Viscoelastic Worthington jets \& droplets produced by bursting bubbles

Abstract

Bubble bursting and subsequent collapse of the open cavity at free surfaces of contaminated liquids can generate aerosol droplets, facilitating pathogen transport. After film rupture, capillary waves focus at the cavity base, potentially generating fast Worthington jets that are responsible for ejecting the droplets away from the source. While extensively studied for Newtonian fluids, the influence of non-Newtonian rheology on this process remains poorly understood. Here, we employ direct numerical simulations to investigate the bubble cavity collapse in viscoelastic media, such as polymeric liquids, examining how their elastic modulus $G$ and their relaxation time $\lambda$ affect jet and droplet formation. We show that the viscoelastic liquids yield Newtonian-like behavior as either parameter $G$ or $\lambda$ approaches zero, while increasing them suppresses jet formation due to elastic resistance to elongational flows. Intriguingly, for some cases with intermediate values of $G$ and $\lambda$, smaller droplets are produced compared to Newtonian fluids, potentially enhancing aerosol dispersal. By mapping the phase space spanned by the elastocapillary number (dimensionless $G$) and the Deborah number (dimensionless $\lambda$), we reveal three distinct flow regimes: (i) jets forming droplets, (ii) jets without droplet formation, and (iii) absence of jet formation. Our results elucidate the mechanisms underlying aerosol suppression versus fine spray formation in polymeric liquids, with implications for pathogen transmission and industrial processes involving viscoelastic fluids.