With a tiny magnet, researchers have designed a simple system to make a little drop of water move around.
To create a “Hydrobot”, the team – led by Yifan Si of the City University of Hong Kong – used a 1-mm-wide iron bead with an extremely hydrophilic (water-loving) surface. Because the surface strongly attracts water, droplets wrapped around the bead to form a Hydrobot that could then be manipulated with magnetism.
Since liquid can change shape to reach small and hard-to-reach places, Hydrobots could be used to clean out minute spaces or deliver dissolved substances to tricky spots.
This video shows how Hydrobot can move and stop on demand following an external magnet. Credit: Yifan Si.
“Hydrobot may offer some new ideas to soft robot design,” says Si. “Currently, most soft robots use solid materials. Even though they are flexible, they would not be as flexible as liquids.
“Liquids also have characteristics that can be advantageous, including their ability to change shapes and vaporize.
With more studies, these features can make Hydrobot even more versatile.”
They put the bead on a hydrophobic (water-hating) surface that was dotted with water droplets. Using a magnet on the other side of the surface, the team drove the bead around, and it picked up more water droplets along the way. It could speed along at two metres per second!
“The idea of Hydrobot was inspired by little fish bouncing on and off lotus leaves,” says Si. “We have a pond on campus with many lotus plants, and occasionally I would see fish get trapped on these big, hydrophobic leaves. When they manage to escape and jump back in the pond, the water puddle around the fish on the leaves will also be taken away.”
This is because the fish scales are hydrophilic and the water clings to them instead of the leaves.
The little bead was able to carry one millilitre of water with it – for comparison, a teaspoon holds about 5 millilitres.
“One advantage of Hydrobot is that the materials involved are easily accessible. If a task requires controlling a larger amount of water, we can simply use more beads to increase the surface area,” says Si.
The team also tried driving the Hydrobots upside down – they put the bead under a surface and the magnet on top, and the Hydrobot reached speeds of two centimetres per second.
Hydrobot still requires a special surface to work, but the team plan to investigate other real-world applications.
The paper was published in Cell Reports Physical Science.
The video demonstrates Hydrobot’s ability to collect dust (blue grains) on a superhydrophobic surface. Credit: Yifan Si.
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