Usual 3D buoyancy in fluids results form a difference in density between the immersed object and its surroundings. A simple question is : what happens in a nearly 2D systems such as soap bubbles or soap films ? In such a situation, it is observed that some particles experience an ascending motion and that those particles are always thinner than their surroundings . In order to explain this behavior, a 2D density has to be defined, accounting for the confined geometry .
We have performed a series of experiments evidencing that the apparent 2D buoyancy in soap films can be reinterpreted in terms of the surface tension profiles characterizing these films. Small circular objects were introduced into the film for generating a hole without breaking the film. A careful study of the ascending motion of these objects allowed us to propose  a model based on the surface tension difference present between the top and the bottom of the objects. This surface tension difference results from the thickness profile in the film, which is counterbalanced by the capillary force.
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 N.Adami and H.Caps, Capillary-driven two-dimensional buoyancy in vertical soap films, EPL 106, 46001 (2014) – PDF
Keywords3D printing antibubble biosensor bouncing capillary forces chaos cohesion collision colloids cracks droplet foam granular matter hypergravity instability Leidenfrost locomotion magnetism melting microfluidics microgravity monopole nanoparticle optofluidics packing percolation powder rheology segregation self-assembly soap film