Microfluidic devices have attracted much interest in the fields of biology, biotechnology, and analytical and synthetic chemistry with applications as varied as protein crystallization, analyte diagnostics, cytometry, and combinatorial chemistry. These miniaturized fluidic systems have many advantages over their macroscale equivalents and have made feasible the integration of multiple processes on one device – the so-called lab on a chip or micro total analysis system. Many attempts have been made to characterize mixing performance in microfluidic systems with a view to optimizing their design and operation. Both flow and concentration mapping have typically been achieved by using optical methods. Despite the achievements of workers using optical techniques, there remain some inherent limitations of these methods. For example, the applicability of optical methods is limited to systems that have been fabricated and sealed with optically transparent materials. Optical methods are also often limited with respect to the type of device geometry that can be studied. More recently, researchers have begun to explore MRI as a tool for the study of microscale systems^1,2,3.

In this work, MRI has been used for the first time to obtain both cross-sectional velocity and concentration maps of flow through an optically opaque Y-shaped microfluidic sensor. Images of 23 μm × 23 μm resolution were obtained for a channel of rectangular cross section (250 μm × 500 μm) fed by two square inlets (250 μm × 250 μm). Both miscible and immiscible liquid systems have been studied. These include a system in which the coupling of flow and mass transfer has been observed, as the diffusion of the analyte perturbs local hydrodynamics. Our motivation for developing MRI tools for the study of microchannels extends beyond experimental visualization of mixing performance. The data thereby obtained can be used in the validation of numerical codes for simulating micromixing processes. Such codes will play a key role in the design and optimization of microfluidic systems. Images presented in this work have been used by Sullivan et al.⁴ to validate a lattice-Boltzmann code capable of simulating coupled diffusive mass transfer and hydrodynamics in 3D.        

References:

(1) C. Hilty et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 14960. (2) E. Harel et al. Phys. Rev. Lett. 2007, 98, 017601. (3) S. Ahola et al. Lab Chip 2006, 6, 90. (4) S.P. Sullivan et al. Sens. Actuators, B: Chem. 2007, 123, 1142.