  Solvent exchange (also called solvent shifting or Ouzo effect) is a generally
used bottom-up process to mass-produce nanoscale droplets. In this process, a
good solvent for some oil is displaced by a poor one, leading to oil
nanodroplet nucleation and subsequent growth. Here we perform this process on a
hydrophobic substrate so that sessile droplets so-called surface
nanodroplets-develop, following the work of Zhang et al. [Zhang, X.; Lu, Z.;
Tan, H.; Bao, L.; He, Y.; Sun, C.; Lohse, D. Proc. Natl. Acad. Sci. U.S.A.
2015, 122, 9253-9257]. In contrast to what was done in that paper, we chose a
very well-controlled Hele-Shaw geometry with negligible gravitational effects,
injecting the poor solvent in the center of the Hele-Shaw cell, and
characterize the emerging nanodroplets as a function of radial distance and
flow rates. We find that the mean droplet volume per area <Vol>_area strongly
depends on the local Peclet number Pe and follows a universal scaling law
<Vol>_area~Pe^(3/4). Moreover, the probability distribution function of the
droplet volume strongly depends on the local Pe as well, regardless of the flow
rates and radial distance, giving strong support to the theoretical model of
the solvent exchange process developed in Zhang et al.'s work.
