Self-assembly of actively changing particles
Harnessing nature's inherent ability for self-assembly, essentially creating nano-sized products that make themselves from reconfigurable and adaptive building blocks, is a sort of Holy Grail for engineers. The majority of studies have focused on components with shapes and interactions that do not change during the assembly process (i.e. they are static). EU-funded scientists working on the project 'Active self-assembly' (ACTSA) are pushing the boundaries to enhance understanding of active self-assembly through theoretical studies, computer simulations and experiments. The densest way to pack objects in space, also known as the packing problem, has intrigued scientists and philosophers for millenia. Today, packing comes up in various systems over many length scales from batteries and catalysts to the self-assembly of nanoparticles, colloids and biomolecules. Despite the fact that so many systems' properties depend on the packing of differently-shaped components, we still have no general understanding of how packing varies as a function of particle shape. In this project, we carried out an exhaustive study of how packing depends on shape by investigating the packings of over 55,000 polyhedra. Our resulting density surface plots can be used to guide experiments that utilize shape and packing in the same way that phase diagrams are essential to do chemistry. The properties of particle shape indeed are revealing why we can assemble certain crystals, transition between different ones, or get stuck in kinetic traps. Entropy modulates the ordering of particles and the phase behaviour of colloids (particles of one substance dispersed in another). ACTSA scientists quantified the directional entropic forces that tend to align neighbouring particles. Most importantly, the team provided a framework to quantify the role of shape in packing and in self-assembly in experimental systems in which other forces contribute to assembly. Finally, the team demonstrated that the mechanism occurs in a wide range of systems. Researchers also studied emergent phenomena in systems of spinners, particles that experience a constant internal torque in the clockwise or counterclockwise direction. They showed that the active motion of the otherwise non-interacting rigid bodies induces an effective interaction that favours rotation in the same direction. This can lead to self-organisational and cooperative behaviours that are not possible in equilibrium systems (without applied activity). Results to date have already led to three publications. The project is thus expected to make a major contribution to understanding of the forces that drive self-assembly in systems of complexly interacting particles. This, in turn, could help engineers and designers create new materials systems with biomimetic self-assembly capacity for a new era of nanodevices.
Keywords
Self-assembly, particles, biomedicine, materials science, entropic forces
