Gels occur everywhere in our everyday life, but the precise way in which they form is not very well understood. Combining experimental observations and numerical models, physicists from the universities of Amsterdam and Cambridge and from Unilever have now shown that gel formation is closely related to another well-known physical process: percolation. The results were published in Nature Communications this week.
Gelation, the formation of gels, is a wide-spread phenomenon. We encounter gels in many realms of life, such as food and cosmetic products. When we boil an egg, or make yoghurt, pudding or cheese, the processes involved are gelation phenomena, rendering complex fluids (egg white, milk) rigid. Gelation even plays a role in life itself: blood clotting is another well-known example, where a solid crust forms to seal the wound. All these seemingly different phenomena are based on the coagulation of the constituent particles: egg or milk proteins or blood plasma proteins that aggregate into space-spanning networks, lending the material rigidity like a spider’s web does to the spider threads.
Gelation is distinct from the usual well-known solidification based on crystallization, occurring when for example water freezes and becomes ice. In such a process the atoms or molecules order at high density on a periodic lattice. When gelation occurs, on the other hand, network structures form at low density and with a structural peculiarity: they are often fractals that exhibit intriguing self-similarity: the structures look identical when zooming into smaller and smaller details.
While the phenomenon of gelation is ubiquitous and occurs in various forms in nature, it is not well understood, and there is no general physical theory that describes all these phenomena. Researchers from the Institute of Physics of the University of Amsterdam, together with their colleagues from the University of Cambridge and Unilever, have now made significant progress on this issue by obtaining new insights into these ubiquitous gelation phenomena. To this end, they used both direct observation of agglomerating particles, whose attraction they could adjust directly, as well as computer simulations and theoretical modelling.
The combined study, published in Nature Communications this week, gives direct insight into how the structures evolve. As the particles agglomerate and the number of connected neighbours grows, the particles slow down and the system becomes more and more rigid, until the largest agglomerated cluster spans the entire space (it ‘percolates’) and the material acquires a system-spanning rigidity.
Percolation phenomena are well-known in our daily life: the most famous example is probably how hot water spreads through ground coffee beans to pick up the soluble compounds that give coffee its colour and flavour. Percolation makes the liquid to come out at the other end of the pressed powder. The researchers have now found that the fully connected state of a gel is approached in way identical to other well-known percolation phenomena in physics and chemistry. When you make breakfast, the physics that boils your egg is the same physics that makes your coffee! The only difference: in the former case, the percolating network formed of aggregated egg proteins is solid, while in the latter, it is liquid hot water.
The findings indicate a new form of thermodynamic transition, besides well-known transitions like freezing (crystallization) and evaporating. In the gelation transition, the system becomes increasingly trapped in a growing connected network state—unlike crystallization, where atoms and molecules rearrange into an ordered state. These results, which are expected to apply to many food and cosmetic products, as well as structures in biology and nanoscience, may have many applications. For example, the findings could help in designing new plant-based protein food products for sustainable nutrition.
J. Rouwhorst, C. Ness, S. Stoyanov, A. Zaccone and P. Schall, Nonequilibrium continuous phase transition in colloidal gelation with short-range attraction, Nature Comm. 11, 3558 (2020). DOI 10.1038/s41467-020-17353-8.