Significance Solvent exchange is a generally used approach for producing many nanoscale droplets on an immersed substrate. In this process, a good solvent is displaced by a poor solvent of oil, leading to oil nanodroplet nucleation and subsequent growth on the substrate. This work is, to our knowledge, the first attempt to quantitatively understand the relationship between the droplet size and the flow conditions during the solvent exchange, and to pave the way for the droplet size control. The experimental results show that the droplet volume increases with increasing Peclet number of the flow as ∝ P e 3 / 4 , in good agreement with our theoretical analysis. We also reveal that the buoyancy effects contribute to the formation of bigger and less homogeneously distributed droplets in less-narrow channels.