The roughness factor equal to the ratio of the solid surface area to its flat projected area. What meant by flat projected area?
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Nanotechnology_17_2006_4970_
Dear Ramakrishna flat projected area means the actual area that you can measure without any roughness. imagine you are measuring a rectangle area of 5*5 microns. your projected area is 25 micron(square). if your surface is atomically smooth you will get the measured surface as exactly as 25 micron(square) yet by increasing roughness your measured surface area will increase also. the ratio of measured surface area to the projected area gives you an idea about roughness.
Thank you Mr. Behzad Vaziri Hassas for your detailed explanation.
Just as a side-note: THIS roughness parameter might not be the quantity you should use to characterize the super-hydrophobic surfaces you're referring to. The area ratio is used in Wenzel's model which refers to a wetting scenario, where a droplet is in full contact with the surface, i.e. penetrates the rough structure. These surfaces are not exactly super-hydrophobic, however. This is achieved in case air is trapped in the rough structure creating a composite surface of substrate material and air. The contact area in this case is SMALLER than the projected area, which is described in the Cassie-Baxter model by a factor giving the fraction of the surface, which is actually wetted.
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