Multi-parameter quantitative mapping of microfluidic devices

Abstract

Fluorescence lifetime imaging microscopy (FLIM) is a powerful technique to non-invasively map the physical and chemical environment within microfluidic devices. In this work FLIM has been used in conjunction with a variety of other techniques to provide a greater insight into flow behaviour and fluid properties at the microscale. The pH-sensitive fluorescent dyes, fluorescein and C-SNARF 1, have been used to generate pH maps of microfluidic devices with a time-gated camera and a time-and-space-correlated single photon counting (TSCSPC) detector, respectively. Using time-gated detection and fluorescein, the fluorescence lifetime images allow for direct reading of the pH. The relative contribution to fluorescence of the acid and basic forms of C-SNARF 1 was spatially resolved on the basis of pre-exponential factors, giving quantitative mapping of the pH in the microfluidic device. Three dimensional maps of solvent composition have been generated using 2-photon excitation FLIM (2PE-FLIM) in order to observe the importance of gravitational effects in microfluidic devices. Two fluidic systems have been studied: glycerol concentration in the microfluidic device was measured using Kiton red; water concentration in a methanolic solution was measured using ANS. The density mismatch between two solutions of different composition induced a rotation of the interface between two streams travelling side by side in a microchannel. The experiment has provided evidence of non-negligible gravitational effects in microflows. 2PE-FLIM has superior capability than methods used previously to assess similar phenomena. FLIM and micro-particle imaging velocimetry (μ-PIV) have been implemented on a custom-built open frame microscope and used simultaneously for multimodal mapping of fluid properties and flow characteristics. It has been shown that viscosity mismatch between two streams induces a non-constant advective transport across the channel and results in a flow profile that deviates from the usual Poiseuille profile, characteristic of pressure driven flow in microfluidic devices.