Time-Varying Oscillatory Response of Burning Gel Fuel Droplets.

Gel fuel droplets exhibit disruptive burning due to the rupture of their gellant shell, which causes the release of unreacted fuel vapors from the droplet interior to the flame in the form of jets. In addition to pure vaporization, this jetting allows convective transport for fuel vapors, which accelerates gas-phase mixing and is known to improve droplet burn rates. Using high-magnification and high-speed imaging, this study found that the viscoelastic gellant shell at the droplet surface evolves during the droplet's lifetime, which causes the droplet to burst at different frequencies, thereby triggering a time-varying oscillatory jetting.

Jetting Dynamics of Burning Gel Fuel Droplets.

Jetting in burning gel fuel droplets is an important process which, in addition to pure vaporization, enables the convective transport of unreacted fuel vapors from the droplet interior to the flame envelope. This aids in accelerating the fuel efflux and enhancing the mixing of the gas phase, which improves the droplet burn rates. In this study, Schlieren imaging was used to characterize different jetting dynamics that govern the combustion behavior of organic-gellant-laden ethanol gel fuel droplets.

Oscillatory bursting of gel fuel droplets in a reacting environment.

Understanding the combustion behavior of gel fuel droplets is pivotal for enhancing burn rates, lowering ignition delay and improving the operational performance of next-generation propulsion systems. Vapor jetting in burning gel fuel droplets is a crucial process that enables an effective transport (convectively) of unreacted fuel from the droplet domain to the flame zone and accelerates the gas-phase mixing process. Here, first we show that the combusting ethanol gel droplets (organic gellant laden) exhibit a new oscillatory jetting mode due to aperiodic bursting of the droplet shell.

Morphological transitions and buckling characteristics in a nanoparticle-laden sessile droplet resting on a heated hydrophobic substrate.

In this work, we have established the evaporation-liquid flow coupling mechanism by which sessile nanofluid droplets on a hydrophobic substrate evaporate and agglomerate to form unique morphological features under controlled external heating. It is well understood that evaporation coupled with internal liquid flow controls particle transport in a spatiotemporal sense. Flow characteristics inside the heated droplet are investigated and found to be driven by the buoyancy effects.

Towards universal buckling dynamics in nanocolloidal sessile droplets: the effect of hydrophilic to superhydrophobic substrates and evaporation modes.

The evaporation of a nanocolloidal sessile droplet exhibits preferential particle assembly, nanoporous shell formation and buckling to form cavities with unique morphological features. Here, we have established many universal trends that explain the buckling dynamics under one umbrella irrespective of hydrophobicity, evaporation mode and particle loading. We provide a regime map explaining the droplet morphology and buckling characteristics for droplet evaporation on various substrates.

Universal buckling kinetics in drying nanoparticle-laden droplets on a hydrophobic substrate.

We provide a comprehensive physical description of the vaporization, self-assembly, agglomeration, and buckling kinetics of sessile nanofluid droplets pinned on a hydrophobic substrate. We have deciphered five distinct regimes of the droplet life cycle. Regimes I-III consists of evaporation-induced preferential agglomeration that leads to the formation of a unique dome-shaped inhomogeneous shell with a stratified varying-density liquid core.

Coupled Mechanisms of Precipitation and Atomization in Burning Nanofluid Fuel Droplets.

Understanding the combustion characteristics of fuel droplets laden with energetic nanoparticles (NP) is pivotal for lowering ignition delay, reducing pollutant emissions and increasing the combustion efficiency in next generation combustors. In this study, first we elucidate the feedback coupling between two key interacting mechanisms, namely, secondary atomization and particle agglomeration; that govern the effective mass fraction of NPs within the droplet. Second, we show how the initial NP concentration modulates their relative dominance leading to a master-slave configuration.

Sphere to ring morphological transformation in drying nanofluid droplets in a contact-free environment.

Understanding the transients of buckling in drying colloidal suspensions is pivotal for producing new functional microstructures with tunable morphologies. Here, we report first observations and elucidate the buckling instability induced morphological transition (sphere to ring structure) in an acoustically levitated, heated nanosuspension droplet using dynamic energy balance. Droplet deformation featuring the formation of symmetric cavities is initiated by capillary pressure that is two to three orders of magnitude greater than the acoustic radiation pressure, thus indicating that the standing pressure field has no influence on the buckling front kinetics.