Nanorods ooze water in high humidity

RESEARCHERS have accidently discovered carbon-rich nanorods which can ‘ooze’ water in high humidity during an experiment to create magnetic nanowires.

A team from the Pacific Northwest National Laboratory (PNNL), US was puzzled to find the nanorods they created were losing half their weight as humidity increased. After closer inspection under a microscope, they found the nanorods were expelling water inside the specimen holder, only from where the rods intersected, as conditions reached 50–80% humidity.

Out of curiosity, the team raised the humidity further and found the nanorods’ weight increased as they began to take on water again. The process is also reversible, with water being ejected and absorbed once more as the humidity was gradually lowered.

This is the first time this phenomenon has been observed in a material as other materials tend not to take on water in low humidity.

This unusual behaviour was first theorised in the 1990s, however, a 2012 research paper found water trapped in cavities of around 1.5 nm wide can spontaneously evaporate. Another paper published in 2013 found that water can condense into the confines of close hydrophobic – water repulsing – materials and quickly turn into vapour due to attractive forces between the surfaces of the two materials facing each other.

The team hypothesised that the water was condensing and forming a bridge between the nanorods, through a process known as capillary condensation. They believe when the water between rods forms a curved cavity, the surface tension pulls the adjacent rods closer together. When two intersecting nanorods reach about 1.5 nm, the water caught between them could be forced to quickly evaporate.

David Lao, a post-doctoral research associate at PNNL, said: “Our unusual material behaves a bit like a sponge; it wrings itself out halfway before it's fully saturated with water.”

The team are now considering the real-world applications of the discovery, including low-energy water harvesting and purification for the developing world and remote deserts where it could collect water from the air and harvest it for human consumption.

Another use would be to create a membrane that takes on and later expels water as humidity changes. The membrane could be used in fabrics for outdoor clothing for greater comfort by removing sweat from inside the garment and emitting it outside as a vapour.

In order to make these potential commercial applications possible, the team wants to find ways for the nanorods to spray the water. The team estimates the material already spits around 10–20% of its water with force. However, in order to perfect this, they will need to be able to control the size and shape of the rods.

The team will also need to scale up production to a few grammes in order to determine if the phenomenon is still present and effective in larger quantities. The team also wants to know if other materials have similar physical and chemical properties, and if other nanomaterials can be developed to collect other liquids such as methanol.

Nature Nanotechnology, DOI: 10.1038/NNANO.2016.91