Droplet Breakup Against an Isolated Obstacle

TL;DR

This study combines experiments and simulations to analyze droplet breakup in microfluidic flow around an obstacle, identifying key parameters influencing breakup probability and defining a nondimensional breakup number.

Contribution

It introduces a nondimensional breakup number Bk that predicts droplet breakup likelihood based on flow and collision parameters, validated by experiments and simulations.

Findings

Droplet breakup probability increases with higher flow velocity, larger size, lower surface tension, and head-on collisions.

Breakup likelihood transitions rapidly around Bk = 1, from no breakup to certain breakup.

The breakup number Bk scales with the collision symmetry parameter S as Bk ~ S^(4/3).

Abstract

We describe combined experiments and simulations of droplet breakup during flow-driven interactions with a circular obstacle in a quasi-two-dimensional microfluidic chamber. Due to a lack of in-plane confinement, the droplets can also slip past the obstacle without breaking. Droplets are more likely to break when they have a higher flow velocity, larger size (relative to the obstacle radius R), smaller surface tension, and for head-on collisions with the obstacle. We also observe that droplet-obstacle collisions are more likely to result in breakup when the height of the sample chamber is increased. We define a nondimensional breakup number Bk ~ Ca, where Ca is the Capillary number, that accounts for changes in the likelihood of droplet break up with variations in these parameters. As Bk increases, we find in both experiments and discrete element method (DEM) simulations of the deformable particle model that the behavior changes from droplets never breaking (Bk << 1) to always breaking for Bk >> 1, with a rapid change in the probability of droplet breakup near Bk = 1. We also find that Bk ~ S^(4/3), where S characterizes the symmetry of the collision, which implies that the minimum symmetry required for breakup is controlled by a characteristic distance h ~ R.