Rapid, Self-driven Liquid Mixing on Open-Surface Microfluidic Platforms

Abstract

Self-driven surface micromixers (SDSM) relying on patterned-wettability technology provide an elegant solution for low-cost, point-of-care (POC) devices and lab-on-a-chip (LOC) applications. We present a SDSM fabricated by strategically patterning three wettable wedge-shaped tracks onto a nonwettable, flat surface. This SDSM operates by harnessing the wettability contrast and the geometry of the patterns to promote mixing of small liquid volumes (µL droplets) through a combination of coalescence and Laplace pressure-driven flow. Liquid droplets dispensed on two juxtaposed branches are transported to a coalescence station, where they merge after the accumulated volumes exceed a threshold. Further mixing occurs during capillary-driven, advective transport of the combined liquid over the third wettable track. Planar, non-wettable "islands" of different shapes are also laid on this third track to alter the flow in such a way that mixing is augmented. Several SDSM designs, each with a unique combination of island shapes and positions, are tested, providing a greater understanding of the different mixing regimes on these surfaces. The study offers design insights for developing low-cost surface microfluidic mixing devices on open substrates.Microfluidic devices capable of achieving complex liquid-handling tasks, such as transport, metering, separation and mixing, have found niche applications in numerous fields, including point-of-care (POC) diagnostics 1 , lab-on-a-chip (LOC) applications 2-5 , and micro total analysis systems (μTAS) 6 . Although most conventional microfluidics devices have historically deployed flow-through systems, a more recent strategy of handling liquid samples in the form of discrete droplets has gained popularity 7 . Droplet-based microfluidics is advantageous over flow-through microfluidics, since the liquid sample handling in the former is relatively free from the common problems of the latter, such as axial dispersion, sample dilution, and cross-contamination 8, 9 . In droplet-based microfluidics, individual (or sequences of) droplets of the sample liquid may be handled either within an immiscible liquid in a closed microchannel 10 or on open surfaces 11 . The dispensed liquid volumes range from 1 μL to 1 mL; the lower range is common to many microfluidic applications 12 , while the larger volumes are relevant to on-chip liquid storage 13 , or some specialized microfluidic applications that require larger samples (e.g. whole-blood assays 14,15 ). Open-surface type microfluidic devices offer the possibility of low-cost fabrication -these devices can be built on low-cost, paper or plastic surfaces and do not require elaborate fabrication of embedded microchannels -and hence, are ideally suited for POC diagnostics 16 .Like the flow-through microfluidic devices, rapid and efficient mixing is also an essential pre-requisite for open-surface microfluidic platforms. Open-surface micromixers may be of active or passive type, depending on whether external energy input is required or not, respectivel...

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