Understanding nature: Physical basis of the Lotus-effect

1. Understanding nature: Physical basis of the Lotus-effect S.C.S. Lai (s.lai@chem.leidenuniv.nl) Leiden University September 10th, 2003

2. Table of contents • Description of the Lotus-effect • Physical background • Synthesis of surfaces with the Lotus-effect (Synthesis of superhydrophobic surfaces) • Conclusion Understanding nature: Physical basis of the Lotus-effect

3. What is the Lotus-effect? • Self-cleaning • Superhydrophobic Understanding nature: Physical basis of the Lotus-effect

4. Young’s equation Understanding nature: Physical basis of the Lotus-effect

5. Young’s equation revisited • Minimizing E with constant volume yields: • Shape: Laplace equation: • Contact angle: Young’s equation, independent of shape Understanding nature: Physical basis of the Lotus-effect

6. Complications • Forces acting in vertical direction not taken into account • Contact line tension • Smaller than unity? • Hysteresis • “Ideal” surfaces Understanding nature: Physical basis of the Lotus-effect

7. Rough surfaces Wenzel: Liquid completely fills the grooves of the solid Cassie and Baxter: Liquid “sits” on the surface roughness Understanding nature: Physical basis of the Lotus-effect

8. Wenzel R.N. Wenzel, 1936 Understanding nature: Physical basis of the Lotus-effect

9. Cassie and Baxter A.B.D Cassie and S. Baxter, 1944 Understanding nature: Physical basis of the Lotus-effect

10. Summary • The contact angle of a liquid drop on a smooth solid is given by Young’s equation • Surface roughness enhances the hydrophobicity (hydrophilicity) of the solid interface Understanding nature: Physical basis of the Lotus-effect

11. The Lotus-effect (1):Superhydrophobicity • Microstructural epidermal cells • Nanostructural wax-crystals 20 μm W. Barthlott, C. Neinhuis; 1997 Understanding nature: Physical basis of the Lotus-effect

12. The Lotus-effect (2):Self-cleaning (1) 1 μm 50 μm Both contamination and water have a small contact area with the leaves Understanding nature: Physical basis of the Lotus-effect

13. The Lotus-effect (2)Self-cleaning(2) Water rolls off the surface taking the contamination along Understanding nature: Physical basis of the Lotus-effect

14. Table of contents • Description of the Lotus-effect • Physical background • Synthesis of surfaces with the Lotus-effect (Synthesis of superhydrophobic surfaces) • Conclusion Understanding nature: Physical basis of the Lotus-effect

15. Synthesis of superhydrophobic surfaces • Several methods, most involving mechanical roughening of the surface • Quéré: Non-stick water • Nakajami: Durable self-cleansing • Erbil: Transformation of a simple plastic • Patankar: Analysis of a robust superhydrophobic surface Understanding nature: Physical basis of the Lotus-effect

16. Quéré: Non-stick water Drop of water covered with very hydrophobic powder Experiments on fluid-mechanical behavior D. Quéré, P. Aussillous; 2001 Understanding nature: Physical basis of the Lotus-effect

17. Nakajami: Durable self-cleansing (1) • Problem: Degredation due to build-up of stain • Lack of metabolism • Other solution needed Add TiO2 to coating! Nakajami et al.; 2000 Understanding nature: Physical basis of the Lotus-effect

18. Nakajami: Durable self-cleansing (2) • Add TiO2 to a superhydrophobic coating • Measured the effect of TiO2 to • Superhydrophobicity • Transparency • Durability Understanding nature: Physical basis of the Lotus-effect

19. Nakajami: Durable self-cleansing (3); Transparency • Transparency decreases as concentration increases • Below 20% acceptable Understanding nature: Physical basis of the Lotus-effect

20. Nakajami: Durable self-cleansing (4); Durability • Durability decreases with increasing concentration • Small amount gives better result than nothing UV-illumination Outdoor exposure Understanding nature: Physical basis of the Lotus-effect

21. Erbil: Transformation of a simple plastic • Procedure: • Dissolve polypropylene in x-xylene • Put solution onto object to be coated • Remove x-xylene, either by evaporation or precipitation Grade of superhydrophobicity highly dependant on evaporation rate! Erbil et al; 2003 Understanding nature: Physical basis of the Lotus-effect

22. Patankar: Analysis of a robust superhydrophobic surface (1) • Problem: Experiments both validated Wenzel’s model and Cassie’s model in certain circumstances. • The angle predicted by Wenzel differ a lot from the angle predicted by Cassie • Is it possible to model a surface in such a way that Wenzel’s angle equal Cassie’s angle? N.A. Patankar, 2003 Understanding nature: Physical basis of the Lotus-effect

23. Patankar: Analysis of a robust superhydrophobic surface (2) Energy analysis: • Both model predict local minima in energy • Smallest of the two is the global minimum => Robust superhydrophobic surface if both angles are the same and near 180 degrees Understanding nature: Physical basis of the Lotus-effect

24. Patankar: Analysis of a robust superhydrophobic surface (3) Example: Consider Understanding nature: Physical basis of the Lotus-effect

25. Patankar: Analysis of a robust superhydrophobic surface (4) Design condition at intersection point! Understanding nature: Physical basis of the Lotus-effect

26. Conclusion • Surface roughness increases hydrophobicity • Superhydrophobic if contact angle > 150° • Superhydrophobicity leads to self-cleansing • Several methods reported to synthesize artificial superhydrophobic surfaces, but no “perfect” self-cleansing surfaces yet. Understanding nature: Physical basis of the Lotus-effect

27. Questions? Understanding nature: Physical basis of the Lotus-effect