Dear Dr. Philipp Selenschik,

for example, most leaves that exhibit strong hydrophobicity have hierarchical surface roughness with micro- and nanostructures made of unwettable wax crystals. The roughness enhances the hydrophobic behavior, so that the water droplets on top tend to become nearly spherical.

For more details, I suggest you to have a look at the following papers:

-Influence of Surface Roughness on Superhydrophobicity

C. Yang, U. Tartaglino,and B. N. J. Persson

PRL97,116103 (2006)

Available at: http://juser.fz-juelich.de/record/53839/files/84512.pdf

-The effect of surface roughness parameters oncontact and wettability of solid surfaces

By Yilei Zhang (2007)

Retrospective Theses and Dissertations - Iowa State University Capstones

Available at: https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=16933&context=rtd

-Size Effects of Surface Roughness to Superhydrophobicity

Quanshui Zheng, Cunjing Lü

Procedia IUTAM, Vol. 10, Pages 462-475 (2014)

Available at: https://www.sciencedirect.com/science/article/pii/S221098381400042X

and at: https://tinyurl.com/yymbl5wc

-Objective quantification of surface roughness parameters affecting superhydrophobicity

Yoonkyung Cho and Chung Hee Park

RCS Advances, 52 (2020)

Available, as full text, at: https://pubs.rsc.org/en/content/articlelanding/2020/ra/d0ra03137b#!divAbstract

Best regards, Pierluigi Traverso.

Hi

In addition, water droplets due to high surface tension, on the way to reach a spherical shape; In the absence of another obstacle force, they minimize their level. In contact of the liquid with a dry surface, depending on the surface structure and surface tension of the liquid, it is the adhesion forces that make the surface wet. Despite the complex micro and nanoscopic structure on the surface, surface adhesion is minimized.The presence of air pockets can also reduce the contact of the liquid with the rough surface.

Dear Dr. Philipp Selenschik,

for example, most leaves that exhibit strong hydrophobicity have hierarchical surface roughness with micro- and nanostructures made of unwettable wax crystals. The roughness enhances the hydrophobic behavior, so that the water droplets on top tend to become nearly spherical.

For more details, I suggest you to have a look at the following papers:

-Influence of Surface Roughness on Superhydrophobicity

C. Yang, U. Tartaglino,and B. N. J. Persson

PRL97,116103 (2006)

Available at: http://juser.fz-juelich.de/record/53839/files/84512.pdf

-The effect of surface roughness parameters oncontact and wettability of solid surfaces

By Yilei Zhang (2007)

Retrospective Theses and Dissertations - Iowa State University Capstones

Available at: https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=16933&context=rtd

-Size Effects of Surface Roughness to Superhydrophobicity

Quanshui Zheng, Cunjing Lü

Procedia IUTAM, Vol. 10, Pages 462-475 (2014)

Available at: https://www.sciencedirect.com/science/article/pii/S221098381400042X

and at: https://tinyurl.com/yymbl5wc

-Objective quantification of surface roughness parameters affecting superhydrophobicity

Yoonkyung Cho and Chung Hee Park

RCS Advances, 52 (2020)

Available, as full text, at: https://pubs.rsc.org/en/content/articlelanding/2020/ra/d0ra03137b#!divAbstract

Best regards, Pierluigi Traverso.

Hello,

In my view and limited knowledge, the variation of contact angle with surface roughness may not be generalised. The contact angles depends on the presence of the valley and peaks on the aggregate surface where the drop of liquid is resting and also on the Surface energies components of liquids compared with substrate surface. Further for uniform surface roughness, the higher SFE of solid surface overpowers the lower SFE of liquid causing it spread and reduced contact angle.

Best Regards

Vinamra

Conceptually, a theoretical completely smooth surface will have 1 cm2 of surface area per 1 cm2 of surface. A rough surface might have 2 cm2 of surface area per 1 cm2 of surface. A water drop that will spread to cover say 0.5 cm2 of a hydrophobic surface will spread to 0.5 cm2 on a theoretical completely smooth surface. On the rough surface, if the roughness is large enough so that no air pockets are left, that same drop will spread to 0.25 cm2 and have a correspondingly steeper contact angle.

Hello Philipp,

1- Generally speaking, the reason is that surface roughness provides a pore (hole) space which refrains the air-water interface to touch the substrate for a certain amount of pressure (bellow the critical pressure).

2- Surface roughness does not always promote superhydrophobicity. Depending on the surface material (chemistry), adding roughness can make the surface more hydrophobic or even more hydrophilic (this is proved from Wenzel state).

3- Also, special care should be taken when referring to the surface wettability. The apparent contact angle (ACA) is typically expected to increase by increasing roughness, while the Young-Laplace contact angle (YLCA) remains unchanged as it is only a material property.

Best,

Ali

Dear Philipp,

Increasing the surface roughness does not necessarily enhance the contact angle. The more important points here are the surface pores fraction and the dimensions of the air cushions trapped on the surface. By having a surface with low energy (e.g., polymer coatings), and developing some micro-textures on the surface, you can have a superhydrophobic coating. For example, in the following study, although the surface roughness has decreased, the dynamic contact angles have significantly increased.

Article A Facile, Fast, and Low-Cost Method for Fabrication of Micro...

Best,

Amir

When surface is very gently roughened (Nano-sized pores) the water cannot enter those pores and just slide off.