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1. Characterization of oscillation modes in levitated droplets using image and non-image based techniques

   https://doi.org/10.1038/s41526-023-00254-7  

   Brosius, Nevin; Livesay, Jason; Karpinski, Zachary; Singiser, Robert; SanSoucie, Michael; Phillips, Brandon; Narayanan, Ranga (December 2023, npj Microgravity)

   n/a (Ed.)

   Abstract The dynamics of levitated liquid droplets can be used to measure their thermophysical properties by correlating the frequencies at which normal modes of oscillation most strongly resonate when subject to an external oscillatory force. In two preliminary works, it was shown via electrostatic levitation and processing of various metals and alloys that (1) the resonance of the first principal mode of oscillation (moden = 2) can be used to accurately measure surface tension and (2) that so-called “higher-order resonance” ofn = 3 is observable at a predictable frequency. It was also shown, in the context of future space-based experimentation on the Electrostatic Levitation Furnace (ELF), a setup on the International Space Station (ISS) operated by Japan Aerospace Exploration Agency (JAXA), that while the shadow array method in which droplet behavior is visualized would be challenging to identify then = 3 resonance, the normal moden = 4 was predicted to be more easily identifiable. In this short communication, experimental evidence of the first three principal modes of oscillation is provided using molten samples of Tin and Indium and it is subsequently shown that, as predicted, an “image-less approach can be used to identify bothn = 2 andn = 4 resonances in levitated liquid droplets. This suggests that the shadow array method may be satisfactorily used to obtain a self-consistent benchmark of thermophysical properties by comparing results from two successive even-mode natural frequencies. 

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2. multiUQ: An intrusive uncertainty quantification tool for gas-liquid multiphase flows

   Turnquist, B.; Owkes, M. (July 2018, 14th Triennial International Conference on Liquid Atomization and Spray Systems)

   Assessing the effects of input uncertainty on simulation results for multiphase flows will allow for more robust engineering designs and improved devices. For example, in atomizing jets, surface tension plays a critical role in determining when and how coherent liquid structures break up. Uncertainty in the surface tension coefficient can lead to uncertainty in spray angle, drop size, and velocity distribution. Uncertainty quantification (UQ) determines how input uncertainties affect outputs, and the approach taken can be classified as non-intrusive or intrusive. A classical, non-intrusive approach is the Monte-Carlo scheme, which requires multiple simulation runs using samples from a distribution of inputs. Statistics on output variability are computed from the many simulation outputs. While non-intrusive schemes are straightforward to implement, they can quickly become cost prohibitive, suffer from convergence issues, and have problems with confounding factors, making it difficult to look at uncertainty in multiple variables at once. Alternatively, an intrusive scheme inserts stochastic (uncertain) variables into the governing equations, modifying the mathematics and numerical methods used, but possibly reducing computational cost. In this work, we extend UQ methods developed for single-phase flows to handle gas-liquid multiphase dynamics by developing a stochastic conservative level set approach and a stochastic continuous surface tension method. An oscillating droplet and a 2-D atomizing jet are used to test the method. In these test cases, uncertainty about the surface tension coefficient and initial starting position will be explored, including the impact on breaking/ merging interfaces. 

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3. Characterizing surface wetting and interfacial properties using enhanced sampling (SWIPES)

   https://doi.org/10.1039/C8SM02317D  

   Jiang, Hao; Fialoke, Suruchi; Vicars, Zachariah; Patel, Amish J. (January 2019, Soft Matter)

   We introduce an accurate and efficient method for characterizing surface wetting and interfacial properties, such as the contact angle made by a liquid droplet on a solid surface, and the vapor–liquid surface tension of a fluid. The method makes use of molecular simulations in conjunction with the indirect umbrella sampling technique to systematically wet the surface and estimate the corresponding free energy. To illustrate the method, we study the wetting of a family of Lennard-Jones surfaces by water. For surfaces with a wide range of attractions for water, we estimate contact angles using our method, and compare them with contact angles obtained using droplet shapes. Notably, our method is able to capture the transition from partial to complete wetting as surface–water attractions are increased. Moreover, the method is straightforward to implement and is computationally efficient, providing accurate contact angle estimates in roughly 5 nanoseconds of simulation time. 

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4. Prediction of the Atomization Process in Respimat® Soft MistTM Inhalers Using a Volume of Fluid-to-Discrete Phase Model

   https://doi.org/10.3390/bioengineering12030264  

   Sperry, Ted; Feng, Yu (March 2025, Bioengineering)

   This study investigates the atomization process in Respimat® Soft MistTM Inhalers (SMIs) using a validated Volume of Fluid (VOF)-to-Discrete Phase Model (DPM) to simulate the transition from colliding liquid jets to aerosolized droplets. Key parameters, including colliding jet inlet velocity, surface tension, and liquid viscosity, were systematically varied to analyze their impact on the atomization, i.e., aerosolized droplet size distributions. The VOF-to-DPM simulation results indicate that higher jet inlet velocities enhance ligament fragmentation, producing finer and more uniform droplets while reducing total atomized droplet mass. The relationship between surface tension and atomization performance in colliding jet atomization is not monotonic. Reducing surface tension plays a complex dual role in the atomization process. On the one hand, lower surface tension enhances the likelihood of liquid jet breakup into a liquid sheet, leading to the formation of smaller ligaments under the same airflow conditions and shear forces. This increases the probability of generating more secondary droplets. On the other hand, reduced surface tension also destabilizes the liquid surface shape, decreasing the formation of fine, high-sphericity droplets in regimes where surface tension is a dominant force. Viscosity also influences atomization through complex mechanisms, i.e., lower viscosity reduces resistance to ligament breakup but promotes droplet interactions and coalescence, while higher viscosity suppresses ligament fragmentation, generating larger droplets and reducing atomization efficiency. The validated VOF-to-DPM framework provides critical insights for enhancing the performance and efficiency of inhalation therapies. Future work will incorporate nozzle geometry, jet impingement angles, and surfactant effects to better understand and optimize the atomization process in SMIs, focusing on achieving preferred droplet size distributions and emitted doses for enhanced drug delivery efficiency in human respiratory systems. 

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5. Computational Analysis of Secondary Droplet Breakup at Transcritical Conditions with Surface Tension Effects

   https://doi.org/10.2514/6.2024-2380  

   Jangale, Prajesh A; Boyd, Bradley; Jarrahbashi, Dorrin (January 2024, American Institute of Aeronautics and Astronautics)

   In the pursuit of enhanced engine performance and reduced emissions, the design of liquid-fueled propulsion systems is shifting towards much higher combustor pressures, surpassing the nominal critical pressure of the fuel and air. This trend leads to the adoption of supercritical conditions, wherein the liquid fuel is injected into the ambient air at supercritical pressure and temperature, causing the fuel temperature to exceed its nominal critical point. This transition from a liquid-like to a gas-like behavior, known as "transcritical behavior," is a crucial aspect governing the operation of modern high-pressure propulsion and energy conversion systems. In these systems, the primary liquid jet breakup and the subsequent break-up of the resulting droplets into smaller droplets, namely secondary breakup, significantly impact mixing and combustion processes. Despite its importance, there has been a limited focus on droplet breakup at supercritical conditions, particularly at higher flow speeds relevant to high-speed liquid-fuel propulsion systems. Surface tension effects are often neglected in the simulation of transcritical flow, assuming surface tension vanishes beyond the critical point, while recent experiments and molecular dynamics simulations suggest that surface tension effects persist at transcritical conditions. To gain insight into the effects of surface tension on transcritical flows, we have developed a fully compressible multiphaseDirect Numerical Simulation (DNS) approach that accounts for decaying surface effects. The diffuse interface method is employed to represent transcritical interfaces, accounting for surface tension effects calculated using molecular dynamics simulations. This approach is employed to investigate the behavior of subcritical n-dodecane droplets in a supercritical nitrogen environment interacting with a shockwave, aiming to identify the governing breakup regimes at transcritical conditions. The development of quantitative measures enables the generalization of droplet breakup modes for transcritical droplets. The insights gained from this study contribute to advancing the understanding of transcritical liquid breakup, providing valuable knowledge for designing and optimizing high-speed propulsion systems 

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