Acoustic wave sensors are being developed for many emerging applications such as in semiconductor fabrication, biological diagnostics and polymer characterization. Traditional acoustic wave sensing devices such as quartz crystal microbalance (QCM) rely on polymer thin films coated on quartz plates to detect chemical and biological agents. It has been found that significant sensitivity enhancement of QCM devices can be achieved by simply attaching a polymer micropillar film onto the QCM substrate (QCM-P) to enable a unique coupled resonance between the micropillars and quartz substrate. In the present work, an equivalent circuit model integrating mechanical vibration of micropillars and electrical load impedance of piezoelectric substrate was developed to predict the frequency shift and Q-factor of the QCM-P devices when operating in air and liquid environments. In the model, the vibration of micropillars was solved simultaneously with the liquid loading on the pillar surface. The resultant hydraulic force was integrated into the circuit model to predict the load impedance on the sensor surface. The developed model was validated by experimental results for QCM-P devices operating in air and water with different micropillar heights. It will serve as a powerful tool to predict the performance of the QCM-P devices for different applications.

Development of new sensing techniques to achieve the lowest detection limit of analyte is vital for clinical measurements, such as drug testing, tumor indicators, thyroid-stimulating hormone, and similar immunoassay examinations. It has been reported that a polymer micropillar array appended to a quartz crystal microbalance substrate (QCM-P) forms a two-degree-of-freedom vibration system that was able to significantly improve the sensitivity of conventional QCM systems. This paper focuses on the development of a theoretical model for sensitivity determination of the QCM-P devices. To validate the model, a commercially available QCM platform, qCell T, was utilized to operate the QCM-P device and the adsorption of bovine serum albumin (BSA) protein with varied concentrations was measured with the system. A five-fold enhancement in sensitivity over a traditional film-based QCM (QCM-F) was demonstrated. In addition, it shows that the QCM-P had the capability to detect the BSA protein at very low concentrations approaching 0.3 mu g/mL.

Traditional acoustic wave based sensing devices such as quartz crystal microbalance (QCM) rely on the thin films coated on the piezoelectric substrates as the sensing films for chemical and biological detection. This study demonstrates that significant sensitivity enhancement over traditional film based QCM (QCM-F) devices can be achieved by simply attaching a poly(methylmethacrylate) (PMMA) micropillar film onto a QCM substrate (QCM-P) by using a UV-curable glue. Humidity absorption measurements shows that the unique resonance occurred between the micropillars and the piezoelectric substrate improved the mass sensitivity of the QCM sensor by more than eight-fold. In addition, the newly developed QCM-P sensor and traditional QCM-F were utilized to detect bovine serum albumin (BSA) protein immobilization on PMMA surfaces. It was found that the glued QCM-P was capable of measuring BSA at a much lower concentration (200 nM) in comparison to QCM-F (1500 nM). The glue-based micropillar QCM device showed great potential for improving the sensitivity, simplifying the fabrication process, and reducing the cost of QCM sensors for various biosensing and chemical usages.

The spontaneously jumping motion of condensed droplets by coalescence on superhydrophobic surfaces has been an active area of research due to its great potential for enhancing the condensation efficiency. Despite a considerable amount of microscopic observations, the interfacial wetting characterization during jumping-droplet condensation is still notably lacking. This work focuses on applying a novel acoustic sensor - quartz crystal microbalance (QCM), to characterize the interfacial wetting on nanostructured surfaces during jumping-droplet condensation. Copper oxide nanostructures were generated on the surface of QCM with a chemical etching method. Based on the geometry of the nanostructures, we modified a theoretical model to reveal the relationship between the frequency shift of the QCM and the wetting states of the surfaces. It was found that the QCM is extremely sensitive to the penetrated liquid in the structured surfaces. Then, the QCM with nanostructured surface was tested on a customed flow condensation setup. The dynamic interfacial wetting characteristics were quantified by the normalized frequency shift of the QCM. Combined with microscopic observation of the corresponding drop motion, we demonstrated that partial wetting (PW) droplets with an about 25% penetrated area underwent spontaneously jumping by coalescence. However, the PW droplets no longer jumped when the penetrated area exceeds 50% due to the stronger adhesion between liquid and the surface. It shows that the characterization of the penetrated liquid in micro/nanostructures, which is very challenging for microscopic observation, can be easily carried out by this acoustic technique.

A durable superhydrophobic coating formulation with epoxy binder thermoset was used to coat on surfaces, which provide high quality for corrosion protection, reduced biofouling and improved hydrodynamic behavior. The single and double layers coating of these nanostructured epoxy were fabricated and coated on a novel quartz crystal microbalance (QCM) technique to investigate their hydrophobic properties. Different static and dynamic wettability were obtained and characterized by evaluating the electrical impedance of QCM coated with nanostructured epoxy in air and DI water. It was found that QCM is able to quantitatively characterize the hydrophobicity of these nanostructured polymer surfaces. For double layer coating, the frequency shift in DI water was smaller in comparison to the single layer one. The reduction in mechanical impedance of QCM clearly demonstrates the effect of enhanced hydrophobicity for both single and double layers. The experimental results show that the hydrophobic surface resulted in smaller mechanical impedance loading, while the hydrophilic surface exerted much larger mechanical impedance. The outcome of this research will build a solid foundation for the further improvement of vehicles coated with superhydrophobic surfaces operating in water and increased equipment life.

A new acoustic wave resonance device induced by coupling micropillars with a quartz crystal microbalance (QCM-P) was developed for potential use in a wide range of applications such as drug discovery and development. The effect of wetting states of liquid on the micropillars of the QCM-P devices becomes essential in understanding the frequency signals from QCM in these applications. Euler-Bernoulli beam theory-based models were developed to establish the relationship between the resonance frequency shift of QCM-P and different wetting states of liquid on the micropillar surface including Cassie and Wenzel states. To validate the models, micropillars were fabricated on a QCM substrate by using nanoimprinting lithography (NIL) and liquids with different viscosity and surface tension, as well as different surface treatments were utilized to achieve Cassie and Wenzel states. The experimental results show that both wetting states and micropillar height have profound impact on the frequency shift of the QCM devices and the model can accurately capture the resonance of the QCM-P device and predict the surface-liquid interactions with a reasonable accuracy. (C) 2018 Elsevier B.V. All rights reserved.

This work focuses on the study of the effect of hydrophobicity on the water flow in carbon nanotubes（CNTs）using a molecular dynamics（MD）approach for a wide range of potential applications such as water purification and high efficiency of nanofluid energy absorption systems（NEAS）.The hydrophobicity between liquid water and surface of CNTs was characterized by interaction-energy-coefficient（IEC）—a parameter describing the energy interaction strength between water molecules and carbon atoms.It is shown that the static contact angles between water and carbon surface decrease from 155° to 44°when the values of IEC increase from 0.042 kJ/mol to 2.196 kJ/mol.In addition,the pressure drops in CNT became independent of IEC when the IEC value was higher than 1.192 kJ/mol for a given flow rate.It was found that the hydrophobicity of CNT surface has a significant impact on the pressure drop of water flow in the CNTs and MD method provides a quantitative evaluation of the impact.

The high conductive nickel (Ni) nanoparticles mixed with paraffin wax at two different volume ratios were prepared to investigate thermal conductivity enhancement of Phase Change Material (PCM) under random and aligned particle distribution. For each particle concentration, two samples were prepared. After mixing of the particles into the melted paraffin through sonication, one sample was placed in a static magnetic field to align the nanoparticles while the PCM was allowed to solidify; whereas, the second sample was solidified immediately after sonication to obtain a randomly distributed nanoparticles in the solid PCM. The thermal conductivity of both nanoPCM samples along with a pure paraffin sample were measured experimentally. The conductivity of both nanoPCM samples were substantially higher than the pure wax and the sample with magnetically aligned nanoparticle exhibited significantly higher thermal conductivity in comparison to the randomly distributed nanoPCM sample. It was anticipated that the configuration of the metallic fillers that are parallelly aligned with the applied heat flux direction does enhance the heat dissipation through the particle chains. However, the magnitude of thermal enhancement and sample fabrication in larger scales require further research efforts.

Herein, we demonstrate a facile, rapid, and scalable method to fabricate polymer-based gratings for surface-enhanced Raman spectroscopy (SERS) sensors. To accomplish this, epoxy nanostripe arrays on silicon substrates were prepared using thermal annealing and UV-cross-linking. After preparation of the nanostripe arrays, the surface was briefly treated with oxygen plasma, which decreased the surface energy and enabled the growth of AgNPs on the polymer surface using a simple, low-cost, aqueous-based synthesis procedure. The SERS substrates exhibited a detection limit of ∼1 pM using rhodamine 6G (R6G). In addition, preliminary work with E. coli DH5 showed that the nanoimprinted substrates can be used to obtain Raman spectra of washed bacteria cells.

Enhanced wettability, known as superhydrophobicity or superhydrophilicity has drawn extensive attention in the past for wide range potential applications such as superhydrophobic surfaces for self-cleaning, anti-icing, dropwise condensation, and drag reduction. This research focuses on the investigation of the frequency responses of quartz crystal microbalance (QCM) devices coated with micropillars to the different wetting states of drops. A theoretical model was developed to correlate the resonant frequency shifts of QCMs with the penetrated (Wenzel state) and suspended (Cassie state) states based on the Euler-Bernoulli beam theory. In the experimental validation of the theory, Poly(methyl methacrylate) (PMMA) micropillars were fabricated on the QCMs using nanoimprint lithography (NIL) method and the different wetting states were generated by plasma treatment and chemical coating. The frequency shifts of the QCM device were measured by a network analyzer. A good agreement between experimental measurements and theoretical predictions was obtained. It was found that the micropillars operating in the penetrated state results in one order of magnitude higher frequency shift of QCM than the micropillars in suspended state. There exists a highly nonlinear vibrating behavior of micropillars with different heights in both penetrated and suspended states. The QCM based technology is a valuable tool for studying the wettability of different superhydrophobic or superhydrophilic surfaces.

There has been a rapidly increasing attention to study nanofluidic devices due to their broad potential applications such as water desalination and purification, biosensing and energy saving. CNTs are often used to achieve the transport of gases and liquids in the ultrafiltration devices and energy efficient water filters. In this study, the water flow inside a CNT which is connected by two reservoirs at the two ends of nanotube is investigated. Two movable wall pistons of graphene are used to drive the water molecules through the CNT. Our results show that the velocity profile in the nanotube is similar to a plug flow instead of a fully developed flow and a large pressure difference is required between two reservoirs to drive the water flow in the system. Also, the local pressure distribution and mass flow rate in the CNT is analyzed in details.

Naonofluidics is increasingly attracting more attention for their wide range of potential applications such as water desalination and purification, biosensing, osmotic energy conversion, drug delivery and DNA analysis. It is critical to understand the behavior of the water fluid in nanochannels in order to better design nanofluidic-based systems for these applications. Most applications use Carbon Nanotubes (CNT), boron nitride nanotubes, graphene and graphene oxide. CNTs are good pore models for studying the transport of gases and liquids through nanoporous materials to design ultrafiltration devices and energy efficient water filters. It should be mentioned that fluids confined in nanoscale tubes exhibit significantly different behaviors compare to fluids in the macroscale and microscale. As experimental study in nanoscales is still a challenging task facing scientific society, different numerical technologies such as Molecular Dynamics (MD) method are becoming powerful tools for understanding the fluid behaviors at molecular level in nanofluidics. In the present study, MD simulation method, which is based on Newton's second law, is employed to study the water flow through smooth CNT. The effect of CNT diameter on density and velocity profiles are investigated. Our results show that by increasing the diameter of CNT, the results are approaching to the continuum condition.

A new sensing device was developed to achieve ultrahigh sensitivity, by coupling polymer micropillars with a quartz crystal microbalance (QCM) substrate to form a two-degree- of-freedom resonance system (QCM-P). The sensitivity of these QCM-P devices was evaluated by measuring mass changes for both deposited gold film and adsorption of bovine serum albumin (BSA), respectively, on poly(methyl methacrylate) (PMMA) micropillar surfaces, as well as assessing ligand-analyte binding interactions between anti-human immunoglobulin G (anti-hIgG) and human immunoglobulin G (hIgG). The anti-hIgG and hIgG binding results show QCM-P achieved an eightfold improvement in sensitivity relative to conventional QCM sensors. In addition, the binding affinity obtained from the QCM-P device for anti-hIgG and hIgG proteins was found in good agreement with that measured by surface plasmon resonance (SPR) for the same binding reaction.

Quartz crystal microbalance (QCM) device is a highly sensitive mass sensor (sensitivity: 0.5 ng/cm2) with a wide range of applications including biosensing, thin film deposition, surface chemistry, volatile organic compounds (VOC) and gaseous analytes detection. A recent study shows that several orders of magnitude improvement in sensitivity can be achieved by attaching microscale Polymethyl methacrylate (PMMA) pillars onto the surface of the QCM (QCM-P) to form a two-degree of freedom coupled resonant system. In this research, the effects of residual layer from the nanoimprinting process of micro-pillars and polydispersity index (Pd) of PMMA molecules on the sensitivity of QCM-P devices are investigated both experimentally and theoretically. The results show the residual layer behaves as an additional mass and significantly reduces the frequency shift of QCM-P sensor while a low polydispersity of PMMA improves the sensor responses. The outcome of this research leads to an in-depth understanding of the effects of material and fabrication process on QCM-P sensors which will build a solid foundation for the further improvement of QCM-P devices for a variety of applications such as protein binding measurement in drug discovery, gas detection for environmental monitoring and protection.

In this research, Polydimethylsiloxane (PDMS)/nickel (Ni) composites with embedded Ni spherical particle columns were studied for thermal conductivity enhancement. The volume fraction of Ni particles ranged from 2% to 20% while the strength of the applied magnetic field was fixed at 0.45 Tesla. The distribution and morphology of the column structures were quantitatively analyzed using optical microscope, Scanning Electron Microscope (SEM) and digital image processing. A reusable 3 omega measurement technique was applied to measure the effective thermal conductivity of PDMS/Ni composites in the parallel direction to the magnetic field. The measured thermal conductivity was compared with the prediction from a finite element model built on the observed microscopic structures. Under a static magnetic field, Ni particles align parallel to the field forming columns. The results illustrated that the diameter of Ni columns increased with increasing particle volume fraction while the center to center spacing between columns did not change substantially under the fixed magnetic strength. The magnetically aligned particle columns significantly enhanced the thermal conductivity of PDMS compared to the randomly distributed particles by about two fold. However, the point contacts between magnetically aligned spherical fillers are the major limiting factor for the further improvement of thermal conductivity. (C) 2016 Elsevier Ltd. All rights reserved.

This work reports a novel Quartz Crystal Microbalance (QCM) based method to analyze the droplet-micropillar surface interaction quantitatively during dropwise condensation. A combined nanoimprinting lithography and chemical surface treatment approach was utilized to directly fabricate the micropillar based superhydrophobic surface on the QCM substrate. The normalized frequency shift of the QCM device and the microscopic observation of the corresponding nucleation, drop growth, and drop coalescence processes clearly demonstrate the different characteristics of these condensation states. In addition, a synchrosqueezed wavelet spectrum based multi-resolution technique was utilized to analyze the resonant signal from the QCM sensor in both time and frequency domains simultaneously. An integrated discrete system modeling along with a hybrid signal and image processing approach was adopted to identify the response of the micropillars under different stages of dropwise condensation (DWC). The outcome of this signal processing research leads to a fundamental understanding of DWC spanning multiple time and length scales. The proposed study will also contribute to an in-depth understanding of different hydrophobic surfaces and DWC through this advanced signal processing and surface treatment. The developed QCM system provides a valuable tool for the dynamic characterization of different condensation processes.

Dropwise condensation (DWC) on hydrophobic surfaces is attracting attention for its great potential in many industrial applications, such as steam power plants, water desalination, and de-icing of aerodynamic surfaces, to list a few. The direct dynamic characterization of liquid/solid interaction can significantly accelerate the progress toward a full understanding of the thermal and mass transport mechanisms during DWC processes. This work reports a novel Quartz Crystal Microbalance (QCM) based method that can quantitatively analyze the interaction between water droplets and micropillar surfaces during different condensation states such as filmwise, Wenzel, and partial Cassie states. A combined nanoimprinting lithography and chemical surface treatment approach was utilized to fabricate the micropillar based superhydrophobic and superhydrophilic surfaces on the QCM substrates. The normalized frequency shift of the QCM device together with the microscopic observation of the corresponding drop motion revealed the droplets growth and their coalescence processes and clearly demonstrated the differences between the three aforementioned condensation states. In addition, the transition between Cassie and Wenzel states was successfully captured by this method. The newly developed QCM system provides a valuable tool for the dynamic characterization of different condensation processes.