A Caltech Library Service

The Effects of Confinement in Active Matter: the Casimir Effect, Partitioning, and Hindered Diffusion


Kjeldbjerg, Camilla Maria (2022) The Effects of Confinement in Active Matter: the Casimir Effect, Partitioning, and Hindered Diffusion. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/avfw-fh81.


Active matter describes a class of materials for which constituent "particles" convert chemical energy into mechanical motion leading to self-propulsion (swimming). The origins of this swimming motion for both biological and synthetic constituents is a thriving area of research. However, here we focus on the physical properties and mechanics of the active matter systems. We model active particles using the active Brownian particle (ABP) model that is the simplest model that captures the essential physics, where a particle translates with a swim speed U0 in a direction q for a characteristic reorientation time τR; the average length they move between each reorientation is called the run, or persistence, length ℓ = U0τR. Owing to this persistent swimming, the ABPs distribute non-homogenously near surfaces, accumulating at no-flux boundaries leading to a concentration boundary layer near solid surfaces. Active particles often have an effective size—their run length—which can be much larger than their geometric size such that they experience confinement in geometries whose size is on the order of the run length. Active systems are inherently far from equilibrium, and we cannot appeal to properties of equilibrium thermodynamic such as the chemical potential to predict the partitioning. Fortunately, active particles are still subject to the laws of mechanics, and in this work, we present a simple macroscopic balance that allows one to predict behavior without detailed calculations. We predict the attractive force between two parallel plates in a reservoir (also called the Casimir effect) and find that the average concentration between the plates equals that in the bulk reservoir independent of the degree of confinement (ratio of run length to the spacing between the plates). We then examine the confinement effects in a channel geometry, where the behavior is fundamentally different, and the average concentration grows linearly with the degree of confinement. The understanding of these fundamental geometries motivated us to look into more complex geometries such as porous media. Based on dimensional analysis and our predictive model, we explain the transient behavior and steady-state partitioning of active particles between a fluid reservoir and a porous medium. Lastly, we discuss the hindered diffusion in periodic porous media and how the diffusion depends not only on the porosity of the medium but also on the degree of confinement. We believe that utilizing the insights in effects of confinement for these fundamental geometries and the porous media will be valuable in designing optimal structures for enhancing or isolating active particles.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Active Matter; Soft Matter; Partitioning ; Diffusion
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Brady, John F.
Thesis Committee:
  • Wang, Zhen-Gang (chair)
  • Brady, John F.
  • Seinfeld, John H.
  • Shapiro, Mikhail G.
Defense Date:9 July 2021
Record Number:CaltechTHESIS:12102021-231944176
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Ch. II adapted for Ch. III
Kjeldbjerg, Camilla Maria0000-0003-2224-0534
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14445
Deposited By: Camilla Kjeldbjerg
Deposited On:10 May 2022 23:54
Last Modified:10 Nov 2022 16:27

Thesis Files

[img] PDF - Final Version
See Usage Policy.


Repository Staff Only: item control page