| 1. |
- Esmaeildoost, Niloofar, et al.
(author)
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Anomalous temperature dependence of the experimental x-ray structure factor of supercooled water
- 2021
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 155:21, s. 214501-214501
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Journal article (peer-reviewed)abstract
- The structural changes of water upon deep supercooling were studied through wide-angle x-ray scattering at SwissFEL. The experimental setup had a momentum transfer range of 4.5 Å-1, which covered the principal doublet of the x-ray structure factor of water. The oxygen-oxygen structure factor was obtained for temperatures down to 228.5 ± 0.6 K. Similar to previous studies, the second diffraction peak increased strongly in amplitude as the structural change accelerated toward a local tetrahedral structure upon deep supercooling. We also observed an anomalous trend for the second peak position of the oxygen-oxygen structure factor (q2). We found that q2 exhibits an unprecedented positive partial derivative with respect to temperature for temperatures below 236 K. Based on Fourier inversion of our experimental data combined with reference data, we propose that the anomalous q2 shift originates from that a repeat spacing in the tetrahedral network, associated with all peaks in the oxygen-oxygen pair-correlation function, gives rise to a less dense local ordering that resembles that of low-density amorphous ice. The findings are consistent with that liquid water consists of a pentamer-based hydrogen-bonded network with low density upon deep supercooling.
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| 2. |
- Esmaeildoost, Niloofar, et al.
(author)
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Effects of beam temperature and plasma frequency on the radiation growth rate of a FEL with a laser wiggler
- 2017
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In: Laser and particle beams (Print). - : Hindawi Limited. - 0263-0346 .- 1469-803X. ; 35:2, s. 241-251
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Journal article (peer-reviewed)abstract
- A linearly polarized laser pulse has been employed as a wiggler in a free-electron laser (FEL) in the presence of a plasma background for generating short wavelength radiation down to the extreme ultraviolet ray and X-ray spectral regions. Introducing plasma background in the FEL interaction region would lessen the beam energy requirement and also enhance both the beam current and the electron-bunching process. This configuration affords the possibility of scaling the device to more compact FELs and would have a higher tunability by changing the plasma density and the temperature of the electron beam. Electron trajectories have been analyzed using single-particle dynamics. The effect of plasma density on electron orbits has been investigated. A polynomial dispersion relation considering longitudinal thermal motion has been derived, by employing perturbation analysis. Numerical studies indicate that by increasing plasma density, the growth rate for groups I and II decreases, while the growth rate for group III increases. In addition, the effect of beam temperature and cyclotron frequency on the growth rate has been discussed. It has been found that by increasing the thermal velocity of the electron beam, the growth rate for groups I and III trivially decreases, while it increases for group II orbits. Besides, an increase in cyclotron frequency cause growth enhancement for group I orbits, while it present a growth decrement for group II and III orbits.
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| 3. |
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| 5. |
- Esmaeildoost, Niloofar, et al.
(author)
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Heterogeneous Ice Growth in Micron-Sized Water Droplets Due to Spontaneous Freezing
- 2022
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In: Crystals. - : MDPI AG. - 2073-4352. ; 12:1
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Journal article (peer-reviewed)abstract
- Understanding how ice nucleates and grows into larger crystals is of crucial importance for many research fields. The purpose of this study was to shed light on the phase and structure of ice once a nucleus is formed inside a metastable water droplet. Wide-angle X-ray scattering (WAXS) was performed on micron-sized droplets evaporatively cooled to temperatures where homogeneous nucleation occurs. We found that for our weak hits ice grows more cubic compared to the strong hits that are completely hexagonal. Due to efficient heat removal caused by evaporation, we propose that the cubicity of ice at the vicinity of the droplet’s surface is higher than for ice formed within the bulk of the droplet. Moreover, the Bragg peaks were classified based on their geometrical shapes and positions in reciprocal space, which showed that ice grows heterogeneously with a significant population of peaks indicative of truncation rods and crystal defects. Frequent occurrences of the (100) reflection with extended in-planar structure suggested that large planar ice crystals form at the droplet surface, then fracture into smaller domains to accommodate to the curvature of the droplets. Planar faulting due to misaligned domains would explain the increased cubicity close to the droplet surface.
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| 6. |
- Esmaeildoost, Niloofar
(author)
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Optical and x-ray studies of ice growth in water
- 2022
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Doctoral thesis (other academic/artistic)abstract
- The key purpose of this thesis is to study the structure of metastable water and its transformation into ice upon deep supercooling using x-ray scattering as well as optical microscopy. In all experiments, micrometer-sized water droplets were evaporatively cooled in vacuum and probed either by x-rays or optical illumination. In addition to these, an infrared (IR) heating pulse was employed in one of the experiments to introduce a temperature jump in water droplets and achieve ultrafast calorimetry, which can measure specific heat capacity upon supercooling. The second peak of the structure factor presented a maximum at 236 K. The anomalous decrease in peak positions below 236 K was related to a repeat spacing in the tetrahedral network, associated with the intermediaterange correlations in water. The decrease in temperature makes the paircorrelation function change in a similar manner to that of low-density amorphous ice (LDA), meaning that the structure moves towards a less dense local ordering. This is in consistency with a low-density pentamer-bonded tetrahedral network that shifts continuously towards an LDA structure as it cools down. The x-ray scattering data showed that there is a maximum in the specific heat capacity of water at about 229 K and it increases from 88 J/mol/K at 244 K to 218 J/mol/K at 229 K upon cooling. Homogeneous ice nucleation showed that there is a mechanism of freezing based on the rates at which different frozen stages, i.e., partially frozen, liquid extrusion and fractured droplets, are seen in the microscopic images. Experimental nucleation data at temperatures as low as ∼230 K resulted in a nucleation fitting curve that shows a slower nucleation rate increase upon supercooling. Using self-diffusion data that was experimentally measured through wide-angle x-ray scattering and ultrafast calorimetry, we can assess the interfacial energy as a function of temperature. This resulted in a minimum in interfacial energy at around 236 K. Moreover, within the droplet, ice tends to form different structures after it has nucleated based on where in the droplet it is growing. It was observed that for crystals inside the bulk and close to the center of the droplets, ice crystallizes with hexagonal structure whereas on the surface it crystallizes with stacking-disorder containing a considerable amount of cubic structure. This can come from the fact that planar growth of crystals at the surface breaks down into a faulty structure that needs to accommodate the curvature of the droplet’s surface.
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| 8. |
- Pathak, Harshad, et al.
(author)
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Enhancement and maximum in the isobaric specific-heat capacity measurements of deeply supercooled water using ultrafast calorimetry
- 2021
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In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 118:6
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Journal article (peer-reviewed)abstract
- Knowledge of the temperature dependence of the isobaric specific heat (Cp) upon deep supercooling can give insights regarding the anomalous properties of water. If a maximum in Cp exists at a specific temperature, as in the isothermal compressibility, it would further validate the liquid–liquid critical point model that can explain the anomalous increase in thermodynamic response functions. The challenge is that the relevant temperature range falls in the region where ice crystallization becomes rapid, which has previously excluded experiments. Here, we have utilized a methodology of ultrafast calorimetry by determining the temperature jump from femtosecond X-ray pulses after heating with an infrared laser pulse and with a sufficiently long time delay between the pulses to allow measurements at constant pressure. Evaporative cooling of ∼15-µm diameter droplets in vacuum enabled us to reach a temperature down to ∼228 K with a small fraction of the droplets remaining unfrozen. We observed a sharp increase in Cp, from 88 J/mol/K at 244 K to about 218 J/mol/K at 229 K where a maximum is seen. The Cp maximum is at a similar temperature as the maxima of the isothermal compressibility and correlation length. From the Cp measurement, we estimated the excess entropy and self-diffusion coefficient of water and these properties decrease rapidly below 235 K.
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