Playing with Phobia

Hydro(oleo)phobic surfaces and in particular their super-phobic analogues have attracted much interest due to their potential applications such as self-cleaning, water-oil separation or the condensation of dew into droplets. The movie shows a film that was casted from a dispersion of perfluorosilane coated titania particles. Droplets of water and of a reduced graphene oxide dispersion were deposited on the half which was masked (right side) and on the half which was exposed to ambient light in the lab (left side). Gentle air flow moves the droplets on the right.

However, "phobia" may also cause the dewetting of coatings which leaves largely varying patterns on the substrate depending on the surface energies, the coatings viscosity, the phase behaviour of the coating and not least the substrate surface structure. Brief descriptions of the shown examples are given in the lightbox.

An apparently related phenomenon is the Marangoni effect, named after the Italian physicist (1840 - 1925) who studied surface and interface properties of liquids. He found that for two neighboring liquids with different surface tensions, a net force is exerted by the one with the higher tension. This can be observed as “tears of wine”. Ethanol (γ = 23 mN/m) evaporates faster from the adherent liquid and consequently, the film which is enriched with water (γ = 42 mN/m) lifts tears from the bulk. In the movie water rushes out of the central spot after the addition of a drop of ethanol. Macroscopic prolate objects form temporary obstacles for the proceeding contact line in the refilling phase. The phenomenon was observed during the cleaning of a petri dish. The solid objects are the scraped reaction product of a sol-gel synthesis.

This can be perceived as a macroscopic analogue of the dewetting in the interstitial space between hydrophobic squares (d < 1.5 nm, L = 2.5 nm). Molecular dynamics simulations suggest that this is promoted by low ethanol concentrations in water (X. Ren et al., Soft Matter 9 (2013) 4655).
  • aqueous coating with clay particles got phase separated during film formation, baked on glass

    My Album 2_dewet

  • droplets of silylated resin on aluminum after baking

    My Album 3_dewet

  • baked film of an aqueous coating with dispersed reduced graphene oxide, polypropylene substrate

    My Album 1_dewet

  • baked film of an aqueous coating with dispersed reduced graphene oxide, polypropylene substrate

    My Album 0_dewet

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