Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session S22: Drops: Instability and Breakup II |
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Chair: Larry Li, HKUST Room: 604 |
Tuesday, November 26, 2019 10:31AM - 10:44AM |
S22.00001: Deformation and breakup of droplets in an oblique continuous air stream Surendra Kumar Soni, Pavan Kumar Kirar, Pankaj Kolhe, Kirti Sahu We experimentally investigate the deformation and breakup of droplets interacting with an oblique continuous air stream. A high-speed imaging system is employed to record the trajectories and topological changes of the droplets of different liquids. The droplet size, the orientation of the air nozzle to the horizontal and fluid properties are varied to study different breakup modes. We found that droplet possessing initial momentum prior to entering the continuous air stream exhibits a variation in the required Weber number for the vibrational to the bag breakup transition with a change in the angle of the air stream. The critical Weber numbers for the bag-type breakup are obtained as a function of the Eotvos number, angle of inclination of the air stream and the Ohnesorge number. It is found that although the droplet follows a rectilinear motion initially that transforms to a curvilinear motion at later times when the droplet undergoes topological changes. The apparent acceleration of the droplet and its size influence the critical Weber number for the bag breakup mode. The departure from the cross- flow arrangement shows a sharp decrease in the critical Weber number for the bag breakup which asymptotically reaches to a value associated with the in-line flow configuration. [Preview Abstract] |
Tuesday, November 26, 2019 10:44AM - 10:57AM |
S22.00002: Dynamics of impulsively induced viscoelastic jets Emre Turkoz, Howard Stone, Craig Arnold, Luc Deike Understanding the physics of viscoelastic liquid jets and their breakup is relevant to jet-based printing and deposition techniques. In this study, we study the behavior of jets induced from viscoelastic liquid films. We use the mechanical impulse provided by a laser pulse to actuate jet formation. The parameter space governing the maximum jet length and the droplet size of the resulting viscoelastic liquid jets is investigated using simulations, which are validated with experiments. To investigate the effect of viscoelasticity, we present direct numerical simulations and solve the two-phase axisymmetric momentum equations together with the volume-of-fluid technique for interface tracking and the log-conformation transformation to solve the viscoelastic constitutive equation. We show that changing the Deborah number, which denotes the ratio between the elastic and capillary time scales, can change the resulting droplet size and the maximum jet length, so that it is possible to increase the resolution of jet-based printing and deposition techniques by modifying the elasticity of the liquid material to be printed. [Preview Abstract] |
Tuesday, November 26, 2019 10:57AM - 11:10AM |
S22.00003: Jet atomization of brine to achieve zero liquid discharge Christian Machado, Youhua Jiang, Kyoo-Chul Park Achieving zero liquid discharge in brine management facilities is critical to solving the environmental problems associated with returning high salinity water to its source. One such pathway for reducing the amount of discharged brine is by utilizing evaporation. To accelerate the evaporation of water in brine, the bulk brine solution should be atomized into microdroplets (i.e., fog) using a high pressure flow. In this study, brine with systematically varying concentrations of salt (NaCl) and surfactant (cetrimonium bromide (CTAB)) was atomized at different compressed air pressures. Results show that the rate of fog droplet generation decreases with increasing salinity. To combat this effect, adding a low surface tension surfactant and increasing compressed air pressure have shown to improve the rate of atomization. Quantitative data analysis was performed to understand the effects of the brine's physico-chemical properties, such as density, surface tension, and viscosity, as well as external variables such as compressed air pressure on overall droplet generation. This study introduces a new approach of brine evaporation using recovered thermal energy, and provides insights in jet atomization. [Preview Abstract] |
Tuesday, November 26, 2019 11:10AM - 11:23AM |
S22.00004: Thin-Film Breakup Dynamics of a Binary Mixture Drop during Evaporation H. Alex Guo, Thomas P. Witelski, Chuan-Hua Chen When a drop consisting of two volatile liquids evaporates on a solid substrate, the parent drop frequently bursts into tiny droplets. Although the bursting originates from solutal Marangoni stresses, like the wine-tear phenomenon, the binary-drop setup is distinct with its moving contact line and the dramatic bursting of the entire parent drop before complete evaporation. For a drop of water and isopropanol, a ridge develops at the contact line when the drop spreads on the silicon substrate. When evaporation drives the contact line to eventually recede, the ridge formed during spreading fragments into tiny droplets. We have developed a numerical model for the thin-film evolution of the binary drop. The model accounts for the liquids’ differential evaporation, which generates Marangoni stresses. Our model captures the ridge formation process, and predicts a threshold mixing ratio for the Marangoni bursting that is experimentally observed. [Preview Abstract] |
Tuesday, November 26, 2019 11:23AM - 11:36AM |
S22.00005: Evaporation-Induced Breakup of a Droplet in a Shallow Well Kris F. Wiedenheft, H. Alex Guo, Thomas P. Witelski, Chuan-Hua Chen Droplet evaporation is most frequently studied on a flat substrate, for which surface defects are typically accounted for by contact angle hysteresis. To directly investigate the effect of surface defects, we study droplet evaporation in a shallow circular well, created by etching a smooth silicon substrate. An inkjet printed water droplet first spreads over the entire well, and then evaporates into the air until its contact line is pinned at the top edge of the well. From this point on, the droplet evaporation shows complex patterns, including an annular breakup away from the walls of the well. This annular breakup presents a drop evaporation mode that is distinct from the widely reported modes with constant contact angle or constant contact radius. We have developed a thin-film evaporation model that captures the annular breakup pattern as well as the range of aspect ratio conducive to the breakup. [Preview Abstract] |
Tuesday, November 26, 2019 11:36AM - 11:49AM |
S22.00006: Role of initial perturbation on the droplet breakup characteristics levitated in an electrodynamic balance Mohit Singh, Neha Gawande, Y.S Mayya, Rochish Thaokar Experimental observations are reported on the effect of the initial perturbation on the mechanism of Rayleigh breakup phenomenon of a charged droplet (diameter\textasciitilde 100-250 \textmu m), levitated in an electrodynamic (ED) balance. As the droplet undergoes evaporation, the droplet size decreases with a corresponding increase in the surface charge density near to the Rayleigh limit, finally leading to its breakup.~ All the successive events such as droplet deformation, breakup and relaxation of the drop after jet ejection have been captured using a high- speed camera at around 200 thousand~fps. It is observed that the droplet surface exhibits finite amplitude of oscillations with higher prolate deformation on account of unbalanced gravity. These perturbations lead to subcritical Rayleigh breakup of the droplet. There exists a ``$\pi $'' phase shift between the centre of mass motion and applied field which causes the asymmetric breakup in such a way that the droplet breaks in the upward direction. The experimental observations are validated with BEM simulations and a reasonable~agreement is observed between the two. [Preview Abstract] |
Tuesday, November 26, 2019 11:49AM - 12:02PM |
S22.00007: Thermal Atomization during Droplet Impingement on Superhydrophobic Substrates Preston Emerson, Julie Crockett, Daniel Maynes Water droplets impinging superheated substrates may be characterized by dynamic droplet boiling, causing an upward ejection of miniscule secondary droplets, called thermal atomization. In this study, droplets impact superheated, superhydrophobic substrates of varying microstructure configuration for a range of superheat temperatures between 120 and 320 degrees Celsius. Thermal atomization is captured using a high-speed camera and is quantified by estimating the amount of liquid spray present for each impingement event using a 2D image processing technique. The start time, quantity, and velocity of the atomization spray is shown to depend on the microstructure configuration of the substrate (most notably the height and center-to-center spacing of the structures) and the Weber number of the impinging droplet. The droplet boiling regimes are identified for the range substrate temperatures and the Leidenfrost temperature is estimated for each scenario. [Preview Abstract] |
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