Archive for June, 2007
Shear-induced migration of suspensions in 3D microfluidic geometries
We investigated shear-induced migration of 1 micron Brownian particles in 1D, 2D, 3D steady microfluidic flows generated in straight, herringbone, and staggered herringbone channels respectively. The transverse flows induced by recessed herringbone structures in the top of the channels interplay with particle migration to the low shear regions of the pressure-driven flow. Using high-speed confocal laser scanning microscopy, we were able to image directly flowing particles inside the channels. Moreover, we located the 3D positions for each particle and obtained 2D concentration and 2D velocity profiles to better understand of the effects from the underlying flow topology, colloidal hydrodynamics, and Reynolds and Pclet number on particle migration.
Study of Monomer Droplets in Miniemulsions
Miniemulsion technology offers the ability to produce latexes that cannot be prepared via conventional methods, such as those using highly water-insoluble monomers, or encapsulates of pigments, oils, and polymers. Fundamental understanding of miniemulsions has been hindered by the inability to measure and monitor their droplet size distribution (DSD), which is thought to lie in the range of 50 to 500 nm. The goals of this work are to characterize the DSD of miniemulsions, understand the mechanisms that determine it, and seek methods to control it. › Continue reading
Deposition of microsphere monolayers for microlens arrays
Colloidal silica microspheres of 0.5 and 1 micron were deposited into thin films on a glass substrate via a rapid convective deposition method. By varying deposition rate and contact angle, the optimal operating ranges in which 2D closed-pack of silica existed were obtained. Using a confocal laser scanning microscope, dynamic self assembly of colloidal particles under capillary force during solvent evaporation was revealed. In addition, interaction between substrate and colloidal particles played an important role in formation of ordered crystalline arrays. The interaction was altered by varying pH (2-11) and salt concentration of either substrate rinsing solution or colloidal suspension. Using the same technique, stacked layers of 1 micron silica monolayer on top of 1.1 micron polystyrene monolayers and subsequent melting of the polystyrene to partially wet the silica microspheres were deposited on GaN layer. This process was implemented on the top p-GaN layer of InGaN quantum wells light emitting diode (LEDs) device structure, resulting in the formation of a microlens array for enhancing its light extraction efficiency. This approach led to ~230% increase of the LEDs output power.
Supervisory Control of a Multi-Echelon Supply Chain: Structure, Modeling, Performance Measures and System Analysis for Inter-organizational Control
Petri nets are frequently utilized to model system dynamics due to their ability to handle concurrencies and sequential dependence. In this paper a portion of the Supply Chain Operations Reference (SCOR) model has been extracted and modeled using Petri nets for the purpose of exerting supervisory control upon a multi-echelon supply chain. The activities of source, make and deliver, inherent in the SCOR model form the basis of the representation of the Petri Net model for each echelon considered in the supply chain model. A supervisor is placed above the base model of each echelon to exert local constraints. These constraints are at the tactical and operational levels. An Enterprise level SCM is added which enforces additional constraints consisting of long term planning goals at the strategic level. Invariant analysis is used to create the supervisors. Performance measures of the supply chain as one entity are formulated to determine the effectiveness of any partnership. An efficient method for finding the current state of the system is developed which is used to determine the performance measures of each echelon. This paper presents an approach to the overall structure and Petri Net modeling for the system and is intended to extend the use of supervisory control from a shop-floor level to an inter-organizational facility and enterprise level.