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International audience; In flow-coating processes at low substrate velocity , solvent evaporation occurs during the film withdrawal and the coating process yields directly a dry deposit. In this regime, often referred to as the evaporative regime, several works performed on blade-coating-like configurations have reported a deposit thickness h d proportional to the inverse of the substrate velocity V. Such a scaling can be easily derived from simple mass conservation laws, assuming that evaporation occurs on a constant distance, referred to as the evaporation length, noted L ev in the present paper and of the order of the meniscus size. However the case of colloidal dispersions deserves further attention. Indeed, the coating flow leads to a wet film of densely-packed colloids before the formation of the dry deposit. This specific feature is related to the porous nature of the dry deposit, which can thus remain wet when capillary forces are strong enough to prevent the receding of the solvent through the pores of the film (the so-called pore-emptying). The length of this wet film may be possibly much larger than the menis-cus size, therefore modifying the solvent evaporation rate, as well as the scaling h d ∼ 1/V. This result was suggested recently by different groups using basic modeling and assuming for simplicity an uniform evaporation rate over the wet film. In this article, we go a step further and investigate the effect of multi-dimensional vapor mass transfer in the gas phase on L ev and h d in the specific case of colloidal dispersions. Using simplified models, we first provide analytical expressions in asymptotic cases corresponding to 1D or 2D diffusive vapor transport. These theoretical investigations then led us to show that L ev is independent of the evaporation rate amplitude, and roughly independent of its spatial distribution. Conversely, h d strongly depends on the characteristics of vapor mass transfer in the gas phase, and different scaling laws are obtained for the 1D or the 2D case. These theoretical findings are finally tested by comparison with experimental results supporting our theoretical simplified approach.

Shape-controlled synthesis of uniform noble metal nanoparticles (NPs) is crucial for the development of future plasmonic devices. The use of nanocrystals with well-defined morphologies and crystallinity as seed particles is expected to provide excellent shape control and monodispersity. We report the aqueous-based seed-mediated growth of monodisperse gold octahedra with wide range of sizes (50–150 nm in side length) by reducing different amounts of HAuCl4 on preformed single crystalline gold nanorods using butenoic acid as reducing agent. Butenoic acid plays a key role as a mild reducing agent as well as favoring the thermodynamic control of the reaction. The uniformity of the as-prepared Au octahedra combined with the use of a microfluidic technique based on microevaporation will allow the self-assembly of octahedra into uniform 3D supercrystals. Additionally, these plasmonic substrates exhibit high and uniform SERS signals over extended areas with intensities increasing with the Au nanoparticle size.

We investigate the dynamics of solidification of a charged colloidal dispersion using an original microfluidic technique referred to as micropervaporation. This technique exploits pervaporation within a microfluidic channel to extract the solvent of a dilute colloidal dispersion. Pervaporation concentrates the colloids in a controlled way up to the tip of the channel until a wet solid made of closely packed colloids grows and invades the microfluidic channel. For the charged dispersion under study, we however evidence a liquid to solid transition (LST) preceding the formation of the solid, owing to the presence of long-range electrostatic interactions. This LST is associated with the nucleation and growth of domains confined in the channel. These domains are then compacted anisotropically up to forming a wet solid of closely packed colloids. This solid then invades the whole channel as in directional drying with a growth rate which depends on the microfluidic geometry. In the final steps of the solidification, we observed the occurrence of cracks and shear bands, the delamination of the wet solid from the channel walls, and its invasion by a receding air front. Interestingly, this air front follows specific patterns within the solid which reveal different microscopic colloidal organizations.

6 pages; International audience; This article reports a one-step approach for the fabrication of highly uniform, spherical Ag particles with tailored dimensions ranging from 10 to 30 nm. Coated with silica shell, the high uniformity of the particles allows their spontaneous assembly into millimeter-long extended 3-D arrays with transverse dimensions of tens of micrometers, using a microfluidic evaporation-based process.

In this work, we developed a microfluidic equivalent of the classical droplet method for investigating nucleation kinetics. Our microfluidic device allows us to store hundreds of droplets of small volume ( approximately 100 nL) and to accurately control their temperature. We also monitor directly all the stored droplets, and thus perform statistical measurements on a large number of nucleation events. In the case of aqueous solutions of potassium nitrate, we manage to investigate nucleation kinetics and polymorphs and quantify the influence of impurities. The use of small droplets is crucial in such experiments, since it allows the sample to reach high supersaturations and to separate all the nucleation events. Moreover, we compare our results to the classical nucleation theory, and we demonstrate unambiguously using direct observations of the droplets that nucleation in aqueous solutions of potassium nitrate always occurs using heterogeneous mechanisms.

Microfluidic devices face presently a tremendous interest, especially for the development of labs-on-a-chip systems. One of the primary challenges for such applications is the ability to perform local chemical detection and analysis from various species. In this paper, we investigate the use of confocal Raman spectroscopy from both qualitative and quantitative sides, to obtain spatially resolved concentration maps of chemically reactive fluids flowing in different channels networks. As a model chemical reaction, we used the isotopic exchange reaction between D(2)O and H(2)O, which is diffusion-controlled and whose equilibrium states exhibit distinct Raman signatures depending on the composition. Two types of chip technologies were studied, which are typical of those used for chemical kinetics investigations. In the first one, reagent mixing occurs by molecular interdiffusion of the two streams (H(2)O and D(2)O) flowing side by side in the same channel; in the second one, reagents are hosted in droplets moving in winding channels that enhance the mixing. In the first series of experiments, we were able to extract Raman images of H(2)O, D(2)O, and HOD concentrations in the main channel together with an estimate of an interdiffusion coefficient, and in the second one, we evidenced the influence of channel wiggles on mixing efficiency.

In this work, a viscosimeter implemented on a microfluidic chip is presented. The physical principle of this system is to use laminar parallel flows in a microfluidic channel. The fluid to be studied flows side by side with a reference fluid of known viscosity. By using optical microscopy, the shape of the interface between both fluids can be determined. Knowing the flow rates of the two liquids and the geometrical features of the channel, the mean shear rate sustained by the fluid and its viscosity can thus be computed. Accurate and precise measurements of the viscosity as a function of the shear rate can be made using less than 300 microL of fluid. Several complex fluids are tested with viscosities ranging from 10(-)(3) to 70 Pa.s.