Yen-Wen Lu

NANOSCALE SURFACE MODIFICATION TECHNIQUES FOR POOL BOILING ENHANCEMENT - A CRITICAL REVIEW AND FUTURE DIRECTIONS

New discoveries presented in the last decade for enhancing boiling performance utilizing nanoscale structures on surfaces are critically examined in this paper. Since the mechanism for such a phenomenon is not fully understood, this review mainly focuses on the experimental studies reported in the literature on the boiling phenomena on nanostructures, and implementation of nanostructures on various substrates. The paper also focuses on the interpretation of underlying phenomena for enhancing the boiling performance. The main influencing parameter in controlling is seen as the change in the surface energy of the boiling surface, which is characterized by the contact angle of the liquid and vapor phase interface at the heating surface. The nanostructures are seen to alter the contact angle. Design consideration and theoretical developments are also discussed, followed by practical aspects of nanostructure manufacturing. The issues related to performance, ease of fabrication, and durability (whenever available) are reviewed and recommendations are made for future research in this emerging area.

Microhand for biological applications

Micromanipulators, miniature devices capable of handling tiny objects, are useful in biomedical applications, as many biological entities exhibit dimensions on the micrometer scale. The applications of such devices include cell handling for research purposes, sample preparations for inspection, and microsurgical operations in clinical use. Some of these devices have been developed using traditional manufacturing methods, while others were produced with exploratory micro- and nanotechnologies. 1‐8

A SURFACTANT-MEDIATED RELAXED SI0.5GE0.5 GRADED LAYER WITH A VERY LOW THREADING DISLOCATION DENSITY AND SMOOTH SURFACE

A method to grow a relaxed Si0.5Ge0.5 graded layer with a very smooth surface and a very low threading dislocation density using solid-source molecular-beam epitaxy is reported. This method included the use of Sb as a surfactant for the growth of a 2 μm compositionally graded SiGe buffer with the Ge concentration linearly graded from 0% to 50% followed by a 0.3 μm constant Si0.5Ge0.5 layer. The substrate temperature was kept at 510 °C during the growth. Both Raman scattering and x-ray diffraction were used to determine the Ge mole fraction and the degree of strain relaxation. Both x-ray reflectivity and atomic force microscopy measurements show a surface root mean square roughness of only 20 A. The threading dislocation density was determined to be as low as 1.5×104 cm−2 as obtained by the Schimmel etch method. This study shows that the use of a Sb surfactant and low temperature growth is an effective method to fabricate high-quality graded buffer layers.

Observation of inter-sub-level transitions in modulation-doped Ge quantum dots

The inter-sub-level transitions in modulation-doped Ge quantum dots are observed. The dot structure is grown by molecular-beam epitaxy, and consists of 30 periods of Ge quantum dots sandwiched by two 6 nm boron-doped Si layers. An absorption peak in the midinfrared range is observed at room temperature by Fourier transform infrared spectroscopy, which is attributed to the transitions between the first two heavy-hole states of the Ge quantum dots. This study suggests the possible use of modulation-doped Ge quantum dots for improved infrared detector applications.

Pool Boiling Heat Transfer Enhancement Through Nanostructures on Silicon Microchannels

Uniform silicon nanowires (SiNW) were successfully fabricated on the top, bottom, and sidewall surfaces of silicon microchannels by using a two-step electroless etching process. Different microchannel patterns with the channel width from 100 to 300 lm were first fabricated in a 10 mm 10 mm silicon chip and then covered by SiNW with an average height of 10–20 lm. The effects of the microchannel geometry, micro/ nano-hierarchical structures on pool boiling were studied and the bubble dynamics on different sample surfaces were compared. It was found that the combination of the micro/nanostructures promoted microbubble emission boiling under moderate heat fluxes, and yielded superior boiling heat transfer performance. At given wall superheats, the maximum heat flux of the microchannel with SiNW was improved by 120% over the microchannel-only surface, and more than 400% over a plain silicon surface. These results provide a new insight into the boiling mechanism for micro/nano-hierarchical structures and demonstrate their potential in improving pool boiling performance for microchannels. [DOI: 10.1115/1.4007425]

Fabrication of nanowires on orthogonal surfaces of microchannels and their effect on pool boiling

A novel microfabrication technique was developed to create silicon nanowires (SiNWs) on orthogonal surfaces of microchannels machined on top of a silicon wafer. Using a two-step metal-assisted etching process, the SiNW for the first time could be selectively fabricated on two different crystalline directions—the channel top and bottom surfaces oriented in the (100) direction while the sidewall surfaces in the (110) direction. Different SiNW etching conditions were investigated to get the optimal height, density, morphology and orientation for the SiNWs on the microchannels. The resulting samples were tested as heat sinks with water for their pool boiling applications. The sidewall SiNWs affected bubble behaviors and played important roles during the boiling process. The critical heat flux and the heat transfer coefficient both were thus improved compared to a plain silicon surface and a silicon chip with only microchannels. (Some figures may appear in colour only in the online journal)

Pneumatically Driven Microcage for Microbe Manipulation in a Biological Liquid Environment

This paper describes the design, fabrication, and force analysis of a pneumatically driven microcage. Pneumatic actuation, suitable to function in a wide range of environments, is especially useful for operation in a small liquid space. The capture of single motile microbes (Volvox and Stentor ~400 mum) in aqueous environments is demonstrated with the microcage. The setup for the capture experiment includes a visualization scheme, a robotic micropositioner, and an electropneumatic control system that facilitates the underwater tracking of the moving microbes and the operation of the microcage for subsequent capture

A Foldable Microplasma-Generation Device on a Paper Substrate

We report the fabrication of a plasma-generating device on a paper substrate. This device was fabricated using a screen print process. Plasmas were ignited between two parallel electrodes with a plasma gap of 237 to 710 μm using a dc power source. We demonstrated that a stable helium plasma can be sustained when the substrate is flat, rolled, and folded along various orientations. When the plasma was ignited with a 0.2-μL salt solution droplet with 1.4, 4.6, and 7.8 ng of Li, Na, and K respectively applied to the discharge gap, clear metallic emission lines emanated from the plasma. Our result demonstrates that this paper-based plasma device can be used in analytical applications.

Enhancement of microfluidic particle separation using cross-flow filters with hydrodynamic focusing

A microfluidic chip is proposed to separate microparticles using cross-flow filtration enhanced with hydrodynamic focusing. By exploiting a buffer flow from the side, the microparticles in the sample flow are pushed on one side of the microchannels, lining up to pass through the filters. Meanwhile a larger pressure gradient in the filters is obtained to enhance separation efficiency. Compared with the traditional cross-flow filtration, our proposed mechanism has the buffer flow to create a moving virtual boundary for the sample flow to actively push all the particles to reach the filters for separation. It further allows higher flow rates. The device only requires soft lithograph fabrication to create microchannels and a novel pressurized bonding technique to make high-aspect-ratio filtration structures. A mixture of polystyrene microparticles with 2.7 μm and 10.6 μm diameters are successfully separated. 96.2 ± 2.8% of the large particle are recovered with a purity of 97.9 ± 0.5%, while 97.5 ± 0.4% of the small particle are depleted with a purity of 99.2 ± 0.4% at a sample throughput of 10 μl/min. The experiment is also conducted to show the feasibility of this mechanism to separate biological cells with the sample solutions of spiked PC3 cells in whole blood. By virtue of its high separation efficiency, our device offers a label-free separation technique and potential integration with other components, thereby serving as a promising tool for continuous cell filtration and analysis applications.

Constant‐current‐density model for the anomalous Hall effects in Hg0.8Cd0.2Te

This letter presents a simple model developed for calculating the Hall coefficient and Hall mobility in inhomogeneous narrow‐band‐gap semiconductors, including inclusions with opposite conduction type. The model is based on one assumption that the current density is a uniform constant in the sample in spite of the inhomogeneities. This assumption is supported by a variational calculation based on the principle of least entropy production rate. The model is then applied to explain the anomalous Hall data in the n‐type Hg0.8Cd0.2Te, and a good fit is obtained when p‐type Hg0.8Cd0.2Te is considered to be the inclusions. The results show that some assumptions in a previous layer model are not necessary in order to explain the observed phenomena. By using this simplified treatment of material inhomogeneity, it will be easier to design experiments to investigate the cause of such inhomogeneity.