First ever observation of Brazil nut effect for colloidal particles

Experimental and theoretical physicists at Utrecht University have observed an unexpected phenomenon in mixtures of colloidal particles: the Brazil nut effect. The discovery is important for knowledge about matter, including colloids, polymers, liquid crystals and surfactants. But developers of paints and inks, and completely different disciplines, such as geology, will also benefit from understanding this phenomenon. The study recently appeared in the scientific journal PNAS. 

You've probably noticed it before: when you shake a nut mixture vigorously, the largest nuts, such as Brazil nuts, float to the top. The same goes for the coarser pieces in a pack of granola. It’s funny really, because you would expect the heavier components to sink to the bottom due to gravity. Nevertheless, it is a well-known phenomenon that is quite appropriately called the Brazil nut effect.

Theoretical and experimental

The Brazil nut effect has now been observed for the first time in mixtures of charged colloidal particles. These are particles that behave in many ways like molecules, but are much larger. The research was conducted by PhD candidates Marjolein van der Linden and Jeffrey Everts, under the supervision of Utrecht professors Alfons van Blaaderen of the Debye Institute and René van Roij of the Institute for Theoretical Physics. The researchers observed the Brazil nut effect in physical experiments, but theory could also predict the phenomenon. Both showed that the heavier, charged colloidal particles appeared at the surface.

The knowledge can be directly applied in industry, for example, to stabilize paints and inks

Paint and Earth’s strata

The discovery of the Brazil nut effect in mixtures of colloidal particles is important for the study of soft condensed matter, such as liquids, gels and foams. In addition, the knowledge can be directly applied in industry, for example, to stabilize paints and inks. Even outside of materials science, understanding the Brazil nut effect is useful. After all, understanding sedimentation processes leads to fundamental insights into, for example, the structure of Earth's strata in geology.

Electrical charge

The Brazil nut effect in mixtures of colloidal particles of equal charge is caused by the presence of negative or positive electrical charge on the surface of the colloidal particles. Larger (and therefore heavier) particles have more charge, so they repel each other harder, sometimes even so hard that they can push each other to greater heights than the smaller (lighter) particles despite their greater mass. Adding enough salt to the liquid removes this effect, as salt ions form a charge-shielding electrical double layer around the particles, making the mutual repulsion much less strong. As a result, the heaviest particles simply sink to the bottom.   

Incidentally, the Brazil nut effect in the aforementioned nut mixture has a very different explanation: when you shake the mixture, the smaller particles fall into the holes created at the bottom, pushing up the larger particles.