Bao Quoc Ta

Geometric effects on mixing performance in a novel passive micromixer with trapezoidal-zigzag channels

A novel passive micromixer, called a trapezoidal-zigzag micromixer (TZM), is reported. A TZM is composed of trapezoidal channels in a zigzag and split–recombine arrangement that enables multiple mixing mechanisms, including splitting–recombining, twisting, transversal flows, vortices, and chaotic advection. The effects of geometric parameters of the TZM on mixing performance are systematically investigated by the Taguchi method and numerical simulations in COMSOL Multiphysics. The number of mixing units, the slope angle of the trapezoidal channel, the height of the constriction element, and the width ratio between the middle-trapezoidal channel and the side-trapezoidal channel are the four parameters under study. The mixing performance of the TZM is investigated at three different Reynolds number (Re) values of 0.5, 5, and 50. The results showed that a TZM with six mixing units, a trapezoidal slope angle of 75°, a constricting height of 100 µm, and a width ratio of 0.5 has the highest mixing efficiency. This optimal TZM has a mixing efficiency greater than 85% for Re values from 0.1 to 80. In particular, for Re ≤ 0.9 and Re ≥ 20, the mixing efficiency of the optimal TZM is greater than 90%. The proposed TZM has a higher mixing efficiency and a smaller footprint than previously reported micromixers.

Electrical control of synthesis conditions for locally grown CNTs on polysilicon microstructure

Carbon Nanotubes (CNTs) have been extensively studied in recent years for various applications using their extraordinary properties. Integration of CNTs into microsystems and CMOS circuits enables devices that use CNTs as nano-functional components. This work investigates the local synthesis and integration of CNTs into a microsystem fabricated by a commercial process - PolyMUMPs. The proposed synthesis process is automated, scalable to wafer-level, and performed at ambient temperature. The obtained results show great promise for a low-cost, wafer-level process for the integration of CNTs into microsystems and CMOS circuits.

Carbon nanotube based gas sensor for expiration detection of perishable food

With increasing pressure on food industry to reduce waste while meeting the demand for high food quality and safety, there is a strong motivation for research into sensors for monitoring freshness and general food quality. Degradation of fresh meat and produce is related to metabolic activities of microorganisms which commonly result in formation of gaseous compounds such as CO2. Monitoring the freshness and quality of the food can therefore be accomplished by monitoring the release of these gases in the food package. In this study, an approach to create sensors for detecting gases released by perished food is described. Sensor test is conducted with CO2 which is a common gas related to the degradation process of food. The sensor is based on Carbon Nanotubes (CNTs) as sensing elements. A local synthesis technique is used to directly integrate the CNTs onto silicon-based circuits at room temperature. Fabricated CNT-based structures have successfully demonstrated the ability to detect CO2. The sensitivity and selectivity of gas sensors to different gas compounds were also studied. The results indicate that this approach is promising for fabrication of CNT-based gas sensors at low cost for determination of food freshness and quality, making possible complete gas sensors including CNT sensing elements, CMOS signal processing and RF communication on the same chip, manufactured and integrated at the wafer-level scale.

Low-Cost Fabrication of Hollow Microneedle Arrays Using CNC Machining and UV Lithography

In order to produce disposable microneedles for blood-collection devices in smart homecare monitoring systems, we have developed a simple low-cost scalable process for mass fabrication of sharp-tipped microneedle arrays. The key feature in this process is a design of computer numerical control-machined aluminum sample (CAS). The inclined sidewalls on the CAS enable microfabricated traditional-shaped microneedles (TMNs) to be produced in the desired shape. This process provides significant advantages over other methods that use inclined lithography or anisotropic wet etching. TMNs with a length of 1510 μm, a hollow diameter of 120 μm, and the tip radius of 16 μm were successfully fabricated. Theoretical study and practical measurements of fracture force verified the improved mechanical strength of TMNs for safe skin insertion. In addition, the penetration tests on cadaver pork skin demonstrated that the TMNs could pierce the pork skin without breaking, and create the transport conduits through microneedle lumens.

Diameter dependency for the electric-field-assisted growth of carbon nanotubes

Controlling the growth orientation of carbon nanotubes (CNTs) is a topic of great importance in the integration of CNTs into micro/nanosystems. The electric field has been exploited to control the growth orientation of CNTs. In this letter, the diameter dependency of the effect of an electric field on the growth orientation of CNTs is reported. Statistical analysis of the direction and the straightness of CNTs show that the electric field has a positive effect on small-diameter CNTs, but not on large-diameter CNTs. An electrostatic and thermodynamic model is proposed for an explanation of this phenomenon.

Integration of Carbon Nanotubes in Microsystems: Local Growth and Electrical Properties of Contacts

Carbon nanotubes (CNTs) have been directly grown onto a silicon microsystem by a local synthesis method. This method has potential for wafer-level complimentary metal-oxide-semiconductor (CMOS) transistor-compatible integration of CNTs into more complex Si microsystems; enabling, e.g., gas sensors at low cost. In this work, we demonstrate that the characteristics of CNTs grown on specific locations can be changed by tuning the synthesis conditions. We also investigate the role of the contact between CNTs and the Si microsystem; observing a large influence on the electrical characteristics of our devices. Different contact modes can render either an ohmic or Schottky-like rectifying characteristics.

Annealing nano-to-micro contacts for improved contact resistance

Nano-to-micro electrical contact resistance between carbon nanotubes (CNTs) and larger-scale silicon systems are investigated using both low-and high-power annealing techniques. Carbon nanotubes locally synthesized and suspended between two silicon microbridges are used as the test platform. The annealing technique involves Joule heating of either the CNT/silicon system or the secondary silicon bridge only at low or high input power for various times. Of the 15 samples tested, results show that the contact resistance decreases for 60% of the samples and two of the 15 samples show a decrease in contact resistance greater than 50%. Higher power and longer time anneals show the greatest improvement in reducing the contact resistance. This technique can potentially reduce the contact resistance for integration of CNTs with MEMS or microelectronics systems.

Deposition of palladium on suspended and locally grown carbon nanotubes using thermal evaporation

We present the potential of using a simple thermal evaporation process to deposit Palladium (Pd) nanoparticles on suspended and locally grown CNTs. Functionalization of CNTs with Pd is known to enhance the sensitivity and recovery towards detection of H2 and CH4. The functionalization processes reported previously in literature are generally dispersion and sonication. Such processes are destructive and not compatible to systems where CNTs are suspended and span two Si electrodes. Functionalization of CNTs using thermal evaporation represents an opportunity to fabrication of CNT-based sensors with a high scalability and CMOS/MEMS compatibility.

Observations on defects and contact modes for locally grown CNTs

Local synthesis and direct integration of Carbon Nanotubes (CNTs) with Si microstructures at room temperature has opened the prospects of a low-cost, wafer-level and CMOS-compatible processing of CNT-based devices. However, the performance of such devices depends upon the quality of CNTs and the contact between CNTs and the silicon structure. Therefore, the defects of CNTs and the CNT-silicon contacts need to be examined. In this paper, Scanning Electron Microscopy (SEM) observations are reported. Three types of CNT defects were observed: voids, branches, and coating flakes. The varying density and diameter distribution of CNTs grown along the microstructure were also revealed. The observations provide a better understanding of the local synthesis process.

A novel design of passive split and recombination micromixer with trapezoidal zigzag channels

In this paper, we proposed a novel passive micromixer to provide effective mixing performance even at very low Reynolds number (Re). The micromixer consists of trapezoidal channels that facilitate the mixing between top-fluid and bottom-fluid. In addition, the zigzag arrangement of trapezoidal channels creates transversal flows, thus enhancing the mixing in transversal direction. The micromixer also creates high distortion of fluid streams based on chaotic advection, producing very high mixing efficiency. We conducted numerical study to evaluate mixing performance of our micromixer using COMSOL Multiphysics based on Finite Element Method. The simulation results indicated that the micromixer has a high mixing efficiency over 81% for an entire range of Reynolds number from 0.1 to 80. A mixing efficiency over 90% was achieved when Re ≤ 0.9 and Re ≥ 20. When operating with Re = 20, our micromixer achieved a mixing efficiency that are 4.07 and 5.58 times higher than those of 3-split rhombic micromixer and T-shaped micromixer. Furthermore, our micromixer has a small footprint, which is advantageous to integrate into various microfluidic systems.