The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Image Analysis as a Tool to Study Electrokinetics of Heterogeneously Charged Colloids

by Lena Asavathiratham



Many synthetic and natural colloids have heterogeneous surfaces and are highly nonspherical. The charge heterogeneity and the geometry of the particle play important roles in the processing of these colloids. Conventional methods in characterizing the surface charge of colloidal particles rely on the assumption that the particle has homogeneous surface properties. Recent theories predict that a particle with a distribution of zeta potential having a dipole moment will rotate into alignment with the applied field. In this thesis, an experimental method was developed to study particle motion; in particular, particle rotation and alignment caused by nonuniformity of the surface charge. The new experimental setup and techniques allowed recording of the experimental observations of particle motion on a videotape. The images were then digitized for image processing and analysis.
The alignment effect was studied through sedimentation studies which involved single particle measurement of sedimentation velocity (the component of the velocity parallel to gravity) before and after the application of an electric field for kaolinite and latex particles. The disk-like kaolinite and the spherical latex displayed very similar sedimentation behavior which was neither a simple, steady sedimentation nor an accelerating motion with a terminal velocity parallel to gravity. The sedimentation velocity after the application of an electric field was found to be lower than before the field was applied, but after a period of time the particle accelerated and approached its sedimentation velocity equal to the value before application of the field. An observed time-dependent component of the sedimentation velocity for kaolinite was attributed to natural convection because it was also observed with latex. The sedimentation velocity measured for latex before the application of the electric field was in agreement with the hydrodynamic predictions, based on Stokes law.
The combination of video microscopy and digital image analysis in the experimental setup allowed determination of the y-coordinate (i.e., in the direction of gravity) on the order of +1 μm, an accuracy superior to the classical electrophoresis apparatus. Such accurate data provides a means of testing hydrodynamic models of particle motion far more accurately then previous techniques. The "by-hand" tracking of single particle used in this thesis sets the stage for computer algorithms which simultaneously track many particles and allow a large number of data manipulations on the same particle.