Microeddies as microfluidic elements: Reactors and cell traps

Microfluidic applications generally seek to control fluids, reagents, and objects at the microscale, and the development of individual components to either mimic traditional processes or to realize novel processes remains important to development in the field. This work focuses on microscopic acoustic streaming eddies as hydrodynamic microreactors and traps for microscopic objects including motile cells.; Four microeddies were created around a stationary cylinder (radius 406 μm) by oscillating the surrounding fluid (audible frequency). Concentration images measured using Raman spectroscopy show that eddies act as hydrodynamic “vessels” for reagents dosed from the cylinder (an electrode), and the oscillation amplitude and reagent dosing rate quantitatively controlled the eddy composition. These “vessels” were used to quantify the antioxidant properties of vitamin C against an electrogenerated oxidant. Material balances over the eddy yield a reactor model identical to a two-input CSTR (i.e., perfect backmixing model); and the mean reactor residence time, Damköhler number, and reagent feed ratio are quantitatively related to eddy properties. The CSTR model fit to data for a range of reactor conversions gives the homogeneous rate constant for vitamin C oxidation, showing that the composition of microeddy reactors can be controlled quantitatively.; The cylinder and oscillating fluid were incorporated into microscale channels to provide a route to integration with more conventional microfluidic applications. Detailed flow measurements describe the three-dimensional acoustic streaming flow structure, and theory relates measured flow features to frequency and geometry through simple scaling. These channel-based microeddies show an impressive ability to trap microscopic objects at fixed positions in three-dimensions. Microeddies formed in a microchannel (425 μm depth) collect and trap motile phytoplankton (P. micans) and microspheres (∼20–0 μm diameter). The trap strength (≤50 pN) is controlled via the oscillation amplitude, and estimated shear stress (≤1 N/m2) is appropriate for many cells (comparable to arterial shear stress). Trapped objects are surrounded by fluid, and traps are generally applicable to any typical cell medium.; In all, this work demonstrates a novel approach to controlling fluids, reagents, and objects at the microscale. The quantitative reagent control and trapping ability could allow treatment and dynamic analysis of single cells.