| dc.description.abstract | Aerobreakup of a liquid droplet refers to the process of breaking the droplet into smaller fragments by subjecting it to a sufficiently high-speed stream of gas (generally air). This process lies at the core of various natural and industrial processes like fuel atomization in IC engines, breakup of falling raindrops, spray atomization of paints and pharmaceutical chemicals, breakup of sneezed ejecta, etc. The present study investigates the aerobreakup of a polymeric droplet. A polymeric liquid exhibits viscoelastic nature due to the presence of long-chain polymer molecules, and the aerobreakup of such viscoelastic droplets can be drastically different from that of a purely viscous (Newtonian) droplet. Studying the aerobreakup of a polymeric droplet draws special attention because of two reasons. First, many liquids of practical importance are inherently viscoelastic, and second, polymers can be added as a rheological modifier to gain control over the aerobreakup process. While extensive research has been conducted on the aerobreakup of Newtonian droplets, studies on viscoelastic droplets are scarce, and the mechanism through which liquid elasticity influences the process remains elusive. The present study delves into both the mechanism and the impact of liquid elasticity on the aerobreakup of a polymeric droplet. The study is structured into three major parts.
The first part provides an experimental study on the shock-induced aerobreakup of a polymeric droplet. Here, we explore the role of liquid elasticity on the aerobreakup process for a wide range of Weber number (∼10^2 - 10^4) and elasticity number (∼10^(-4) - 10^2) by subjecting polymeric droplets (aqueous solutions of polyethylene oxide) of different concentrations to a strong airflow induced behind a moving shock wave. Experiments revealed that the impact of liquid elasticity is negligible in the early stages of droplet breakup, which involves droplet deformation and growth of different hydrodynamic instabilities (Kelvin-Helmholtz and Rayleigh-Taylor). However, a dominant role of liquid appears in the final stages of droplet breakup in terms of the morphology of the liquid mass. In this first part of our study, the viscosity of the polymeric solutions differed from that of the Newtonian solvent (DI water), and some of these polymeric solutions displayed shear-thinning behavior. Therefore, isolating the effect of elasticity from the viscosity and shear-thinning behavior remains an unresolved issue. This forms the basis for the second part of the research work. Here, we employ Boger fluids, which do not show a strain rate-dependent viscosity in shear flows (like Newtonian liquids) but exhibit viscoelastic properties. By comparing the shock-induced aerobreakup of a Boger fluid droplet with that of a Newtonian droplet having similar shear viscosity, we eliminate any effect that may come from the shear-thinning behavior.
The third part of this work provides an experimental study on the aerodynamic bag breakup of a polymeric droplet at a fixed Weber number of ≈15, while the elasticity number is varied in the range of ∼ 10^(-4) - 10^(-2). Here we attempt to address a very fundamental aspect of studying aerobreakup, i.e., to predict fragmentation. Experiments revealed that the initial deformation dynamics of a polymeric droplet are similar to the Newtonian solvent droplet. However, in the later stages, the actual fragmentation of liquid mass is resisted by the presence of polymers. Depending upon the liquid elasticity, fragmentation can be completely inhibited in the timescale of experimental observation. We provide a framework to study this problem, identify the stages where the role of liquid elasticity can be neglected and where it must be considered, and finally, establish a criterion that governs the occurrence or the absence of fragmentation in a specified time period. | en_US |
