More Like this

1. Detailed numerical investigation of the drop aerobreakup in the bag breakup regime

   https://doi.org/10.1017/jfm.2023.708  

   Ling, Y; Mahmood, T (October 2023, Journal of Fluid Mechanics)

   Aerobreakup of drops is a fundamental two-phase flow problem that is essential to many spray applications. A parametric numerical study was performed by varying the gas stream velocity, focusing on the regime of moderate Weber numbers, in which the drop deforms to a forward bag. When the bag is unstable, it inflates and disintegrates into small droplets. Detailed numerical simulations were conducted using the volume-of-fluid method on an adaptive octree mesh to investigate the aerobreakup dynamics. Grid-refinement studies show that converged three-dimensional simulation results for drop deformation and bag formation are achieved by the refinement level equivalent to 512 cells across the initial drop diameter. To resolve the thin liquid sheet when the bag inflates, the mesh is refined further to 2048 cells across the initial drop diameter. The simulation results for the drop length and radius were validated against previous experiments, and good agreement was achieved. The high-resolution results of drop morphological evolution were used to identify the different phases in the aerobreakup process, and to characterize the distinct flow features and dominant mechanisms in each phase. In the early time, the drop deformation and velocity are independent of the Weber number, and a new internal-flow deformation model, which respects this asymptotic limit, has been developed. The pressure and velocity fields around the drop were shown to better understand the internal flow and interfacial instability that dictate the drop deformation. Finally, the impact of drop deformation on the drop dynamics was discussed. 

   more » « less
2. Experimental evidence for both progressive and simultaneous shear during quasistatic compression of a bulk metallic glass

   https://doi.org/10.1063/1.4942004  

   Wright, Wendelin_J; Liu, Yun; Gu, Xiaojun; Van_Ness, Katherine_D; Robare, Steven_L; Liu, Xin; Antonaglia, James; LeBlanc, Michael; Uhl, Jonathan_T; Hufnagel, Todd_C; et al (February 2016, Journal of Applied Physics)

   Two distinct types of slip events occur during serrated plastic flow of bulk metallic glasses. These events are distinguished not only by their size but also by distinct stress drop rate profiles. Small stress drop serrations have fluctuating stress drop rates (with maximum stress drop rates ranging from 0.3–1 GPa/s), indicating progressive or intermittent propagation of a shear band. The large stress drop serrations are characterized by sharply peaked stress drop rate profiles (with maximum stress drop rates of 1–100 GPa/s). The propagation of a large slip is preceded by a slowly rising stress drop rate that is presumably due to the percolation of slipping weak spots prior to the initiation of shear over the entire shear plane. The onset of the rapid shear event is accompanied by a burst of acoustic emission. These large slips correspond to simultaneous shear with uniform sliding as confirmed by direct high-speed imaging and image correlation. Both small and large slip events occur throughout plastic deformation. The significant differences between these two types require that they be carefully distinguished in both modeling and experimental efforts. 

   more » « less
3. Making droplets from highly viscous liquids by pushing a wire through a tube

   https://doi.org/10.1063/5.0082003  

   Sun, Yueming; Ma, Jianxing; Peng, Fei; Kornev, Konstantin G. (March 2022, Physics of Fluids)

   Drop-on-demand (DOD) printing is a versatile manufacturing tool, which has been widely used in applications ranging from graphic products to manufacturing of ceramics, even for cell engineering. However, the existing DOD methods cannot be applied for highly viscous materials: the printing technologies are typically limited to the inks with the water level viscosity and fall short to eject jets from thick fluids and break them into droplets. To address this challenge, a new wire-in-a-tube technology for drop generation has been developed replacing the nozzle generator with a wire-in-a-tube drop generator. We successfully formed droplets on demand from highly viscous (∼10 Pa s) liquids and studied the mechanisms of drop formation in the wire-in-a-tube drop generators. These mechanisms couple unique fluid mechanics, capillarity, and wetting phenomena providing a new platform that can be used in different microfluidic applications. 

   more » « less
4. Vertically oriented fiber arrays suppress splashing by restricting spreading of impacting drops

   https://doi.org/10.1063/5.0286271  

   Rible, Gene_Patrick S; Raza, Syed Jaffar; Watkins, Joshua T; Lin, Abbey; Chakpuang, Visalsaya; Dickerson, Andrew K (September 2025, Physics of Fluids)

   This experimental work builds on our previous studies on the post-impact characteristics of drops striking three-dimensional-printed fiber arrays by investigating the highly transient characteristics of impact. We measure temporal changes in drop penetration depth, lateral spreading, and drop dome height above the fiber array as the drop impacts. Liquid penetration of vertical fibers may be divided into three sequential periods with linearly approximated rates of penetration: (i) an inertial regime, where penetration dynamics are governed by inertia; (ii) a transitional regime exhibiting inertial and capillary action; and (iii) a capillary regime characterized purely by downward wicking. Horizontal fibers exhibit only the inertial and transitional stages, with wicking only observed horizontally along the direction of fibers. In horizontal hydrophilic fiber arrays, the time duration to reach the maximum lateral deformation of the drop is proportional to We1/4, as observed in drops impacting solid surfaces. There exists a critical Weber number below which the drop shows no radial deformation, and the critical value increases with decreasing fiber density. At large Weber numbers, drops splash. In contrast, vertical fibers restrict the lateral spreading of the drop, thereby suppressing a splash for all tested drop velocities, even those exceeding 5 m/s. 

   more » « less
5. Dynamic drop penetration of vertically oriented fiber arrays

   https://doi.org/10.1063/5.0246986  

   Rible, Gene_Patrick S; Chakpuang, Visalsaya; Holihan, Aidan D; Sebek, Hannah P; Osman, Hannah H; Brown, Kyle R; Wang, Wei; Dickerson, Andrew K (February 2025, Physics of Fluids)

   This experimental work investigates the impact dynamics of drops on vertically oriented, three-dimensional-printed (3D-printed) fiber arrays with variations in packing density, fiber arrangement, and wettability. These fiber arrays are inspired by mammalian fur, and while not wholly representative of the entire morphological range of fur, they do reside within its spectrum. We define an aspect ratio, a modified fiber porosity relative to the drop size, that characterizes various impact regimes. Using energy conservation, we derive a model relating drop penetration depth in vertical fibers to the Weber number. In sparse fibers where the Ohnesorge number is less than 4×10−3, penetration depth scales linearly with the impact Weber number. In hydrophobic fibers, density reduces penetration depth when the contact angle is sufficiently high. Hydrophilic arrays have greater penetration than their hydrophobic counterparts due to capillarity, a result that contrasts the drop impact-initiated infiltration of horizontal fibers. Vertical capillary infiltration of the penetrated liquid is observed whenever the Bond number is less than 0.11. For hydrophilic fibers, we predict that higher density will promote drop penetration when the contact angle is sufficiently low. Complete infiltration by the drop is achieved at sufficient times regardless of drop impact velocity. 

   more » « less