Interferometric Spectra Research Articles

Wavelength-Dependent Bragg Grating Sensors Cascade an Interferometer Sensor to Enhance Sensing Capacity and Diversification through the Deep Belief Network

Fiber-optic sensors, such as fiber Bragg grating (FBG) sensors and fiber-optic interferometers, have excellent sensing capabilities for industrial, chemical, and biomedical engineering applications. This paper used machine learning to enhance the number of fiber-optic sensing placement points and promote the cost-effectiveness and diversity of fiber-optic sensing applications. In this paper, the framework adopted is the FBG cascading an interferometer, and a deep belief network (DBN) is used to demodulate the wavelength of the sampled complex spectrum. As the capacity of the fiber-optic sensor arrangement is optimized, the peak spectra from FBGs undergoing strain or temperature changes may overlap. In addition, overlapping FBG spectra with interferometer spectra results in periodic modulation of the spectral intensity, making the spectral intensity variation more complex as a function of different strains or temperature levels. Therefore, it may not be possible to analyze the sensed results of FBGs with the naked eye, and it would be ideal to use machine learning to demodulate the sensed results of FBGs and the interferometer. Experimental results show that DBN can successfully interpret the wavelengths of individual FBG peaks, and peaks of the interferometer spectrum, from the overlapping spectrum of peak-overlapping FBGs and the interferometer.

The size-luminosity relation of local active galactic nuclei from interferometric observations of the broad-line region

By using the GRAVITY instrument with the near-infrared (NIR) Very Large Telescope Interferometer (VLTI), the structure of the broad (emission-)line region (BLR) in active galactic nuclei (AGNs) can be spatially resolved, allowing the central black hole (BH) mass to be determined. This work reports new NIR VLTI/GRAVITY interferometric spectra for four type 1 AGNs (Mrk 509, PDS 456, Mrk 1239, and IC 4329A) with resolved broad-line emission. Dynamical modelling of interferometric data constrains the BLR radius and central BH mass measurements for our targets and reveals outflow-dominated BLRs for Mrk 509 and PDS 456. We present an updated radius-luminosity (R-L) relation independent of that derived with reverberation mapping (RM) measurements using all the GRAVITY-observed AGNs. We find our R-L relation to be largely consistent with that derived from RM measurements except at high luminosity, where BLR radii seem to be smaller than predicted. This is consistent with RM-based claims that high Eddington ratio AGNs show consistently smaller BLR sizes. The BH masses of our targets are also consistent with the standard MBH-σ* relation. Model-independent photocentre fitting shows spatial offsets between the hot dust continuum and the BLR photocentres (ranging from ∼17 μas to 140 μas) that are generally perpendicular to the alignment of the red- and blueshifted BLR photocentres. These offsets are found to be related to the AGN luminosity and could be caused by asymmetric K-band emission of the hot dust, shifting the dust photocentre. We discuss various possible scenarios that can explain this phenomenon.

Combined Interrogation Algorithm of Phase Function and Minimum Mean Square Error for Fiber-Optic Fabry-Perot Micro-Pressure Sensors Based on White Light Interference

In this paper, a demodulation algorithm using the phase function of the Fabry-Perot (FP) cavity combined with the minimum mean squared error (MMSE) is proposed for the pressure measurement with low-pressure sensitivity fiber-optic FP micro-pressure sensors, which exhibits reasonable demodulation results. In this algorithm, the initial phase variation during the measurement process is focused, which is usually disregarded in traditional demodulation, and then, use the phase change features of phase function to achieve precise pressure demodulation. A functional relation between the reference pressure and the amount of phase function displacement is developed for demodulation based on interferometric spectra of reference pressures at 0 kPa and 35 kPa. The algorithm has been investigated theoretically and experimentally. The experiment results demonstrate that the measurement accuracy is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.2 kPa in the pressure measurement range from 0 kPa to 35 kPa, the pressure resolution is up to 0.0218 kPa, and the reading error is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm 3\%$</tex-math></inline-formula> in the pressure range from 6.7 kPa to 35 kPa, which indicate that the proposed algorithm can meet medical pressure analysis standards in the cardiovascular sector for low-pressure sensitivity sensors. Moreover, this work illustrates that inaccurate reference pressure calibration would cause substantial demodulation errors. These errors can be mitigated by appropriately increasing the number of reference pressures to keep the reading error at a reasonable level according to the specific application.

The effect of alkylation degree in hyperbranched polyester on modified heavy oil fluidity

As flow improvers (FIs) for Xinjiang heavy oil with a high content of resins, a series of hyperbranched polyesters with varying degrees of alkylation (FI-1 to FI-7) were synthesized from poly (propylene glycol) (PPG) with trimesic acid (TA) and different dosages of nonanoic acid (NA). Here, the effect of alkylation degree in the flow improver on modified heavy oil fluidity was investigated. Firstly, the evaluation test indicated that the performance of FI-3 was optimal with an appropriate dosage of alkylation modifier, and a rate of viscosity reduction of 56.16%. Supportingly, the chemical structures of three flow improvers with different alkylation degree were compared by Fourier transform infrared spectroscopy (FTIR) and 1H-nuclear magnetic resonance spectra (1HMR). Importantly, the results of Ultraviolet visible spectrophotometer (UV–Vis) showed that FI-3 with an appropriate alkylation degree can better promote the solubility of resins in toluene. The results of simulating studies expounded that the interaction of FI-3 and resin clusters was more sufficient, which made the aggregation degree of resin clusters lower. The results of scanning electron microscope (SEM), white-light interferometer and X-ray diffraction spectra (XRD) revealed that flow improvers can significantly impact the structure of resins. Collectively, this study make known that this type of hyperbranched polyesters with alkylated branched chains can inhibit the aggregation of resins to modified heavy oil fluidity. An appropriate alkylation degree could improve the performance by strengthening the synergistic effect of polar groups and lipophilic alkyl chains. This work has important implications for comprehending the interaction between amphiphile and heavy oil, which could be exploitable for modified polymers used as flow improvers.

Reducing Biases in Thermal Infrared Surface Radiance Calculations Over Global Oceans

Thermal infrared (IR) environmental satellite data assimilation and remote sensing of the surface and lower troposphere depend on accurate specification of the spectral surface emissivity within clear-sky forward calculations. Over ocean surfaces, accurate modeling of surface-leaving radiances over the sensor scanning swaths is complicated by a quasi-specular bidirectional reflectance distribution function (BRDF). Recent findings at the Joint Center for Satellite Data Assimilation (JCSDA) have also revealed significant zonally varying systematic biases (≈ |0.5| K) on a global scale over cold ocean waters, these the result of temperature dependence in the thermal IR optical constants. This paper proposes practical solutions to these problems by modeling thermal IR “effective emissivity” in a manner that accounts for both surface emission and quasi-specular reflectance, along with temperature dependence, while meeting the latency and computational constraints of operational global data assimilation and retrieval systems. We overview the theoretical basis of the model and validate it against ship-based Marine Atmospheric Emitted Radiance Interferometer (MAERI) spectra obtained from cold and warm water ocean campaigns.

Interstellar detection and chemical modeling of iso-propanol and its normal isomer

Context.The detection of a branched alkyl molecule in the high-mass star forming protocluster Sagittarius (Sgr) B2(N) permitted by the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) revealed a new dimension of interstellar chemistry. Astrochemical simulations subsequently predicted that beyond a certain degree of molecular complexity, branched molecules could even dominate over their straight-chain isomers.Aims.More generally, we aim to probe further the presence in the interstellar medium of complex organic molecules with the capacity to exhibit both a normal and iso form, via the attachment of a functional group to either a primary or secondary carbon atom. Methods. We used the imaging spectral line survey ReMoCA performed with ALMA at high angular resolution and the results of a recent spectroscopic study of propanol to search for the iso and normal isomers of this molecule in the hot molecular core Sgr B2(N2). We analyzed the interferometric spectra under the assumption of local thermodynamical equilibrium. We expanded the network of the astrochemical model MAGICKAL to explore the formation routes of propanol and put the observational results in a broader astrochemical context.Results.We report the first interstellar detection of iso-propanol, ¿-C3H7OH, toward a position of Sgr B2(N2) that shows narrow linewidths. We also report the first secure detection of the normal isomer of propanol, n-C3H7OH, in a hot core. Iso-propanol is found to be nearly as abundant as normal-propanol, with an abundance ratio of 0.6 which is similar to the ratio of 0.4 that we obtained previously for iso- and normal-propyl cyanide in Sgr B2(N2) at lower angular resolution with our previous ALMA survey, EMoCA. The observational results are in good agreement with the outcomes of our astrochemical models, which indicate that the OH-radical addition to propylene in dust-grain ice mantles, driven by water photodissociation, can produce appropriate quantities of normal- and iso-propanol. The normal-to-iso ratio in Sgr B2(N2) may be a direct inheritance of the branching ratio of this reaction process.Conclusions.The detection of normal- and iso-propanol and their ratio indicate that the modest preference for the normal form of propyl cyanide determined previously may be a more general feature among similarly sized interstellar molecules. Detecting other pairs of interstellar organic molecules with a functional group attached either to a primary or secondary carbon may help in pinning down the processes that dominate in setting their normal-to-iso ratios. Butanol and its isomers would be the next obvious candidates in the alcohol family, but their detection in hot cores will be challenging.

Highly sensitive Mach-Zehnder interferometric micromagnetic field sensor based on 3D printing technology.

A two-photon 3D printed polymer magnetic sensing device based on a Mach-Zehnder interferometer (MZI) is proposed. One arm of the MZI contains a hollow cavity and two connecting open channels that can be filled with magnetic fluids (MFs) and sealed by the UV curable adhesive, forming a magneto-optical component of the interferometer. As the magnetic field changes, the refractive index (RI) of the MF changes, and the effective RI of the guiding mode of the waveguide changes accordingly, which results in a change in the phase of the MZI. The interferometric spectra can be used to evaluate the sensing sensitivity. The MZI structure with a hollow length of 40 µm is fabricated, and the microstructure is encapsulated with MF, demonstrating a highly sensitive magnetic field device. The experimental results show that the magnetic field sensitivity of the fabricated magnetic field device is -1.675nm/Oe. For a spectrometer with a resolution of 1 pm, the minimal detectable magnetic field resolution of the sensor is up to 59.7 nT with good stability.

Simultaneous measurement of refractive index and temperature or temperature and axial strain based on an inline Mach-Zehnder interferometer with TCF-TF-TCF structure.

A refractive index (RI) and temperature or a temperature and axial strain sensor based on an inline Mach-Zehnder interferometer with thin core fiber (TCF)-thin fiber (TF)-TCF structure is proposed and experimentally demonstrated, requiring only the cleaving and fusion splicing methods. The operation principle depends on the effect that the TF cladding modes interfere with the core mode as an optical coupler. The RI, temperature, or axial strain variations can lead to resonance dip variations in the interferometer spectra, and the RI, temperature, or axial strain sensitivity can be measured by monitoring the wavelength shifts of resonance dips. Then we can measure both RI and temperature, or temperature and axial strain through the demodulation matrix. Four sensors with different TF lengths are fabricated based on numerical simulation. A 15 mm long TF sensor displays an RI sensitivity as high as -174.357nm/RIU, temperature sensitivities in the glycerin solution and the air of 12.47 and 26.19 pm/°C, and axial strain sensitivity of -3.43×10-4nm/µε. Moreover, due to its simple manufacture, high cost-effectiveness and compactness, the proposed sensor has a broad application prospect in physical, chemical, and biological sensing.

The spatially resolved broad line region of IRAS 09149−6206

We present new near-infrared VLTI/GRAVITY interferometric spectra that spatially resolve the broad Brγ emission line in the nucleus of the active galaxy IRAS 09149−6206. We use these data to measure the size of the broad line region (BLR) and estimate the mass of the central black hole. Using an improved phase calibration method that reduces the differential phase uncertainty to 0.05° per baseline across the spectrum, we detect a differential phase signal that reaches a maximum of ∼0.5° between the line and continuum. This represents an offset of ∼120 μas (0.14 pc) between the BLR and the centroid of the hot dust distribution traced by the 2.3 μm continuum. The offset is well within the dust sublimation region, which matches the measured ∼0.6 mas (0.7 pc) diameter of the continuum. A clear velocity gradient, almost perpendicular to the offset, is traced by the reconstructed photocentres of the spectral channels of the Brγ line. We infer the radius of the BLR to be ∼65 μas (0.075 pc), which is consistent with the radius–luminosity relation of nearby active galactic nuclei derived based on the time lag of the Hβ line from reverberation mapping campaigns. Our dynamical modelling indicates the black hole mass is ∼1 × 108 M⊙, which is a little below, but consistent with, the standard MBH–σ* relation.

An active fiber sensor based on modal interference in few-mode fibers for dual-parameter detection

We propose a dual-parameter active fiber sensor (DP-AFS) based on a fiber ring laser, which uses a single-mode–two-mode-single-mode fiber interferometer (STS-FI) and a two-mode Fiber Bragg Grating (TMFBG) as the intracavity DP sensing element. The effects of different lateral-offset splicing parameters on the interferometric spectra of the STS-FI are investigated. The excitation ratio of the LP11 mode in the STS-FI is the highest when the lateral-offset distance is 6μm, which achieves the best sensing performance. By measuring the output wavelength and intensity of the fiber laser, DP sensing of strain and stress with high linearity is achieved simultaneously. The sensor has a strain sensitivity of −0.0167 dB/με and a stress sensitivity of 7.67×10−3 nm/N. The proposed DP-AFS can be applied for reliable and precise measurements of strain and stress simultaneously.

Optically computed optical coherence tomography for volumetric imaging.

We describe an innovative optically computed optical coherence tomography (OC-OCT) technology. The OC-OCT system performs depth resolved imaging by computing the Fourier transform of the interferometric spectra optically. The OC-OCT system modulates the interferometric spectra with Fourier basis function projected to a spatial light modulator and detects the modulated signal without spectral discrimination. The novel, to the best of our knowledge, optical computation strategy enables volumetric OCT imaging without performing mechanical scanning and without the need for Fourier transform in a computer.

An optical fiber strain sensor by using of taper based TCF structure

We propose an optical fiber strain sensor consisting of a spliced twin-core fiber (TCF). The sensor structure has two fiber tapers, which are weak taper and T-shaped taper. The taper waist of T-shaped taper is about half-size of the weak taper, but the length of the T-shaped taper is also about half- length of the weak taper. In the sensing structure, core mode and cladding modes of the TCF are excited simultaneously. Two tapered regions around the splicing interfaces can play the roles of splitting and combining the modes in the fiber. In experiment, the proposed device has good quality interferometric spectra, and the highest extinction ratio of 20 dB is achieved. When the stress increases from 0 με to 841.5 με, different wavelength sensitivities of 3.31 pm/με and 6.11 pm/με are achieved. Different intensity sensitivities of 7.5 × 10−3 dB/με and 9.9 × 10−3 dB/με are also achieved, respectively. The sensor has shown good test stability and low temperature sensitivity, which is good for precise strain measurement in practical applications.

Modeling of Nonlocal Thermodynamic Equilibrium Effects in the Classical and Principal Component‐Based Version of the RTTOV Fast Radiative Transfer Model

AbstractThe direct assimilation in 4D‐Var of principal component (PC) scores derived from Infrared Atmospheric Sounding Interferometer (IASI) spectra has recently been demonstrated. To maximize the exploitation of the IASI instrument, a future step is to consider the extension of the PC approach to the extraction of information from the 4.3‐μm CO2‐absorbing region. Shortwave IASI channels are currently underused compared to similar longwave channels because of day‐night variations in data usability due to departures from local thermodynamic equilibrium (LTE). In this paper, we document the introduction of non‐LTE (NLTE) effects in the PC‐based version of the radiative transfer for TIROS operational vertical sounder (RTTOV) fast radiative transfer model (PC‐RTTOV). The inclusion of NLTE effects in PC‐RTTOV has required the development of a parameterized scheme that allows the fast computation of a NLTE correction to LTE radiances. The fast NLTE model is general enough to be applied to any sensor and can be utilized to add a fast and accurate NLTE correction to polychromatic LTE spectra computed by any general radiative transfer model, including RTTOV, which now incorporates the fast NLTE model developed in this study. The accuracy of the NLTE correction is such that daytime and nighttime radiances can be simulated to almost the same degree of accuracy. The comparison with IASI observations shows that the fast NLTE model presented here performs significantly better than the fast NLTE model incorporated in the previous version of RTTOV and also that improvements have to be made to the simulation of NLTE effects at winter high latitudes.

Energy-resolved attosecond interferometric photoemission from Ag(111) and Au(111) surfaces

Photoelectron emission from solid surfaces induced by attosecond pulse trains into the electric field of delayed phase-coherent infrared (IR) pulses allows the surface-specific observation of energy-resolved electronic phase accumulations and photoemission delays. We quantum-mechanically modeled interferometric photoemission spectra from the (111) surfaces of Au and Ag, including background contributions from secondary electrons and direct emission by the IR pulse, and adjusted parameters of our model to energy-resolved photoelectron spectra recently measured at a synchrotron light source by Roth et al. [J. Electron Spectrosc. 224, 84 (2018)]. Our calculated spectra and photoelectron phase shifts are in fair agreement with the experimental data of Locher et al. [Optica 2, 405 (2015)]. Our model's not reproducing the measured energy-dependent oscillations of the Ag(111) photoemission phases may be interpreted as evidence for subtle band-structure effects on the final-state photoelectron-surface interaction not accounted for in our simulation.

Double‐clad fiber Michelson interferometer for measurement of temperature and refractive index

AbstractAn all‐fiber Michelson interferometer based on double‐clad fiber (DCF) is proposed for temperature and refractive index (RI) sensing. The sensor is formed by continuous splicing between single mode fiber, multimode fiber, and DCF. Temperature measurement is achieved by monitoring the dip wavelength shift of the interferometer spectra resulting from thermo‐optic effect on core and first cladding of DCF. RI measurement is established from the power change in spectra due to the Fresnel's reflection at the DCF tip. Experimental result on 18.5 mm sensor manifests that temperature and RI responses are distinguishable. For instance, power level at 1540.5 nm exhibits quadratic shift to RI changes while unaffected to temperature changes. Dip wavelengths of sensor spectra demonstrate linear shift to temperature changes while unaffected by RI changes. The proposed sensor is cost effective as sensor fabrication only requires conventional splicing. Sensor capability to distinguish temperature and RI parameters makes it practical for broad range applications including for biomedical and food industries.

Miniature optical fiber temperature sensor based on FMF-SCF structure

Abstract We proposed and experimentally demonstrated a miniature optical fiber temperature sensor consisting of a seven core fiber (SCF) and a few mode fiber (FMF). The device is fabricated by splicing a section of FMF with a segment of SCF to form a FMF-SCF based sensing structure, and during the FMF region, few modes can be excited and will propagate within the SCF. In experiment, the proposed device has good quality interferometric spectra, and the highest extinction ratio of 27 dB was achieved. When the temperature increases from room temperature to 110 °C, the temperature response properties of the sensor have been investigated, the wavelength sensitivity of about 91.8 pm/°C and the amplitude sensitivity of about 1.57 × 10−2 a.u./°C are obtained, respectively. Due to its easy and controllable fabrication, the sensor can be a suitable candidate in temperature sensing applications.

Mid-infrared interferometric variability of DG Tauri: Implications for the inner-disk structure

Context. DG Tau is a low-mass pre-main sequence star, whose strongly accreting protoplanetary disk exhibits a so-far enigmatic behavior: its mid-infrared thermal emission is strongly time-variable, even turning the 10 $\mu$m silicate feature from emission to absorption temporarily. Aims. We look for the reason for the spectral variability at high spatial resolution and at multiple epochs. Methods. We study the temporal variability of the mid-infrared interferometric signal, observed with the VLTI/MIDI instrument at six epochs between 2011 and 2014. We fit a geometric disk model to the observed interferometric signal to obtain spatial information about the disk. We also model the mid-infrared spectra by template fitting to characterize the profile and time dependence of the silicate emission. We use physically motivated radiative transfer modeling to interpret the mid-infrared interferometric spectra. Results. The inner disk (r<1-3 au) spectra exhibit a 10 $\mu$m absorption feature related to amorphous silicate grains. The outer disk (r>1-3 au) spectra show a crystalline silicate feature in emission, similar to the spectra of comet Hale-Bopp. The striking difference between the inner and outer disk spectral feature is highly unusual among T Tauri stars. The mid-infrared variability is dominated by the outer disk. The strength of the silicate feature changed by more than a factor of two. Between 2011 and 2014 the half-light radius of the mid-infrared-emitting region decreased from 1.15 to 0.7 au. Conclusions. For the origin of the absorption we discuss four possible explanations: a cold obscuring envelope, an accretion heated inner disk, a temperature inversion on the disk surface and a misaligned inner geometry. The silicate emission in the outer disk can be explained by dusty material high above the disk plane, whose mass can change with time, possibly due to turbulence in the disk.

Single-camera full-range high-resolution spectral domain optical coherence tomography.

We developed spectral domain optical coherence tomography using a dual-channel spectrometer for complex conjugate artifacts (CCA) suppression. We used a three-line charge coupled device to simultaneously detect two interferometric spectra with 2π/3 phase difference. The complex interferometric signal was reconstructed by trigonometric manipulation of two real interferometric spectra, and then full-range images were obtained by use of inverse Fourier transform. Artifacts at direct current (DC) and the ghost remnant of the CCA are common issues with the previously reported two-spectrometer method because the slight mismatching between two spectral detection channels had strong negative effects on CCA suppression and appeared to be the limiting factor on system performance. This novel dual-channel spectrometer uses the same spectrometer optics for the two spectral detection channels and, therefore, achieves better matching between two spectral detection channels and consequently better performance in CCA suppression as compared with the dual spectrometer solution. Full-range imaging with CCA suppression up to ∼25 dB was demonstrated when imaging an attenuated reflector. The efficacy of both CCA and DC suppressions also was validated by imaging the anterior segment of a rat eye ex vivo. The quality of CCA-suppressed images was significantly improved with regard to those obtained with the dual-spectrometer design.

Adsorption phase change and wetting condition at solid-vapor interface

Adsorption is the process in which atoms or molecules from a substance adhere to a surface of a adsorbent. Adsorption is a typical surface-based process, which has been a hot research topic in interfacial science and attracts great attention in recent years. Adsorption at a solid-vapor interface is not only important in many industry technologies, but also a phenomenon commonly encountered in nature. The first isotherms model namely Langmuir assumes that one gas molecule could be adsorbed on one adsorption site. Later, the well known BET isotherm formulation was introduced, the concept is that adsorption takes place in layer. However, these models of adsorption isotherms all indicate infinity in the limit of pressure approaching the saturation vapor pressure. Recently, the ζ adsorption isotherm was proposed by approximating the adsorbated as molecular clusters with a maximum of one cluster adsorbed at an adsorption site. This model has the essential characteristic of predicting a finite amount adsorbed at the saturation-vapor pressure. When the Gibbsian thermodynamics is combined with the equilibrium adsorption isotherm, the expression for the solid-vapor surface tension can be established. The ζ adsorption isotherm has been tested successfully for some materials in the limit of pressure approaching the saturation pressure. When a smooth, rigid, homogeneous solid surface is exposed to a vapor at pressure equal or greater than the saturation pressure, the ζ isotherm indicates that an adsorbed vapor film forms. However, no experimental investigation has been reported to examine the validity. To investigated the adsorption behaviors at the solid-vapor interface and to examine the ζ isotherm model, the amount of adsorption are obtained by means of two sets of experiments. First, when the pressure is less than the saturation pressure, the gravimetric measurements of vapor adsorption on silicon powder are conducted. Then the interferometer spectra analysis is used to measure the adsorbed thickness when the pressure is greater than the saturation pressure. Based on the ζ adsorption isotherm, the phase change and wetting condition are discussed. Results show that the isotherms predicted by ζ adsorption equation agree well the experimental measurements. When the pressure ratio is greater than a certain value, the amount of adsorption undergoes a sharp increase and then tends to a constant. The adsorbed vapor is approximated as a collection of molecular clusters. When the pressure is near zero, the adsorbate consists primarily of single molecules, and the number of occupied sites is small. As the pressure ratio progressively increases, the clusters with a larger number of molecules appear in the adsorbate and the number of occupied sites decreases. When the pressure ratio reaches 1.14, the number of unoccupied adsorption sites goes to zero, all the adsorption sites are occupied and the clusters merge to initiate the liquid phase. When the pressure ratio exceeds 1.14, only the large liquid-like clusters are present, which suggests a phase change takes place in the adsorbate and wetting occurs. The surface tension of the solid-vapor are found to be decreased as the adsorption increases, and the predicted wetting condition is in good agreement with the experimental measurements.

European VLBI Network imaging of 6.7 GHz methanol masers

Methanol masers at 6.7 GHz are well known tracers of high-mass star-forming regions. However, their origin is still not clearly understood. We aimed to determine the morphology and velocity structure for a large sample of the maser emission with generally lower peak flux densities than those in previous surveys. Using the European VLBI Network we imaged the remaining sources (17) from a sample of sources that were selected from the unbiased survey using the Torun 32 m dish. Together they form a database of a total of 63 source images with high sensitivity, milliarcsecond angular resolution and very good spectral resolution for detailed studies. We studied in detail the properties of the maser clouds and calculated the mean and median values of the projected size (17.4 au and 5.5 au, respectively) as well as the FWHM of the line (0.373 km s$^{-1}$ and 0.315 km s$^{-1}$ for the mean and median values, respectively), testing whether it was consistent with Gaussian profile. We also found maser clouds with velocity gradients (71 per cent) that ranged from 0.005 km s$^{-1}$ au$^{-1}$ to 0.210 km s$^{-1}$ au$^{-1}$. We tested the kinematic models to explain the observed structures of the 6.7 GHz emission. There were targets where the morphology supported the scenario of a rotating and expanding disk or a bipolar outflow. Comparing the interferometric and single-dish spectra we found that, typically, 50-70 per cent of the flux was missing. This phenomena is not strongly related to the distance of the source. The EVN imaging reveals that in the complete sample of 63 sources the ring-like morphology appeared in 17 per cent of sources, arcs were seen in a further 8 per cent, and the structures were complex in 46 per cent cases. The UC HII regions coincide in position in the sky for 13 per cent of the sources. They are related both to extremely high and low luminosity masers from the sample.