Changcheng Shi

Molecular dynamics simulations of conformation and chain length dependent terahertz spectra of alanine polypeptides

Terahertz absorption spectra of alanine polypeptides in water were simulated with classical molecular dynamics at 310 K. Vibrational modes and oscillator strengths were calculated based on a quasi-harmonic approximation. Absorption spectra of Alan (n = 5, 15, 30) with different chain lengths and Ala15 in coiled and helical conformations were studied in 10–40 cm− 1 bandwidth. Simulation results indicated both the chain length and the conformation have significant influences on THz spectra of alanine polypeptides. With the increase of chain length, the average THz absorption intensity increases. Compared with the helical Ala15 polypeptide, the THz spectra of coiled one shows stronger absorption peaks. These results were explained from different numbers of hydrogen bonds formed between polypeptides and the surrounding water molecules.

Nondestructive Evaluation of Carbon Fiber Reinforced Polymer Composites Using Reflective Terahertz Imaging

Terahertz (THz) time-domain spectroscopy (TDS) imaging is considered a nondestructive evaluation method for composite materials used for examining various defects of carbon fiber reinforced polymer (CFRP) composites and fire-retardant coatings in the reflective imaging modality. We demonstrate that hidden defects simulated by Teflon artificial inserts are imaged clearly in the perpendicular polarization mode. The THz TDS technique is also used to measure the thickness of thin fire-retardant coatings on CFRP composites with a typical accuracy of about 10 micrometers. In addition, coating debonding is successfully imaged based on the time-delay difference of the time-domain waveforms between closely adhered and debonded sample locations.

Spectroscopic study of terahertz reflection and transmission properties of carbon-fiber-reinforced plastic composites

Abstract. Carbon-fiber-reinforced plastic (CFRP) composites are widely used in aerospace and concrete structure reinforcement due to their high strength and light weight. Terahertz (THz) time-domain spectroscopy is an attractive tool for defect inspection in CFRP composites. In order to improve THz nondestructive testing of CFRP composites, we have carried out systematic investigations of THz reflection and transmission properties of CFRP. Unidirectional CFRP composites with different thicknesses are measured with polarization directions 0 deg to 90 deg with respect to the fiber direction, in both reflection and transmission modes. As shown in the experiments, CFRP composites are electrically conducting and therefore exhibit a high THz reflectivity. In addition, CFRP composites have polarization-dependent reflectivity and transmissivity for THz radiation. The reflected THz power in the case of parallel polarization is nearly 1.8 times higher than for perpendicular polarization. At the same time, in the transmission of THz wave, a CFRP acts as a Fabry–Pérot cavity resulting from multiple internal reflections from the CFRP–air interfaces. Moreover, from the measured data, we extract the refractive index and absorption coefficient of CFRP composites in the THz frequency range.

Nondestructive evaluation of glass fiber honeycomb sandwich panels using reflective terahertz imaging

The glass fiber honeycomb sandwich panel is an efficient structural configuration which has been widely used in several designs related to air, sea, and land transport systems. Terahertz time domain spectroscopy technology is an effective supplementary method for nondestructive evaluation of composites. Terahertz time domain spectroscopy imaging technology in the reflection configuration was employed to detect the defects of glass fiber honeycomb sandwich panels, such as a skin-core debonding and a foreign inclusion. It was shown in systematic experimental investigation that different defects were detected through terahertz imaging based on different parameters, which were obtained by comparing and analyzing the waveforms in time and/or frequency domain between normal and defective regions of the sample.

Determination of Critical Micelle Concentrations of Surfactants by Terahertz Time-Domain Spectroscopy

A novel detection method is presented to determine the critical micelle concentrations (CMCs) of surfactants by using a simple and convenient terahertz time-domain spectroscopy (THz-TDS) technology which has been successfully used to study the picosecond dynamics of water in reverse micelles. Absorption coefficients or refractive indices of aqueous solutions of an ionic surfactant sodium dodecyl sulfate and a nonionic surfactant nonaethylene glycol monododecyl ether in 0.2-1.8 THz frequency range at different concentrations are measured through the THz-TDS. A clear intersection of THz absorption coefficient or refractive index caused by the micelle formation is observed. This phenomenon is strongly associated with the change of component ratio between surfactant, bulk water and hydration water during the formation of micelles. The corresponding concentration is determined as the CMC of the surfactant which is in good agreement with the most existing results using traditional methods. For comparison, absorption coefficients of the two surfactants in hydrochloric acid solutions are also measured and differences in contrast to those in aqueous solutions are observed and explained. Our findings indicate that THz spectroscopy technology can be applied to determine the CMCs of surfactants in a sensitive, accurate, and label-free way.

Void and crack detection of polymethacrylimide foams based on terahertz time-domain spectroscopic imaging

Terahertz reflection imaging is considered as a potential diagnostic tool for the investigation of polymethacrylimide foam defects. Using terahertz time-domain spectroscopy (TDS) and detection methods based on terahertz spectroscopic analysis, the reflection imaging results of different thicknesses of polymethacrylimide foam with two kinds of detections (cracks and voids) are studied. The samples (Degussa Rohacell WF71) are planar slabs of polymethacrylimide foams with thicknesses of 35 mm, 60.5 mm and 10 mm. It is found that the same kinds of polymethacrylimide foam defects with different foam thicknesses have similar spectral characteristics, with marked differences only in the amplitude and phase of the reflected wave. In view of this, we focus our study on the defective spectral characteristics of one of the foams (35 mm thickness). The characteristics of void and crack defects are analyzed in the time domain, which is based mainly on the variation of the reflected waveform. In particular, the imaging and clear identification of voids of less than 2.4 mm in diameter, fine cracks (0.3 mm wide), and the quantification of defects can be readily achieved using the terahertz non-destructive testing technique described here.

Characterization of Glucosamine and Collagen crystallization by terahertz time-domain spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) has been demonstrated as a promising solid-state analytical tool which can probe intermolecular interactions through crystal lattice vibrations. However, the current studies commonly investigate the interactions between two biological or pharmaceutical molecules with relatively low molecular weights. The interaction between small biomolecule and biomacromolecule in the crystallization process has not been reported. In this study, THz-TDS was used as a novel technique to characterize the interaction between small biomolecule (Glucosamine) and biomacromolecule (Collagen) in the crystallization process. The absence of THz absorption peak in solvent mixed sample with weight ratio of 1:1 showed the crystallization process of glucosamine was disturbed by collagen. The recurrence of THz absorption peak in solvent mixed sample with weight ratio of 7:1 may be related to the saturation of interaction between glucosamine and collagen. The dissolving and lyophilizing processes were validated to hardly affect the crystal formation of glucosamine. This study further demonstrates THZ-TDS has the capability to characterize the interaction between small biomolecule and biomacromolecule in solid-state.

Moisture Content Determination of Polyethylene by Using Terahertz Time-domain Spectroscopy

The moisture content of polyethylene powder mixed with different percentages of water was determined by using terahertz time-domain spectroscopy. The relationship between moisture content and terahertz dielectric properties was explained by using effective medium theory.