Insights into bubble droplet interactions in evaporating polymeric droplets

TL;DR

This study investigates bubble droplet interactions during evaporation of high viscoelastic polymer droplets in an acoustic levitation setup, revealing new bubble nucleation behaviors and membrane formation mechanisms influenced by laser intensity and concentration.

Contribution

It uncovers bubble nucleation in high viscoelastic polymer droplets and analyzes their growth dynamics, extending understanding beyond previous semi-dilute regime observations.

Findings

Bubble nucleation occurs in dilute regimes, contrary to prior semi-dilute findings.

Bubble growth follows Plesset-Zwick criteria, independent of viscoelasticity.

Membrane formation without rupture indicates strong interfacial skin development.

Abstract

Polymer droplets subjected to a heated environment have significance in several fields ranging from spray drying and powder formation to surface coating. In the present work, we investigate the evaporation of a high viscoelastic modulus aqueous polymeric droplet in an acoustically levitated environment. Depending on the laser irradiation intensity, we observe nucleation of a bubble in the dilute regime of polymer concentration, contrary to the previously observed bubble nucleation in a semi-dilute entangled regime for low viscoelastic modulus polymer droplets. After the bubble nucleation, a quasi steady bubble growth occurs depending on the laser irradiation intensity and concentrations. Our scaling analysis reveals that bubble growth follows Plesset-Zwick criteria independent of the viscoelastic properties of the polymer solution. Further, we establish that the onset of bubble growth has an inverse nonlinear dependence on the laser irradiation intensity. At high concentrations and laser irradiation intensities, we report the expansion and collapse of polymer membrane without rupture, indicating the formation of an interfacial skin with significant strength. The droplet oscillations are primarily driven by the presence of multiple bubbles and, to some extent, by the rotational motion of the droplet. Finally, depending on the nature of bubble growth, different types of precipitate form contrary to the different modes of atomization observed in low viscoelastic modulus polymer droplets.