Moisture harvesting surfaces Extracting Liquid water out of moist air by utilizing wetting properties and surface structuring 3.2.2016

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

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

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)

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.

Example

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

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