Dripping, jetting and tip streaming

TL;DR

This review comprehensively links fluid mechanics and microfluidics perspectives on dripping, jetting, and tip streaming, covering theoretical models, stability, mechanisms, and applications in droplet production.

Contribution

It provides an integrated overview connecting fundamental theories with practical microfluidic configurations for droplet and bubble generation.

Findings

Analysis of jet stability affected by viscoelasticity, electric fields, and surfactants

Scaling laws for predicting droplet sizes in microfluidic devices

Identification of parameter windows for tip streaming phenomena

Abstract

Dripping, jetting and tip streaming have been studied up to a certain point separately by both fluid mechanics and microfluidics communities, the former focusing on fundamental aspects while the latter on applications. Here, we intend to review this field from a global perspective by considering and linking the two sides of the problem. In the first part, we present the theoretical model used to study interfacial flows arising in droplet-based microfluidics, paying attention to three elements commonly present in applications: viscoelasticity, electric fields and surfactants. We review both classical and current results about the stability of jets affected by these elements. Mechanisms leading to the breakup of jets to produce drops are reviewed as well, including some recent advances in this field. We also consider the relatively scarce theoretical studies on the emergence and stability of tip streaming flows. In the second part of this review, we focus on axisymmetric microfluidic configurations which can operate on the dripping and jetting modes either in a direct (standard) way or via tip streaming. We present the dimensionless parameters characterizing these configurations, the scaling laws which allow predicting the size of the resulting droplets and bubbles, as well as those delimiting the parameter windows where tip streaming can be found. Special attention is paid to electrospray and flow focusing, two of the techniques more frequently used in continuous drop production microfluidics. We aim to connect experimental observations described in this section of topics with fundamental and general aspects described in the first part of the review. This work closes with some prospects at both fundamental and practical levels.