This was also investigated numerically before and the

  • No School
  • AA 1
  • 7

This preview shows page 2 - 5 out of 7 pages.

2. This was also investigated numerically before, and the general conclusion was that roughness will always increase the contact angle if θ 0 > π/ 2, and decrease it if θ 0 < π/ 2 [8]. ( ) E-mail: [email protected] c EDP Sciences
Image of page 2
166 EUROPHYSICS LETTERS Fig. 1 – A water droplet on a leaf of Cotinus coggygria L . The strong water-repellent properties do not change measurably upon drying the leaf, nor upon evacuating the atmosphere. However, it is shown in the present paper that it is well possible to construct a water- repellent surface from a material of any finite contact angle, i.e. , the microscopic contact angle, θ 0 , may be below π/ 2. This may be interesting not only as a possible explanation of the wetting properties of certain plant leaves, but also in view of possible applications in tailoring surfaces which repel not only water, but also oil and other non-polar liquids. Our study differs from the former at least inasmuch as it considers interface configurations which need not represent the thermodynamic equilibrium of the system. In fact, wetting or non- wetting of certain plant leaves can be considered non-ergodic insofar as the same leaf can be found to behave wet or non-wet, depending on experimental history. As an example, we show in fig. 1 a photograph of a water droplet on a leaf of Cotinus coggygria Scop . The contact angle is obviously quite close to π . Drying of the leaf does not change the wetting properties noticeably, suggesting that complex biophysical properties, which require a living organism, are not responsible for the high contact angle. In addition, we have tested whether the air entrained between the water droplet and the leaf plays a significant role [1] by putting the setup into a vacuum chamber. Down to a pressure well below 1 mbar, no change in the wetting properties, both statical and dynamical, could be seen. However, when the leaf was soaked in water at a depth of 20 cm for a few seconds and then removed from it, it appeared wet, and contact angles close to zero were observed. This latter observation suggests that the above-mentioned brush model cannot be appropriate for the leaf, since this would result in non-wetting in all cases, independently of experimental history. Similar observations were made with other leaves, such as Cercis siliquastrum L. or Ginkgo biloba L . Let us try to construct a substrate profile such that it becomes as water repellent as possible. To begin with, consider a single, cylindrically symmetric indentation in a substrate at which the liquid forms a finite contact angle, θ 0 , such as depicted in fig. 2. If the slopes of that indentation are steep enough, there will be a configuration in which a free liquid surface spans the indentation. For θ 0 < π/ 2, some overhang is needed, but this does not disturb the concept to be developed. More precisely, there will be a (circular) intersection, at an angle of θ 0 , of the substrate with a horizontal plane representing the liquid surface. If the cross-section
Image of page 3
S. Herminghaus : Roughness-induced non-wetting 167 Fig. 2 – At given finite contact angle,
Image of page 4
Image of page 5

You've reached the end of your free preview.

Want to read all 7 pages?

  • Fall '19
  • Surface tension, Wetting

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask ( soon) You can ask (will expire )
Answers in as fast as 15 minutes
A+ icon
Ask Expert Tutors