Pengtao Wang

Effects of Particle Surface Charge, Species, Concentration, and Dispersion Method on the Thermal Conductivity of Nanofluids

The purpose of this experimental study is to evaluate the effects of particle species, surface charge, concentration, preparation technique, and base fluid on thermal transport capability of nanoparticle suspensions (nanofluids). The surface charge was varied by changing the pH value of the fluids. The alumina ( Al 2 O 3 ) and copper oxide (CuO) nanoparticles were dispersed in deionized (DI) water and ethylene glycol (EG), respectively. The nanofluids were prepared using both bath-type and probe sonicator under different power inputs. The experimental results were compared with the available experimental data as well as the predicted values obtained from Maxwell effective medium theory. It was found that ethylene glycol is more suitable for nanofluids applications than DI water in terms of thermal conductivity improvement and stability of nanofluids. Surface charge can effectively improve the dispersion of nanoparticles by reducing the (aggregated) particle size in base fluids. A nanofluid with high surface charge (low pH) has a higher thermal conductivity for a similar particle concentration. The sonication also has a significant impact on thermal conductivity enhancement. All these results suggest that the key to the improvement of thermal conductivity of nanofluids is a uniform and stable dispersion of nanoscale particles in a fluid.

An ultrasensitive quartz crystal microbalance-micropillars based sensor for humidity detection

A unique sensing device, which couples microscale pillars with quartz crystal microbalance (QCM) substrate to form a resonant system, is developed to achieve several orders of magnitude enhancement in sensitivity compared to conventional QCM sensors. In this research, Polymethyl Methacrylate (PMMA) micropillars are fabricated on a QCM substrate using nanoimprinting lithography. The effects of pillar geometry and physical properties, tuned by molecular weight (MW) of PMMA, on the resonant characteristics of QCM-micropillars device are systematically investigated. It is found that the resonant frequency shift increases with increasing MW. The coupled QCM-micropillars device displays nonlinear frequency response, which is opposite to the linear response of conventional QCM devices. In addition, a positive resonant frequency shift is captured near the resonant point of the coupled QCM-micropillars system. Humidity detection experiments show that compared to current nanoscale feature based QCM sensors, QCM-mic...

Ultrasensitive quartz crystal microbalance enabled by micropillar structure

We report a method to significantly enhance the mass sensitivity of a quartz crystal microbalance (QCM) device in which Polymethyl Methacrylate (PMMA) micropillars were fabricated on QCM surface to form a two-degrees-of-freedom vibration system. PMMA micropillars were fabricated using nanoimprinting lithography technology. The QCM-micropillar coupled system exhibits a unique resonant frequency, near which the mass sensitivity of QCM can be enhanced by several orders of magnitude. Both numerical simulation and theoretical analysis were conducted to understand this improvement. Thereafter, ultrahigh sensitivity of the QCM-micropillar system was demonstrated by detecting a 1H, 1H, 2H, 2H-perfluorooctyl-trichlorosilane single monolayer film.

Detection of Liquid Penetration of a Micropillar Surface Using the Quartz Crystal Microbalance

A quantitative characterization of the wetting states of droplets on hydrophobic textured surfaces requires direct measurement of the liquid penetration into surface cavities, which is challenging. Here, the use of quartz crystal microbalance (QCM) technology is reported for the characterization of the liquid penetration depth on a micropillar-patterned surface. The actual liquid-air interface of the droplet was established by freezing the droplet and characterizing it using a cryogenically focused ion beam/scanning electron microscope (cryo FIB-SEM) technique. It was found that a direct correlation exists between the liquid penetration depth and the responses of the QCM. A very small frequency shift of the QCM (1.5%) was recorded when the droplet was in the Cassie state, whereas a significant frequency shift was observed when the wetting state changed to the Wenzel state (where full liquid penetration occurs). Furthermore, a transition from the Cassie to the Wenzel state can be captured by the QCM technique. An acoustic-structure-interaction based numerical model was developed to further understand the effect of penetration. The numerical model was validated by experimentally measured responses of micropillar-patterned QCMs. The results also show a nonlinear response of the QCM to the increasing liquid penetration depth. This research provides a solid foundation for utilizing QCM sensors for liquid penetration and surface wettability characterization.

Control of Non-Spherical Particles in Microfluidic Channel

A precise control of orientation and position of micro- and nano-particles is critical for their applications in micro- and nano-scale systems. This research is focused on a numerical investigation of controlling the cylindrical particle/road with transverse electric field in a pressure driven microchannel flow with Finite Element Method (FEM). The particle/rod is initially located at the channel centerline with different initial angles. The electrophoretic force and hydrodynamic force are combined to control particle/rod movement. It was found that pressure, applied voltage and initial angles have different impact on particles dynamic behavior.Copyright

Hydrophobicity of Nanostructured Films Characterized by a Quartz Crystal Microbalance

The hydrophobicity of two types of nanostructured polymer films were fabricated and characterized with a novel quartz crystal microbalance (QCM) technique to investigate their static and dynamic hydrophobic properties. The nanofibrous films of polymethylmethacrylate (PMMA), PMMA/Polydimethylsiloxane (PDMS) and Polyacrylonitril (PAN) were prepared with an electrospinning process and a PMMA film with nanoscale roughness was fabricated using nanoimprint lithography (NIL) technique. Significantly different static and dynamic hydrophobicities (wettability) were found among these films and the correlation between hydrophobicity and the mechanical impedance of QCM to these films were developed both experimentally and theoretically. It was shown that QCM is capable of quantitatively characterizing the hydrophobicity of these nanostructured polymer surfaces. For nanofibrous films, the double layers — a viscoelastic nanofiber film and a liquid layer result in a nonlinear combination of mechanical impedances of QCM. To simplify the analysis, an apparent viscosity was introduced in the analysis to take into account the interactions between liquid and polymer surfaces. For NIL PMMA film, the hydrophobicity was altered by coating nano-roughened surface with a Teflon layer. The reduction in the mechanical impedance of QCM clearly demonstrates the enhancement of hydrophobicity. The experimental results showed that the hydrophobic surface lead to a small mechanical impedance while the hydrophilic surface resulted in a large mechanical impedance of QCM.Copyright

Hydrophobicity of Electrospun Nanofibers Films Characterized by a Quartz Crystal Microbalance

The quartz crystal microbalance (QCM) sensor is used to study the dynamic characteristic of hydrophobic nanofiberous surfaces. The nanofibrous films of polymethyl methacrylate (PMMA), PMMA/ Polydimethylsiloxane (PDMS) and Polyacrylonitrile (PAN) were prepared with an electrospinning process for different hydrophobicity (wettability). The mechanical impedance analysis of DI water on a fibrous coated QCM surface is able to quantitatively characterize the hydrophobicity of these nanofibers surfaces. The two layers including a viscoelastic nanofiber film and a liquid layer result in a nonlinear combination of mechanical impedances. To simplify the analysis, an apparent viscosity was introduced in the analysis to account for the surfacial slip effect. The experimental results showed that the hydrophobic surface resulted in small mechanical impedance loading and low value of apparent viscosity, while the hydrophilic surface generated large mechanical impedance and gave high value of apparent viscosity.Copyright

Investigation of Nanofibrous Film Coating Effect on Surface Acoustic Wave Sensors

Electrospinning is reported in this paper as a new coating approach for surface acoustic wave (SAW) sensor in order to enhance its chemical detection capability. Ultrafine (100–300 nm) polyethylene oxide (PEO) fibrous film with controlled thickness and porosity were electrospun-coated on the surface of a ST-X quartz based SAW sensor. Compared to the conventional solid thin film coating techniques, the nanofiber-coated SAW sensor shows a higher sensitivity and faster response. A theoretical analysis was performed to characterize the SAW sensor response with nanofibrous film coating. The nanofibrous film provides a high surface area to volume ratio, which can not only offer more adsorption sites for vapor molecules, but also shortens the diffusion length of vapor molecules into polymer material. It is concluded that the nanofiber film holds a great potential in enhancing SAW sensor performance for trace level detection of chemical analytes.Copyright

Modeling of Droplet-Based Processing for the Production of High-Performance Particulate Materials Using Level Set Method

This research is focused on a numerical investigation of dynamic and thermal processes of single droplet in the uniform droplet spray (UDS) process. The level set method (LSM) is used to assist in tracking the liquid-gas and solid-liquid interfaces during droplet’s impingement and solidification. UDS process generates mono-size droplets of desired diameter, permits stringent control of the thermal state of the droplet, and produces deposits and materials with distinctly different microstructures including Icosahedral quasicrystalline phase (I-phase) in the Mg-Zn-Y system. The conservative level set function, combined with the Navier-Stokes and energy equations have been adopted to study the deformation and heat transfer of liquid metal droplet when impacting on the substrate under supercooling condition. The effects of surface tension and contact angle on droplet’s deformation are taken into consideration. The developed simulation technique is validated both analytically and experimentally. A rapid solidification model has been integrated with LSM to simulate the rapid solidification within the deformed Mg-Zn-Y droplet predicted in the former model. It is found that the initial temperature fields and latent heat releasing during solidification have significant impact on the solidification process.© 2008 ASME

Analysis of a Vibrating-Beam-Based Micromixer

The mixing of two or more streams in microscale devices is a slowly molecular diffusion process due to the unique laminar flows, and some ‘turbulence’ based mixing technologies which are effective in macroscales become hard to implement in such small dimensions. The chaotic advection based mixing, depending on the stretching and folding of interface, has been proved to be effective for low Reynolds numbers (Re) and is a very promising technology for micro mixing. We propose a new mixing concept based on a vibrating micro-beam in microfluidic channels to generate chaotic advection to achieve an efficient mixing. The simplicity of the proposed mixer design makes microfabrication process easy for practical applications. The feasibility of the concept is evaluated computationally and moving mesh technique (ALE) is utilized to trace the beam movement. The simulation shows that the mixing quality is determined by parameters such as flow velocities, amplitudes and frequencies of vibrating beam. The Reynolds number (Re) is less than 2.0, Pelect number (Pe) ranges from 5 to 1000, and Strohal number (St) 0.3 to 3.0. It was found that vortex type of flows were generated in microchannel due to the interaction between beam and channel wall. The mixing efficiency with this design is well improved comparing with the flows without beam vibration.© 2007 ASME