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Boger fluids are viscoelastic liquids having constant viscosity for a broad range of shear rates. They are commonly used to separate the effects of liquid elasticity from viscosity in any experiment. We present an experimental study on the shock-induced aerobreakup of a Boger fluid droplet in the Shear-induced entrainment (SIE) and catastrophic breakup regime (Weber number ranging from ~ 800 to 5000). The results are compared with the aerobreakup of a Newtonian droplet having similar viscosity, and with shear-thinning droplets. The study aims to identify the role of liquid elasticity without the added complexity of simultaneous shear-thinning behavior. It is observed that at the early stages of droplet breakup, liquid elasticity plays an insignificant role, and all the fluids show similar behavior. However, during the late stages, the impact of liquid elasticity becomes dominant, which results in a markedly different morphology of the fragmenting liquid mass compared to a Newtonian droplet.

The aerodynamic breakup of a polymeric droplet in the bag breakup regime is investigated experimentally and compared with the result of the Newtonian droplet. To understand the effect of liquid elasticity, the Weber number is kept fixed ($\approx$ 12.5) while the elasticity number is varied in the range of $\sim 10^{-4}-10^{-2}$. Experiments are performed by allowing a liquid droplet to fall in a horizontal, continuously flowing air stream. It is observed that the initial deformation dynamics of a polymeric droplet is similar to the Newtonian 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.

The present study uses Galinstan as a test fluid to investigate the shock-induced aerobreakup of a liquid metal droplet in a high Weber number regime (We ~ 400 - 8000). Atomization dynamics is examined for three test environments: oxidizing (Galinstan-air), inert (Galinstan-nitrogen), and conventional fluids (DI water-air). Due to the readily oxidizing nature of liquid metals, their atomization in an industrial scale system is generally carried in inert atmosphere conditions. However, no previous study has considered gas-induced secondary atomization of liquid metals in inert conditions. Due to experimental challenges associated with molten metals, laboratory scale models are generally tested for conventional fluids like DI water, liquid fuels, etc. The translation of results obtained from conventional fluid to liquid metal atomization is rarely explored. Here a direct multi-scale spatial and temporal comparison is provided between the atomization dynamics of conventional fluid and liquid metals under oxidizing and inert conditions. The liquid metal droplet undergoes breakup through Shear-Induced Entrainment (SIE) mode for the studied range of Weber number values. The prevailing mechanism is explained based on the relative dominance of droplet deformation and KH wave formation. The study provides quantitative and qualitative similarities for the three test cases and explains the differences in morphology of fragmenting secondary droplets in the oxidizing test case (Galinstan-air) due to rapid oxidation of the fragmenting ligaments. A phenomenological framework is postulated for predicting the morphology of secondary droplets. The formation of flake-like secondary droplets in the Galinstan air test case is based on the oxidation rate of liquid metals and the properties of the oxide layer formed on the atomizing ligament surface.

The two-sensor depth from defocus (DFD) technique for the measurement of drop sizes in a spray is further developed to achieve higher spatial and temporal resolution, to improve estimates of size and number concentration, and to provide additional guidelines for the calibration and design of the optical system for a specific application. The technique and these improvements are demonstrated using the case of secondary atomization when a shock wave interacts with a single drop. This is an application in which both high spatially and temporally resolved number density and size distributions of secondary droplets generated in the wake of the original drop are necessary.

The present work investigates the role of contact angle hysteresis at the liquid-liquid-solid interface (LLS) on the rod climbing effect of two immiscible Newtonian liquids using experimental and numerical approaches. Experiments revealed that the final steady state contact angle, $\theta_{w}$ at the LLS interface varies with the rod rotation speed, $\omega$. For the present system, $\theta_{w}$ changes from $\sim$69$^{\circ}$ to $\sim$83$^{\circ}$ when the state of the rod is changed from static condition to rotating at 3.3 Hz. With further increase in $\omega$, the $\theta_{w}$ exceeds 90$^{\circ}$ which cannot be observed experimentally. It is inferred from the simulations that the input value of $\theta_{w}$ saturates and attains a constant value of $\sim$120$^{\circ}$ for $\omega>$ 5 Hz. Using numerical simulations, we demonstrate that this contact angle hysteresis must be considered for the correct prediction of the Newtonian rod climbing effect. Using the appropriate values of the contact angle in the boundary condition, an excellent quantitative match between the experiments and simulations is obtained in terms of: the climbing height, the threshold rod rotation speed for onset of climbing, and the shape of liquid-liquid interface. This resolves the discrepancy between the experiments and simulations in the existing literature where a constant value of the contact angle has been used for all speeds of rod rotation.

Droplet atomization through aerobreakup is omnipresent in various natural and industrial processes. Atomization of Newtonian droplets is a well-studied area; however, non-Newtonian droplets have received less attention despite their frequent encounters. By subjecting polymeric droplets of different concentrations to the induced airflow behind a moving shock wave, we explore the role of elasticity in modulating the aerobreakup of viscoelastic droplets. Three distinct modes of aerobreakup are identified for a wide range of Weber number ($\sim 10^2-10^4$) and Elasticity number ($\sim 10^{-4}-10^2$) variation; these modes are: vibrational, shear-induced entrainment and catastrophic breakup mode. Each mode is described as a three stage process. Stage-I is the droplet deformation, stage-II is the appearance and growth of hydrodynamic instabilities, and stage-III is the evolution of liquid mass morphology. It is observed that elasticity plays an insignificant role in the first two stages, but a dominant role in the final stage. The results are described with the support of adequate mathematical analysis., Comment: 32 pages, 10 figures

Our experiments on the rod-climbing effect with an oil-coated rod revealed two key differences in the rod-climbing phenomena compared to a bare rod. On one hand, an enhancement in magnitude of climbing height for any particular value of rod rotation speed and second, a decrease in threshold rod rotation speed required for the appearance of the rod-climbing effect were observed. Observed phenomena is explained by considering the contact line behavior at the rod-fluid interface. Transient evolution of meniscus at the rod-fluid interface revealed that the three-phase contact line was pinned for a bare rod and de-pinned for an oil-coated rod. We modelled the subject fluid as a Giesekus fluid to predict the climbing height. The differences in the contact line behaviour were incorporated via the contact angle at the rod-fluid interface as a boundary condition. An agreement was found between the observed and predicted climbing height establishing that contact line behaviour may modulate rod-climbing effect.