Thin-Film Frustrated Total Internal Reflection Filter with Plasmonic Nanoparticle Inclusions in the Layers

The influence of plasmonic nanoparticles embedded in the central and side layers of the frustrated total internal reflection filter on the resonant transmission of light is analyzed. It is shown that the frequency dispersion causes the splitting of the filter bandwidth and the angular splitting of the incident beam into several output beams.

Attractive forces slow contact formation between deformable bodies underwater

Thermodynamics tells us to expect underwater contact between two hydrophobic surfaces to result in stronger adhesion compared to two hydrophilic surfaces. However, the presence of water changes not only energetics but also the dynamic process of reaching a final state, which couples solid deformation and liquid evacuation. These dynamics can create challenges for achieving strong underwater adhesion/friction, which affects diverse fields including soft robotics, biolocomotion, and tire traction. Closer investigation, requiring sufficiently precise resolution of film evacuation while simultaneously controlling surface wettability, has been lacking. We perform high-resolution in situ frustrated total internal reflection imaging to track underwater contact evolution between soft-elastic hemispheres of varying stiffness and smooth–hard surfaces of varying wettability. Surprisingly, we find the exponential rate of water evacuation from hydrophobic–hydrophobic (adhesive) contact is three orders of magnitude lower than that from hydrophobic–hydrophilic (nonadhesive) contact. The trend of decreasing rate with decreasing wettability of glass sharply changes about a point where thermodynamic adhesion crosses zero, suggesting a transition in mode of evacuation, which is illuminated by three-dimensional spatiotemporal height maps. Adhesive contact is characterized by the early localization of sealed puddles, whereas nonadhesive contact remains smooth, with film-wise evacuation from one central puddle. Measurements with a human thumb and alternatively hydrophobic/hydrophilic glass surface demonstrate practical consequences of the same dynamics: adhesive interactions cause instability in valleys and lead to a state of more trapped water and less intimate solid–solid contact. These findings offer interpretation of patterned texture seen in underwater biolocomotive adaptations as well as insight toward technological implementation.

Attenuated Total Reflection at THz Wavelengths: Prospective Use of Total Internal Reflection and Polariscopy

Capabilities of the attenuated total reflection (ATR) at THz wavelengths for increased sub-surface depth characterisation of (bio-)materials are presented. The penetration depth of a THz evanescent wave in biological samples is dependent on the wavelength and temperature and can reach 0.1–0.5 mm depth, due to the strong refractive index change ∼0.4 of the ice-water transition; this is quite significant and important when studying biological samples. Technical challenges are discussed when using ATR for uneven, heterogeneous, high refractive index samples with the possibility of frustrated total internal reflection (a breakdown of the ATR reflection mode into transmission mode). Local field enhancements at the interface are discussed with numerical/analytical examples. Maxwell’s scaling is used to model the behaviour of absorber–scatterer inside the materials at the interface with the ATR prism for realistic complex refractive indices of bio-materials. The modality of ATR with a polarisation analysis is proposed, and its principle is illustrated, opening an invitation for its experimental validation. The sensitivity of the polarised ATR mode to the refractive index between the sample and ATR prism is numerically modelled and experimentally verified for background (air) spectra. The design principles of polarisation active optical elements and spectral filters are outlined. The results and proposed concepts are based on experimental conditions at the THz beamline of the Australian Synchrotron.

Attenuated Total Reflection at THz Wavelengths: Peculiarities of Total Internal Reflection and Polariscopy

Capabilities of the Attenuated Total Reflection (ATR) at THz wavelengths for increased sub-surface depth characterisation of (bio-)materials is presented. The penetration depth of a THz evanescent wave in biological samples is dependent on the wavelength and temperature and can reach 0.1-0.5 mm depth due to strong refractive index change ∼0.4 of the ice-water transition; this is quite significant and important when studying biological samples. Technical challenges are discussed when using ATR for uneven, heterogeneous, high refractive index samples with possibility of frustrated total internal reflection (a breakdown of the ATR reflection-mode into transmission-mode). Local field enhancements at the interface are discussed with numerical/analytical examples. Maxwell’s scaling was used to model behaviour of absorber-scatterer inside materials at the interface with ATR prism for realistic complex refractive indices of bio-materials. Modality of ATR with polarisation analysis is proposed and its principle illustrated, opening an invitation for its experimental validation. The sensitivity of the polarised ATR mode to the refractive index between the sample and ATR prism is revealed. Design principles of polarisation active optical elements and spectral filters are outlined. The results and concepts are based on experiments carried out at the THz beamline of the Australian Synchrotron.

Change of electrical characteristics of rubber in the process of “swelling–deswelling”

Objectives. The main indicator that determines electrical conductivity of rubbers is specific volumetric electrical resistance (ρv ). The purpose of this work is to investigate changes in this indicator during swelling and deswelling of electrically conductive rubbers. When considering the swelling process of rubbers in liquid media at a molecular level, an analogy of this process with mechanical deformation of the material is drawn and common features and differences of these processes are revealed.Methods. For rubber compositions based on paraffinate and alkyl sulfonate nitrile butadiene rubbers, the degree of their swelling and the change in linear dimensions in heptane and in gasoline grades 80, 92, and 95 were determined. The ρ v value was determined by a potentiometric method: the initial value was measured after temperature control of rubbers for 1 h at 120°C, and the second measurement was carried out after these rubbers were swollen in the solvents for 48 h, followed by drying at 20°C to a constant weight and repeated temperature control under the same conditions. Using an IR Fourier spectrometer, spectra of the solvents used were obtained before and after identification of the investigated rubber samples in them.Results. It was shown that the type of rubber and solvent used influence the degree of rubber swelling. Rubber compositions based on natural rubbers with a large amount of attached acrylonitrile, obtained in the presence of an alkyl sulfonate emulsifier, have the highest resistance to swelling. The effect of the used solvent on the change in the degree of swelling is determined by its affinity for rubber and the presence of polar additives that increase the octane number of gasoline. It was established that the linear change of the samples upon swelling in the indicated solvents varies according to the length and thickness of the samples. Results show that depending on the type of rubber used and the degree of its filling, the described rubber processing technology leads to a decrease in the ρ v value by 2 to 20 times. The greatest effect of ρ v reduction is observed in low-filled rubber compositions based on paraffinate nitrile rubbers. The spectra of the frustrated total internal reflection of the solvents after their interaction with the studied rubbers show that particulate extraction of dibutyl phthalate, which was used as a plasticizer in rubber compounding, takes place as a result of rubber swelling.Conclusions. The proposed method of rubber processing reduces the ρ v value by removing dibutyl phthalate from the studied rubbers and forming a more developed carbon–elastomer structure. Furthermore, it solves the problem of the negative effect of the plasticizer on the ρ v value of rubber without excluding it from the rubber composition.

Characterisation of Biological Materials at THz Frequencies by Attenuated Total Reflection: Lard

The penetration depth of an evanescent wave in Attenuated Total Reflection (ATR) is dependent on the wavelength of the radiation utilised. At THz frequencies, the penetration depth into biological tissues is in the order of 0.1 to 0.5 mm; rendered pig lard was used as a model sample in this study. A method for the direct measurement of the evanescent wave penetration depth is presented which allows for the estimation of the dispersion of the complex refractive index by using the reflection of the evanescent wave from varying sample depths. The method employs frustrated total internal reflection, and has been demonstrated by using the THz/Far-IR beamline at the Australian synchrotron, and modelled using finite difference time domain (FDTD) simulations.

Stretchable distributed fiber-optic sensors

Silica-based distributed fiber-optic sensor (DFOS) systems have been a powerful tool for sensing strain, pressure, vibration, acceleration, temperature, and humidity in inextensible structures. DFOS systems, however, are incompatible with the large strains associated with soft robotics and stretchable electronics. We develop a sensor composed of parallel assemblies of elastomeric lightguides that incorporate continuum or discrete chromatic patterns. By exploiting a combination of frustrated total internal reflection and absorption, stretchable DFOSs can distinguish and measure the locations, magnitudes, and modes (stretch, bend, or press) of mechanical deformation. We further demonstrate multilocation decoupling and multimodal deformation decoupling through a stretchable DFOS–integrated wireless glove that can reconfigure all types of finger joint movements and external presses simultaneously, with only a single sensor in real time.

Measurement of Real Contact Area for Rough Metal Surfaces and the Distinction of Contribution From Elasticity and Plasticity

The measurement of the real contact area between rough surfaces is one of the most challenging problems in contact mechanics and is of importance to understand some physical mechanisms in tribology. Based on the frustrated total internal reflection, a new apparatus is designed to measure the real contact area. For metallic samples with various surface topographies, the relation between normal load and the real contact area is measured. The unloading process is first considered to distinguish the contribution of elasticity and plasticity in contact with rough surfaces. It is found that both elasticity and plasticity are involved throughout the continuous loading process, different from some present understanding and assumptions that they play at different loading stages. A quantitative parameter is proposed to indicate the contribution of plasticity. The present work not only provides an experimental method to measure the real contact area but figures out how elastic and plastic deformation works in contact with rough surfaces.