Algae cells offer in-depth look at waves, acoustics and vibrations

This video shows swimming Chlamydomonas reinhardtii cells in an acoustic microfluidic device created in Mark Meacham’s lab. After inserting the cells into the device, researchers gradually vary the frequency of device vibration. At different frequencies, the cells group together to form various shapes.

Engineers often create devices to study forces, motion or other behaviors found in nature. J. Mark Meacham, a mechanical engineer in the McKelvey School of Engineering at Washington University in St. Louis, is doing it in reverse — he's using algae cells to study the devices he creates in his lab.

Meacham, assistant professor of mechanical engineering & materials science, received a five-year, $500,000 CAREER Award from the National Science Foundation to assess how well the acoustic microfluidic devices he develops work by using active, swimming algae cells as measurement probes. CAREER awards support junior faculty who model the role of teacher-scholar through outstanding research, excellence in education and the integration of education and research within the context of the mission of their organization. One-third of current McKelvey Engineering faculty have received the award.

Acoustofluidics combines acoustics, or sound, with fluid mechanics. Meacham will build on existing work to develop a new technique to characterize the acoustic pressure field in his microscale acoustofluidic devices.

"Research-scale demonstrations of these acoustic microfluidic technologies have shown a lot of promise for the biological and biomedical sciences, but they're not yet common in clinical and industrial environments, partly because their operation is inconsistent," Meacham said. "While computer models can be used to improve designs, devices often don't perform as expected. It's hard to assess and compare real-world performance of different devices experimentally, so that's where we come in."

Like the regular patterns of peaks and valleys that can form when two school children shake the ends of a jump rope in sync, the acoustic wave fields in microfluidic channels create regular patterns when the device is shaken, Meacham said. This behavior, termed the device's harmonic response, is well understood for channels with simple rectangular or circular shapes; however, it is challenging to predict for devices with complicated geometry.

Meacham's lab will perform experiments with swimming cells of the algae Chlamydomonas reinhardtii to overcome this challenge. They will insert the cells into the device then gradually vary the frequency of device vibration while observing how the cells respond. At different frequencies, the cells group together to form various shapes. In between these frequencies, they return to swimming randomly as the frequency continuously changes. For example, cells can form one, two, three or more straight lines within a rectangular channel and bullseye-like patterns in a circular chamber. The team will record this data, then use results from these simple geometries to determine the relationship between the cell distribution that they observe and the field shape and strength, which are key metrics of device performance. Finally, the method will be applied to more complex microfluidic devices.

"To our knowledge, there's no other way to continuously assess the performance of any device over such a large frequency band," Meacham said. "Not only does it allow you to find these individual best peaks, or resonances, but it also shows you other non-ideal resonances of the system so those conditions can be avoided."

Meacham compared his work to ultrasound imaging.

"In ultrasound imaging, we send acoustic waves into tissue, then the waves hit tissue with different properties and reflect back, forming an image based on this contrast," he said. "We use the same type of equipment you might use for ultrasound imaging, but instead of sending an acoustic wave into tissue and reading the reflection, we send the vibrational wave into a microfluidic device, and it shakes the whole device. That transfer of energy travels off of the walls of the fluidic channels, reflecting back and forth and creating a standing wave. The acoustic wave field sees the swimming cells due to their contrast with the fluid, and the cells feel a force that can be used to move or hold them in place."

Meacham will share his findings through educational materials and experiments on vibrations, waves and resonances developed for area students in Kindergarten through high school in collaboration with the university's Institute for School Partnership. In addition, he plans to develop a new Engineering course in physical acoustics and another in which students design and create custom musical instruments in the Spartan Light Metal Products Makerspace in Henry A. & Elvira H. Jubel Hall.

"I want to have students learn how instrument shapes, sizes and types of designs affect the music you can make, much like the swimming cells teach us how these aspects affect the frequencies, or pitches, at which our microfluidic devices operate best," he said.

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