Fluorescence microscopy is an essential tool that microfluidics relies on heavily.  Classical bench top microscopes, which allow high quality optics, usually in epifluorescence arrangements, are significantly costly (approximately $10, 000 – 1,000, 000 AUS), have a large mass and size, and therefore practically no portability. While this is still satisfactory for the majority of users, the general trend in instrumentation is to aim for smaller size, portability and mass affordability. In addition, in specific cases microfluidic devices may be attached to large or for other reasons non-portable devices (e.g. complex fluidic designs, MS and other non-portable detection or other equipment). Therefore, we investigated the options available using off-the shelf low-cost instrumentation options for portable low-cost fluorescence microscopy using a commercially available USB microscope.

A lightweight small-size portable USB fluorescence microscope equipped with 570 nm excitation LEDs and a 520 nm emission filter aimed at usage with fluorescein or similar fluorophores was applied for the visualisation of microfluidic experiments. Three microfluidic chip designs were fabricated using a photopolymerisation-based Miicraft® 3D printer with clear PMMA polymer and experiments visualised using the USB microscope as follows: (1) In a liquid-liquid extraction device through aqueous droplet formation in a flow of immiscible organic phase, fluorescent images at the beginning and ending sections of the extraction channel were taken by the USB microscope. An aqueous phase containing a mixture of two fluorescent dyes, coumarin 334 (green) and rhodamine B (red), was imaged, as well as the organic phase (decanol at 1 μL/min). While coumarin 334 is extracted into decanol, rhodamine B remains in the aqueous droplet. (2) In a microfluidic mixer device printed according to a previously used design with 500 µm wide and deep channels, the mixing performance was visualised using aqueous solutions of 0.1 μg/mL fluorescein (green) and 1 μg/mL rhodamine B (red) at a flow rate of 100 μL/min each. (3) In another novel microfluidic mixer device incorporating interstitial triangular mixing elements, the mixing performance was visualised in this experiment using solutions of Rhodamine B and Rhodamine 6G (0.01 mg/ml in methanol). Further numerical simulations were run in ANSYS CFX computational simulation software and compared the computed with the experimental results. A mixing efficiency of over 99% was achieved, confirming the experimental data.