How does viscosity affect the speed of a diffusion-controlled reaction?

In a diffusion-controlled reaction, the speed of the chemical process is limited by the rate at which reactant molecules encounter one another through the process of diffusion. Because these reactions depend on the physical movement of molecules to bring them together, the viscosity of the medium plays a critical role in determining the reaction rate.1

The relationship is inverse: as the viscosity of the medium increases, the ability of molecules to move through the fluid decreases, making it more difficult for them to collide and react. Consequently, a more viscous or "thick" medium results in a slower reaction rate, while a less viscous medium facilitates faster diffusion and, therefore, a faster reaction. In many liquid solutions, this relationship is observed through the product of the diffusivity ($D$) and viscosity ($\eta$), where the product $D\eta$ remains nearly constant at a constant temperature.12