Time-resolved measurement of acoustic density fluctuations using a phase-shifting Mach-Zehnder interferometer.

Phase-shifting interferometry is one of the optical measurement techniques that improves accuracy and resolution by incorporating a controlled phase shift into conventional optical interferometry. In this study, a four-step phase-shifting interferometer is developed to measure the spatiotemporal distribution of acoustic density oscillations of the gas next to a rigid plate. The experimental apparatus consists of a polarizing Mach-Zehnder interferometer with a polarization camera capable of capturing four polarization directions in one shot image and it is used to measure the magnitude and the phase of density fluctuations through a duct of rectangular cross section connected to a loudspeaker. The results are compared with the well-established thermoacoustic theory describing the thermal coupling between acoustic oscillations and rigid boundaries, and the results show a very good agreement for various ratios of the (frequency-dependent) thermal boundary layer thickness to the plate spacing. This measurement technique could be advantageously employed to analyze more complex heat transfer processes involving the coupling of acoustic oscillations with rigid boundaries.

Molecular dynamics simulations for interfacial structure and affinity between carboxylic acid-modified Al2O3 and polymer melts.

Controlling the dispersion state of nanoparticles in a polymer matrix is necessary to produce polymer nanocomposites. The surface modification of nanoparticles is used to enable their dispersion in polymers. Moreover, molecular dynamics (MD) simulations are useful for revealing the interfacial properties between nanoparticles and polymers to aid in the design of materials. In this study, the effect of surface coverage, modifier length, and polymer species on the interfacial structure and affinity between surface-modified Al2O3 and polymer melts were investigated using all-atom MD simulations. Hexanoic, decanoic, and tetradecanoic acids were used as surface modifiers, and polypropylene (PP), polystyrene (PS), and poly (methyl methacrylate) (PMMA) were used as polymers. The work of adhesion Wadh and the work of immersion Wimm were selected as quantitative measures of affinity. Wadh was calculated using the phantom-wall approach, and Wimm was calculated by simply subtracting the surface tension of polymers γL from Wadh. The results showed that Wadh and Wimm were improved by surface modification with low coverage, owing to a good penetration of the polymer. The effect of modifier length on Wadh and Wimm was small. Whereas Wadh increased in the following order: PP < PS < PMMA, Wimm increased as follows: PMMA < PS < PP. Finally, the trend of Wadh and Wimm was organized using the Flory-Huggins interaction parameter χ between the modifier and the polymer. This study demonstrates that the interfacial affinity can be improved by tuning the surface coverage and modifier species depending on the polymer matrix.

Evaluation of the work of adhesion at the interface between a surface-modified metal oxide and an organic solvent using molecular dynamics simulations.

Advancing the practical applications of surface-modified nanoparticles requires that their dispersion in solvents can be controlled. The degree of dispersion depends on the affinity between surface-modified nanoparticles and solvents, which can be quantified using the work of adhesion at the interface. Herein, the affinity between a surface-modified inorganic solid and an organic solvent was evaluated by calculating the work of adhesion at the interface. The phantom-wall method, which is a thermodynamic route for evaluating the work of adhesion at an interface using molecular dynamics simulations, was applied to the decanoic acid-modified Al2O3/hexane interface. Molecular dynamics simulations were performed for flat interface systems to focus on the interactions between substances that affect the affinity on the surface. As a result, the surface coverage of decanoic acid was found to affect the work of adhesion, with a maximum value of 45.66 ± 0.75 mJ/m2 at a surface coverage of 75%. An analysis of the mass density profiles of Al2O3, decanoic acid, and hexane in the vicinity of the interface showed that the increase in the work of adhesion with the surface coverage was due to the penetration of hexane molecules into the decanoic acid layer on the Al2O3 surface. At a surface coverage of 75%, some hexane molecules were trapped in the layer of oriented decanoic acid molecules. These results suggested that the interfacial affinity can be enhanced by controlling the surface modification so that the solvent can penetrate the layer of the modifier.

High-speed phase-shifting interferometry using triangular prism for time-resolved temperature measurement.

This study proposes a high-speed phase-shifting interferometer with an original optical prism. This phase-shifting interferometer consists of a polarizing Mach-Zehnder interferometer, an original optical prism, a high-speed camera, and an image-processing unit for a three-step phase-shifting technique. The key aspect of the application of the phase-shifting technique to high-speed experiments is an original prism, which is designed and developed specifically for a high-speed phase-shifting technique. The arbaa prism splits an incident beam into four output beams with different information. The interferometer was applied for quantitative visualization of transient heat transfer. In order to test the optical system for measuring high-speed phenomena, the temperature during heat conduction was measured around a heated thin tungsten wire (diameter of 5 µm) in water. The visualization area is approximately 90 µm×210 µm, and the spatial resolution is 3.5 µm at 300,000 fps of the maximum temporal resolution with a high-speed camera. The temperature fields around the heated wire were determined by converting phase-shifted data using the inverse Abel transform. Finally, the measured temperature distribution was compared with numerical calculations to validate the proposed system; a good agreement was obtained.

Control of the temperature responsiveness of poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) copolymer using ultrasonic irradiation.

Poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) (poly(NIPAM-co-HEMA)) is a temperature-responsive copolymer that is expected to be applicable as an advanced functional polymeric material in various fields. In this study, a novel method was developed to control the responsive temperature of poly(NIPAM-co-HEMA) using an ultrasonic polymerization technique. Initially, the behavior of the reaction was investigated using NIPAM and HEMA monomers under ultrasonic irradiation. A high ultrasonic power was found to produce a high reaction rate and low number average molecular weight of the copolymer. The polydispersity of the synthesized copolymer was approximately 1.5 for all ultrasonic powers examined. In the early stage of the reaction, the molar fraction of NIPAM in the copolymer was lower than the initial molar fraction of the monomers. It was concluded that ultrasonic irradiation affected the initiation reaction and polymer degradation, but did not affect the propagation reaction. Furthermore, the effect of the ultrasonic irradiation conditions on the temperature responsiveness of the copolymer was investigated. The lower critical solution temperature (LCST) of the copolymer was found to increase with increasing ultrasonic irradiation time. In addition, in the early stages of the reaction, the measured values of the LCST were higher than the estimated values using copolymer composition. This can be attributed to some parts of the copolymer chain possessing a higher NIPAM fraction than the overall fraction due to different reactivities of the monomers and terminated radicals. This hypothesis was indirectly verified by the synthesis of a block copolymer from the PNIPAM homopolymer and HEMA monomer.