Kam Chuen Ng

Developing flow of a power-law liquid film on an inclined plane

Developing flow of a liquid film along a stationary inclined wall is analyzed for a power-law constitutive equation. For films with appreciable inertia and therefore small interfacial slopes, the boundary-layer approximation may be used. The boundary-layer equations are solved numerically through the von Mises transformation that gives a partial differential equation over a semi-infinite strip and approximately by the method of von Karman and Polhausen that gives an ordinary differential equation for the film thickness, called a film equation. Film equations derived from self-similar velocity profiles fail when the film thickens and the flow undergoes a supercritical to subcritical transition; a nonremovable singularity arises at the critical point, the location of the flow transition. A film equation is developed that accommodates this transition. Predictions exhibit a standing wave where hydrostatic pressure becomes important and opposes inertia. This thickening effect is accentuated for small angles of inclination at moderate Reynolds numbers. In the limit of small film thickness in which gravitational effects are negligible, the thickness profile is nonlinear in agreement with an independent and new similarity solution. This result contrasts with the established linear thickness profile for a Newtonian liquid. The circumstances in which the film equation gives results close to the full boundary layer equation are identified.

Developing Film Flow on an Inclined Plane With a Critical Point

Viscous, laminar, gravitationally-driven flow of a thin film on an inclined plane is analyzed for moderate Reynolds number under critical conditions. A previous analysis of film flow utilized a momentum integral approach with a semiparabolic velocity profile to obtain an ordinary differential equation for the film thickness for flow over a round-crested weir, and the singularity associated with the critical point for a subcritical-to-supercritical transition was removable. For developing flow on a plane with a supercritical-to-subcritical transition, however, the same approach leads to a nonremovable singularity. To eliminate the singularity, the film equations are modified for a velocity profile of changing shape

MEMS-Based Microfluidic Devices

Micro-Electro-Mechanical Systems (MEMS) technology can be integrated with microfluidic functionality to enable the generation of microdrops with unprecedented throughput and precise control of drop volume, speed, and placement. The most prominent examples of microdrop generators are in the field of inkjet printing where printheads with thousands of nozzles produce steady streams of microdrops at kilohertz repetition rates. In this paper, we discuss a proposed MEMS-based microfluidic drop generator that operates on the basis of a thermally induced Marangoni effect. We describe the physics of droplet generation and discuss operating performance relative to the fluid rheology, thermal modulation, and wavelength dependencies.© 2010 ASME

P‐183: The Barrier Problem in Flexible OLED Display: A Modeling Approach

One of the major challenges in the development of flexible OLED display is the protection of the OLED materials from damage by ambient moisture and oxygen. This requires the use of polymeric substrates with exceedingly low levels of moisture and oxygen permeabilities. This problem is typically addressed through the use of multi-layer barrier coatings comprising alternating organic/inorganic layers, an approach developed and promoted by Vitex Corp. The multi-layer approach is critically examined through a numerical model based on a defect-dominated diffusion process combined with experiments involving face-to-face lamination of two barrier films. The modeling results identify two regimes that correspond to two distinct permeation mechanisms, and provide general scaling relationships and design criteria for producing multi-layered coatings with superior barrier performance. The results suggest that the most significant gain in barrier performance can be realized when the thickness of the organic/adhesive layer(s) is less than the average pinhole (defect) size in the inorganic barrier layer(s).