Sascha Hilgenfeldt

Sascha Hilgenfeldt harnesses the surprising power of bubbles

“Bubbles are powerful little machines,” says Sascha Hilgenfeldt, associate professor of engineering sciences and applied mathematics and of mechanical engineering. To prove his point, Hilgenfeldt cites the well-known example of damage to huge metal ship propellers caused by cavitation, the collapse of bubbles flowing around the propellers at high speed.

Even when they do not cavitate, warns Hilgenfeldt, those bubbles can pack a punch simply by inducing shear forces in the fluid around them. Intact bubbles as power tools? That paradox becomes stranger still as bubbles become smaller: “As you miniaturize the set-up, the shear forces become stronger,” reports Hilgenfeldt, who uses high-speed cameras to study the flow generated by microscale bubbles. This phenomenon opens up a world of possible applications, especially in medicine, promising advances in drug delivery, gene therapy, and the diagnosis of diseases like cancer.

To harness the power of these tiny bubbles, Hilgenfeldt and fellow researchers are working to better understand the mechanisms that govern their behavior, a focus that has surprised Hilgenfeldt himself. After doing undergraduate work in physics and math and earning a PhD in physics in his native Germany, Hilgenfeldt went to Harvard University as a postdoctoral fellow to continue his research on sonoluminescence, the emission of light by bubbles in a liquid excited by sound. In 2000 he joined the faculty of the University of Twente in the Netherlands, where he began to study how bubbles interact with biomaterials. He joined Northwestern's faculty in fall 2004.

Researchers knew that bubbles are capable of delivering drugs and genes to cell membranes but did not understand the mechanism of delivery. Possible explanations included shock waves, heat, jet impact, and shear flow. Of those possibilities Hilgenfeldt and his colleagues decided to study shear flow first. “We thought that would be the boring part and that we'd get it out of the way,” he says. “Needless to say, I've been on it ever since.”

What captivated Hilgenfeldt was the movement of the bubbles. “We knew that bubbles focus ultrasound energy and that an ultrasonic field would excite small bubbles to oscillate,” explains Hilgenfeldt, “but instead of periodic jiggling, we observed a steady flow.” That shear flow promised to offer more control in applications, but the mechanism of control eluded Hilgenfeldt until he made a fortuitous observation during an experiment: “A new player entered, a solid particle, which turned out to be a piece of quartz debris.” That debris interacted with the bubbles and completely changed the flow. Breaking the flow symmetry, the particle created a steady transport motion that Hilgenfeldt believes could be used to penetrate cell walls to deliver drugs or gene therapy. Interestingly, the bubbles themselves do not burst even as they penetrate vesicle membranes.

The next step for Hilgenfeldt has been to recreate that accident through microfabrication, etching onto a substrate such as a silicon wafer a bump that acts as a stationary solid particle as well as tiny holes that retain air pockets. When these “holes and bumps” are submerged in water and excited by ultrasound, they act “as little directional motors,” says Hilgenfeldt, who dubs the effect “dancing bubbles.”

Hilgenfeldt is working to solve problems posed to him by researchers from Northwestern's Feinberg School of Medicine and interacts regularly with faculty from mechanical engineering, materials science, and chemical and biological engineering. “I try to talk to people from many different backgrounds and see what their needs are,” he says. Hilgenfeldt sees many potential applications for his research, from a chemical-lab-on-a-chip system to a way of mixing and sorting materials on the microscale — all from unleashing the power of tiny bubbles.

—Leanne Star

Northwestern University