Step-emulsification in nanofluidic device

TL;DR

This paper investigates the step-emulsification process in nanofluidic devices, combining theory and experiments to understand droplet formation and transition conditions for high-throughput production of monodisperse microdroplets.

Contribution

It introduces a Hele-Shaw based theoretical model for droplet shape and transition conditions, validated by experiments, for nanofluidic step-emulsification devices.

Findings

The theory accurately predicts the critical capillary number for transition.

The droplet size formula matches experimental data well.

The device enables high-throughput production of uniform microdroplets.

Abstract

In this paper we present a comprehensive study of the step-emulsification process for high-throughput production of (sub-)$\mu$m-size monodisperse droplets. The microfluidic device combines a Hele-Shaw nanofluidic cell with a step-like outlet to a deep and wide reservoir. The proposed theory based on Hele-Shaw hydrodynamics provides the quasi-static shape of the free boundary between the disperse liquid phase engulfed by the co-flowing continuous phase prior to transition to oscillatory step-emulsification at low enough capillary number. At the transition the proposed theory anticipates a simple condition for critical capillary number as a function of the Hele-Shaw cell geometry. The transition threshold is in excellent agreement with experimental data. A simple closed-form expression for the size of the droplets generated in step-emulsification regime derived using simple geometric arguments also shows a very good agreement with the experimental results.