Insights into bubble–droplet interactions in high-viscoelastic evaporating polymer droplets

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