Bubbles can be useful tools for microscopic robots, helping them move, steer, create flows of liquid and perform other functions. But designs based on trapping a bubble, a common approach, are unwieldy and unpredictable because bubbles can be hard to catch and control.
“Why are we trying to trap these bubbles?” said Itai Cohen, the Josephson Family Professor of physics in the College of Arts and Sciences (A&S). “Let’s just make them where we want them, when we want them.”
Researchers in Cohen’s lab have developed a way to create and direct microbubbles on demand, without requiring trapping. Set within a 3-millimeter area on a chip, the device produces hydrogen gas bubbles on a grid of electrodes, then powers the bubbles up with acoustic energy through ultrasound. Bubbles resonating on the electrodes can work alone, in pairs or in larger groups to create and direct flows of fluid.
It’s all programmable in real time, said Zhiyuan Zhang, KIC experiment postdoctoral fellow in the Laboratory of Atomic and Solid State Physics and the Kavli Institute at Cornell for Nanoscale Science, first author of the new study.
“With this bubble array, we can control the bubble size, numbers and position in real time, so we can program the flow direction,” he said.
The technology is flexible and forgiving – if one bubble malfunctions, it can simply be released and a new one formed, Cohen said, increasing control over microbubble size and placement. It opens the door to reconfigurable microfluid devices that could be used in medical applications such as super-targeted drug delivery.
“Ultrasound-Driven Electronically Programmable Microbubbles” published Oct. 14 in Science Advances. Cohen is co-corresponding author with Zhang.
In experiments, Zhang used a setup with nine electrodes arranged in a square on the chip, fabricated at the Cornell NanoScale Science and Technology Facility. Each electrode is six microns in diameter and produces bubbles that are 10-100 microns; bubble size can be key to its function.
First, he forms a bubble on the tip of an electrode by adding electrons to break up a water molecule, leaving one hydrogen gas bubble. The longer voltage is applied, the larger the bubble gets.
Then he adds sound energy to the system; ultrasound causes the bubble or multiple bubbles to resonate. The oscillating bubbles move the fluid around them.
“The size of the bubble determines the resonant frequency of the bubble,” Cohen said. “If you’re applying ultrasound at 40 kilohertz and the bubble is too small or too big, then it’s not going to oscillate significantly. But if it’s just the right size to resonate at 40 kilohertz, you get a lot of flow.”
A single bubble can be set to resonate, and the movement of the bubble surface transfers to the fluid around it, creating a flow. Two bubbles on neighboring electrodes can perform additional functions, working together. Bubbles of the same size resonate at the same frequency and create a symmetrical flow, Zhang said. Bubbles of different sizes have different frequencies – one will push fluid harder than the other, creating a flow toward one direction.
Further programmable flow manipulation is possible by placing bubbles on several of the electrodes. For example, three bubbles in a horizontal row can create a flow from left to right. Another three bubbles in a diagonal line create a flow up to the left. Five bubbles arranged in an L-shape direct a flow in a curve.
There’s also a “bubble shooting” function, Zhang said. While experimenting with the voltage that inflates each hydrogen gas bubble, he discovered that a higher voltage generates a cloud of bubbles in a rapid flow. By turning on the ultrasound, he can shoot the bubbles in a specific direction.
The device turns the bubble’s ephemerality into an opportunity for regeneration, Cohen said: “If, for some reason, one of the bubbles isn’t working, you just inflate it until it detaches. And it goes away and then you build another bubble.”
Next steps could include designing circuits that create the bubbles through this mechanism on robots, Cohen said. Co-author Alyosha Molnar, the Ida and Charles Lee Professor of Engineering in the School of Electrical and Computer Engineering, Cornell Duffield Engineering, contributed to this study and will continue the collaboration.
Postdoctoral researcher Jinsong Zhang also contributed to the study, which was supported by the National Science Foundation and the Army Research Office.
Kate Blackwood is a writer for the College of Arts and Sciences.
