The objective of the paper is to present an optical technique that allows to characterize in situ concentration fields in microfluidic systems. The technique is based on the measurement of the refraction index in the liquid phase, in the immediate vicinity of the walls that confine the fluids. We call this method microchannel edge refractometry. The method is non-intrusive. We describe the refractometry technique, using theory, numerics and experiments. We show a resolution and sensitivity on the order of 3×10−3 and 2×10−3, respectively. We apply the technique to analyze diffusion phenomena in T sensors, measure supersaturations and establish phase diagrams of ternary systems in dedicated microdevices.

The overall traffic of droplets in a network of microfluidic channels is strongly influenced by the liquid properties of the moving droplets. In particular, the effective hydrodynamic resistance of individual droplets plays a key role in their global behavior. Here we propose two simple and low-cost experimental methods for measuring this parameter by analyzing the dynamics of a regular sequence of droplets injected into an "asymmetric loop" network. The choice of a droplet taking either route through the loop is influenced by the presence of previous droplets that modulate the hydrodynamic resistance of the branches they are sitting in. We propose to extract the effective resistance of a droplet from easily observable time series, namely, from the choices the droplets make at junctions and from the interdroplet distances. This becomes possible when utilizing a recently proposed theoretical model based on a number of simplifying assumptions. Here we present several sets of measurements of the hydrodynamic resistance of droplets, expressed in terms of a "resistance length." The aim is twofold: (1) to reveal its dependence on a number of parameters, such as the viscosity, the volume of droplets, their velocity as well as the spacing between them. At the same time (2), by using a standard measurement technique, we compare the limitations of the proposed methods. As an important result of this comparison, we obtain the range of validity of the simplifying assumptions made in the theoretical model.

When immiscible fluids flow in microchannels, monodisperse droplets or jet are obtained depending upon the flow rate of the aqueous phase and the oil phase. In this work, we present experimental data showing that the confinement and the shape of the geometry play a fundamental role. We analyze the stability of the jet in the framework of the lubrication at low Reynolds number. In cylindrical geometry, we relate the transition between the droplets regime and the jet regime to the absolute/convective transition of the Rayleigh Plateau instability and reach a remarkable agreement with the data. In rectangular geometry, we follow the same analysis and point out the existence of an absolutely stable bidimensional jet. To cite this article: P Guillot et al., C R. Chimie 12 (2009). (C) 2008 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

In a previous paper we presented a way to measure the rheological properties of complex fluids on a microfluidic chip (Guillot et al., Langmuir 22:6438, 2006). The principle of our method is to use parallel flows between two immiscible fluids as a pressure sensor. In fact, in a such flow, both fluids flow side by side and the size occupied by each fluid stream depends only on both flow rates and on both viscosities. We use this property to measure the viscosity of one fluid knowing the viscosity of the other one, both flow rates and the relative size of both streams in a cross-section. We showed that using a less viscous fluid as a reference fluid allows to define a mean shear rate with a low standard deviation in the other fluid. This method allows us to measure the flow curve of a fluid with less than 250 μL of fluid. In this paper we implement this principle in a fully automated set up which controls the flow rate, analyzes the picture and calculates the mean shear rate and the viscosity of the studied fluid. We present results obtained for Newtonian fluids and complex fluids using this set up and we compare our data with cone and plate rheometer measurements. By adding a mixing stage in the fluidic network we show how this set up can be used to characterize in a continuous way the evolution of the rheological properties as a function of the formulation composition. We illustrate this by measuring the rheological curve of four formulations of polyethylene oxide solution with only 1.3 mL of concentrated polyethylene oxide solution. This method could be very useful in screening processes where the viscosity range and the behavior of the fluid to an applied stress must be evaluated.

We have studied the phase behavior of the poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymer in a mixture of two miscible solvents, water and tetrahydrofuran (THF). The techniques used to examine the different polymers, structures and phases formed in mixed solvents were static and dynamic light scattering, small-angle neutron scattering, nuclear magnetic resonance and fluorescence microscopy. By lowering the water/THF mixing ratio X, the sequence unimers, micron-sized droplets, polymeric micelles was observed. The transition between unimers and the micron-sized droplets occurred at X = 0.75, whereas the microstructuration into core-shell polymeric micelles was effective below X = 0.4. At intermediate mixing ratios, a coexistence between the micron-sized droplets and the polymeric micelles was observed. Combining the different aforementioned techniques, it was concluded that the droplet dispersion resulted from a solvent partitioning that was induced by the hydrophobic blocks. Comparison of poly(n-butyl acrylate) homopolymers and poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymers suggested that the droplets were rich in THF and concentrated in copolymers and that they were stabilized by the hydrophilic poly(acrylic acid) moieties.

This work describes a new microfluidic device developed for the rapid screening of solubility diagrams. In several parallel channels, hundreds of nanolitre volume droplets of a given solution are first stored with a gradual variation in the solute concentration. Then, the application of a temperature gradient along these channels enables us to read directly and quantitatively phase diagrams, concentration vs. temperature. We show, using a solution of adipic acid, that we can measure ten points of the solubility curve in less than 1 hr and with only 250 microL of solution.

We have developed an original microfluidic system to study fast kinetics of temperature-dependent processes in an emulsion. Using flow focusing geometries [Anna et al., 2003], aqueous droplets are continuously formed in an oil flow. These droplets, acting as microreactors (100 nL), contain the solution to be investigated, and are formed at high temperature. They flow in a microchannel to a cooled area through a controlled temperature gradient (typically from 60 to 10 degrees C in a few seconds). Along the microchannel, the distance being equivalent to the time thanks to the use of droplets, the kinetics of the process can be followed (from 10 to 300 s). In particular the microdevice has been used to study the kinetics of crystal nucleation of a solute dispersed in water after a temperature quench.