Authors:	Shea, Jeanine
Advisor:	Schmid, Friederike
Title:	Effective equation of motion of a passive particle immersed in an active fluid
Online publication date:	9-Jun-2023
Year of first publication:	2023
Language:	english
Abstract:	Implicit models of passive, equilibrium systems have been used for many years to study and understand the physical behavior of such systems. Given the success of understanding equilibrium systems through such models, recent studies have focused on coarse-graining non-equilibrium systems. However, non-equilibrium systems are highly dependent on the system’s dynamic properties, which are necessarily altered in a coarse-grained model. Furthermore, certain assumptions which are made in equilibrium coarse-grained modeling are no longer valid beyond equilibrium. Therefore, coarse-graining of non-equilibrium systems must be done with extreme care and consciousness of the true system dynamics. Perhaps the most ubiquitous example of a coarse-grained model is the Brownian particle model, in which the motions of fluid particles surrounding a much larger, immersed particle are only implicitly represented to eliminate these degrees of freedom. In this thesis, we address the question: what happens when the fluid is far from equilibrium? We explore the dynamics of a passive probe particle immersed in an active bath and its implications for coarse-grained modeling. We use effective generalized Langevin equations, which explicitly include memory effects, to examine the immersed probe dynamics. In the first part of this thesis, we classify the behaviors that signify the system’s non- equilibrium nature. Although the probe adopts many active-particle-like behaviors, the trajectory of the probe does not exhibit obvious non-equilibrium signatures. To tell that the probe is out of equilibrium requires examination of its behavior in tandem with that of the active fluid. Alternatively, applying a small perturbation to the probe, reveals a violation of the first fluctuation dissipation theorem. In the second part of this thesis, we determine the mechanism behind the active-particle-like behavior of the probe. This behavior cannot simply be attributed to the convective motion of the active bath. Instead, the boundary of the probe contributes significantly to these adopted dynamics by causing active bath particles to accumulate behind the probe with respect to its instantaneous velocity. This gathering of active bath particles then pushes the probe, which in turn promotes its active-particle-like behavior. In the final part of this thesis, we map the dynamics of a probe immersed in an active bath and subject to an external force onto an equilibrium coarse-grained model. We find that this system can be mapped onto a physically meaningful coarse-grained model. However, due to the activity of the bath, the external force in such an equation is not equal to the physical external force, but rather a renormalized external force.
DDC:	530 Physik 530 Physics
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 08 Physik, Mathematik u. Informatik
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-9094
URN:	urn:nbn:de:hebis:77-openscience-46597675-832c-4188-ac7d-b78082482d122
Version:	Original work
Publication type:	Dissertation
License:	CC BY
Information on rights of use:	https://creativecommons.org/licenses/by/4.0/
Extent:	118 Seiten ; Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen