The temperature-dependent viscosity of nanofluids consisting carbon-based nanoparticles (CBNs), such as graphite, graphene oxide modified with oleic acid, and reduced graphene oxide, were investigated with steady-shear viscometry. The viscosity of nanofluids relative to that of the base fluid decreased until 30 °C and increased thereafter. Moreover, the CBN nanofluids had lower dynamic viscosity than the base fluid. Particle size distribution analysis revealed a polydispersed system of nanofluids, and intrinsic viscosity studies investigated the contribution of completely nanosized particles to the viscous behavior. Based on their lateral size, the CBNs played the role of percolation participants or Brownian motion participants. It was found that the total numbers of percolation participants varied among different types of CBN nanofluids. Furthermore, the size of the percolation participants influenced to optimize the number of nanoparticles undergoing Brownian motion. Regarding size effects and the affinity of the CBNs with the base fluid, the interaction between nanoparticles with different lateral sizes induced not only temperature-dependent viscosity in the nanofluids but also a reduction in dynamic viscosity. The mechanism underlying this lower viscosity was explained by a combination of percolation effects and nanoparticle structuring.

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J. Lee et. al., Journal of Molecular Liquids 325, pp.114659, 2021

Micro/nanotribology for biomedical applications

As a promising material for biomedical application, bio-nanocomposites have attracted great attention due to their excellent mechanical properties, low density, and enhanced chemical inertness. Understanding tribological properties of bio-nanocomposites, namely adhesive force, frictional force, and wear (scratch) resistance, is one of the critical issue in designing and developing biomedical applications such as surgery tools and artificial joints. However, tribological issues of biomedical bio-nanocomposites have not been properly studied. We have worked to develop micro/nanocrystalline diamond coated implant materials having reduced friction which is suitable for hemiarthroplasty. Additionally, the crucial design factors of nanohoneycomb composites for reducing the capability of bacterial adhesion have been studied. We demonstrate mechanically robust, tribologically suitable, and biocompatible nanocomposites using metals, ceramics, and various carbon materials including graphene, graphene oxide, and diamond-like carbon (DLC) in conjunction with the self-assembly techniques for biomedical applications. This research will also explore how bio-nanocomposites can improve and optimize the lifetime of structures and part quality for biomedical devices.

Micro/nanotribology for ultra-durable nanocomposites

Securing durability in nanocomposite-based structures/devices has been a problem for further improvement due to their unexpected interfacial failures. To eliminate such interface failures, research is needed on the tribological properties of nanocomposites, namely adhesive force, frictional force, and wear (scratch) resistance. Attempts have been made to study the fundamental tribological performance of nanocomposites. However, tribological issues of tunable nanocomposites have not been properly studied. This research area is aimed at studying how nanocomposites can improve the lifetime of structures and part quality and can optimize the settings for a given structure. Specifically, with nano-honeycomb composites, it is found that applying electrical potential to nanocomposites results in manipulation of friction and adhesive force, attributable to changes in the real contact area. We will focus on developing and engineering new nanocomposites designed for dynamic manipulation of contact areas, thus yielding more sustainable and robust micro-/nano-structures and devices.

S. Kim et. al., J. Appl. Phys., 2015

H. Xiao, S. Kim et. al., Carbon, 2014

S. Kim et. al., APL materials, 2013