  Solvent exchange is a simple method to produce surface nanodroplets on a
substrate for a wide range of applications by displacing a solution of good
solvent, poor solvent and oil (Solution A) by a poor solvent (Solution B). In
this work, we show that the growth and coalescence of nanodroplets on a
homogeneous surface is mediated by the viscosity of the solvent. We show that
at high flow rates of viscous Solution B, the final droplet volume deviates
from the scaling law that correlates final droplet volume to the flow rate of
non-viscous Solution B, reported in previous work. We attribute this deviation
to a two-regime growth in viscous Solution B, where transition from an initial,
fast regime to a final slow regime influenced by the flow rate. Moreover,
viscous solution B hinders the coalescence of growing droplets, leading to a
distinct bimodal distribution of droplet size with stable nanodroplets, in
contrast to a continuous size distribution of droplets in non-viscous case. We
demonstrate that the group of small droplets produced in high viscosity
environment may be applied for enhanced fluorescence detection with higher
sensitivity and shorter response time. The finding of this work can potentially
be applied for mediating the size distribution of surface nanodroplets on
homogeneous surface without templates.
