Speeding up biphasic reactions with surface nanodroplets

TL;DR

This study combines experimental and theoretical approaches to accelerate biphasic reactions on nanodroplet surfaces by analyzing flow rate and reactant concentration effects, revealing key scaling laws and interaction effects.

Contribution

It introduces a combined experimental and theoretical framework to understand and optimize biphasic reactions on nanodroplets under flow conditions, highlighting the impact of flow and product interactions.

Findings

Reaction rate scales with flow Peclet number as Pe^{-3/2}.

Reaction lifetime inversely proportional to reactant concentration.

Product from upstream reactions can delay downstream droplet reactions.

Abstract

Biphasic chemical reactions compartmentalized in small droplets offer advantages, such as streamlined procedures for chemical analysis, enhanced chemical reaction efficiency and high specificity of conversion. In this work, we experimentally and theoretically investigate the rate for biphasic chemical reactions between acidic nanodroplets on a substrate surface and basic reactants in a surrounding bulk flow. The reaction rate is measured by droplet shrinkage as the product is removed from the droplets by the flow. In our experiments, we determine the dependence of the reaction rate on the flow rate and the solution concentration. The theoretical analysis predicts that the life time $\tau$ of the droplets scales with Peclet number $Pe$ and the reactant concentration in the bulk flow $c_{re,bulk}$ as $\tau\propto Pe^{-3/2}c_{re,bulk}^{-1}$, in good agreement with our experimental results. Furthermore, we found that the product from the reaction on an upstream surface can postpone the droplet reaction on a downstream surface, possibly due to the adsorption of interface-active products on the droplets in the downstream. The time of the delay decreases with increasing $Pe$ of the flow and also with increasing reactant concentration in the flow, following the scaling same as that of the reaction rate with these two parameters. Our findings provide insight for the ultimate aim to enhance droplet reactions under flow conditions.