Strain rate effects on front propagation in advection reaction diffusion systems*

The growth of a reactive scalar in a flowing fluid is known as an advection-reaction-diffusion (ARD) system. In this talk I will focus on effects of flow strain rate (deformation) within ARD systems - specifically chemical systems - through experiments measuring front propagation. Fronts in this context are the borders of reacted regions. Fronts are often modelled as moving with velocity equal to the sum of flow and chemical front speed in stagnant fluid. We find strain rate to have profound effects in multiple contexts. For example, I will present how reactive mixing in a two-dimensional (2D) experimental flow does not behave like a 2D ARD system due to strain rate. We also perform experiments on reactions in the presence of straining flows to demonstrate how chemical front speed is altered, how bulk reaction is changed in turn, and how in special situations stirring can actually inhibit reaction. We identify various processes by which shear strain changes front speed, and make predictions about the size of these changes. These results may explain part of why mixing changes reactions and how flow sometimes creates reaction barriers.