In micro-dispensing applications, printhead activation mechanism, its design and operating parameters are integrated together to affect the droplet generation process. These factors give each printhead advantages and limitations over the others in specific fabrication. Hence, multiple printheads on micro level fabrication are preferred to perform multi-material dispensing task. In this paper, the mechanisms of two commonly used micro printheads, solenoid actuated micro-valve and piezoelectric printhead are discussed. Comprehensive experiments are conducted to characterize their performance and the results are analyzed so as to explore optimal droplet formation condition. With regards to the operational parameters' influence on droplet formation, micro-valve is investigated in terms of pressure, and operational on time, and piezoelectric printhead is investigated based on pulse amplitude, and width of driving pulse. Nozzle size, a key design parameter in the two printheads, is also studied according to its influence on dispensing capability. To facilitate dispenser selection, the two printheads are compared based on droplet size, droplet stability, droplet velocity, and dispensing viscosity of successful ejection. Other factors such as chemical compatibility, time consumption in determining optimal condition and reliability of dispensing process are also reported to play an essential role in this selection. Our investigation on the relationship between related parameters and dispensing performance will not only benefit dispenser selection in multi-material dispensing application, but also build a solid background to develop multiple printhead system for fabrication of bioengineering components.

No abstract

In this paper, numerical simulations are performed on the interaction of vortices with a longitudinal corrugated wall in a Taylor-Couette (TC) setting with the inner smooth surface cylinder rotating and the outer corrugated surface cylinder stationary. The motivation of the study is to shed light on how such an interaction affects the drag/torque with respect to two geometric parameters of the corrugations, namely, the wavelength λc* and amplitude A*, where * indicates a normalization by the gap width d. Results show that in the circular Couette flow regime, the secondary vortices induced by the corrugations cause the torque to increase. When λc*<1, there is a linear relationship between torque and λc*, and when λc*>1, there is a steeper increase of torque due to the interaction of the growing secondary vortices and the opposite wall. In the Taylor-vortex flow regime, the interaction between the Taylor vortices and the corrugations produces three distinct behaviors characterized by λc*. As the wavelength increases, our results show that the stronger modulation effects can override the inherent TC flow dynamics, which in turn leads to a wide range of flow structures that can have a significant impact on the resulting drag/torque characteristics. Generally, a torque reduction is achieved when λc*≤1, while forcing the Taylor vortices to stay on the crests of the corrugations can lead to significant improvement in torque reduction. Finally, the geometrical shape of the corrugations mainly alters the wall shear stress distribution on the corrugated wall, with a negligible effect on the flow dynamics when compared to λc*.

No abstract

No abstract