Oscillative Trapping of a Droplet in a Converging Channel Induced by Elastic Instability

Permanent trapping of an oscillating, nonwetting droplet is observed in a converging-diverging microchannel when aqueous, viscoelastic fluids are injected. Classical theories based on the balance between capillary and viscous forces suggest that the droplet size should decrease with increasing flow rates of a displacing Newtonian fluid, and the droplet should be completely displaced at high injection rates. However, droplets in viscoelastic fluids cannot be removed by increasing flow rates due to the oscillation. The oscillation amplitude linearly increases with the Deborah number (De), which further inhibits the droplet's passing through the constriction, permanently. Our microfluidic experiments show that the onset of oscillation is determined by a critical De, which is near 1. We derive a linear relationship for the trapped droplet length with Ec(1/3), where Ec is the elastocapillary number, by introducing the elastic force into the force balance in addition to the capillary and viscous forces.

Automated summary

Trapping of droplets in microchannels can occur when viscoelastic fluids are injected, and this behavior cannot be explained by classical theories. The onset of droplet oscillation is determined by a critical Deborah number, which indicates when the droplet will be permanently trapped.