AsianScientist (Jun. 27, 2016) – Many of us have fond childhood memories of poking the ubiquitous Mimosa pudica plant’s leaflets, then exclaiming in delight when they fold up at even the slightest touch.
This spontaneous, protective motion, which is more complicated than it looks, is triggered by a cascade of reactions and pressure waves. Now, researchers from Hong Kong and Australia have drawn inspiration from the ever-so-sensitive plant to develop self-organizing soft materials that fold themselves into predetermined shapes when wet. The study is published in the journal Science Advances.
The Mimosa plant responds to touch by undergoing self assembly, which is how many organisms in nature adapt to their environments. One such example is the crystallization-driven formation of seashells.
“This is a bit like your skin reacting automatically to cold temperatures,” corresponding author Associate Professor Antonio Tricoli from Australian National University told Asian Scientist Magazine. “Imagine a material that becomes waterproof if it gets even slightly wet.”
Led by Tricoli and Associate Professor Wang Zuankai from the City University of Hong Kong, the research team engineered a unique material made out of a stack of multi-functional layers, some hydrophilic and some hydrophobic. The material incorporates Janus nanoparticles, named after the two-faced god Janus, as they have two distinct types of chemistry on the same particle.
In response to contact with a water droplet, the engineered material efficiently transforms surface energy into directional kinetic and elastic energies that propel its self-assembly into a quasi-cylindrical ‘straw’ shape.
“It starts reorganizing very fast, with an initial reaction time of less than 33 milliseconds. It folds around the water droplet, and at the same time, propels some of the water, further causing a chain of folding-water propulsion reactions,” said Tricoli.
“As a result, we are able to engineer materials that can form specific maze-like tubes and other geometries once one of their ends get in contact with water.”
Notably, this self-assembly is reversible: the material is capable of unfolding and recovering its initial surface properties.
According to Tricoli, the material may someday be utilized for numerous wearable and portable applications that do not require power or have significantly reduced power consumption. These include wearable sensors capable of collecting and analyzing sweat and other body fluids; portable paper-based microfluidic devices; fog harvesting for water recollection; and micro-robotic devices.
The article can be found at: Wong et al. (2016) Mimosa Origami: A Nanostructure-Enabled
Directional Self-Organization Regime of Materials.
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Copyright: Asian Scientist Magazine; Photo: Mon Œil/Flickr/CC.
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