The Effect of Supercooling on the Properties of Thin Saline Ice Layers

Transcription

1 The Effect of Supercooling on the Properties of Thin Saline Ice Layers Johan Wåhlin 1, Alex Klein-Paste 1,* 1 Department of Civil and Transport Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway * Corresponding author. alex.klein-paste@ntnu.no The effect of supercooling on the strength and microstructure of thin saline ice layers formed on asphalt substrates was investigated. Ice layers formed from solutions at large supercooling froze more rapidly and got finer microstructure than ice formed from solutions at a lower supercooling. In addition, the slowly grown ice seemed to have a more distinct separation between brine and ice, with some observations suggesting that the brine was expelled towards the substrate, forming a thin layer between it and the ice. The faster frozen ice incorporated most of the brine between its subgrains. The data analysis showed that within the ice fraction range 0.68 F i 0.78, a higher supercooling lead stronger ice and more salt was needed to weaken it enough to let it fail under the imposed load. 1. INTRODUCTION A wet road or runway usually cause little problems for traffic safety, but when the water freezes it causes hazardous slippery conditions. To prevent this from happening, maintenance engineers typically perform anti-icing operations by applying salt. Sodium chloride often used for anti-icing on roads, whereas on airports it is more common to use chloride-free alternatives such as potassium formate. Recently, Klein-Paste and Wåhlin 1 investigated the hypothesis that the practice of anti-icing on roads and runways is not only based on preventing the formation of ice, but also on controlling the microstructure when ice is formed. It was shown that anti-icing agents play an important role in weakening the ice, allowing traffic itself to break down the ice and reach the underlying pavement texture. The saline ice layers, consisting of pure ice and brine (also called mushy layer), were too weak to withstand the simulated traffic load when the equilibrium ice fraction, F i was less than At higher ice fractions, the microstructure of the layer determines if it fails or remains intact. At ice fractions above 0.78 most specimens remained intact. At F i > 0.85 all specimens were too strong to be removed by the simulated traffic, irrespective of the microstructure. The microstructure of thin saline ice layers is not only determined by the equilibrium ice fraction, but also by other factors such as the magnitude and direction of the heat flow and the amount of supercooling prior to the freezing. The goal of this paper is to investigate the role of supercooling on the microstructure and to what extent it influences the mechanical properties of thin saline ice layers within the ice fraction range 0.68 F i SUPERCOOLING AND FREEZING Like most liquids, water passes through a thermodynamically unstable supercooled state before freezing. In this supercooled state, ice nuclei that form are too small to induce crystal growth in the entire liquid. As the temperature decreases, the critical size needed for a nucleus to survive and grow decreases 2. Ice can be formed by random thermal fluctuations without help from any foreign objects. This is called homogeneous nucleation and requires the water to be supercooled down to about -38 C, a temperature that is depressed even further by the addition of solutes 3. In conditions encountered on roads such a level of supercooling is highly unlikely, hence ice forms by nucleation on a foreign object, called heterogeneous nucleation. This takes place at temperatures much closer to the freezing point. In the paper by Ayel et al 2, supercooling is defined as the difference between the freezing point, T f, of the liquid and the temperature at which the

2 nucleation takes place, called the crystallization temperature T c. T = T f T c (1) The crystallization temperature depends on stochastic phenomena, but is dependent on factors such as sample volume, the presence of foreign objects and the cooling rate. Freezing in a supercooled liquid is always dendritic, and dendrites are growing from a locally heated (due to ice formation) area into a colder liquid 4. The freezing speed of dendrites increases with the amount of supercooling and it decreases by the presence of a solute 2. Ice found under normal conditions on Earth, has a hexagonal structure 5 and it grows slowly along the c-axis compared to along the basal plane, something that results in ice dendritic arms growing perpendicular to the c-axis. Ayel et al 2 describes the temperature response of a freezing event. During the freezing of an aqueous solution, the temperature-time curve goes through four characteristic stages. First the sensible cooling stage, zone A in Figure 1, where the liquid is cooled down to the crystallization temperature. After this follows the recalescence, zone B, where the temperature increases rapidly as latent heat is released due the growth of dendrites into the liquid. As the heat sink in form of the supercooled liquid is used up, the freezing rate decreases and the temperature drops to that of the surroundings, zone C. An ice-brine mixture in quasi equilibrium has now formed and from this stage freezing continues gradually, zone D, down to the eutectic temperature where all liquid eventually is solidified. On the time derivative, the constant cooling in zone A is seen as a constant value. When recalescence occurs, it increases suddenly (zone B) to then decrease to a value lower than in zone A. This decrease and the slow recovery to a stable cooling rate is zone C. Once the time derivative again reaches a constant (but lower than in zone A due to the continuous release of latent heat) value, t eq is reached and zone D starts. As the temperature in the experiments of this paper always is above the eutectic temperature, the brine will never solidify completely. The freezing time in this paper is therefore defined as the time from recalescence, t rec in Figure 1, until the formed ice Figure 1. The temperature signal of a freezing event and its time derivative. layer is at quasi-equilibrium with the slowly cooling surroundings, marked t eq Figure 1. In other words: t f = t eq t rec (2) It is important to note that with this definition of freezing time, it is the local (the area near the temperature sensors) transition from a non-equilibrium supercooled state to a quasi-equilibrium ice-brine mixture which is measured, rather than the freezing time of the entire specimen. 2. EXPERIMENTAL SETUP AND METHOD The details of the freezing procedure and testing of the specimens are presented elsewhere 1. The testing procedure was extended with microstructural observations and the temperature data was analyzed. Supplements and deviations from the original experimental setup are described below. 2.1 Freezing time One substrate was equipped with 8 thermocouples which were evenly distributed along the circumference. The sampling rate was set to 1 Hz. This setup was used to (1) investigate how the freezing time of a solution varied with the amount of supercooling, and (2) determine the spatial variation of supercooling and freezing time. Seven specimens at -15ºC were produced with this arrangement, all with the same 5w% NaCl concentration dyed with 0.05w% eosine-b. The freezing time, as defined in

3 equation (2) was identified by differentiating the temperature signals with respect to time. 2.2 Microstructural investigations Two sets of eight specimens were prepared with different cooling rates to study how the supercooling affects the microstructure of the ice. These specimens were prepared on substrates with one thermocouple each, logging at 1 Hz. The cooling rate was varied by starting one set with cold substrates (0ºC) and the other set with warm substrates (+15ºC). The heat capacity of the warm substrates reduced the cooling rate sufficiently to alter the crystallization temperature. All substrates were prepared with 7.5w% KCOOH solution, dyed with 0.05w% eosine-b and cooled to -15ºC. This results in an equilibrium ice fraction of The eosine-b, just as KCOOH, is not accepted into the ice lattice and will color the brine red, making it easy to distinguish from the ice after freezing. The specimens were photographed and samples, approximately 5-10 mm in diameter, were taken from different sections. The samples were carefully cut loose, placed on a thin glass plate and studied and photographed under an optical microscope. The dimensions of the ice structure were measured from the images. By rotating the samples under cross-polarized light, the orientation (vertical or not-vertical) of the c-axis was determined. In addition to the data collected in these dedicated tests, photos and microscope images were acquired during the collection of the KCOOH data set presented in Klein-Paste and Wåhlin 1, and the freezing time experiment described above. 2.3 Mechanical properties The last part of the investigation was to analyze the dataset from Klein-Paste and Wåhlin 1 to get an impression of how much supercooling altered the mechanical properties of the ice. In the original experiment on NaCl the temperature sampling time was 5 minutes. This is not sufficient to determine the crystallization temperature. During the collection of the KCOOH data, the sampling time was reduced to 10 seconds, to accommodate for this analysis. The selected range for KCOOH data, 0.68 F i 0.78, contained 133 specimens. For each specimen, the crystallization temperature was identified manually and the supercooling was calculated using equation (1). In five cases of very low supercooling, the crystallization temperature could not be determined, leaving 128 specimens. 3. RESULTS 3.1 Freezing time The maximum difference in local supercooling measured at eight locations on the same substrate was 1.4ºC. However, in general the difference was much smaller. Over the seven tests, the average standard deviation between the eight thermocouples was 0.27ºC. The relation between freezing time and supercooling is shown in Figure 2. There is large variability present in the data, with different freezing times for the same supercooling, but there exists a clear trend: A higher amount of supercooling leads to more rapid freezing. 3.2 Microstructural observations At high supercooling, above 6 C, the ice formed in a fine dendritic pattern (Figure 3a). It covered the entire specimen and consisted of small ice cells; typically mm wide and mm long (Figure 3b). The brine was kept between the ice cells and little, if any, free liquid was present. The c-axis was randomly oriented. With decreasing supercooling, areas with an undulated surface began to appear (Figure 3c). The lower the supercooling, the larger area this ice type covered, and there was a tendency for the undulations to increase in size. This kind of ice consisted of roundish platelets 0.25 mm in diameter when they first appeared at high supercooling, as seen in Figure 3d. Figure 2. The freezing time as a function of supercooling

4 Figure 3. The microstructure of different ice types. Normal photos on the left hand side and microscope photos of the same ice on the right.

5 As the supercooling decreased, these platelets grew larger and they developed a dendritic appearance, with stem and branches (Figure 3f). This ice type eventually, dominated entire substrates, no longer looking like small undulated areas but as larger dendrites on the surface of the liquid, as seen in Figure 3e. For very low supercooling (below 1.5 C), these could be so large that a single dendrite with its secondary arms could cover an entire specimen. The c-axis of the smallest platelets was close to vertical while on the larger platelets it was fully vertical. For all but the lowest amounts supercooling, the fine dendritic ice with random c-axis was found near the nucleation point, even when the rest of the specimen had undulated ice or large horizontal dendrites. This phenomenon is shown in Figure 3c. The brine is this type of plate ice seemed not to be incorporated in the microstructure of the ice in the same way as in the finer cell structure. It rather formed a layer between the ice and the substrate. This observation is supported by the fact that it was much easier to lift from the substrate when cut for study in the microscope. For specimens with low concentrations and low supercooling, the ice showed a tendency to grow in a needle-shaped fashion as shown in Figure 3g. These needles consisted of primary dendrites with serrated sides or very short secondary arms (see Figure 3h). The needles were thick, typically 1-3 mm wide and often more than 0.5 cm long. They had a vertical c-axis. Between the needles were areas with very little or even no liquid or ice. 3.3 Strength analysis The amount of supercooling from all the KCOOH specimens is plotted against ice fraction in Figure 4. The specimens that failed under the imposed load are marked and with symbol and the specimens that remained intact are marked with o. Partly failed samples are marked with. A line was drawn manually to mark the transition between failed and intact samples. The figure shows that there is a clear correlation between the ice fraction at which the specimens fail and the amount of supercooling. Figure 4. The distribution of failed vs. not-failed specimens as a function of ice fraction and supercooling 4. DISCUSSION Freezing time is clearly dependent of the degree of supercooling at nucleation, as seen in Figure 2. At a large supercooling, the liquid as well as the surroundings are further below the freezing point than for a small supercooling. This means that released latent heat is much more rapidly removed. The freezing is therefore faster, and the quasi equilibrium with the surroundings is reached quicker. The small standard deviation of supercooling on a single substrate shows that there are no large spatial temperature variations in the water film. The larger variations in freezing time show that supercooling alone does not control freezing. Other factors, such as solute redistribution and heat transfer could be possible reasons for these variations. The finer microstructure formed from high supercooling and faster freezing is consistent with earlier reports on dendritic growth in aqueous solutions, where it was found that the spacing of secondary dendrites decreases with an increased freezing rate 6. At high supercooling, even though the freezing in a supercooled solution always is dendric, no dendrites could be identified at the microscopic scale. The ice rather appeared as collection of cell-like subgrains, as seen in Figure 3b and 3d. However, it should be noted that we observe the microstructure hours after freezing. Initially the structure may have been finer, but due to a process called Ostwald ripening the larger structure will grow on the expense of finer structure, causing a coarsening of the microstructure 4. At the same time, long narrow dendrites would become

6 unstable and disintegrate into rounded subgrains. When attempting to separate the subgrains from each other using a piece of a thin glass, the force required suggests that they are bond to each other by ice-ice bonds, and not only capillary forces from the brine contained in the veins between subgrains. As ice grows faster along the basal plane an ice nucleus would, given enough space and time, form thin plates which due to buoyancy forces would float with their c-axis vertical. This happens when lake or ocean water freezes under calm conditions 7. The mostly vertical c-axis found in ice forming on the substrates with low supercooling suggests that this also happens here in our experiment, even though the water film only was 0.3 mm thick. A long freezing time would also give dissolved salt a longer time to diffuse away from the freezing boundary. This could give a larger scale to the phase segregation, something that could explain why brine and ice seemed to be separated to a higher degree at low supercooling. The needle shaped ice present at low concentrations and low supercooling is expected from Rohatgi et al 6, who reports that for a constant freezing rate there is a threshold concentration after which dendrites start to develop side arms. The areas empty of ice and brine between needles is probably an effect occurring due to slow growth rates. As the needles start to form, capillary effects pump more solution to the growing needles, emptying the areas in between. Figure 4 shows that there is a clear correlation between the ice fraction at which the specimens fail and the amount of supercooling. The drawn line can be interpreted as the transition ice fraction 1, and it shows that a high supercooling give stronger ice, meaning that more salt have to be added in order to make it fail to the pendulum load. 95% of all measurements from KCOOH specimens had a supercooling between 1.5 and 7.1 degrees. For this supercooling range, the line in Figure 4 would stretch from F i =0.68 to F i =0.79, which is the entire transition zone for KCOOH bottom cooling in Klein-Paste and Wåhlin 1. The supercooling conditions therefore give an explanation for the variability of the transition ice fraction within this zone. 5. CONCLUSIONS The strength of a thin saline ice layer formed on asphalt substrates is to a large degree dependent on the relative amounts of ice-brine present. However, the microstructure that forms under different freezing conditions also plays a role. Ice forming from a highly supercooled solution freezes more rapidly and forms a finer microstructure than ice formed from a less supercooled solution. The freezing rate also seems to influence the brine redistribution. At high freezing rate the brine is to a high degree contained within the ice structure, between the subgrains. For slower freezing it seems like the brine forms a thin layer between the ice and the substrate. Within the transition zone for KCOOH there is a clear tendency for ice frozen at a higher supercooling to need more salt in order to become weakened enough to fail to the pendulum load. ACKNOWLEDGEMENTS The authors thank Øystein Larsen, Norwegian Public Roads Administration and Armann Norheim, Avinor ASA for the financial support for this study. The authors thank Lars Arnberg at the metallurgical department at NTNU for valuable discussion. REFERENCES (1) A. Klein-Paste, J. Wåhlin, Controlling the Properties of Thin Ice Layers on Pavement Surfaces - An Alternative Explanation for Anti-icing, 12 th International Conference on the Physics and Chemistry of Ice, Sapporo, Japan, September 5-10, (2) V. Ayel, O. Lottin, M. Faucheux, D. Sallier, H. Peerhossaini, International Journal of Heat and Mass Transfer, 49, 11-12, (2006). (3) T. Koop, Zeitschrift für physikalische Chemie, 218, 11, (2004). (4) W. Kurz, D.J. Fisher, Fundamentals of Solidification, 4 th Ed., Trans Tech Publications LTD, (5) P.V. Hobbs, Ice Physics, Clarendon Press, Oxford, 1974, p (6) P. Rohatgi, C.M. Adams, Journal of Glaciology, 6, 47, (1967). (7) W. Weeks, S. Ackley, The Growth, Structure and Properties of Sea Ice, in N. Untersteiner (ed.) The Geophysics of Sea Ice, Plenum Press, New York 1986, p.9-164