Diffusiophoresis at the macroscale

Diffusiophoresis, a ubiquitous phenomenon that induces particle transport whenever solute concentration gradients are present, was recently observed in the context of microsystems and shown to strongly impact colloidal transport (patterning and mixing) at such scales. In the present work, we show experimentally that this nanoscale mechanism can induce changes in the macroscale mixing of colloids by chaotic advection. Rather than the decay of the standard deviation of concentration, which is a global parameter commonly employed in studies of mixing, we instead use multiscale tools adapted from studies of chaotic flows or intermittent turbulent mixing: concentration spectra and second and fourth moments of the probability density functions of scalar gradients. Not only can these tools be used in open flows, but they also allow for scale-by-scale analysis. Strikingly, diffusiophoresis is shown to affect all scales, although more particularly the small ones, resulting in a change of scalar intermittency and in an unusual scale bridging spanning more than seven orders of magnitude. By quantifying the averaged impact of diffusiophoresis on the macroscale mixing, we explain why the effects observed are consistent with the introduction of an effective P\'eclet number.Comment: 13 page

Fabrication and Flow Dynamics Analysis of Micromixer for Lab-on-a-Chip Devices

The miniaturized systems designed for lab-on-a-chip (LOC) technologies are generally implemented with a micro-scale mixer to provide intimate contact between the reagent molecules for interactions and chemical reactions. The exponential increase of research in microfabrication and microfluidic applications highlights the importance of understanding the theory and mechanism that governs mixing at the microscale level. In this study, the fabrication of an active and passive micromixer was discussed. The optimized state of art soft lithography and 3D printing was used as a microfabrication technique. The challenges at different fabrication steps were presented along with the modifications. Microelectrodes were integrated with the active microfluidic mixer to create an electrokinetic effect. The fluid flow field inside the micromixerwas characterized by the Micro Particulate Image Velocimetry (Micro-PIV) system. Besides, numerical simulations were performed on 2D and 3D micromixers. Finally, results obtained in experiment and numerical simulations were analyzed to get a better understanding of the micromixer design

Simulations and Experimental Analysis of High-Aspect-Ratio Diffusive Micro-Mixers

Passive (diffusional) mixing has been used in designing high-aspect-ratio micro-mixers for the purpose of performing the Liagase Detection Reaction (LDR). A simple model was used to design such mixers optimized for pressure drop or time required to deliver a prescribed volume of mixture. The types of mixers considered are simple, cheap, and durable and can perform over a broad range of volumetric flow rates at reasonably modest pressure drops. The fluids typically have a very low diffusion coefficient of=1.2x10^10m^2/s, and thus diffusional mixing can only be effective in high-aspect-ratio micro-channels. A realizable aspect ratio of 6 has been considered initially because it is easily releasable using the LIGA technique. Numerical simulations were performed on various diffusional-based micromixer configurations. Two variants of a Y-type mixer with contraction and several variants of a mixer employing jets in cross-flow have been simulated. The various mixers have been evaluated in terms of volumetric mixing efficiencies and maximum pressure drops. One of the mixers with jets-in-cross-flow was found to perform best. In addition, the effect of jet width and expansion after the mixing were assessed. Experimental validations for the jets-in-cross-flow mixer were performed. The mixer was manufactured using a micromilled brass mold insert hot embossed into a Polymethyl-methacrylate (PMMA) substrate, which was then covered with 0.125mm PMMA coverslip. A chemiluminescence technique was applied for the first time to make Qqualitative observations of the mixing zones. Quantitative mixing efficiency experiments were performed by using Rhodamine B fluorescent dye solution and de-ionized water. The experimental results show good agreement with numerical simulations