Emulating the beetle’s water harvesting capabilities
Tilting its body forward, the Stenocara beetle of the Namib Desert in Namibia is perfectly poised to collect and drink small water droplets from the light fog that drifts over the desert early each morning. Fit for survival in a region that experiences less than half an inch of rain each year, this organism was studied by scientists at MIT in the USA – they wanted to develop a material that could emulate the beetle’s water harvesting capabilities.
The wings on the Stenocara beetle’s back are patterned with an array of superhydrophilic bumps, about 100µm in diameter, on a waxy superhydrophobic background.
As the fog-laden wind sweeps across the surface, water droplets of about 15 to 20 microns are attracted to and accumulate on these bumps. When the droplets get large enough, they overcome the binding hydrophilic force and roll down the water-repelling surface into the beetle’s mouth.
Forseeing potential applications, for harvesting water, open-air microfluidic channels and controlled drug release coatings, the team at MIT wanted to create a new material based on this contrast of textures.
According to MIT Professor Robert Cohen, although coexisting hydrophilic and hydrophobic patterned surfaces have been developed previously, ‘to the best of my knowledge, we are the first to demonstrate the coexistence of extremes – superhydrophilic and superhydrophobic behaviour’.
The former demonstrates a water droplet advancing contact angle of less than five degrees, while the latter exhibits a contact angle of 150º and above.
By repeatedly dipping glass substrates into solutions of charged polymer chains of polyallylamine hydrochloride (PAH) or polyacrylic acid (PAA), a multilayer porous material was produced. Its superhydrophobicity was increased with a coating of silica nanoparticles that created a rough texture, followed by a layer of semi-fluorosilane molecules.
Superhydrophilic patterns are then created on this surface by applying sporadic spots of about 750µm with water/2-propanol solutions of PAA.
Cohen explains, ‘Post-doctoral researcher Lei Zhai found just the right composition that would allow the PAA molecules to wet and penetrate our superhydrophobic starting material.’ By spraying a fine mist from a plant sprayer, it was found that the material could mimic the beetle. Cohen describes how water accumulates on the patterned regions. He says that when the glass microscope slide is tilted the ‘pinned droplets release and roll along the dry regions. Smaller droplets remained pinned until sufficient water accumulates for the selected tilt angle,’.
Further research focuses on employing other flexible and rigid substrate materials, while additional investigation is required on the time taken for the water droplets to coalesce, such as by studying the velocity and water density of the mist.
Funded by the Defense Advanced Research Projects Agency and the National Science Foundation in the USA, the research has already attracted the interest of the US military for developing a self-decontaminating surface that could channel and collect harmful substances.
Cohen says, ‘Our technique might be amenable to rapid large-scale production [as] inexpensive technologies akin to ink-jet printing could be used to create the type of patterned surfaces we have reported.’