Influence of temperature on linear stability in buoyancy-driven fingering of reaction-diffusion fronts

Analysis of a microfluidic device for diffusion coefficient determination of high molecular weight solutes detectable in the visible spectrum

We developed a procedure to measure diffusion coefficients using microfluidic devices that contributes to the transport analysis of high molecular weight solutes with low diffusion coefficient. This procedure allows a quick determination of diffusion coefficients and a precise evaluation of measurement errors. Making use of color variation of a pH indicator, we determined its diffusion coefficient in its own solvent (water). The value obtained was compared with previously published ones and was found to be similar to those cited. The microfluidic device has a serpentine-shaped channel that allows monitoring the solution evolution in different regions of the path in a single visual field without the need to move the camera or the microchip. This kind of device also allows the spatial and temporal tracking of the diffusion process. The solution color intensity is used to determine solute concentration; therefore, this method presents an advantage compared to those based on fluorescence detection. A complete analysis of the diffusive behavior along the channel path was performed in order to test the accuracy of these kinds of methodologies. This analysis can be used with similar devices, and the techniques employed for diffusion analysis can be applied to a µTAS-type microfluidic platform, allowing obtain variations of the diffusion coefficient as a function of time due to variations in external factors, e.g., temperature, etc.

Introduction to the focus issue: chemo-hydrodynamic patterns and instabilities

Pattern forming instabilities are often encountered in a wide variety of natural phenomena and technological applications, from self-organization in biological and chemical systems to oceanic or atmospheric circulation and heat and mass transport processes in engineering systems. Spatio-temporal structures are ubiquitous in hydrodynamics where numerous different convective instabilities generate pattern formation and complex spatiotemporal dynamics, which have been much studied both theoretically and experimentally. In parallel, reaction-diffusion processes provide another large family of pattern forming instabilities and spatio-temporal structures which have been analyzed for several decades. At the intersection of these two fields, "chemo-hydrodynamic patterns and instabilities" resulting from the coupling of hydrodynamic and reaction-diffusion processes have been less studied. The exploration of the new instability and symmetry-breaking scenarios emerging from the interplay between chemical reactions, diffusion and convective motions is a burgeoning field in which numerous exciting problems have emerged during the last few years. These problems range from fingering instabilities of chemical fronts and reactive fluid-fluid interfaces to the dynamics of reaction-diffusion systems in the presence of chaotic mixing. The questions to be addressed are at the interface of hydrodynamics, chemistry, engineering or environmental sciences to name a few and, as a consequence, they have started to draw the attention of several communities including both the nonlinear chemical dynamics and hydrodynamics communities. The collection of papers gathered in this Focus Issue sheds new light on a wide range of phenomena in the general area of chemo-hydrodynamic patterns and instabilities. It also serves as an overview of the current research and state-of-the-art in the field.