By Bulbul Chakraborty

Gels are one of the most puzzling of all solids. Originally coined as a short form of gelatin, gels can be jelly-like as in Jello, or quite hard as in silica gels. They appear in suspensions of particles at extremely low volume fractions, and yet they are rigid. The conventional wisdom is that gels are a consequence of arrested phase separation of the suspended particles from the fluid. A natural mechanism for the arrest is attraction between the particles, which leads to the formation of filamentous networks of particles weaving through the suspending fluid.

Attraction has been viewed as being essential to the formation of gels. However, a new study published in Physical Review Research led by Carl Merrigan from the Chakraborty group, shows that “active particles” can gel even in the absence of physical attraction. Active matter, composed of particles that convert ambient energy to directed motion, has emerged as an important model for the collective behavior of biological matter such as bacterial suspensions. Using a combination of theoretical analysis and numerical simulations, the collaboration between the groups of Chakraborty and Shokef (Tel Aviv University) showed that the directed motion acts like an effective attraction, leading to gelation of the active particles.

The figure below shows the structure of these gels. As the particles become more active, they jam into clusters of immobile particles (red) surrounded by fluid regions (blue), and often opening up voids. Intriguingly, these active particles, which repel each other also show a transition from a dense glassy solid to a gel as the speed of directed motion is increased. The remarkable similarity between the behavior of passive particles with attraction and active particles suggests that biological entities could form solid-like aggregates without any physical or chemical attraction, purely as a consequence of their dynamics.