1. Moisture harvesting surfaces
Extracting Liquid water out of moist air by utilizing wetting properties and surface structuring

2. Introduction to moisture harvesting
Even in dry areas, there is usually some moisture in the air
Fog, dew
Many organisms collect water from ther atmospheric air
Beetles: hydrophilic back bumps within hydrophobic areas
Cactus: clusters of conical spines and trichomes
Lizards: honeycomb structure: superhydrophilic skin surface + capillary network
Spider silk: periodic spindle-knots and joints
What do they have in common?
Applications
Water collection
Droplet motion
Condensation of water, accumulation and collection of water droplet
General phases
Deposition
Collection
Transport
Absorption

3. Surface Properties Surface wettability Surface nanostructure
Superhydrophilic surface
Superhydrophilic patterns on superhydrophobic surfaces
Nanostructures of the cactus
Surface nanostructure
Create hydrohilicity and hyrdophobicity
Direct the water droplets to wanted place
Grooves, lines
Bumps or areas
Knots and joints
Hyrdophilic and hydrophobic areas

4. Phenomena Continuous condensation of moisture from air to surface
Driving force for directional movement of water drops
Low friction → does not slow the motion induced by:
Surface energy gradient
Difference of surface roughness (Wenzel’s equation)
Driving force for the water droplets alonside the strcture
Directional water collection by using Laplace pressure
Wenzel’s equation: description of roughness
gradient of roughness -> gradient of wettability -> gradient of surface free energy
(width of microgrooves broader at base and narrower at tip but width of sub-microgrooves remain same creating roughness gradient)

5. Phenomena – Laplace Pressure
Difference in Laplace pressure
Opposite sides of droplet
Anisotropic contact angle hysteresis:
in the direction parallel or perpendicular to the grooves improving directional movement of water along the grooves.

6. Example

7. Making a water collecting structure
Superhydrophobic background: TiO2
Superhydrophilic patterns: ZnO
Constructing the surfaces
Dropet formation into the hyrdophilic surface
TiO2 ZnO structure

8. Water collecting example
Effect of hydrophobicity
Best results with dot-pattern
Effect of the surface structure
Superhydrophilic, Superhydrophobic and dot-pattern
Dot, Mesh, Line, Branch