How do Quasi-Liquid Layers Emerge from Ice Crystal Surfaces?

Feb 18, 2013 - However, revealing the dynamic behavior of QLLs, which dominates the surface properties of ice crystals at temperatures near the meltin...
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How do Quasi-Liquid Layers Emerge from Ice Crystal Surfaces? Gen Sazaki,*,†,‡ Harutoshi Asakawa,† Ken Nagashima,† Shunichi Nakatsubo,† and Yoshinori Furukawa† †

Institute of Low Temperature Science, Hokkaido University, N19−W8, Kita-ku, Sapporo 011-0819, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

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W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: Ice crystal surfaces melt at temperatures below 0 °C, and then quasi-liquid layers (QLLs) are formed. However, revealing the dynamic behavior of QLLs, which dominates the surface properties of ice crystals at temperatures near the melting point, remains an experimental challenge. Here, we demonstrate the similarities and differences in the generation mechanisms of two types of QLL phases, which show different morphologies and dynamics. We directly visualized the appearance of round liquidlike droplets (αQLLs) and thin liquidlike layers (β-QLLs) on ice basal faces by advanced optical microscopy, which can allow visualization of the individual elementary steps on basal faces. We found that αQLLs always appear at outcrops of dislocations, and that β-QLLs emerge from crystal surfaces where many microdefects are embedded. These results clearly demonstrate the similar function that strain induces the appearance of both types of QLLs. We also found that β-QLLs are spontaneously formed at interfaces between basal faces and α-QLLs, when the diameter of the αQLLs becomes larger than several tens of micrometers. This result arose from the different structures of α- and β-QLLs: the βQLLs probably have a structure intermediate between those of basal faces and α-QLLs, resulting in a reduction of the total interfacial free energy. temperatures of α- and β-QLLs show slight variations, βQLLs always appear at a higher temperature than α-QLLs in the same run. The different morphologies of α- and β-QLLs clearly demonstrate that they have different structures and also different interactions with ice crystal surfaces. To reveal such differences, one way is examining the similarities and differences in the generation mechanisms of α- and β-QLLs on ice crystal surfaces. Hence, in this study, we directly observed how α- and β-QLLs emerged from a basal face of an ice crystal by LCM-DIM and demonstrated the effects of strain and the wettabilities of interfaces (Figure S1 of the Supporting Information).

1. INTRODUCTION Surface melting (premelting) occurs on crystal surfaces of various materials, such as metal, semiconductor, and inorganic/ organic materials, at temperatures below the melting point,1 resulting in the formation of quasi-liquid layers (QLLs) on crystal surfaces. Since QLLs dominate the properties of crystal surfaces,1−4 QLLs play crucially important roles in crystal growth of various materials in a wide variety of fields. Among various materials, ice is the only one for which the formation of QLLs has been studied extensively,2−4 because of the easiness of experimentally obtaining a surface melting state and also the close linkage to our daily life. Hence, we also tried to obtain further understanding of the behavior of QLLs on ice crystal surfaces, utilizing our advanced optical microscopy. Although surface melting of ice crystals was first proposed by M. Faraday in the 1850s,5 it was not until very recently that QLLs on ice crystal surfaces could be visualized directly.6 We have visualized surface melting processes in situ on a basal face of an ice crystal below 0 °C by laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM),7 by which the 0.37 nm thick elementary steps on ice crystal surfaces can be visualized directly.8 Then, we found that two types of QLL phases appear that exhibit different morphologies and dynamics. With increasing temperature, round liquidlike droplets (α-QLLs) first appear from −1.5 to −0.4 °C (white arrowhead in Figure 1). In addition, we found that thin liquidlike layers (β-QLLs) appear at −1.0 to −0.1 °C (red arrowhead in Figure 1). Although the appearance © 2013 American Chemical Society

2. EXPERIMENTAL METHODS A confocal system (FV300, Olympus Optical Co. Ltd.) was attached to an inverted optical microscope (IX70, Olympus Optical Co. Ltd.), as previously explained.7,8 A super luminescent diode (Amonics Ltd., model ASLD68-050-B-FA, 680 nm) was used for LCM-DIM observations. The LCM-DIM system used in this study (Figure S2A of the Supporting Information) contained all the improvements reported in our recent study8 of the observation of elementary steps on ice crystal surfaces. Received: January 15, 2013 Revised: February 7, 2013 Published: February 18, 2013 1761

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Figure 1. (A) A photomicrograph and (B) schematic illustration of two types of quasi-liquid layers (QLLs) that appeared on a basal face of an ice crystal during surface melting.6 With increasing temperature, round liquidlike droplets (α-QLLs) first appeared from −1.5 to −0.4 °C. In addition, thin liquidlike layers (β-QLLs) appeared from −1.0 to −0.1 °C. The appearance temperatures of α-QLLs and β-QLLs exhibited slight variations, but β-QLLs always appeared at a higher temperature than α-QLLs in the same run. White, red, and black arrowheads indicate α-QLLs, β-QLLs and elementary steps, respectively. Black arrows show the growth direction of elementary steps.

Figure 2. Repetitive appearances (at −0.5 °C) and disappearances (at −1.0 °C) of round liquidlike droplets (α-QLLs) on a basal face of an ice crystal. The temperature of a basal face was repeatedly increased from −1.0 to −0.5 °C (B to C and D to E) and decreased from −0.5 to −1.0 °C (C to D), keeping the water vapor supersaturated. We processed the original LCM-DIM image A, according to the recipe explained in Figure S3 of the Supporting Information, to obtain the image B. We also obtained the images C−F according to the same recipe. The positions from where α-QLLs appeared during the three successive temperature rises are summarized in F: white circles (the first rise), white triangles (the second rise) and white squares (the third rise). Blue arrowheads correspond to the positions from where concentric steps appeared. Other arrowheads and arrows are the same as those in Figure 1. A video of this process is available (Video 1).

An observation chamber had upper and lower Cu plates, whose temperatures were separately controlled using Peltier elements (Figure S2B of the Supporting Information). At the center of the upper Cu plate, a cleaved AgI crystal (a kind gift from emeritus professor G. Layton of Northern Arizona University), known as an ice nucleating agent, was attached using heat grease. On this AgI crystal, Ih sample ice crystals were grown at −15 °C from supersaturated water vapor in a nitrogen environment. To supply water vapor to the sample ice crystals, other ice crystals were prepared on the lower Cu plate, as a source of water vapor. Other details of the observation chamber were reported in our recent study.8 By separately controlling the temperatures of the sample and source ice crystals, the growth temperature of the sample ice crystals and the supersaturation of the water vapor were adjusted independently. After the lateral size and height of the sample ice crystals reached several hundred μm at −15 °C, the temperature was increased to a final value of −0.1 °C, at rates of ∼0.1 °C/min (from −15.0 to −2.0 °C) and ∼0.02 °C/min (from −2.0 to −0.1 °C). All through this process, the sample ice crystals were kept growing, by carefully changing the supersaturation and confirming the growth by LCM-DIM observations. Then the behavior of QLLs on the surfaces of sample ice crystals was observed.

°C. We processed the raw image in Figure 2A to obtain the image in Figure 2B, according to the recipe explained in Figure S3 of the Supporting Information: we subtracted the timeaveraged image from the original image, then adjusted a gain and an offset, and smoothed the image using a Gaussian filter that was one pixel in size. We also obtained Figures 2C−F according to the same recipe. In Figures 2B−F, the differential interference contrast was adjusted to appear as if the ice crystal surface was illuminated by a light beam slanted from the upperleft to the lower-right direction, as explained in Figure S4 of the Supporting Information. Figure 2B shows concentric elementary steps (black arrowhead) growing in the black arrow directions at −1.0 °C. The concentric steps appeared repeatedly from the location marked by a blue arrowhead. Hence, the source of the concentric steps was a screw dislocation, although the steps did not show a spiral shape since the screw dislocation was located at the edge of the crystal surface. Then, we increased the growth temperature to −0.5 °C. As shown in Figure 2C, we

3. RESULTS AND DISCUSSION 3.1. The Appearance of Round Liquidlike Droplets (αQLLs). To reveal how α-QLLs emerge from a basal face of an ice crystal, we first increased and then decreased the growth temperature of a sample ice repeatedly from −1.0 to −0.5 °C and from −0.5 to −1.0 °C, keeping the water vapor supersaturated. Then we observed a basal face by LCM-DIM. Figure 2A shows a raw LCM-DIM image of a basal face at −1.0 1762

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found the appearance of α-QLLs (white arrowhead). As discussed in our previous paper,6 once α-QLLs had emerged, they functioned as sources of steps. When we decreased the growth temperature to −1.0 °C (Figure 2D), the α-QLLs disappeared and concentric steps (black arrowhead) emerged from the dislocation marked by a blue arrowhead. After we again increased the growth temperature to −0.5 °C (Figure 2E), α-QLLs appeared reproducibly. In Figure 2F, we summarized the positions from where αQLLs appeared during the repeat experiments involving three successive temperature rises from −1.0 to −0.5 °C, using white circles (the first rise), white triangles (the second rise), and white squares (the third rise). α-QLLs appeared repeatedly from the three locations at the edge of the basal face. The concentric steps also emerged from these three locations, as shown in Figure 2 (panels B and D). Hence, we concluded that the dislocations functioned as the sources for generation of the α-QLLs, as well as spiral steps. The increase in free energy caused by strain around the dislocations probably resulted in the repeated appearances of the α-QLLs. We have observed more than several hundred basal faces on which concentric steps appeared. In all cases, outcrops of screw dislocations were always located at the edges of basal faces, as marked by blue arrowheads in Figure 2 (panels B and D). We could never observe screw dislocations located in central regions of basal faces. Screw dislocations that emerged from basal faces do not have energetically stable configurations.9 This probably results in the appearance of screw dislocations only at the edges of basal faces. In contrast, in the central region of the basal face (Figure 2F), α-QLLs appeared in a spatially random manner, as marked by white circles, triangles, and squares. This result strongly suggests that the central region of the basal face was strained in a spatially homogeneous way. In the early stage of the heteroepitaxial growth of ice crystals on an AgI substrate crystal, ice crystals usually possess a larger amount of strain than in the later stages. Hence, the central region of the basal face probably more easily inherited strain that was generated during the early growth stage. At this moment, the origin of the strain is still unclear, although the lattice deformation by heteroepitaxy and the formation of misfit dislocations are most probable. Hence, in the near future, we will find heteroepitaxial substrate crystals other than AgI and will carry out similar experiments using different substrates with different lattice mismatches. 3.2. The Appearance of Thin Liquidlike Layers (βQLLs). In addition to α-QLLs, it is also important to reveal how β-QLLs emerge from a basal face. To understand this issue, at −1.5 °C, we first prepared a basal face on which only elementary steps existed. Then, we increased the temperature of the basal face from −1.5 to −0.1 °C, keeping the water vapor supersaturated. Figure 3 demonstrates an in situ observation of the appearance of β-QLLs on a basal face. Figure 3A shows a raw LCM-DIM image taken 19.6 s after the increase in the growth temperature. In the upper left area marked by a dotted circle, many white dots were observed. Irrespective of the direction of the differential interference contrast (Figure S4 of the Supporting Information), these dots always looked white. From this observation, we concluded that these white dots did not correspond to asperities on the basal face but corresponded to scattered light from microdefects (e.g., gas inclusions, aggregates of point defects, aggregates of dislocations, etc.) that were embedded in the basal face during the growth. At this

Figure 3. The appearance of thin liquidlike layers (β-QLLs) with increasing temperature from −1.5 to −0.1 °C. Images A, B, C, D, E, and F were taken 19.6, 19.6, 26.1, 32.7, 42.5, and 49.0 s, respectively, after the temperature was set at −0.1 °C. We processed an original LCM-DIM image A, according to the recipe explained in Figure S3 of the Supporting Information, to obtain image B. We also obtained the images C−F, according to the same recipe. Many white dots marked by a dotted circle shown in A indicate many microdefects that were embedded in the basal face, resulting in the promotion of the nucleation of β-QLLs. Red arrowheads correspond to the border of the area in which many small β-QLLs appeared. (C and D) Such a border was propagated in the direction of the red arrow. Other arrowheads and arrows are the same as those in Figures 1 and 2. A video of the process B−F is available (Video 2).

moment, real identity of the microdefects is still unclear. As in the case of Figure 2 (panels B−F), we also processed the raw LCM-DIM images to obtain a series of successive images shown in Figure 3 (panels B−F), according to the standard recipe (Figure S3 of the Supporting Information). As shown in Figure 3B, the lateral growth of elementary steps (black arrowhead) indicates that the basal face was bare in the early stages. After 26.1 s (Figure 3C), many small β-QLLs nucleated in the upper left area of the basal face. A red arrowhead shows the border of the area in which many such small β-QLLs appeared. This border propagated toward the lower right (red arrow) (Figure 3D, 32.7 s), demonstrating that the nucleation of β-QLLs in the upper left area was faster than in the other area. The β-QLLs coalesced with each other (Figure 3E, 42.5 s), and then finally fully covered the basal face (Figure 3F, 49.0 s). As demonstrated in Figure 3, when many microdefects were embedded in a basal face, β-QLLs always emerged reproducibly 1763

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from such areas. This result strongly suggests that the increase in free energy of a basal face caused by strain around microdefects induces the appearance of β-QLLs, as in the case of α-QLLs. Hence, from the similarity in behavior found for both α- and β-QLLs, it is reasonable to assume that surface melting starts from crystal surfaces having a higher free energy caused by strain. The central strained area of the basal face can also promote two-dimensional (2D) nucleation and subsequent lateral growth of 2D island steps, as reported previously.8 In addition, the strain around dislocations can also function as the source of α-QLLs as well as spiral steps (Figure 2). Therefore, we can conclude that strain plays vital roles in both the surface melting and growth of ice crystals. Many studies have so far been carried out to measure the relation between temperature and thickness of QLLs by various methods (see papers of ref 6 cited in Table S1 of the Supporting Information). These studies reported that QLLs appeared even at −10 °C and their thickness significantly increased with increasing temperature. But in this study, we could observe the appearances of QLLs, only at a temperature higher than −1.5 °C, from basal faces that include lattice defects. Comparison between the previous studies and this study suggests that lower-quality ice crystal surfaces with larger amounts of lattice defects induce the appearance of QLLs at a lower temperature. Higher-index faces, which in particular appear at grain boundaries in polycrystalline ice, would also provoke the appearance of QLLs at a lower temperature. In the case of β-QLLs, we found that there exists another different mechanism for their generation. To explain this, first we will show the coalescence behavior of α-QLLs on a basal face. Figure 4 presents a series of successive LCM-DIM images (left) and their schematic cross-sectional drawings (right) during the coalescence at −0.3 °C. Two adjacent α-QLLs, marked by white arrowheads in Figure 4A, coalesced just like liquid droplets on a bare basal face. In contrast, after the α-QLLs became larger, a different phenomenon took place. Figure 5A shows an LCM-DIM image taken 452 s after the image of Figure 4D on the same basal face. During that 452 s, the α-QLL grew from 15 (Figure 4D) to 35 μm (Figure 5A) in diameter. Figure 5 demonstrates that βQLLs existed beneath the α-QLLs. When two adjacent α-QLLs (white arrowheads in Figure 5A) coalesced, the β-QLL located beneath the α-QLL (red arrowhead in Figure 5B) was first extended in the direction toward the adjacent α-QLL. After two α-QLLs on the β-QLL coalesced (Figure 5C), the β-QLL was exposed (Figure 5D). From the exposed β-QLL, a new α-QLL formed (Figure 5E). Such emergence of β-QLLs from the interfaces between a basal face and α-QLLs was frequently observed when the α-QLLs became larger than tens of micrometers in diameter. The results shown in Figure 5 indicate that the contact between a basal face and α-QLLs through β-QLLs is energetically more favorable than the direct contact between a basal face and α-QLLs. The flat morphology of β-QLLs suggests that they have higher wettability (a more favorable interaction, i.e., smaller interfacial free energy) with a basal face than α-QLLs, supporting the observation shown in Figure 5. βQLLs probably have a property (structure) intermediate between α-QLLs and solid ice (basal faces). It is still unclear why α-QLLs showing lower wettability appear at a lower temperature than β-QLLs. However, as α-QLLs increase in size,

Figure 4. A series of successive LCM-DIM images (left) and their schematic cross-sectional drawings (right) during the coalescence of round liquidlike droplets (α-QLLs) at −0.3 °C. Images A−D were taken 0.0, 1.0, 3.0, and 7.9 s, respectively, after the temperature was set at −0.3 °C. Arrowheads are the same as those in Figures 1−3. α-QLLs coalesced with each other just like liquid droplets on a bare basal face.

β-QLLs appear at the interfaces between a basal face and αQLLs, to decrease the total interfacial free energy. In this paper, we reported how QLLs emerge from a basal face of an ice crystal. However, an ice crystal grown from water vapor exhibits another important facet: a prism face. To obtain comprehensive understanding about the surface melting, we are planning to perform similar direct observation also on a prism face, using LCM-DIM, in the near future.

4. CONCLUSIONS In this study, we carried out in situ observation of ice crystal surfaces by LCM-DIM. We found that on basal faces, both QLL phases (α and β) similarly emerged from areas that were strained and, hence, had a higher free energy. This result strongly suggests that QLLs appear at lower temperature from ice crystals of lower quality. In addition, we discovered another 1764

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α-QLLs, implying that β-QLLs have the intermediate structure between a basal face and α-QLLs. Insights into the nature of QLLs obtained in this study may provide a clue to unlocking mechanisms of surface melting that play vital roles in crystal growth of various materials.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic cross-sectional drawing, schematic drawing of the experimental setup, image processing, differential interference contrast, and captions for the videos. This material is available free of charge via the Internet at http://pubs.acs.org. W Web-Enhanced Feature *

Videos of repetitive appearances and disappearances of round liquidlike droplets on a basal face of an ice crystal, the appearance of thin liquidlike layers with increasing temperature, and the coalescence of round liquidlike droplets after the diameter of α-QLLs became larger than several tens of micrometers are available in the HTML version of this paper.

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AUTHOR INFORMATION

Corresponding Author

*Address: Institute of Low Temperature Science, Hokkaido University, N19−W8, Kita-ku, Sapporo 011-0819, Japan. Email: [email protected]. Tel/Fax: +81-11-706-6880. Author Contributions

G.S. and Y.F. designed the research performed by G.S., H.A., and K.N. S.N. produced the experimental system. G.S. and Y.F. wrote the paper. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Y. Saito and S. Kobayashi (Olympus Engineering Co., Ltd.) for their technical support of LCMDIM, G. Layton (Northern Arizona University) for the provision of AgI crystals, and T. Sei (Aichi Gakuin University) for valuable discussions. G.S. is grateful for the partial support by JSPS KAKENHIs (Grants 23246001 and 24656001) and by the JST PRESTO program.

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ABBREVIATIONS LCM-DIM, Laser Confocal Microscopy combined with Differential Interference contrast Microscopy; QLL, Quasi-Liquid Layer

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REFERENCES

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Figure 5. A series of successive LCM-DIM images (left) and their schematic cross-sectional drawings (right) during the coalescence of round liquidlike droplets (α-QLLs) at −0.3 °C. Image A was taken 452 s after the image of Figure 4D, on the same basal face. Images B− D were also taken 14.9, 25.9, 37.8, and 45.7 s, respectively, after the image A. Arrowheads and an arrow are the same as those in Figures 1−3. After the diameter of α-QLLs became larger than several tens of micrometers, we could frequently observe the emergence of β-QLLs from the interfaces between a basal face and α-QLLs. A video of this process is available (Video 3).

generation mechanism of β-QLLs that is different from that of α-QLLs. β-QLLs arose from interfaces between a basal face and 1765

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(7) Sazaki, G.; Matsui, T.; Tsukamoto, K.; Usami, N.; Ujihara, T.; Fujiwara, K.; Nakajima, K. In situ observation of elementary growth steps on the surface of protein crystals by laser confocal microscopy. J. Cryst. Growth 2004, 262 (1−4), 536−542. (8) Sazaki, G.; Zepeda, S.; Nakatsubo, S.; Yokoyama, E.; Furukawa, Y. Elementary steps at the surface of ice crystals visualized by advanced optical microscopy. Proc. Natl. Acad. Sci. U.S.A. 2010, 107 (46), 19702−19707. (9) Hondo, T. An overview of microphysical processes in ice sheets: Toward nanoglaciology. Low Temperature Science 2009, 68 (Supplement Issue), 1−23.

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