Microfluidics Nanofluidics manuscript No. (will be inserted by the editor) Quantitative comparison of thermal and solutal transport in a T-mixer by FLIM and CFD David-A. Mendels 1 ⋆ , Emmelyn M. Graham 2,3 , Steven W. Magennis 2 , Anita C. Jones 2 , Franc¸ois Mendels 4 1 National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK 2 Collaborative Optical Spectroscopy, Micromanipulation and Imaging Centre (COSMIC) and the School of Chemistry, The University of Edinburgh, King’s Building, Edinburgh EH9 3JZ, UK 3 present address: TUV NEL, East Kilbride, Glasgow G75 0QU, UK 4 Cognoscens, 35 Crs d’Herbouville, 69004 Lyon, F The date of receipt and acceptance will be inserted by the editor Abstract The development and adoption of lab-on-a-chip and micro-TAS (total analysis system) techniques requires not only the solving of design and manufacturing issues, but also the introduction of reliable and quantitative methods of analysis. In this work, two complementary tools are applied to the study of thermal and solutal transport in liquids. The experimental determination of the concentration of water in a water-methanol mix- ture and of the temperature of water in a microfluidic T-mixer are achieved by means of fluorescence lifetime imaging microscopy (FLIM). The results are compared to those of finite volume simulations based on tabulated properties and well-established correlations for the fluid properties. The good correlation between experimental and modelled results demonstrate without ambiguity that i- the T-mixer is an adiabatic system, ii- buoyancy ef- fects influence the mixing of liquids of different densities at moderate flow rates, and iii- the combination of FLIM and computational fluid dynamics (CFD) has the potential to be used to measure the thermal and solutal diffusion coefficients of fluids for a range of tem- peratures and concentrations in one single experiment. As such, it represents a first step towards the full-field monitoring of both the extent and the kinetics of a chemical reaction. 1 Introduction Over the past ten years, the application of micro-manufacturing technologies to the produc- tion of fluidic devices have led the rapid development of now widely accepted micro-fluidic devices. As it is often the case, the early promises - emphasizing cheaper, faster, and more reliable devices with smaller volumes of liquids - have only been partially fulfilled [1,2]. Manufacturing and design intricacies are issues that micro-fluidics share with their more mature parent technology, micro-electromechanical systems (MEMS). Despite recent ad- vances in terms of computer assisted design (CAD) and modelling integration (a growing number of finite volume and finite element packages commercially available), the scien- tific community has only made a limited use of advanced computational tools to optimise the design and operation of micro-fluidic devices [3]. The reasons are numerous, and the lack of quantitative comparisons and correlations between microfluidic experiments and simulation results is certainly an important factor. In this work, quantitative comparisons between fluorescence lifetime imaging microscopy (FLIM) and computational fluid dy- namics (CFD) results are established for the basic case of mixing in a T-junction. ⋆ To whom correspondence should be addressed, e-mail: david.mendels@npl.co.uk