Dropwise condensation is highly desirable for thermal management, water harvesting, and desalination applications. Recently, microstructured hybrid hydrophilic-hydrophobic surfaces with discretized nucleation sites have shown efficient dropwise condensation. However, high sessile droplet mobility on hybrid surfaces depends strongly on the final wetting state (Cassie or Wenzel state), which can lead to poor droplet shedding or filmwise condensation after successive coalescence of droplets.  External excitation, such as surface vibrations, has been used to prevent droplets from reaching a Wenzel state, while repelling droplets from the  surfaces. Though the approach seems to be promising, a comprehensive understanding of drop dynamics on microstructured hybrid surfaces with external excitation remains to be established. When the vibration frequencies of the surface match with the natural frequencies, the energy required to excite the drop will be lower, thanks to the resonant effect. The contact line dynamics has a strong impact on the natural frequencies. In particular, when the contact line is pinned, there exists an additional lateral modes, beyond the longitudinal axisymmetric modes. A comprehensive understanding of the oscillation dynamics and an accurate theory to predict the natural frequencies of different modes on different surfaces remain to be established. The present work is a joint theoretical-numerical study. The focus will be on predicting the natural frequencies at two asymptotic limits: 1) the contact line moves freely with the static contact angle and 2) the contact line is fully pinned, during drop oscillation. These two limits represent the least and most resistant surfaces and thus the natural frequencies will serve as the lower and upper bounds for general cases. Two-dimensional axisymmetric simulations will be performed to identify the natural frequencies of the longitudinal modes; while three dimensional direct numerical simulations will be run to identify the lateral modes in the limit of pinning contact lines. High-fidelity simulations with gas-liquid interfaces fully resolved are performed by the adaptive multiphase flow solver, Basilisk, in which the interfaces are captured by a geometric volume-of-fluid method. The height-function method is used to calculate curvature and also to specify the contact angle on the boundary of the computational domain. Grid-refinement studies with different maximum adaptive mesh refinement levels are conducted to guarantee the key findings are mesh independent. Theoretical models will also be developed based on the potential flow theory and linear spherical harmonic mode decomposition to predict the natural frequencies of different modes for both of the contact-line limits. The theoretical predictions will be validated against our DNS results.


Resonance Oscillation Frequency of Sessile Drops on Hydrophobic Surfaces

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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