|Ph.D Student||Nikola Nikolai|
|Subject||Forces in Active Systems|
|Department||Department of Physics||Supervisors||Professor Yariv Kafri|
|Professor Dov Levine|
|Full Thesis text|
Active systems are a class of non-equilibrium systems where ambient energy is converted into systematic motion on a microscopic scale. They are the focus of much attention due to a host of interesting physical phenomena, their ubiquity in biological systems - where they play crucial roles on scales ranging from the microscopic to the macroscopic - as well as their potential utility for self-assembly and microscopic engineering applications, offering novel perspectives beyond those of equilibrium systems. In this thesis, we study, from first principles, theoretically and in simulations, dry active systems consisting of spherical self-propelled particles, which convert energy into motion that is directed along an internally defined, stochastically changing polarity. We focus on the interactions of such systems with confining boundaries and passive inclusions, represented by external potentials. We show that despite non-trivial phenomenology near structured walls, including shear currents and stresses, and a non-uniform pressure profile, the total force exerted by active systems on a general boundary may satisfy an equilibrium-like equation of state. The pressure inhomogeneity on a curved surface is shown to lead to a modulational instability of flexible boundaries, as well as the spontaneous motility of short filaments. We find a general relation between the force exerted on objects immersed in active systems and the surrounding particle flux. We demonstrate how asymmetric objects experience a net steady-state force, and characterize the density and current fields generated by such objects, and how these lead to long range interactions between two or more objects that decay as a power law with the distance, are anisotropic, non-additive, and do not obey an action-reaction principle. These interactions may result in rich dynamics, such as the spontaneous synchronization and phase-locking of pinned rotors, and the system-wide ordering of large collections of rotors. Lastly, we calculate the drag-force exerted on soft objects in an active fluid, and demonstrate the viability of such objects exhibiting negative mobility, leading to the enhanced diffusion and the spontaneous motility of otherwise passive and symmetric objects.