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 Q37: Drops: Levitation |
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Chair: Panagiota Angeli, UCL Room: 619 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q37.00001: Superwalking Droplets Rahil Valani, Anja Slim, Tapio Simula A \textit{walker} is a droplet of liquid that can self-propel on the free surface of a vibrating bath of the same liquid through feedback between the droplet and its wave field. We have studied walking droplets in the presence of two driving frequencies and have observed a new class of walking droplets, which we coin \textit{superwalkers}. Superwalkers may be more than double the size of the largest walkers, may travel at more than triple the speed of the fastest ones, and enable a plethora of novel multi-droplet behaviors. Physical insights from numerical simulations into the emergence of the superwalking behavior are also discussed. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q37.00002: Experimental and theoretical investigation of capillary oscillations of a charged droplet levitated using various applied waveforms Neha Gawande, Mohit Singh, Y.S. Mayya, Rochish Thaokar A conducting charged droplet levitated in a quadrupole trap becomes unstable when the surface charge on the droplet exceeds its Rayleigh limit. However, when the charge on the droplet is in the sub-Rayleigh limit, it exhibits capillary oscillations whose amplitude and frequency depend upon the magnitude of the charge as well as on the applied electric field. In this work, we present the high-speed imaging of the surface oscillations of a highly charged droplet levitated using various applied waveforms such as sine, square and ramp wave. The surface dynamics is characterized by Fast Fourier Transformation (FFT) analysis and it is observed that the droplet oscillates with the applied frequency irrespective of the type of the waveform. To understand the experimental observation an asymptotic theory is carried out for a perfectly conducting charged droplet in the potential flow limit with viscous corrections. The analysis suggests that the droplet oscillation behavior is the result of complex interplay between the dipolar charge distribution on the drop and the local uniform electric field acting due to off-centered position of the droplet within the trap. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q37.00003: Rheological measurements of gels via ultrasonic levitation of gel drops J. R. Saylor, Xingchen Shao, Steven Fredericks, Joshua Bostwick The application of ultrasonic levitation to the measurement of surface tension of liquid drops has a rich history. However this method has not been extended to gels which, unlike liquids, have a finite elasticity. Herein a method is presented for obtaining measurements of elasticity, surface tension, and viscosity of ultrasonically levitated gel drops. Agarose, a hydrogel, was the material explored. This approach is a significant development given that gels are of growing importance due to their relevance to biomedical applications and exhibit behaviors partially determined by their elasticities. Moreover, obtaining surface tension of gels is important but challenging since measurements cannot be made using standard Wilhelmy plate or DuNuoy ring methods, each of which cannot be applied without breaking the gel. Herein a theoretical development is presented which enables obtaining elasticity, surface tension, and viscosity of a gel drop from characteristics of its response to ultrasonic excitation. Measurements of surface tension and viscosity obtained using this approach are obtained for gel drops having elasticities ranging from 12.2 Pa to 200.3 Pa. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q37.00004: Delayed coalescence of drops at moving liquid/liquid interfaces Weheliye Hashi Weheliye, Teng Dong, Fei Wang, Panagiota Angeli In this work, the delayed coalescence of drops with moving interfaces between two immiscible liquids were experimentally investigated. The aqueous phase was 78{\%} Glycerol/water solution and the organic phase was Exxsol D80 oil. It was found that the coalescence was delayed when the interface moved and the delay time increased with the speed of the interface. The delay was attributed to the lubrication pressure in the film trapped between the drop surface and the bulk interface. Particle Image Velocimetry (PIV) was used to study the velocity fields while the structure of the thin trapped film was investigated with the Planar Laser Induce Fluorescence (PLIF) technique. The film was found to form a dimple symmetrical to the centerline when the interfaces had low velocity, while the dimple was less obvious when the interface velocity increased. Numerical simulations were carried out to investigate the profile of the lubrication pressure along the film of varying thickness. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q37.00005: Interfacial Instability in Spheres by Resonance Nevin Brosius, Kevin Ward, Takehiko Ishikawa, Satoshi Matsumoto, Mike SanSoucie, Ranga Narayanan When a levitated spherical liquid drop is subjected to continuous periodic forcing at a frequency equal to one of its natural frequencies, it can undergo resonance and form modal structures at the surface. The natural frequencies of a liquid sphere directly depend on the modal structure, the mass of the liquid, and the surface tension. By deliberately resonating a sphere at its natural frequency we can therefore obtain the surface tension. The work presented herein compares the analytical result for natural frequency of a liquid sphere in a ``self-gravitational field'' by Rayleigh (1879) to experimental observations using levitated water in ambient conditions and molten metals for varying modal structures. The natural frequency of two normal modes are obtained to verify the values of surface tension. Comparisons and contrasts between experiments and theory are explained. A method for the measurement of interfacial tension of high temperature liquid metals is introduced. Acknowledgments: NASA 80NSSC18K1173, NASA NNX17AL27G, FSGC08/NNX15025, UFIC Research Abroad for Doctoral Students Award [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q37.00006: Levitation of a non--volatile drop by an evaporating pool: the inverse Leidenfrost effect S.J.S. Morris, Meng Shi Assuming axisymmetry, zero gravity, and uniform surface tension $\gamma$ and vapour properties (viscosity $\eta$, conductivity $k$ and density $\rho$), we determine the maximum value of the force $F$ with which a heated sphere (radius $b$) can be pressed against the pool surface without rupturing it. The Laplace--Young and Reynolds equations form a coupled system of ODEs determining, in particular, film thickness $h_0/b$ at the sphere bottom as a function of $F/(2\pi \gamma b)$ with $\varepsilon= \frac{\eta k \Delta T}{\gamma b\rho H_{lv}}$, as a parameter (latent heat $H_{lv}$). Numerical solutions for fixed small $\varepsilon$ show that as $F/(2\pi \gamma b)$ is increased from zero, $h_0/b$ first decreases to a minimum. With further increase in $F$, $h_0$ increases until a turning point is reached. There, the slope ${\rm d} h_0/{\rm d} F\to \infty$, and the response curve doubles back on itself to form an upper branch. Near the turning point, the interface shows an apparent contact line with apparent contact angle $\pi$ (on the liquid side). The turning point corresponds to the contact line moving from the lower hemisphere to the upper; during this process, $F/(2\pi \gamma b)$ reaches its maximum (unity). This result is consistent with work by Adda--Bedia et al.(2016). [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q37.00007: Aqueous droplets in a Leidenfrost state on near room temperature sulphuric acid Stoffel Janssens, Mohamed Abdelgawad, Eliot Fried Droplets that hover above a condensed phase of matter, on a cushion of gas, are in the Leidenfrost state [1]. A typical example of this is a water droplet levitated by its own vapor above a heated pan [2,~3]. In the last few decades more exotic systems such as oil droplets hovering above warm oil [4,~5] or self-propelled acetone droplets hovering above warm water have attracted attention [6]. Acknowledging the dangers of adding water to an acid, we demonstrate that aqueous droplets at room temperature can be prepared in a Leidenfrost state above sulphuric acid at slightly higher temperatures. Guided and supported by experiments, possible mechanisms underlying non-coalescence of the droplets with the acid are discussed.\\ \noindent [1] D.\ Qu\'er\'e, Annu.\ Rev.\ Fluid Mech.\ \textbf{45}, 197--215 (2013).\\ \noindent [2] J.\ G.\ Leidenfrost, Ovenius (1756).\\ \noindent [3] A.-L.\ Biance \textit{et al.}, Phys.\ Fluids \textbf{15}, 1632--1637 (2003).\\ \noindent [4] G.\ P.\ Neitzel and P. Dell'Aversana, Annu.\ Rev.\ Fluid Mech.\ \textbf{34}, 267--289 (2002).\\ \noindent [5] M.\ Geri \textit{et al.}, J.\ Fluid Mech.\ \textbf{833}, R3, (2017).\\ \noindent [6] S.\ D.\ Janssens \textit{et al.}, Phys.\ Fluids \textbf{29}, 032103 (2017). [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q37.00008: The Leidenfrost Effect in Liquid Helium Peter Taborek, Matthew Wallace, Michael Milgie, David Mallin, Kenneth Langley, Andres Aguirre-Pablo, Sigudur Thoroddsen We present the results of our investigation of the Leidenfrost effect in liquid helium droplets impacting on a solid dry surface in an optical cryostat at temperatures between 3.5 K and 5.2 K at saturated vapor pressure. We use high-speed video to image the impacting drops and record the minimum temperature difference Delta T necessary to levitate the drops, and also to observe the lifetime, changing radius, and termination of levitation. The Delta T needed to levitate the drops is much smaller than has been predicted by previous authors examining film boiling in helium, requiring only 1-70 mK for levitation. We observe that the Leidenfrost onset temperature TL is a function of the ambient temperature and runs approximately parallel to the vapor pressure curve, with a lower Delta T needed to levitate the drop at higher temperatures. We compare our results to previous models for TL, and we calculate the vapor film thickness to be 700-1250 nm, much thinner than for experiments with conventional fluids. We observe that helium drops levitate over both a warmer solid surface and a warmer thin layer of liquid helium. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q37.00009: ABSTRACT WITHDRAWN |
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