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In this review, we consider and unify all aspects of the dynamics of Newtonian and viscoelastic liquid drops in high-speed gas flows, including shock waves. The path to understanding is opened by novel, laser-induced fluorescence visualizations at spatial resolutions of up to 200 pixels for millimeter and exposure times as low as 5 ns. The central role of the competition between Rayleigh-Taylor and Kelvin-Helmholtz instabilities is assessed in the frame of rich aerodynamics, from low subsonic to supersonic, and the multitude of characteristic length scales and timescales at play with varying liquid properties. Acceleration and liquid redistribution (drop deformation) early in the evolution set the stage for this competition, and we insist on an interpretation of the drag coefficient that is physically meaningful. Two principal breakup regimes (patterns of bodily loss of coherence) are identified depending on whether the gas finds its way through the liquid mass, causing gross disintegration, or goes around to induce, through shear, a surface-layer peeling-and-ejection action. Corresponding criticalities are quantified in terms of key physics, consistent with experiments. This covers in a unified fashion all liquids, independent of viscosity and elasticity, and the potential role of direct numerical simulations in supporting further advances is forecast. The resulting particle-size distributions (in a final equilibrium cloud) depend crucially on the pattern of breakup, although in this respect the role of elasticity obtains a special significance in terms of the underlying entangled-polymer-chain dynamics. From a more general perspective, we explain the canonical significance of this fundamental problem and summarize the wide range of its practical relevance, including the recently renewed interest in predicting shock-induced fluidization (or high-speed, atmospheric dissemination) of large masses of liquid agents (so-called weapons of mass destruction).

We extend the work of Theofanous and Li [“On the physics of aerobreakup,” Phys. Fluids 20, 052103 (2008)] on aerobreakup physics of water-like, low viscosity liquid drops, to Newtonian liquids of any viscosity. The scope includes the full range of aerodynamics from near incompressible to high Mach number flows. The key physics of Rayleigh–Taylor piercing (RTP, first criticality) and of shear-induced entrainment (SIE, second and terminal criticality) are verified and quantified by new viscosity- and capillarity-based scalings for fluids of any viscosity. The relevance and predictive power of linear stability analysis of the Rayleigh–Taylor and Kelvin–Helmholtz problems (both including viscosity) is demonstrated for the RTP and the SIE regimes, respectively. The advanced stages of breakup and of the resulting particle-clouds are observed and clear definition and quantification of breakup times are offered.

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By using laser-induced fluorescence to visualize liquid drops that are suddenly exposed to supersonic gas streams, we show that the previously available experimental results, which are based on the shadowgraph method, allowed misinterpretations that have lead to inappropriate conceptualizations (and theory) of the physics that govern breakup at high Weber numbers (We>103)—instead of the Rayleigh–Taylor piercing, the dominant mechanism is shear-induced motion with a significant radial component and instabilities on the so-generated, stretched liquid sheet. At low Weber numbers (We<102), the new data reveal the quantitative features of multiwave piercing by Rayleigh–Taylor instabilities. The highly resolved images provide uniquely suitable benchmarks for direct numerical simulations of interfacial instabilities in general and of drop breakup in particular.

We present new experimental results on the interfacial instabilities and breakup of Newtonian liquid drops suddenly exposed to rarefied, high-speed (Mach 3) air flows. The experimental approach allows for the first time detailed observation of interfacial phenomena and mixing throughout the breakup cycle over a wide range of Weber numbers. Key findings are that Rayleigh-Taylor instability alone is the active mechanism for freestream Weber numbers as low as 28 for low viscosity liquids and that stripping rather than piercing is the asymptotic regime as We→∞. This and other detailed visual evidence over 26<We<2,600 are uniquely suitable for testing Computational Fluid Dynamics (CFD) simulations on the way to basic understanding of aerobreakup over a broad range of conditions.

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