Droplet impact upon solid surfaces is a widespread phenomenon and has been investigated for over a century. Important technological applications include: rapid cooling, delayed freezing, crop spraying, inkjet printing, and fog harvesting.  For topographically complex surfaces, the impacting droplet can either penetrate into the surface fine structure, or become suspended on the asperities creating air pockets underneath giving a composite solid-air interface. These are respectively the Wenzel and Cassie-Baxter states.  The impact and wetting of water droplets onto fabricated and naturally occurring three-dimensional hierarchical surfaces has been investigated.             Hierarchical superhydrophobic surfaces have been prepared by solvent casting polybutadiene films under controlled humidity to create two-dimensional hexagonally ordered honeycomb surfaces (10 micron scale) followed by CF4 plasmachemical fluorination to introduce roughness on the micron and nano length scales. This yields sessile drop water contact angle values exceeding 170�, with negligible contact angle hysteresis. High-speed imaging shows that variation of surface topography length scales can be employed to control the dynamics of droplet impact.            In the case of naturally occurring three-dimensional hierarchical structures, the Cotula fallax plant is an example whose hierarchical length scales comprise stems, leaves, and fine hairs.  Of particular note is that during fog episodes, water is retained throughout the entirety of the plant including the foliage (rather than accumulating in localised regions).  Water droplets attach and grow on individual plant hairs.  The overall 'mat-like' hydrophobic hair structure supports the growing water droplets during continued fog exposure (akin to the Cassie-Baxter state), until eventually the hairs become wrapped around large water droplets to effectively secure the water droplets via their nanoscale grooves (thus preventing the captured water droplets from falling towards the ground).  This previously unreported water collection mechanism offers a promising new approach for fog harvesting in arid and semi-arid regions of the world to provide drinking water.

Droplet impact onto 3D hierarchical surfaces

246th National Meeting (2013)

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