Transport in passive systems such as molecular liquids and colloidal suspensions is controlled by temperature and/or particle packing fraction. In contrast, active systems (e.g. suspensions of E. coli bacteria or organelles moving inside the cytoplasm of animal cells) present a new control parameter in the form of a non-thermal energy of metabolic origin. This energy, used by the active particles to drive their motion, is accompanied by a rich variety of particle features, including different particle shapes (interactions). However, despite the deep biological implications, it is unclear how the presence of non-thermal energy conditions transport and phase transitions in these systems.
In particular, this project covers the general problem of transport in systems near a phase transition from a liquid state to an amorphous solid state, known as glass. We tackle this problem for passive systems (i.e. those systems where glass transition (GT) is controlled by packing fraction and/or temperature) and active systems (i.e. those systems where GT is controlled by an energy of non-thermal origin). Apart from that, we consider different defining shapes (interactions) for the particles constituting the studied systems as well as particle polarity, and both isotropic and non-isotropic particle interactions. The overall aim of this project is to explore transport for a collection of canonical models using molecular dynamics simulations and experimental techniques (in particular light scattering). The project was conceived to provide a foundation for exploring much more complex biological environments where distinct primary entities coexist and cooperate. We also aim to favour a transfer of knowledge between physicists and biologists. The project also promotes some of the aspirations of the MSCA: the engagement of the general public through a pedagogical dissemination and the establishment of interdisciplinary interchanges.
As overall objectives, this project considers the study of transport in real systems and computational models with: isotropic interactions, non-isotropic interactions, and polarity. The original objectives of this project were twofold: theoretical/computational and experimental.
Objective 1: Study of the GT in active systems with isotropic interaction.
Objective 2: Study of the GT in active systems with non-isotropic interactions.
Objective 3: Study of the GT in active systems with polar interactions.
Objective 4: Study of collective dynamics in active systems near the GT.
Objective 5: Study of static properties in active systems near the GT.
As a result we have first developed and tested computational models of: i) particles with isotropic interaction in 2D and 3D; ii) particles with non-isotropic interactions in 3D; iii) elongated particles in 3D. Second, we performed dynamic light scattering experiments using suspensions of E. coli bacteria.
With the models presenting isotropic and non-isotropic interactions we have obtained remarkable results: i) the rare events dynamics in these systems, which is the precursor mechanism anticipating the glass transition, depends not only on the system dimension but also on the temperature; ii) we have also shown that, contrary to the classical theory of Brownian motion, these systems present distributions of displacements whose mean squared displacement evolves linearly (diffusively) with time, being the probability density of displacements non-Gaussian; iii) the observed Brownian yet non-Gaussian transport is a manifestation of collective dynamics. We also probed our model for elongated particles and saw that it satisfactorily explains the diffusion observed in real E. coli bacterial colonies.
Some of these findings contrasts with the current theoretical explanations of the Brownian yet non-Gaussian transport observed in a large variety of systems, opening new experimental perspectives on the class of molecular and supramolecular systems whose dynamics is ruled by rare events. We indeed expect to see our results manifested in other systems such as colloidal glasses and gels, granular systems, complex biological media controlled by pH, and other non-equilibrium active systems such as cells in migration processes or organelles moving in the cytoplasm of animal cells. Our model for elongated particles also showed the capability for capturing some relevant features manifested in the transport of real E. coli bacteria, making it an optimal candidate to study the collective dynamic and structural behaviour observed in these microbiological systems.
The communication of these results, and the dissemination of other topics related to the project, have reached different expert and non-expert audiences (including soft matter physicists, microbiologists, undergraduate students in physics and biology, and secondary schools students) and have had a presence in the media (see for details Publications and Miscellanea).