  We show the emergence of reaction hotspots induced by three-dimensional (3D)
vortices with a simple $A+B \rightarrow C$ reaction. We conduct microfluidics
experiments to visualize the spatial map of the reaction rate with the
chemiluminescence reaction and cross-validate the results with direct numerical
simulations. 3D vortices form at spiral saddle type stagnation points, and the
3D vortex flow topology is essential for initiating reaction hotspots. The
effect of vortices on mixing and reaction becomes more vigorous for
rough-walled channels, and our findings are valid over wide ranges of channel
dimensions and Damk\"{o}hler numbers.
