报告人 Chien C. Chang （张建成）

Professor, Institute of Applied Mechanics , Center for Mechanics, Research Center

for Applied Sciences, Academia Sinica, Taiwan

 

主持人： 苏先樾  教授

 

时  间：3月27日（周二）下午2:30点

 

地  点：北京大学力学楼四层434会议室

 

 

欢迎广大师生光临！要求力学系研究生必须参加。

联系人：刘才山，62756177

Abstract ：

Part A: In this talk, we examine the band structures of phononic crystals with particular emphasis on the effects of the mass density ratio and of the contrast of elastic constants. The phononic crystals consist of arrays of different media embedded in a rubber or epoxy. It is shown that the density ratio rather than the contrast of elastic constants is the dominant factor that opens up phononic band gaps. The physical background of this observation is explained by applying the theory of homogenization to investigate the group velocities of the low-frequency bands at the center of symmetry of the first Brillouin zone. The theory is first illustrated for one-dimensional layered structures, which is simple and easy to understand. The two- and three-dimensional theory establishes a more sophisticated connection between the fine periodic microstructures and the group velocities of the lowest-frequency dispersion. Interesting comparisons will be made with band gaps of photonic materials in which the composed materials have different dielectric constants.

Part B: In this talk, I will describe a nano-liter mixer fully driven by capillary force, and explain its mechanics. It is known that surface tension-capillary pumping is an effective driving force in a microchannel, however a power-free mixer that uses only surface tension has not yet been achieved. In the present study, a power-free method is explored to perform mixing in a microchannel without any external active mechanisms such as pumps, valves or external energies like electrostatic or magnetic fields. The mixer is cost effective as the channel is designed to have no sidewalls with the liquid being confined to flow between a bottom hydrophilic stripe and a top-covered hydrophobic substrate. It is found from both theoretical analysis and experiments that for a given channel width, the flow rate solely due to capillary pumping can be maximized at an optimal channel height. The flow rate is in the order of nanoliters per second, for example, the flow rate is 0.65 nL s(-1) at the optimal channel height 13 mu m, given the channel width 100 mu m. It is most crucial to this power-free mixing device that two liquid species must be well mixed before the liquids are transported to exit to a reservoir. For this purpose, asymmetric staggered grooved cavities are optimally arranged on the bottom substrate of the channel to help mixing two different liquid species. It is shown that maximum mixing occurs when the depth of the grooved structures is about two-thirds of the total channel height.

 

Biography ：

Ph.D. Mathematics, University of California, Berkeley (1985)

M.S. Applied Mechanics, National Taiwan University (1982)

B.S. Chemical Engineering, National Taiwan University (1980)

Fields of Interest: Multi-scale Mechanics; Photonics and Plasmonics, Biomedical Mechanics, Multi-body Fluid Mechanics