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Oscillating and star-shaped drops levitated by an airflow.
Wilco Bouwhuis1, Koen G Winkels, Ivo R Peters
1Physics of Fluids Group, Faculty of Science and Technology, University of Twente, 7500 AE Enschede, The Netherlands.
Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
|September 17, 2013
Summary
Hydrodynamic forces, not heat, cause "star drops" when drops levitate on air. Experiments and simulations reveal critical flow rates for instability, suggesting a new mechanism for Leidenfrost star formation.
Area of Science:
- Fluid dynamics
- Hydrodynamics
- Instability phenomena
Background:
- Drops levitating on air cushions exhibit spontaneous oscillations.
- These oscillations can lead to axisymmetry breaking and the formation of "star drops."
- This phenomenon is similar to Leidenfrost stars observed with heated substrates.
Purpose of the Study:
- To investigate the formation of star drops using a controlled airflow system, eliminating thermal effects.
- To understand the role of fluid dynamics in star drop formation.
- To compare experimental observations with numerical simulations.
Main Methods:
- Levitating drops of varying viscosities above an air cushion with a constant airflow rate.
- Observing and recording phenomena such as stable states, oscillations, and chimney instabilities.
- Performing numerical simulations using potential flow for the drop and a viscous lubrication layer for the gas.
Main Results:
- Star drop formation was observed above a critical airflow rate, dependent on drop size.
- Smaller drops required higher critical flow rates for instability.
- Numerical simulations qualitatively reproduced experimental observations, though quantitative flow rates differed.
Conclusions:
- Thermal effects are not essential for the formation of star drops.
- The study strongly suggests a purely hydrodynamic mechanism drives the formation of star drops and potentially Leidenfrost stars.
- This work provides a foundation for further research into hydrodynamic instabilities in levitated drops.
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