Droplet cluster

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Self-assembled droplet clusters
Self-assembled droplet cluster
Chain droplet cluster
Ring droplet cluster
Hierarchical droplet cluster
Hierarchical droplet cluster
Small droplet clusters
Small droplet clusters

Droplet cluster is a self-assembled levitating monolayer of microdroplets usually arranged into a hexagonally ordered structure over a locally heated thin (about 1 mm) layer of water. The droplet cluster is typologically similar to

colloidal crystals. The phenomenon was observed for the first time in 2004,[1] and it has been extensively studied after that.[2][3]

Growing condensing

droplets with a typical diameter of 0.01 mm – 0.2 mm levitate at an equilibrium height, where their weight is equilibrated by the drag force of the ascending air-vapor jet rising over the heated spot. At the same time, the droplets are dragged towards the center of the heated spot; however, they do not merge, forming an ordered hexagonal (densest packed) pattern due to an aerodynamic repulsive pressure force from gas flow between the droplets. The spot is usually heated by a laser beam or another source of heat to 60 °C – 95 °C, although the phenomenon was observed also at temperatures slightly above 20 °C.[4] The height of levitation and the distance between the droplets are of the same order as their diameters.[5]

Due to complex nature of

ice crystals. The droplets pack near the center of heated area where the temperature and the intensity of the ascending vapor jets are the highest. At the same time, there are repulsion forces of aerodynamic nature between the droplets. Consequently, the cluster packs itself in the densest packing shape (a hexagonal honeycomb structure) with a certain distance between the droplets dependent on the repulsion forces.[5]

By controlling the temperature and temperature gradient one can control the number of droplets and their density and size. Using infrared irradiation, it is possible to suppress droplet growth and stabilize them for extended periods of time.[6]

It has been suggested that the phenomenon, when combined with a spectrographic study of droplets content, can be used for rapid biochemical in situ analysis.[7] Recent studies have shown that the cluster can exist at lower temperatures of about 20 °C, which makes it suitable for biochemical analysis of living objects.[4]

Clusters with an arbitrary small number of droplets can be created. Unlike the clusters with a large number of droplets, small clusters cannot always form a hexagonally symmetric structure. Instead, they produce various more or less symmetric configurations depending on the number of droplets. Tracing individual droplets in small clusters is crucial for potential applications. The symmetry, orderliness, and stability of these configurations can be studied with such a measure of self-organization as the Voronoi entropy.[8]

Since the most common hexagonal (honeycomb shaped) droplet cluster was observed for the first time in 2004, new types of types of levitating droplet clusters were discovered. In a chain droplet cluster, rotating droplets may be very close to each other but viscosity of the thin gas layer between the droplets prevents them from coalescing. There is a reversible structural transition from the ordered hexagonal cluster to the chain-like structure.

Dynkin diagrams.[12]

The phenomenon of the droplet cluster is different from the Leidenfrost effect because the latter occurs at much higher temperatures over a solid surface, while the droplet cluster forms at lower temperatures over a liquid surface. The phenomenon has also been observed with liquids other than water.

See also

References

  1. S2CID 189769894
    .
  2. doi:10.1016/j.physleta.2011.10.032
    .
  3. PMID 25623086
    .
  4. ^
    doi:10.1016/j.ijheatmasstransfer.2017.06.015
    .
  5. ^
    PMID 28507295
    .
  6. doi:10.1016/j.infrared.2015.12.020
    .
  7. S2CID 122022390
    .
  8. PMID 29087715
    .
  9. S2CID 204967374
    .
  10. S2CID 244478224
    .
  11. S2CID 221467795
    .
  12. S2CID 218759409
    .

External links
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