Droplet motion on flexible superhydrophobic porous sponge surface

Millimetre-sized droplets are able to bounce multiple times on flat solid substrates irrespective of their wettability, provided that a micrometre-thick air layer is sustained below the droplet, limiting $\mathit{We}$ to ${\lesssim}4$. We study the energy conversion during a bounce series by analysing the droplet motion and its shape (decomposed into eigenmodes). Internal modes are excited during the bounce, yet the viscous dissipation associated with the in-flight oscillations accounts for less than 20 % of the total energy loss. This suggests a significant contribution from the bouncing process itself, despite the continuous presence of a lubricating air film below the droplet. To study the role of this air film we visualize it using reflection interference microscopy. We quantify its thickness (typically a few micrometres) with sub-millisecond time resolution and ${\sim}30~\text{nm}$ height resolution. Our measurements reveal strong asymmetry in the air film shape between the spreading and receding phases of the bouncing process. This asymmetry is crucial for effective momentum reversal of the droplet: lubrication theory shows that the dissipative force is repulsive throughout each bounce, even near lift-off, which leads to a high restitution coefficient. After multiple bounces the droplet eventually hovers on the air film, while continuously experiencing a lift force to sustain its weight. Only after a long time does the droplet finally wet the substrate. The observed bounce mechanism can be described with a single oscillation mode model that successfully captures the asymmetry of the air film evolution.

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Experimental and Numerical Investigation of a Large Cylindrical Oil-Fired Combustor

Volume 3: Coal, Biomass and Alternative Fuels; Combustion and Fuels; Oil and Gas Applications; Cycle Innovations ◽

10.1115/83-gt-184 ◽

1983 ◽

Author(s):

F. M. ElMahallawy ◽

E. E. Khalil ◽

O. Abdel Aal

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Gas Phase ◽

Combustion Model ◽

Steam Boiler ◽

Discrete Ordinate ◽

Radiation Model ◽

Droplet Motion ◽

Transfer Rates ◽

Lagrangian Equations

The present work presents measurements of velocity, temperature and heat transfer rates carried out on a segmented water-cooled cylindrical oil-fired flame tube typical of a 0.56 kg/s packaged fire-tube steam boiler. A prediction procedure, that solves the conservation equations of the various entities, was successfully used to produce computed velocities, temperatures and heat flux distributions. A two-equation turbulence model, a combustion model and a discrete ordinate radiation model were used to approximate the various characteristics of the flow. The combustion model solved the Eulerian equations of the gas phase, and the Lagrangian equations of the droplet motion, heating, evaporation and combustion.

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Sliding Dynamics of a Water Droplet On Silicon Oil Film Surface

Journal of Fluids Engineering ◽

10.1115/1.4050347 ◽

2021 ◽

Author(s):

Bekir Sami Yilbas ◽

Anwaruddin Siddiqui Mohammed ◽

Abba Abdulhamid Abubakar ◽

Saeed Bahatab ◽

Hussain Al-Qahtani ◽

...

Keyword(s):

Water Droplet ◽

High Speed ◽

Film Surface ◽

Maximum Velocity ◽

Optical Transmittance ◽

Gravitational Energy ◽

Droplet Motion ◽

Droplet Dynamics ◽

Oil Film ◽

Silicon Oil

Abstract A sliding droplet over the silicon oil film is examined and the dynamics of droplet motion are explored. The solution crystallized wafer surfaces are silicon oil impregnated and the uniform thickness oil film is realized. A recording facility operating at high-speed and the tracker program are used to monitor and evaluate the droplet dynamics during droplet sliding. The sliding behavior and flow generated in the droplet fluid are predicted by adopting the experimental terms. Findings revealed that the crystallized surface possesses the texture composing of spherules and fibrils, which give rise to 132o ± 4o contact angle and 38o ± 4o hysteresis. Oil impregnation on the crystalized surface improves the optical transmittance by three times for 250 nm to 500 nm wavelength range and almost 1.5 times after 500 nm to 850 nm wavelengths of the optical spectrum. The oil rim and ridges are developed in sliding water droplet vicinity while influencing droplet motion; however, this influence is estimated as almost 12% of droplet gravitational energy change during sliding. A circulatory flow is developed inside the droplet fluid and the maximum velocity in the droplet fluid changes as the droplet location changes on the oil surface during its sliding.

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