Researchers at Cornell University (USA) have created a device that forms microbubbles in a desired location and controls them without needing to hold them in place. The work was published in the journal Science Advances.
Usually bubbles are caught and fixed in place, but this is difficult and unpredictable. "Why are we trying to catch these bubbles? Let's just create them where we need them, and when we need them," says Itai Cohen, a professor of physics and one of the authors of the study.
The new chip, measuring 3 by 3 millimeters, contains a grid of nine electrodes. Each electrode, six micrometers in diameter (a micrometer is one thousandth of a millimeter), creates a hydrogen bubble: under applied voltage, a water molecule splits and gas is released. The longer the voltage is applied, the larger the bubble, from 10 to 100 micrometers.
Then ultrasound is switched on. Sound waves make the bubbles oscillate (resonate), and these oscillations are transmitted to the liquid, creating a flow. "The size of the bubble determines its resonant frequency," Cohen explains. "If you apply ultrasound at a frequency of 40 kilohertz and the bubble is too small or too large, it will not oscillate noticeably. But if its size exactly matches the resonance at that frequency, you get a strong flow."
The flow can be controlled by arranging the bubbles in a certain way. Identical bubbles create a symmetric current, while bubbles of different sizes create a flow directed to one side. For example, three bubbles in a row direct the liquid from left to right, three along a diagonal direct it up and to the left, and five in the shape of the letter G direct it along a curve. In addition, at a higher voltage a cloud of bubbles forms that can be "fired" in a desired direction by switching on ultrasound.
An important advantage is the ability to refresh: if a bubble stops working, it can be "blown away" and a new one created. The scientists note that in the future such bubbles could become the driving force of microscopic robots and find applications in medicine, for example for ultra-precise drug delivery. However, for now this is a laboratory prototype, and no timeline for the appearance of finished devices is being discussed.