behavior and you make your material more hydrophilic.
This is what happens generally in the case of plasma. You change both topography and surface chemistry.
Hi Marina,
Morphology effet depends on the scale of roughness. Sometimes you can not even detect subnanomter changes in surface morphology with SEM after plasma treatment. In this regard, AFM will give you a local and yet 3D and high resolution (resolution depends on many factors of course) picture of plasma-treated surfaces. XPS, FTIR and contact angle may be used for surface chemistry change.
For the AFM study of plasma etched sapphire surfaces, you may be interested in our recent paper: D. Zhang, Y. Gan*, "Effects of Plasma Treatment on Evolution of Surface Step-Terrace Structure of Critically Cleaned C-Plane Sapphire Substrates: An AFM Study, Applied Surface Science", 285B (2013) 211-214. Here is the official link to it and you may also find the full-text on my RG home page: http://dx.doi.org/10.1016/j.apsusc.2013.08.038.
Hope this helps.
Yang
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