  Compartmentalizing reagents within small droplets is promising for highly
efficient conversion and simplified procedures in many biphasic chemical
reactions. In this work, surface nanodroplets (i.e., less than 100 nm in their
maximal height) were employed to quantitatively understand the size effect on
the chemical reaction rate of droplets. In our systems, a surface-active
reactant in pure or binary nanodroplets reacted with the reactant in the bulk
flow. Meanwhile, the product was removed from the droplet surface. The
shrinkage rate of the nanodroplets was characterized by analyzing the lateral
size as a function of time, where the droplet size was solely determined by
chemical reaction rate at a given flow condition for the transport of the
reactant and the product. We found that the overall kinetics increases rapidly
with the decrease of droplets lateral radius R, as dR/dt ~ R^(-2). The faster
increase in the concentration of the product in smaller droplets contributes to
accelerating reaction kinetics. The enhancement of reaction rates from small
droplet sizes was further confirmed when a non-reactive compound presented
inside the droplets without reducing the concentrations of the reactant and the
product on the droplet surface. The results of our study improve the
understanding of chemical kinetics with droplets. Our findings highlight the
effectiveness of small droplets for the design and control of enhanced chemical
reactions in a broad range of applications.
