'Phase' transitions and pattern formation in wet granular matter
A schematic ‘phase’ diagram for a thin layer of wet granular matter under vertical vibrations. ‘Melting’ arises as external driving force overcomes capillary forces induced by the formation of liquid bridges between neighboring particles, while ‘vaporizing’ occurs as the energy injection is sufficient to break the liquid bridges. This series of investigations suggests a ‘bridge’ between ‘micro-‘ and ‘macroscopic’ scales in wet granular systems, and triggers the questions on the universal aspects of such a ‘bridge’.
Check here for the pattern formation aspect.
Surface melting
Bright patterns are from the light emitted from Ruby tracers, which are embedded in the wet granular sample as peak acceleration increases step by step
Phase separation
As the energy injection is sufficient to break capillary bridges between adjacent particles, a wet granular ‘gas bubble’ emerges.
Surface melting has been a topic of interest since Michael Faraday’s observations on regelation, welding of two blocks of ice after contact below 0 degree. After more than a century’s investigations, it becomes clear that melting is a continuous process that tends to start from the free surface (see e.g. a review of premelted ice). In a non-equilibrium system such as wet granular matter, it is obvious from the above video that melting also tends to starts from the surface. Again, the question is:
How does the analogue of surface melting in a random packing of spheres compare to that in thermal systems?
Merging of wet granular ‘gas bubbles’ suggests the existence of interfacial tension. The questions are:
How to define the surface tension in such a nonequilibrium system?
From the fluctuations of gas bubbles, can we learn something about viscosity of a wet granular liquid?
References
- “Phase transitions far from equilibrium in wet granular matter”, Axel Fingerle, Klaus Roeller, Kai Huang and Stephan Herminghaus, New J. Phys. 10, 053020 (2008) doi: 10.1088/1367-2630/10/5/053020
- “Universal and non-universal aspects of wet granular matter under vertical vibrations”, Kai Huang, Klaus Roeller and Stephan Herminghaus, Euro. Phys. J. – Special Topics 179, 25 (2009), doi: 10.1140/epjst/e2010-01191-5
