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April 20th 2019

In 2013, FOM programme researchers from Utrecht University have developed a new imaging technique to prove that minute particles on the surface of a liquid are no longer subject to gravity. Does an object float on a liquid or not? The researchers studied this question for particles just several nanometres in size. They did this by making a three-dimensional image of nanoparticles on the surface of a liquid. This type of imaging is a world first.

No Archimedes
For large objects, such as a boat, Archimedes' principle explains whether an object will or will not float. This principle states that the upward force that allows an object to float equals the weight of the fluid displaced. Objects with dimensions of just a few millimetres appear to be able to violate this rule. An example is a paperclip or a water flea. They both carry on floating even though this appears to fly in the face of Archimedes' principle. They can actually float thanks to the surface tension of water: the water surface is stretched as a result of which an extra upward force arises.

For objects that are one million times smaller still, with dimensions of just a few nanometres, gravity no longer plays a significant role. For these objects, only the molecular interactions between object, liquid and air are important. These interactions determine whether or not the object adheres to the surface of the liquid.

By analysing the microscopic images made, the researchers could calculate exactly how strongly the particles adsorb to the boundary between liquid and air. This research is a collaboration between the two groups for nanomaterials science at the Debye Institute of Utrecht University. The researchers published their results in the journal Physical Review Letters.