Investigation of a system with a tunable anharmonic interaction potential using paramagnetic colloids
Biswal, Sibani L
Doctor of Philosophy
Colloidal physics dictates properties of many small-scale and large-scale systems with applications ranging from drug delivery to catalysis. Colloidal systems can also be used as emulsion stabilizers or used in liquid crystal displays. Scientists and engineers have studied colloids to model the behavior of macroscopic systems at the atomic level. Colloids are uniquely suited for this application because the associated length scales are large enough to observe under an optical microscope, yet small enough so that their dynamics are driven by thermal motion. In this work, we manipulate magnetically induced and negatively charged colloidal particles in which we can control their interactions by applying a tunable magnetic field. Magnetically tunable particles provide us the ability to control the structure and phase behavior of the colloidal dispersions. Tunability allows for precise manipulation of particle interactions and thus, particle assembly into colloidal agglomerates that exhibit both fluid-like and crystal-like properties. These collections of particles nucleate, coalesce, and grow over time, which are properties of a model system for non-equilibrium and quasi-equilibrium behaviors. At quasi-equilibrium, these systems do not exhibit great changes in morphology, but can still coarsen over time. A non-equilibrium state is characterized by dynamics such as nucleation, decomposition, and coalescence. The system changes at a scale large enough to affect the energetics and morphological properties of the system. This dissertation is divided into two parts, one that focuses on the characterization of what we call colloidal clusters which are finite-sized aggregates that form in a sample with low particle concentration. These aggregates are individual islands of particles with adequate spacing such that we are able to examine them individually. The second half of this thesis describes the kinetics that take place when a colloidal dispersion undergoes quenching, causing behavior analogous to that of phase separating systems. We quantify the stability and kinetics of the system by measuring thermodynamic properties and morphological features as a function of three main parameters: time, t , field strength, B, and particle concentration. We characterize the phase behavior by considering both the bulk and interfacial properties of colloidal aggregates. We also find a scaling relationship between the three parameters to predict the aggregation kinetics governing systems that undergo quenching caused by long-range interactions.
Magnetism; colloids; controlled assembly; energetics.