Too small for Archimedes
Better imaging throws new light on colloidal nanoparticles
With a new imaging technique, chemists from Utrecht University and the FOM Foundation have discovered that minuscule particles on the surface of a liquid are no longer subject to gravity. The results were published on 3 September in the journal Physical Review Letters.
"Does an object remain floating on a liquid or sink? That is the question we studied for miniscule solid particles just several nanometres in size," says Dr. Ben Erné. Erné is the associate supervisor of FOM PhD researcher Jos van Rijssel, the first author of the article. "We are the first to have imaged these minuscule particles in three dimensions on the surface of a liquid."
Evading Archimedes
"The floating of large objects is explained by Archimedes' principle," explains Erné. "A piece of wood remains floating. A brick sinks. Some small objects just several millimetres in size seem to break Archimedes' principle. A classic example is the well-known children's experiment of carefully placing a drawing pin or a paperclip on water. Thanks to the water's surface tension it remains floating despite Archimedes: the water surface is stretched out as a result of which a force develops that opposes the force of gravity."
Molecular interaction
"For objects that are one million times smaller still, just several nanometres, gravity no longer plays a role," says Erné. "Now the molecular interactions between object, liquid and air determine whether the object remains attached to the surface of the liquid. By analysing microscopic images we could precisely calculate how strongly the solid particles adsorbed to the liquid/air interface."
Quantum dots
"This study is relevant for research into quantum dots," says Erné. These are miniscule semiconducting crystals with quantum mechanical properties. Under UV light these emit a colour, which is dependent on the size of the sphere. "On the surface of the liquid those spheres automatically form a clear lattice," explains Erné. "We can then, as it were, scoop them out of the liquid phase so that they can be used for other applications. The particles directly on the liquid surface have now for the first time been successfully imaged in three dimensions."
Stability of emulsions
The research of Erné and Van Rijssel also contributes to our understanding of how colloids function in food. Emulsions, such as milk or mayonnaise, are colloidal mixtures in which two immiscible substances are still mixed together. "Adsorption of particles on liquid interfaces is also important in food, as it influences the stability of emulsions. And in detergents adsorbed molecules ensure that the surface of fat particles become hydrophilic," says Erné.