Jit Kai Chin

Coupled effects of hydrophobic layer and vibration on thermal efficiency of two-phase closed thermosyphons

Condensed droplets in a two-phase closed thermosyphon (TPCT) are subject to two competing forces: contact line pinning force between the droplet and the wall of the TPCT and the body force due to gravity. Either reducing the contact line pinning force or increasing the body force can lead to significant enhancement in the heat transport capability. This study aims to scrutinize the coupled effects of hydrophobic surface coating at the condenser wall and high-acceleration induced vibration on the thermal efficiency of a TPCT. We explore an approach to reduce the contact line pinning force by applying a thin layer of hydrophobic coating, which also facilitates dropwise condensation to further increase the heat transport capability. The body force of the condensed droplets can be increased by introducing a low-frequency (f ∼ 102 Hz) high-acceleration ( ∼ 103 m s−2) vibration. The formation of elongated liquid jets and entrainment of droplets induced by the high-acceleration vibration counteracts the enhancing effect from the increased body force of condensed droplets. Nanofluid with distinguished thermo-physical properties is charged to the TPCT to further enhance the thermal efficiency. By incorporating the coupled effects of hydrophobic layer and vibration, we can obtain a maximum augmentation in the heat transfer coefficient exceeding 47.7%. The factors contributing to the enhancement of thermal efficiency of a TPCT are identified and the underlying physical significance of the coupled effects is delineated.

Progress on Development of Al2O3-SiO2 Ceramic MEMS- based Monopropellant Micropropulsion System

An Al2O3-SiO2 ceramic MEMS-based micropropulsion system was designed and fabricated using new fabrication route. The micropropulsion system consists of five major components: propellant reservoir, injector, electrodes, combustion chamber, and micronozzle; all integrated into a single volume of 20 mm × 20 mm × 5 mm. 85-15 wt% of HAN-water mixture was self-prepared and used as propellant along with electrolytic mechanism for ignition. Performance of the system was predicted numerically. The system was fabricated by microcasting the well-dispersed suspension of submicron Al2O3 powder and PSZ preceramic resin into a lithography-prepared PDMS soft mold and sintering at 1200°C to form the ceramic composite. Shrinkage of about 20% was noticed after sintering. TGA analysis reveals good thermal stability of the composite as weight loss at temperature of 1000°C is as small as 1.3wt%. SEM-EDX analysis verifies the formation of Al2O3-SiO2 composite as only elements of Al, O, and Si exist with a negligible amount of residual carbon. Current fabrication route hold the promise to fabricate MEMS micropropulsion system using different ceramic composite materials to meet different design requirements.

Analysis of Liquid–Liquid Droplets Fission and Encapsulation in Single/Two Layer Microfluidic Devices Fabricated by Xurographic Method

This paper demonstrates a low cost fabrication approach for microscale droplet fission and encapsulation. Using a modified xurography method, rapid yet reliable microfluidic devices with flexible designs (single layer and double layer) are developed to enable spatial control of droplet manipulation. In this paper, two different designs are demonstrated, i.e., droplet fission (single layer) and droplet encapsulation (double layer). In addition, the current fabrication approach reduces the overall production interval with the introduction of a custom-made polydimethylsiloxane (PDMS) aligner. Apart from that, the fabricated device is able to generate daughter droplets with the coefficient of variance (CV) below 5% and double emulsions with CV maintained within 10% without involvement of complex surface wettability modification.

The effect of different structures of embroidery patch antenna on polymer substrate with identical embroidery properties

This paper presents a novel class embroidery patch antenna on polymer composite - polydimethylsiloxane (PDMS). By lamination and polymer integration, different structures of fully embroidered polymer patch antenna with ground plane have been designed, fabricated and tested. Analysis of the effect of conductive patch weight, conductive characteristics using different embroidery structure on antenna performance has been carried out. The measured results show that although double embroidered layer on one side of fabric has similar conductivity and identical embroidery properties as two-sided embroidery, the antenna performs better using two-sided embroidery structure.

Enhancing Electroconductivity of Yytria-Stabilised Zirconia Ceramic Using Graphene Platlets

A simple method to produce graphene-Yytria stabilised zirconia (YSZ) ceramic composite with significant improvement in electrical properties is reported here. The material was consolidated by annealing in presence of Argon gas that allowed densification of the ceramics. A detailed x-ray diffraction (XRD) analysis was used to study the phases and crystallinity of graphene-YSZ ceramic composite. XRD patterns of the sintered composite showed that graphene diffraction peaks were detected at 2θ≈27°. Furthermore, experimental results indicate that electrical conductivity of YSZ composites drastically increased with the addition of graphene platelets, and it reached a value of 2.8 S/cm at 2 wt.%.

Droplet Breakup Dynamics in Bi-Layer Bifurcating Microchannel

Breakup of droplets at bi-layer bifurcating junction in polydimethylsiloxane (PDMS) microchannel has been investigated by experiments and numerical simulation. The pressure drop in bi-layer bifurcating channel was investigated and compared with single-layer bifurcating channel. Daughter droplet size variation generated in bi-layer bifurcating microchannel was analyzed. The correlation was proposed to predict the transition between breakup and non-breakup conditions of droplets in bi-layer bifurcating channel using a phase diagram. In the non-breakup regime, droplets exiting port can be switched via tuning flow resistance by controlling radius of curvature, and or channel height ratio. Compared with single-layer bifurcating junction, 3-D cutting in diagonal direction from bi-layer bifurcating junction induces asymmetric fission to form daughter droplets with distinct sizes while each size has good monodispersity. Lower pressure drop is required in the new microsystem. The understanding of the droplet fission in the novel microstructure will enable more versatile control over the emulsion formation, fission and sorting. The model system can be developed to investigate the encapsulation and release kinetics of emulsion templated particles such as drug encapsulated microcapsules as they flow through complex porous media structures, such as blood capillaries or the porous tissue structures, which feature with bifurcating junctions.

Recent developments in ceramic microthrusters and the potential applications with green propellants: a review

Conventional chemical propellants such as hydrazine and ammonium perchlorate have been used within the realm of contemporary space propulsion devices and are well established owing to their rich heritage. However, their limitations such as toxicity, difficulty in operational handling and environmental impacts have raised concerns. In view of these limitations, the significance of green propellants such as hydroxylammonium nitrate, hydrogen peroxide (H2O2) and ammonium dinitramide has become more pronounced. In this paper, recent developments in ceramic microthrusters and the associated ceramic microfabrication techniques are reviewed. The characteristics of green propellants are examined, followed by the evaluation of previous attempts to incorporate green propellants into ceramic microthrusters. This has further unveiled the possibilities of green and clean space missions in the future.

Investigation of embroidery conductive layer dyed with graphene and ZnO

This paper presents a method to improve the conductivity of embroidered patch antenna by using graphene and zinc oxide (ZnO) nanopowder. By dispersion and immersion of graphene and ZnO, the embroidered conductive patch layers have been designed, fabricated and tested. Analysis of the effect of graphene and ZnO solution on patch resistance has been carried out. The measured results show that the conductivity performs better with ZnO dyed embroidered patch layer. In contrast, graphene improves the sheet resistance while ethanol solvent reacts with silver thread.

Study on Mechanical Properties of Zirconia-Alumina Based Ceramics

This paper describes the characterisations of ceramic composites consisting of different compositions of alumina and zirconia. The material characterisations were performed from the aspects of densification, hardness and fracture toughness. The surface morphology and elemental composition of the composite were studied using SEM and EDX respectively. As for physical properties, the highest attainable hardness and fracture toughness were 11.35 GPa and 3.41 MPa m0.5 respectively for ceramic composite consisted of 80 wt % Zr and 20 wt% Al. Sintering at 1150oC assisted in the densification of ceramics.