Lately we’re hearing more and more about tiny medical and environmental diagnostic devices, that can perform a variety of tests using very small fluid samples. Working with such small samples does present a challenge, however – how do you thoroughly mix tiny amounts of different fluids, or wrangle individual drops for analysis? According to a team of scientists from the University of Washington, the answer lies in the lotus leaf.

The leaves of the lotus plant have a unique microstructure, consisting of tiny bumps that are topped with even tinier projecting hairs. When a drop of liquid lands on one of the leaves, the hairs support its weight, leaving an air pocket underneath. This makes the drop bead up, at which point the slightest disturbance or slope will cause it to roll off. In recent years, this effect has been mimicked in things such as self-cleaning plastic, more efficient solar cells, and dust shields for spacecraft.

The U Washington researchers roughly copied this structure, by using “nanotechnology manufacturing techniques” to create a silicon surface that consisted of many tiny posts, varying in spacing and height. As with the lotus, drops that were added to this surface sat lightly upon it, forming into almost perfect spheres.

Using audio speakers or another machine, that surface was then vibrated at a frequency of 50 to 80 times per second. It was observed that doing so caused the drops to move along specific paths, predetermined by the properties of the posts. Following these paths required the drops to move uphill and downhill, in circles or upside down. Additionally, two drops could be made to merge together, and then moved as one. Changing the frequency of the vibrations allowed the scientists to control the speed at which the drops moved, or let them target drops of a certain size or weight.

Drops of red and blue liquid move along the upper and lower surface of the vibrating platform at speeds up to one inch per second, and merge as they meet in the middle

Although a smartphone’s speaker wasn’t powerful enough to provide sufficient vibrations, the team plan on creating another surface incorporating posts that are 100 times smaller than the current batch – they believe that a smartphone’s low-energy output will be able to move drops across it.

“All you need is a vibration, and making these surfaces is very easy. You can make it out of a piece of plastic,” said study leader Karl Böhringer. “I could imagine this as a device that costs less than a dollar – maybe much less than that – and is used with saliva or blood or water samples ... We envision a device that you plug into your phone, it’s powered by the battery of the phone, an app generates the right type of audio vibrations, and you run your experiment.”

A paper on the research was recently published in the journal Advanced Materials. One of the shimmying droplets can be seen in the video below.

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