Impact of a microfluidic jet onto a pendant droplet

TL;DR

This study investigates how microfluidic jets impact pendant droplets, predicting the critical velocity for traversal and examining effects of liquid properties, with implications for needle-free injection technologies.

Contribution

The paper introduces a simple model to predict the critical impact velocity for jet traversal and validates it through experiments varying liquid properties.

Findings

Good agreement between model predictions and experiments.

Surfactants and viscoelastic effects influence impact velocity.

Systematic variation of target properties reveals their effects on impact dynamics.

Abstract

High speed microfluidic jets can be generated by a thermocavitation process: from the evaporation of the liquid inside a microfluidic channel, a rapidly expanding bubble is formed and generates a jet through a flow focusing effect. Here, we study the impact and traversing of such jets on a pendant liquid droplet. Upon impact, an expanding cavity is created, and, above a critical impact velocity, the jet traverses the entire droplet. We predict the critical traversing velocity (i) from a simple energy balance and (ii) by comparing the Young-Laplace and dynamic pressures in the cavity that is created during impact. We contrast the model predictions against experiments, in which we vary the liquid properties of the pendant droplet and find good agreement. In addition, we asses how surfactants and viscoelastic effects influence the critical impact velocity. Our results are relevant for the study of needle-free injections, where jets of similar velocities and dimensions are being used. Given the simplicity of our system we can systematically vary the target properties and unravel their effect on the impact dynamics, a true challenge when injecting into real skin.