In microfluidics, the efficient and reliable sorting of small particles by size and other properties is a challenge encountered in cell sorting, genetic cell assays, or lab-on-a-chip devices handling droplets and suspensions. The objective of the research is to develop a sorting device that is (i) passive, i.e., it does not rely on active feedback from the particles; (ii) tunable, i.e., a critical particle size can be selected at will; and (iii) does not compromise an efficient throughput through the device by introducing obstacles. The centerpiece of this device is an oscillating microbubble, driven by ultrasound, which establishes a steady streaming flow field that can be superimposed with an external flow through the device. By varying flow rate and ultrasound amplitude, the device is tuned to a critical size of particle: if a particle's diameter is below this value, it will pass through the device; if it is above this value, its trajectory will (passively) deflect it off the bubble to be collected in a secondary flow circuit. No physical constriction is introduced into the channel, and the particles are separated where the flow is fastest, not where it is slowest, as in traditional passive sorting devices. The same device can also be applied to sort particles by deformability, including macromolecules with conformational changes.
Among laminar flows on the microscale, the proposed bubble streaming flows show fundamentally novel qualities. They demonstrate the concept of virtual flow confinement: narrow conduits for cargo-laden fluid are produced that neither slow the flow down nor lead to excessive shear forces on the cargo. Moreover, these flows are under quantitative, non- invasive control from easily adjustable external parameters and show size specificity on scales much smaller than any scale of soft-lithography manufacturing. Using macromolecules in the set-up gives new fundamental insight into the dynamics of coiled or stretched macromolecules (such as DNA) and the transition between coiled and stretched states.
The broader impact of the work is societal and educational: Crucially important applications profit from improved-throughput, size-selective microfluidic particle transport, including cytometry and cell sorting, assays of cell deformability in diagnostics of cancer and blood diseases, or purification and enrichment of biomedical samples. The classification and purification of macromolecules is likewise of enormous importance in biotechnology, in genetic assays of DNA, or in the efficient production of pure proteins, a major challenge that at present makes many therapeutic proteins prohibitively expensive. The interdisciplinary nature of the work will attract undergraduate students from both bioengineering and engineering science backgrounds, who will form a research team together with the graduate student on this project, analyzing experimental data and helping with simulations. This project will be connected to existing interdisciplinary initiatives at Illinois, such as the new In3 Innovation Initiative and K-12 outreach, through the development of both focused research projects for university student teams and demonstration projects for high-school students.
