Thursday, May 24, 2012
106 Spalding Lab, Hartley Memorial Seminar Room
Chemcial Engineering Seminar
Tuning 2D and 3D suspension microstructure
James F. Gilchrist, Professor, Chemical Engineering, Lehigh University
Self-organization arises in systems when constituents having local repulsion are confined or have a long range attraction, resulting in rich phase behavior and/or pattern formation. However, it is generally unclear how these systems behave when subjected to deformation or when self-organization is coupled to the underlying flow. Two prototypical systems will be discussed, both of which have practical applications in device fabrication and suspension handling.
Convective deposition of nano- and microscale particles is used as a scalable nanomanufacturing platform to fabricate surface morphologies such as microlens arrays atop light emitting diodes (LEDs) and dye-sensitized solar cells (DSSCs) to considerably enhance the photon transport and various other energy, optical, and BioMEMS applications. The fundamental mechanism behind self-organization is attraction driven by the local capillary interactions of particles confined in a thin film of an advancing meniscus. We will highlight resulting morphology and various instabilities that occur during deposition of uni- and bimodal suspensions.
More generally, microstructure formation in sheared suspensions is key to understanding their rheological behavior. Thus far, limited experimental evidence is available to reveal the subtle details and dynamics of suspension microstructure. In this work, silica microsphere suspensions under pressure-driven flow are studied using dynamic confocal laser scanning microscopy. The pair distribution function, a measure of microstructure, is presented as a function of Péclet number, local volume fraction, and time in fully-developed and reversed flows. Suspensions of various pH values and electrolyte concentrations at flow cessation are also investigated out of an interest on the competition between hydrodynamic and electrostatic forces. These results capture the evolution of suspension microstructure as a consequence of changing shear field and of relaxation due to electrostatic repulsion upon flow cessation. Explanation for this evolution and its implication on suspension rheology is discussed.