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July 2021

What is a Hydrobot?

Researchers in Hong Kong have developed a novel method for moving a water drop along a controlled path on a superhydrophobic surface. The method requires a superhydrophilic steel ball which, due to it's high wetting properties, is attracted to water and will become embedded in a water drop. The water, by contrast, is repelled by the superhydrophobic surface on which it sits. A magnet under the surface is used to control - with an amazing degree of precision - the location and movement of the drop (and can even pickup additional water drops in its path. The diagram below details the system as described in a recent paper entitled Bioinspired magnetically driven liquid manipulation as microrobot in Cell Reports Physical Science.1

Image Credit: Yifan Si

The study of controlled movement of small volumes of liquid is called microfluidics and has become of increased interest to researchers. This particular method permits velocities unmatched by previous microfluidic methods. There are a number of practical applications for this system which include surface cleaning, energy harvesting, and cargo transportation.

Previous methods of "smart" liquid manipulation have relied on light fields, electrical fields, and thermal fields. This method relies on a magnetic field. In addition to exceeding the drop velocity of prior methods, the magnetically driven liquid manipulation strategy also permits drop movements on surfaces that are positioned at any angle - including upside down.

What's noteworthy is that the proposed system fully relies on wetting properties. If the ball were not superhydrophilic and if the surface were not superhydrophobic, it would not be possible to move the drop, or hydrobot, via magnetic force.

Imagine a hard-to-clean surface which is out of reach of traditional cleaning methods. One or more hydrobots, controlled locationally by a magnet, offer the potential for cleaning such a surface quickly and efficiently. In tests, the hydrobots were able to move and stop at speeds up to 2 meters per second. Add the capacity to move the hydrobot along a surface positioned at any angle - even upside down - and you have capacities that are irreplaceable using other methods. Precision manufacturing could benefit from this technology in a narrow array of special applications.

1 https://doi.org/10.1016/j.xcrp.2021.100439

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