## Dissertations and Theses

#### Date of Award

2019

#### Document Type

Dissertation

#### Department

Chemical Engineering

#### First Advisor

Ilona Kretzschmar

#### Keywords

Janus Rates Magnetic Tracking Aggregation Smoluchowski

#### Abstract

Magnetic Janus particles are spherical colloids covered with a thin magnetic cap on one half of their surface. The asymmetric magnetic material gives the particles a magnetic dipole shifted away from the center of mass of the particle. The magnetic dipole either points perpendicular (axially-shifted FexOy caps) or parallel (radially-shifted (Co/Pd)x multilayered particles) to the shift vector. When a magnetic field is applied to a system of magnetic Janus particles, the particles aggregate into chains aligned with the magnetic field. The rate of aggregation is of theoretical and practical interest and the focus of the thesis.

Optically studying samples of these magnetic Janus particles at resolutions fine enough to distinguish individual particles reveals important information about the bulk properties of the fluid, for example, particle area fraction, concentrations of different aggregates, nearest neighbor data, pair interaction forces, and diffusion coefficients. Widely used image processing algorithms are applied in novel ways in the thesis to collect and process individualized data relevant to the aggregating magnetic Janus particle systems. Individual frames are processed to extract path-independent information about the systems such as concentrations and nearest-neighbor data. Paths are drawn from the static data to find path-dependent information such as: trajectories, diffusion coefficients, and particle-pair energy landscapes.

Collectively, the data analyzed in the thesis reveals information about the components of the physics underlying magnetic particle aggregation. The rates at which various pairs aggregate is determined by both the: diffusion coefficients and magnetic dipole forces of the Janus particles along with the nearest neighbor information. From studying multiple experimental magnetic Janus particle systems of varying particle dipole strength, particle dipole shift, and initial concentration profiles; a general theoretical model is developed. The model uses both stochastic and deterministic forces to find the aggregation rates and predict concentration profile evolutions from the individual particle composition and initial concentrations. The theory developed and tested within the thesis has the practical application of being used for microscopic predictions of magnetic Janus particle behavior and bulk properties of the magnetorheological fluids that they make up. The software used to extract the assembly-determining parameters from the system has broad applicability for studying the physical properties of other non-reversible, discrete assembly processes, such as cell phagocytosis and aerial images of crowd movements.

#### Recommended Citation

Long, Thomas W., "Designing Particle Tracking Software for a Theoretical Understanding of Magnetic Janus Particle Aggregation" (2019). *CUNY Academic Works.*

https://academicworks.cuny.edu/cc_etds_theses/859