To miniaturize and automate various laboratory processes, some researchers have focused on manipulating microscale liquid droplets using several mechanisms, such as electrokinetics, acoustics, magnetism, and hydrodynamics. Such approaches could be used to perform logic operations akin to electrons in a digital processor and enable, for example, the small-volume, high-throughput analyses of cellular material. But one challenge for those microfluidic processors is the potential for a droplet to contaminate the one following it on a rewritable fluidic path. Peiran Zhang and Tony Jun Huang from Duke University and their colleagues have now solved the problem by using acoustic streaming vortices (ASVs)—a mechanism that applies multitonal electrical signals to control individual droplets without contact.
In the experimental setup shown below, the microfluidic droplets (red and blue spheres) travel within an oil layer above a piezoelectric substrate and interact with immersed interdigitated transducers (IDT, orange structures), which convert an applied electrical signal to surface acoustic waves. Previous efforts to carefully position droplets acoustically have been disrupted by ASVs created from the circulation of fluid jets hitting the air–oil interface.
In their work, Zhang and coworkers tune the various transducer frequencies to generate several spatially overlapping ASVs in a channel. A droplet is guided between two vortices to an equilibrium position that changes continuously as the electrical signal is varied. In the schematic, sets of transducers use different working frequencies so that a specific excitation voltage can be applied to guide a droplet to the left or the right.
The acoustofluidic process keeps the droplets contact-free above the substrate. With that advantage, the researchers guide one droplet after another along the same path without contamination. A future microfluidic processor based on an ASV design may help move cells and other biological samples around for various biomedical or biophysical applications. (P. Zhang et al., Sci. Adv. 6, eaba0606, 2020.)
