Optical and x-ray studies of ice growth in water

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.Huvudsyftet med denna avhandling är att studera strukturen av metastabilt vatten och dess omvandling till is vid underkylning med hjälp avröntgenspridning samt optisk mikroskopi. I alla experiment kyldes mikrometerstora vattendroppar genom avdunstning i vakuum och undersöktesantingen med röntgenljus eller genom optisk belysning. Utöver dessa användes en infraröd puls i ett av experimenten för att introducera ett temperaturhopp i vattendropparna och på så sätt uppnå ultrasnabb kalorimetrisom möjliggör att mäta specifik värmekapacitet vid underkylning.Den andra toppen av strukturfaktorn för vatten visade ett maximumvid 236 K. Den abnormala minskningen av toppens position vid temperaturer under 236 K tros vara relaterat till ett upprepande avstånd i dettetraedriska nätverket, associerat med korrelationerna på medellånga avstånd i vatten. Minskningen i temperatur gör att parkorrelationsfunktionenförändras på liknande sätt som för amorf is med låg densitet (LDA), vilketinnebär att strukturen rör sig mot en mindre tät lokal ordning. Detta äri överensstämmelse med ett pentamerbaserat tetraedriskt nätverk med lågdensitet som skiftar kontinuerligt mot en LDA-struktur när temperaturenminskar. Röntgenspridning visade att det finns ett maximum i den specifikavärmekapaciteten för vatten vid cirka 229 K och att den ökar vid kylningfrån 88 J/mol/K vid 244 K till 218 J/mol/K vid 229 K.Homogen isbildning fann olika frysta stadier, nämligen delvis frusnadroppar, vätskeutstötningar och frakturer, vilka sågs i de mikroskopiskabilderna. Experimentella isbildningsdata vid temperaturer så låga som 230K resulterade i en anpassningskurva som visar en långsammare ökning avisbildningshastigheten vid underkylning än tidigare trott. Vi kan uppskattaenergin i gränssnittet mellan is och vätska som funktion av temperatur medhjälp av diffusivitetsdata härledd från den specifika värmekapaciteten sommättes experimentellt med ultrasnabb kalorimetri. Detta resulterade i ettminimum i energin i gränssnittet vid cirka 236 K.Slutligen tenderar is att bilda olika strukturer beroende på var i droppenisen växer. Det observerades att för kristaller nära dropparnas mittpunktkristalliserar is med hexagonal struktur medan på ytan kristalliserar is medavsevärd mängd kubisk struktur. Detta kan bero på att tillväxt av plana kristaller vid ytan resulterar i oordning mellan kristallplanen, eftersomstrukturen måste anpassas till krökningen av droppens yta.QC 220905</p

Optical and x-ray studies of ice growth in water

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.Huvudsyftet med denna avhandling är att studera strukturen av metastabilt vatten och dess omvandling till is vid underkylning med hjälp avröntgenspridning samt optisk mikroskopi. I alla experiment kyldes mikrometerstora vattendroppar genom avdunstning i vakuum och undersöktesantingen med röntgenljus eller genom optisk belysning. Utöver dessa användes en infraröd puls i ett av experimenten för att introducera ett temperaturhopp i vattendropparna och på så sätt uppnå ultrasnabb kalorimetrisom möjliggör att mäta specifik värmekapacitet vid underkylning.Den andra toppen av strukturfaktorn för vatten visade ett maximumvid 236 K. Den abnormala minskningen av toppens position vid temperaturer under 236 K tros vara relaterat till ett upprepande avstånd i dettetraedriska nätverket, associerat med korrelationerna på medellånga avstånd i vatten. Minskningen i temperatur gör att parkorrelationsfunktionenförändras på liknande sätt som för amorf is med låg densitet (LDA), vilketinnebär att strukturen rör sig mot en mindre tät lokal ordning. Detta äri överensstämmelse med ett pentamerbaserat tetraedriskt nätverk med lågdensitet som skiftar kontinuerligt mot en LDA-struktur när temperaturenminskar. Röntgenspridning visade att det finns ett maximum i den specifikavärmekapaciteten för vatten vid cirka 229 K och att den ökar vid kylningfrån 88 J/mol/K vid 244 K till 218 J/mol/K vid 229 K.Homogen isbildning fann olika frysta stadier, nämligen delvis frusnadroppar, vätskeutstötningar och frakturer, vilka sågs i de mikroskopiskabilderna. Experimentella isbildningsdata vid temperaturer så låga som 230K resulterade i en anpassningskurva som visar en långsammare ökning avisbildningshastigheten vid underkylning än tidigare trott. Vi kan uppskattaenergin i gränssnittet mellan is och vätska som funktion av temperatur medhjälp av diffusivitetsdata härledd från den specifika värmekapaciteten sommättes experimentellt med ultrasnabb kalorimetri. Detta resulterade i ettminimum i energin i gränssnittet vid cirka 236 K.Slutligen tenderar is att bilda olika strukturer beroende på var i droppenisen växer. Det observerades att för kristaller nära dropparnas mittpunktkristalliserar is med hexagonal struktur medan på ytan kristalliserar is medavsevärd mängd kubisk struktur. Detta kan bero på att tillväxt av plana kristaller vid ytan resulterar i oordning mellan kristallplanen, eftersomstrukturen måste anpassas till krökningen av droppens yta.QC 220905</p

Heterogeneous Ice Growth in Micron-Sized Water Droplets Due to Spontaneous Freezing

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. QC 20220223</p

Anomalous temperature dependence of the experimental x-ray structure factor of supercooled water

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 angstrom(-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 (q(2)). We found that q(2) 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 q(2) 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. (C) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).11Ysciescopu

Enhancement and maximum in the isobaric specific-heat capacity measurements of deeply supercooled water using ultrafast calorimetry

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.QC 20220317</p