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Double Contact During Drop Impact on a Solid Under Reduced Air Pressure.
Er Qiang Li1,2, Kenneth R Langley1, Yuan Si Tian1
1Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.
Physical Review Letters
|December 9, 2017
Summary
Reducing air pressure during droplet impacts shrinks the entrapped air disk. At low pressures, the air disk bifurcates, leading to double contacts and toroidal air entrapment due to rarefied gas effects.
Area of Science:
- Fluid dynamics
- Surface science
- Physics of soft matter
Background:
- Droplet impacts on solid surfaces typically entrap air due to lubrication pressure.
- The behavior of this entrapped air layer is crucial for understanding impact dynamics.
Purpose of the Study:
- To investigate the influence of reduced ambient air pressure on the air layer entrapped during droplet impacts.
- To analyze the structural changes and dynamics of the air layer under varying pressure conditions.
Main Methods:
- Utilizing ultrahigh-speed interference imaging at 5 million frames per second (Mfps).
- Conducting experiments with droplet impacts at reduced ambient air pressures.
Main Results:
- Decreased ambient air pressure leads to a smaller radius and thickness of the central air disk.
- A bifurcation of the air disk's radial extent occurs when the compressibility parameter exceeds approximately 25.
- This bifurcation manifests as a double contact, forming a second ring that entraps an outer toroidal air strip, which subsequently contracts into bubbles.
Conclusions:
- Reduced air pressure significantly alters the air entrapment dynamics during droplet impacts.
- Navier slip effects, prominent in rarefied gas regimes, enhance gas escape, facilitating the observed bifurcation and double contact phenomena.
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