Tiny bubbles can pack a surprising punch. When a liquid filled with small bubbles is subjected to a burst of ultrasound, the bubbles can quickly collapse, releasing concentrated pulses of energy that can reach temperatures of 5,000 kelvin and pressures as high as 100 megapascals. That focused energy surge, called cavitation, offers scientists a clean way to drive not only chemical reactions, but also biological applications—provided that it can be controlled. Tandiono from the A*STAR Institute of High Performance Computing and co-workers from the A*STAR Bioprocessing Technology Institute and Nanyang Technological University are now working towards just that by trapping bubbles within a microfluidic chip1.
The idea for the researchers’ proposed process is that by creating bubbles within sub-microliter volumes of liquid confined inside the narrow channels of a microfluidic chip, the location of the cavitations can be controlled—unlike cavitations in bulk liquid. The confinement of bubbles inside narrow channels, however, poses its own complications. Wall boundaries, for example, can interfere with bubble collapse, dissipating the energy produced.
To test the idea, the researchers built a microfluidic device fitted with an ultrasound-producing transducer. They then pumped liquid—in this case a water-based solution of the luminescent chemical luminol—into the channels of the device, interspersed with small amounts of gas. They found that the energy released by cavitation could tear some of the water molecules apart, generating hydroxide radicals that interact with the luminol to release blue light.
Each burst of ultrasound resulted in a blue glow from the chip (see image). High-speed photography revealed that the gas trapped within the channels broke up into little bubbles that subsequently underwent violent collapse due to the pressure waves caused by the ultrasound vibration.
“The ultimate goal of our work is to understand the characteristics of bubble collapse in the microfluidic channel and its correlation with ‘sonochemistry’—a subfield of chemistry that uses sound,” says Tandiono. The microfluidic device the researchers developed gives them a platform with which to do just that. Because the oscillating bubbles are trapped within a well-defined environment within the microfluidic channel, a systematic study can be carried out.
“The next step of our research is to study the correlations between bubble dynamics, sonochemical reactions and sonoluminescence,” Tandiono says. The microfluidic device also has potential biological research applications, he adds. “We will also explore the use of acoustic bubbles in microfluidics for some biological applications, such as cells disruption and gene delivery.”