Diffusive–layered convection in Y-shaped continuous-flow microreactor

Abstract

Efficient and rapid mixing of fluids in continuous-flow microfluidic devices plays an important role in a wide range of applications. Although there are many different methods to enhance mixing in such reactors, the potential of natural convection to mix fluids has been overlooked because of its possible limitations at the microscale. Recent studies have challenged this statement by demonstrating the significance of both gravity-dependent and gravity-independent types of natural convection in improving mixing efficiency. It was shown that inducing double-diffusive convection, Marangoni convection, or Rayleigh–Taylor convection can greatly enhance the mixing of two aqueous solutions in a continuous-flow microreactor. In this study, we experimentally investigate the influence of another type of differential-diffusion instability, known as diffusive–layered convection, on the mixing of two aqueous solutions of chemically inert substances in a Y-shaped continuous-flow microreactor. We consider the scenario when two miscible solutions flow into the main channel through distinct tubes, in which they come into contact and mix, thus causing the onset of diffusive–layered convection. We used two experimental methods, including laser interferometry and dye visualization, to analyse the mixing process. Surprisingly, despite the vigorous convective motion driven by diffusive–layered convection, the intensity of mixing of pumped fluids remains comparable to the scenario when solutions mix only via a diffusion mechanism. We explain this unexpected result by the unique structure of the convective motion, which prevents system homogenization. A comparison with the results previously obtained for double-diffusive convection is performed, and pure diffusive mixing scenarios are implemented.