ARIADNE · Structure and dynamics in active glass-forming liquids

In this project I plan to investigate the transition between a liquid state and a disordered solid-like state in systems of active particles using numerical simulations and experimental approaches. In contrast to equilibrium (passive) systems, where this transition is controlled by temperature and/or density, active systems incorporate a new control parameter in the form of a locally-absorbed energy of non-thermal origin. Thus, with this investigation, I propose an extension to the canonical Glass Transition (GT) problem to non-equilibrium, actively-stimulated, systems. Active interplay between liquid and solid states manifests in real systems such as the cytoplasm of animal cells, dense bacterial colonies, tissues, and packed crowds. However, despite the deep biological implications, it is unclear how the presence of non-thermal energy affects the emergence of the GT in these systems. This will be the first time that this problem will be addressed with a collection of representative models of active systems. Using molecular dynamics simulations of variable complexity, I will consider part of the rich variety of features present in real active particles. This will include shape, polarity, and anisotropy. I will also explore how these features influence the emergence of collective active behaviour. These computational approaches will be accompanied by an uncommon interplay of experimental techniques which will serve to contrast computational results. I will study rheological material properties and characterise the emergence of the glass solidness for active systems using suspensions of colloids and bacteria. To extend the experimental characterisation to a microscopic level, I will use diffusive wave spectroscopy and optical microscopy. With this novel and comprehensive investigation, I aim to provide a foundation for exploring much more complex biological environments, where distinct primary entities coexist and cooperate.