Lec_2_Superhydrophobic surfacebhfvnbvs.pptx

Biology

1. Superhydrophobic surfaces Lecture 2

2. Not wetting surfaces

3. Key concepts that we shall discuss today What are superhydrophobic surfaces ? What are the key features of superhydrophobic surfaces ? Superhydrophobic surfaces in nature (lotus leaf, pitcher plant) How can we imitate such nature-based solutions in engineering superhydrophobic surfaces ? What are the potential applications for such superhydrophobic surface or property ?

4. In nature we have a variety of surfaces Wettable Non- Wettable

5. What is contact angle ? The contact angle is the angle formed by the intersection of the solid, liquid, and solid-gas interface. The wettability of a surface can be measured by the contact angle water makes with the surface

6. Hydrophobic surfaces are defined by a water CA of >90 degrees, CA > 150 degrees, the surfaces are called superhydrophobic surfaces (SHSs).

7. Angle between 0 ° and 90 ° = surface wettable , hydrophilic Examples: Glass, rusted metal surfaces Angle between 90 ° and 150 ° = surface not wettable , hydrophobic Examples: Teflon, oily surfaces Angle above 150 ° = superhydrophobic surface, completely liquid-repellent, lotus effect

8. Superhydrophobicity in nature Prof. Barthlott , University of Bonn, Germany Lotus leaf-effect ( Nelumbo nucifera )

9. Why should lotuses have this property ? (what could be the adaptive significance ?)

10. Normal plants SEM of leaf of normal plants They have deposits of epicuticular waxes Example: maize, paddy, wheat etc Stomata

11. By Erika Eiser, Complex Fluids Group, UvAmsterdam http://www.botanik.uni-bonn.de/system/lotus/en/lotus_effect_multimedia.html  > 150° Any normal leaf Water stick to leaf surface and glides away Leaves are partially wet

12. https:// www.dnaindia.com / india /report-banana-leaves-replace-plates-at-chennai-joints-2766106

13. What do you expect the surface of lotus leaf to be ? Very smooth Smooth Rough Very Rough

14. Scanning Electron Micrograph of Lotus leaves The lotus leaf has a microstructure  comprising small protuberances or spiked papillae 10 – 20 microns in height and 10 – 15 microns in width

15. Will such surfaces retain water or drain them off ? Potholes in roads : allow water to stay in pools !

16. By Erika Eiser, Complex Fluids Group, UvAmsterdam http://www.botanik.uni-bonn.de/system/lotus/en/lotus_effect_multimedia.html  > 150° Lotus leaf

17. By Erika Eiser, Complex Fluids Group, UvAmsterdam http://www.botanik.uni-bonn.de/system/lotus/en/lotus_effect_multimedia.html  > 150° Lotus leaf Roll off Angle < 10 Self cleaning property

18. Two factors that influence hydrophobicity of surface Surface tension or surface energy Surface roughness ( granularity )

19. Two factors that influence hydrophobicity of surface Surface tension or surface energy Surface roughness ( granularity ) The trapping of air directly results in a short and discontinuous water/solid/air three-phase contact line (TCL) and realizes an ultralow water-adhesive force. The cooperative effect of the hydrophobic natural wax and the micro/nanoscale surface roughness contributes to the high water repellency and selfcleaning ability of the lotus leaf (i.e., the “lotus effect”).

20. Increasing surface tension or reducing surface energy Can lead to hydrophobic surfaces High surface energy = more adhesion forces Low surface energy = less adhesion forces Reduced surface tension Increased surface tension Surface tension is the property of any liquid by virtue of which it tries to minimize its free surface area.  (via cohesive forces among their own molecules ) The surface energy is defined as the sum of all intermolecular forces that are on the surface of a material (via greater adhesive force between the liquid and the surface )

21. A farmer is spraying pesticide/weedicide on plants ? What should he do to increase the contact surface area of the pesticide on the plants ?

22. Surfactants are used to reduce surface tension A farmer is spraying pesticide/weedicide on plants ? What should he do to increase the contact surface area of the pesticide on the plants ?

23. How can you prevent your bathroom mirror from getting foggy ?

24. In rough surfaces, air bubbles are caught in the ups and downs of the surface. When water is put on the surface, it does not contact with all surface points. Due to low energy of surface and the presence of air, water does not penetrate into the valley Consequently, the surface area is reduced which results in a reduction of friction.

25. https:// www.researchgate.net /figure/Schematic-illustration-of-different-wetting-models-a-Youngs-model-b-Wenzels_fig2_333974770 Young’s model

26. In Lotus leaf, both surface chemistry (due to epicuticular waxes) and surface roughness play a role Surface tension of water is increased while surface energy is decreased leading to very high contact angle = superhydrophobicity

27. Using these principles, a hierarchical structured surface can be fabricated to create ultra hydrophobic surfaces

28. Namib beetle: Stenocara

29. End of Class