Understanding nature: Physical basis of the Lotus-effect S.C.S. Lai Leiden University September 10 th, 2003

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Table of contents  Description of the Lotus-effect  Physical background  Synthesis of surfaces with the Lotus-effect (Synthesis of superhydrophobic surfaces)  Conclusion

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect What is the Lotus-effect?  Self-cleaning  Superhydrophobic

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Young’s equation

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Young’s equation revisited Minimizing E with constant volume yields:  Shape: Laplace equation:  Contact angle: Young’s equation, independent of shape

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Complications  Forces acting in vertical direction not taken into account  Contact line tension  Smaller than unity?  Hysteresis  “Ideal” surfaces

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Rough surfaces Wenzel: Liquid completely fills the grooves of the solid Cassie and Baxter: Liquid “sits” on the surface roughness

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Wenzel R.N. Wenzel, 1936

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Cassie and Baxter A.B.D Cassie and S. Baxter, 1944

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect 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

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect The Lotus-effect (1): Superhydrophobicity  Microstructural epidermal cells  Nanostructural wax-crystals W. Barthlott, C. Neinhuis; μm

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect The Lotus-effect (2): Self-cleaning (1) Both contamination and water have a small contact area with the leaves 1 μm50 μm

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect The Lotus-effect (2) Self-cleaning(2) Water rolls off the surface taking the contamination along

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Table of contents Description of the Lotus-effect Physical background  Synthesis of surfaces with the Lotus-effect (Synthesis of superhydrophobic surfaces)  Conclusion

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect 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

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Quéré: Non-stick water Drop of water covered with very hydrophobic powder Experiments on fluid-mechanical behavior D. Quéré, P. Aussillous; 2001

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Nakajami: Durable self- cleansing (1)  Problem: Degredation due to build-up of stain  Lack of metabolism Other solution needed Add TiO 2 to coating! Nakajami et al.; 2000

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Nakajami: Durable self- cleansing (2)  Add TiO 2 to a superhydrophobic coating  Measured the effect of TiO 2 to  Superhydrophobicity  Transparency  Durability

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Nakajami: Durable self- cleansing (3); Transparency  Transparency decreases as concentration increases  Below 20% acceptable

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Nakajami: Durable self- cleansing (4); Durability UV-illumination Outdoor exposure  Durability decreases with increasing concentration  Small amount gives better result than nothing

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect 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

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect 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

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect 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

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Patankar: Analysis of a robust superhydrophobic surface (3) Example: Consider

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Patankar: Analysis of a robust superhydrophobic surface (4) Design condition at intersection point!

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect 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.

S.C.S. Lai, September 10th 2003 Understanding nature: Physical basis of the Lotus- effect Questions?