Fog Collection on Plant Surfaces and Biomimetic Applications Dissertation Zur Erlangung des Doktorgrades (Dr. rer. nat.) der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn vorgelegt von Md. Abul Kalam Azad aus Satkhira, Bangladesh Bonn, Januar 2016 Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn. 1. Gutachter: Prof. Dr. Wilhelm Barthlott 2. Gutachter: Prof. Dr. Kerstin Koch Tag der Promotion: 18.04.2016 Erscheinungsjahr: 2016 I II Dedicated to the martyrs of the liberation war of Bangladesh,1971 and the people living in the water stressed regions of the world III IV Contents 1. General introduction .................................................................................................... 1 1.1. Fog in arid regions: a neglected source of water................................................................. 1 1.2. History of fog collection ....................................................................................................... 3 1.3. Existing technologies for fog collection ............................................................................... 7 1.4. Physics of surface wettability .............................................................................................. 9 1.5. Fog collection on plant surfaces ........................................................................................ 13 2. Aims of the thesis ...................................................................................................... 16 3. Material and methods................................................................................................ 17 3.1. Plant and technical material .............................................................................................. 17 3.2. Sample preparation ........................................................................................................... 18 3.2.1. Plant samples ..................................................................................................................... 18 3.2.2. Technical samples .............................................................................................................. 21 3.3. Contact angle (CA) measurement ...................................................................................... 23 3.4. Light- and Scanning Electron- Microscopy (SEM) .............................................................. 24 3.5. Modification of the chemistry of the sample surfaces ...................................................... 24 3.6. Experimental setup ............................................................................................................ 25 3.7. Effective surface area of samples for fog collection .......................................................... 26 3.8. Statistical analysis .............................................................................................................. 27 3.9. Simulation .......................................................................................................................... 27 4. Fog collection on plant surfaces: Influence of leaf shape modification and directed water transport on the surfaces ................................................................................. 29 4.1. Introduction ....................................................................................................................... 29 4.2. Results ................................................................................................................................ 31 4.2.1. Structural characterization of the samples ........................................................................ 31 4.2.2. Surface wettability ............................................................................................................. 32 4.2.3. Surface microstructure ...................................................................................................... 33 4.2.4. Influence of leaf shape modification ................................................................................. 35 4.2.5. Influence of directed water transport on the surfaces ..................................................... 36 4.3. Discussion .......................................................................................................................... 38 4.3.1. Influence of leaf shape modification ................................................................................. 38 4.3.2. Influence of directed water transport on the surfaces ..................................................... 40 V Contents 4.4. Conclusion .......................................................................................................................... 41 5. Fog collecting biomimetic surfaces: Influence of microstructure and wettability ......... 43 5.1. Introduction ....................................................................................................................... 43 5.2. Results ................................................................................................................................ 45 5.2.1. Surface microstructure ...................................................................................................... 45 5.2.2. Surface wettability ............................................................................................................. 47 5.2.3. Influence of microstructure ............................................................................................... 48 5.2.4. Influence of wettability ...................................................................................................... 52 5.3. Discussion .......................................................................................................................... 54 5.3.1. Influence of microstructure ............................................................................................... 54 5.3.2. Influence of wettability ...................................................................................................... 56 5.4. Conclusion .......................................................................................................................... 58 6. Hierarchical surface architecture of plants as an inspiration for biomimetic fog collectors .................................................................................................................... 59 6.1. Introduction ....................................................................................................................... 59 6.2. Results ................................................................................................................................ 61 6.2.1. Structural characterization of the samples ........................................................................ 61 6.2.2. Surface microstructures ..................................................................................................... 62 6.2.3. Surface wettability ............................................................................................................. 63 6.2.4. Comparison of fog-collection efficiency ............................................................................ 64 6.2.5. Simulation .......................................................................................................................... 71 6.3. Discussion .......................................................................................................................... 74 6.4. Conclusion .......................................................................................................................... 78 7. Trichomes of Ptilotus manglesii: An integrated system for efficient fog collection ...... 79 7.1 Introduction ....................................................................................................................... 79 7.2. Results and discussion ....................................................................................................... 80 7.2.1. Characterization of trichomes ........................................................................................... 80 7.2.2. Fog collection by trichomes ............................................................................................... 81 7.3. Conclusion .......................................................................................................................... 86 8. Fog collection on polymer fibers: Droplet behavior and collection efficiency influenced by cross-section profiles and surface structures ......................................... 87 8.1. Introduction ....................................................................................................................... 87 8.2. Results ................................................................................................................................ 89 8.2.1. Cross-section profiles and surface structures of the fibers ............................................... 89 VI Contents 8.2.2. Surface wettability ............................................................................................................. 93 8.2.3. Influence of cross-section profiles and surface structure on fog collection ...................... 93 8.2.4. Fog droplet behavior on different fiber profiles ................................................................ 97 8.2.5. Transport or drainage of the