Drop impact on thin powder layers: pattern formation by air entrapment.

Area of Science:

• Physics
• Materials Science
• Fluid Dynamics

Background:

• Drop impacts on granular materials typically form craters.
• Previous observations of drop impacts on thin powder layers primarily noted crater formation with outward particle displacement.
• The diversity of patterns resulting from such impacts was not fully explored.

Purpose of the Study:

• To investigate and characterize novel patterns formed by drop impacts on thin powder layers.
• To elucidate the underlying mechanisms responsible for the formation of these patterns.
• To expand the understanding of drop-impact phenomena on granular matter.

Main Methods:

• High-speed imaging to capture dynamic processes during impact.
• Scaling analyses to identify key parameters influencing pattern formation.
• Systematic experiments using various liquids and powder layer properties.

Main Results:

• Drop impacts can generate disc or disc-plus-ring patterns, in addition to craters.
• Air entrapment and subsequent micro-bubble formation are identified as key factors in forming these non-crater patterns.
• The specific pattern observed is dependent on the properties of the powder layer and the impacting liquid.

Conclusions:

• Drop impacts on thin powder layers exhibit a broader range of pattern formation than previously recognized.
• A proposed mechanism involving air entrapment and micro-bubble dynamics explains the formation of disc and disc-plus-ring patterns.
• This research opens new avenues for studying drop-impact interactions with granular materials.