Characterization of Preferred Crystal Nucleation Sites on Mica

Article pubs.acs.org/crystal

Characterization of Preferred Crystal Nucleation Sites on Mica Surfaces James M. Campbell,† Fiona C. Meldrum,‡ and Hugo K. Christenson*,† †

School of Physics and Astronomy, University of Leeds, and ‡School of Chemistry, University of Leeds, Leeds, U.K., LS2 9JT S Supporting Information *

ABSTRACT: Rates of heterogeneous nucleation can be greatly increased not only through control of the chemistry of a surface, but also of its topography. Following previous work in which we showed that scratching a mica surface significantly enhances crystal nucleation from vapor, we here use a new experimental approach to understand better the effect of topography on crystal nucleation. The compounds carbon tetrabromide, camphor, norbornane, and hexachloroethane were deposited from vapor onto mica sheets containing various surface defects, and their nucleation was studied using optical microscopy. Following subsequent evaporation of the crystals, examination of the sites where these had nucleated with a scanning electron microscope enabled the nature of each material’s preferred nucleation sites to be determined, and all four compounds appeared to exhibit a strong preference for sites characterized by delamination of the layered mica structure. Indeed, comparison of the four compounds on the same mica substrates showed that they all favored the same nucleation sites. These observations are attributed to the presence of an acute wedge geometry at the delamination lines, which can provide a thermodynamic reduction in the free energy barrier to nucleation directly from vapor; alternatively, the results are also consistent with a two-step nucleation mechanism via a liquid capillary condensate.

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INTRODUCTION Many crystallization processes, both natural and technological, proceed via heterogeneous nucleation, such that if the critical nucleus has a favorable contact angle1 (150°) do not experience a significant reduction. It can also be seen that for any system where the contact angle is less than 90° − (ϕ/2), where ϕ is the wedge angle, the new phase may grow without a nucleation barrier, forming a capillary condensate. Considering growth from vapor, this would be most likely to happen with liquid phases, which © 2013 American Chemical Society

Figure 1. Graph showing the free energy of a critical nucleus having a contact angle θ with a planar wedge of varying angle (a wedge of 180° corresponds to a flat plane). For all contact angles, figures are relative to an equivalent nucleus on a flat surface. Plotted from the expressions derived by Sholl and Fletcher.7

generally exhibit much lower contact angles than crystalline phases. Consequently, there is a possible second pathway of crystal nucleation from vapor in which a supercooled liquid phase grows first and then freezes after it has grown large Received: November 23, 2012 Revised: March 25, 2013 Published: April 17, 2013 1915

dx.doi.org/10.1021/cg301715n | Cryst. Growth Des. 2013, 13, 1915−1925

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enough for the solid phase to be thermodynamically favorable. This is an extension of the commonly observed macroscopic phenomenon of drops of supercooled liquid condensing onto a surface before freezing. Investigations of the effect with several crystal compounds have been made using an annular wedge formed by two crossed mica cylinders in a saturated atmosphere.8−10 Some materials were seen to form a liquid condensate and then freeze, while others formed a solid too quickly for any preceding liquid condensate to be observed. A number of simulations have looked at this issue. Nucleation has been shown to be facilitated by a twodimensional pit11 and by a three-dimensional planar wedge.12 Simulations of three-dimensional pits have illustrated the formation of a crystal nucleus via a liquid condensate.13 Indepth experimental investigations of these phenomena have been limited but are spread across a number of fields. Several studies have reported higher nucleation densities of diamond on roughened silicon surfaces during chemical vapor deposition.14−17 Nucleation on various polymeric films has also been investigated,18−20 although here the effects of topography are hard to separate from those of the surface chemistry. Experiments on the heterogeneous nucleation of proteins have shown it to be heavily influenced by topography;21−24 observations to this effect have also been made of zinc oxide25 and germanium.26 A greater understanding of these phenomena could enable us to acquire new levels of control over the earliest stages of crystallization. A number of efforts to control crystal polymorph and orientation through topographical controls have been described.27−31 Investigations into topographically aided nucleation take two main forms. First, attempts have been made to mill well-defined nucleation sites into a flat surface,26,30,32 although this approach has proven difficult as the geometry of the feature needs to be known to within the length scale of a critical nucleus, which is typically only a few nanometers. Second, nucleation densities have been compared on chemically similar surfaces of differing roughness levels,15,18,19,33,34 coupled with an attempt to characterize the topography of these surfaces. Such characterization is extremely difficult, due to the great variety of nanoscale features typically evident on each surface. We ourselves have used this approach to demonstrate enhanced nucleation of neopentanol and carbon tetrabromide from vapor on mica surfaces scratched with three different grades of diamond powder,35 although precise characterization of nucleation sites proved impossible. In this paper, we describe an alternative approach to address this problem. Following from our previous work we study the nucleation of crystals (carbon tetrabromide, camphor, norbornane, and hexachloroethane) from vapor on mica, but rather than attempt to manufacture surface features we rely on cleavage defects naturally present on the mica surface. These are much easier to characterize, as after crystals have nucleated on the surface their locations are noted, and the crystals then vaporized. Subsequent characterization of the nucleation sites by electron microscopy allows us to determine which site geometries are most favorable to nucleation and to quantitatively study nucleation at these sites.

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Figure 2. Illustration of the equipment used in the experiments: (a) crystal reservoir; (b) electric cartridge heaters; (c) thermocouple; (d) aluminum base; (e) Teflon walls; (f) mica substrate, pressed under a sheet of glass; (g) thermocouple; (h) inlet/outlet for flushing cell with nitrogen; (i) perspex optical port for use of transmitted light. forms the roof of the cell and is sealed against the Teflon walls under a glass sheet via a rubber O-ring. An inlet and outlet allow the cell to be flushed with nitrogen gas. Two thermocouples measure the temperature of the aluminum base and of the mica. Mica substrates were prepared by cleaving Muscovite mica along its (001) plane. An area of the sheet was then selected, cut to size with scissors, and put in place within the cell. Each substrate was handled within a laminar flow cabinet between cleavage and being sealed within the cell to prevent contamination. The density and types of defects found on mica substrates after cleavage varied considerably between one sample and another. We selected substrates to give a range of defect densities for each compound, from those with no features visible to the naked eye to those with a high density of varied surface features. The compounds selected for study were carbon tetrabromide ( t e t r a b r o m om e t h a n e , A l dr i c h, 99 %) , c am p ho r ( 1 , 7 , 7 trimethylbicyclo[2.2.1]heptan-2-one, (+), Aldrich, 98%), norbornane (Aldrich, 98%), and hexachloroethane (Alfa Aesar, 98%). The properties of each are summarized in Table 1. About 0.3 g was used

Table 1. Physical Properties of the Compounds Useda compound

Tm

ΔHsub

ΔHfus

pv

carbon tetrabromide camphor norbornane hexachloroethane

92.336 178.836 87.536 186.836

5437 5239 4040 4941

3.836 6.838 4.440 9.836

1.0038 0.2138 3940 0.7538

a Here Tm is the melting point in °C, ΔHsub is the enthalpy of sublimation in kJ mol−1, ΔHfus is the enthalpy of fusion in kJ mol−1, and pv is the equilibrium vapour pressure in mm Hg at 300 K.

for each experiment, and crystals were placed evenly around the aluminum trough in the base of the cell. This was replenished each time a new substrate was loaded. Crystal growth was observed using an optical microscope (Vickers M41 Photoplan, fitted with an infrared-filtered halogen light source). The cell was flushed with nitrogen before the start of each experiment to remove water vapor from the chamber, and then the temperature of the base was ramped upward from room temperature (typically 21−22 °C) at 2 °C per minute until crystals were observed growing on the

METHODS AND MATERIALS

Figure 2 shows the cell used in the experiments. An aluminum base holds a reservoir of the compound to be sublimated and is heated by two computer-controlled electric cartridge heaters. The mica substrate 1916



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Figure 3. Scanning electron micrographs of the dominant carbon tetrabromide nucleation site on each substrate. Here and in Figures 4−6 the red circles indicate the position(s) of nucleation, as ascertained through comparison to optical micrographs. The symbols inset into each image correspond to those in Figures S5−S8. Each denotes a different substrate. The final two substrates (◊ and ◆) are the same across all four compounds; all others are different. The letters in the top-right of the images from substrates ◊ and ◆ correspond to the site designations in Figure 9. Optical micrographs of crystals growing at these sites are presented in Figure S1. possible to observe the entire substrate at once. A prerun was therefore performed before the first experimental run on each substrate to find the most favorable area for nucleation. This was identical to an experimental run, except that the view was scanned over the entire substrate. The area first observed to contain crystals in the prerun was then used as the area of study in all subsequent runs, helping to ensure that crystals were unlikely to nucleate just out-of-view significantly before those in-view and deplete the local vapor pressure. As the variation in nucleation time between crystals was typically much shorter than the time required for a crystal to grow to a size large enough to be likely to affect the results, this measure was felt to be sufficient despite the stochastic variation in nucleation times. The temperature of the mica substrate remained close to room temperature, being thermally insulated from the base, but tended to rise over the course of multiple runs. The top of the cell was cooled between runs by contact with a cold metal block if the mica temperature approached 2 °C above room temperature (21−22 °C). For experiments with norbornane, crystallization was seen on the Teflon walls of the cell before it was seen on the mica unless the substrate was kept slightly below room temperature (up to 5 °C). The top of the cell was therefore cooled before every run. None of the other compounds showed any tendency to nucleate anywhere in the cell other than on the mica. Nucleation events were identified by examining the sequence of photographs until two could be conclusively identified as being before

underside of the mica. A digital camera was used to take periodic timestamped micrographs during each ramp, and both top and bottom temperatures were logged. Although the two thermocouples were not directly in contact with either the underside of the mica or with the crystals, tests found them to be representative of the temperatures of these areas to within their 0.1 °C resolution. For camphor, norbornane, and hexachloroethane, nucleation was studied on six separate mica substrates, on each of which 12 experimental runs were performed. Carbon tetrabromide nucleation was studied on only three substrates, on each of which again 12 runs were performed. In addition to these 21 substrates, two substrates were used to study nucleation of all four compounds: on these, eight runs were performed per compound per substrate. For each compound, the different substrates are distinguished in the text, graphs, and electron micrographs by a geometrical symbol; the two substrates used to study all four compounds are designated ◊ and ◆. The first of these two substrates (◊) was studied in this order: carbon tetrabromide; camphor; norbornane; hexachloroethane. The second (◆) was studied in a different order: norbornane; carbon tetrabromide; hexachloroethane; camphor. After crystals were observed, the cell base was cooled back to room temperature, and the chamber was flushed with nitrogen until all crystals had evaporated, before beginning the next run. As the area of substrate visible under the microscope (1.8 mm2) was much smaller than the exposed area of substrate (2.5 cm2), it was not 1917



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Figure 4. Scanning electron micrographs of camphor nucleation sites; see caption to Figure 3 for details. Optical micrographs of crystals growing at these sites are presented in Figure S2.

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and after the first nucleation event. These were not always consecutive photographs if the event was ambiguous, and a time resolution of no finer than five seconds was used, this being longer than the time expected for any crystal to grow to a visible size. The base and substrate temperatures corresponding to the intervening period could then be found, and the positions of recurring nucleation sites were noted. Between four and six nucleation sites were selected for study per substrate, although fewer were studied on some substrates because too few sites were observed. For substrates ◊ and ◆, 14 sites were chosen from each substrate. These nucleation sites were studied using a scanning electron microscope (SEM) (LEO 1530 FEGSEM) after being sputter coated with 5 nm platinum. Sites were located by reference to the characteristic shapes of defects in the mica and with the aid of a small scratch placed near the region of interest after the completion of the final crystallization run.

RESULTS

On every substrate studied, nucleation was observed to occur exclusively at a small number of sites, and crystals were observed repeatedly at these sites on subsequent runs. This was the case even when there were no obvious features to provide favorable sites. Figures 3−6 show scanning electron micrographs of the most dominant nucleation site on each substrate for carbon tetrabromide, camphor, norbornane, and hexachloroethane, respectively. The dominant site was considered to be that at which the first nucleation event was most often observed, although it should be noted that crystals were often seen to appear near-simultaneously at several distinct sites. 1918



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Figure 5. Scanning electron micrographs of norbornane nucleation sites; see caption to Figure 3 for details. Optical micrographs of crystals growing at these sites are presented in Figure S3.

To aid analysis, the observed sites were sorted into seven categories. Illustrative examples of these are shown in Figure 7. Not shown is an example of the first category: apparent nucleation on a flat surface. It should be noted that the lack of optically visible identifying features at these sites made identifying their location with any precision impossible, so we must consider the possibility of unseen small-scale surface features, or surface fragments which detached from the surface during sample preparation for electron microscopy. The simplest and most prevalent type of surface feature on mica is a step edge, as shown in Figure 7a, occurring either singly or more often in parallel rows. However, nucleation on these was not very common, considering their prevalence on most substrates. A class of feature which was more effective at

nucleating crystals was that shown in Figure 7b, where there is a flake of material hanging loose from the surface, or where the top layer of mica otherwise appears to overhang those underneath. Figure 7c shows another effective geometry, that of a cave-like mouth where a trapped air pocket in the material meets a step edge. These occurred on many but not all substrates and were almost invariably effective nucleation sites when they were present. Other effective classes of site were cracks or splits in the mica surface, shown in Figure 7d, and loose fragments on the surface, illustrated in Figure 7e. This latter category does not distinguish between fragments of mica and foreign bodies, only requiring them to be not structurally part of the surface; however, most of the fragments in this class resemble mica in their 1919



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Figure 6. Scanning electron micrographs of hexachloroethane nucleation sites; see caption to Figure 3 for details. Optical micrographs of crystals growing at these sites are presented in Figure S4.

morphology. Two dominant sites, those of norbornane □ and hexachloroethane ▲, occurred on optically visible fragments which detached from the surface during SEM sample preparation. The final category, an example of which is shown in Figure 7f, consists of those sites at which there were too many types of features within the area of interest to make any meaningful classification possible. However, in cases where one site type overwhelmingly dominated the area, they were classified by the dominant site type. It should be noted that site type designations were occasionally ambiguous between one category and another, particularly between caves and hanging flakes. However, the large majority corresponded clearly to one of these seven categories.

Figure 8 shows the counts of each class of site for each compound across all nucleation sites observed with the electron microscope. It is evident that the same pattern is seen for all four compounds: few nucleation sites on the flat surface, a higher number on step edges and more involved geometries accounting for the majority of sites. The saturation S of vapor within the cell, with respect to a crystal on the mica surface, was obtained using this expression which is derived from the Clausius−Clapeyron relation:42 ln S = 1920

ΔHsub ⎛ 1 1⎞ ⎜ − ⎟ R ⎝ Ts Tr ⎠ dx.doi.org/10.1021/cg301715n | Cryst. Growth Des. 2013, 13, 1915−1925


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Figure 7. Scanning electron micrographs of six nucleation sites chosen to be illustrative of the classes used in site categorization: (a) step edges; (b) hanging flakes of mica; (c) cave-like openings; (d) a split or crack in the mica surface; (e) a loose fragment on the surface; (f) a region containing too many types of feature to classify.

exceptions, did not vary significantly between the substrates. Only carbon tetrabromide gave the expected result of requiring a larger supersaturation for nucleation on substrates with fewer effective features present, and here the number of substrates is too small for the result to be conclusive. All results are presented in Figures S5−S8 of the Supporting Information, alongside a more detailed discussion. On the two substrates used to study all four compounds (◊ and ◆), nucleation was principally observed at the same sites for each compound. To study this in detail, a set of sites was chosen from both substrates, and the nucleation frequency was compared across the four crystallizing compounds (Figure 9). Sites were selected by choosing five dominant sites for each compound on each substrate. When compiled, some sites were nominated multiple times, with the result that only 14 independent sites were studied for each substrate. Electron micrographs of all of these sites are available in Figures S9 and S10 of the Supporting Information. From Figure 9, it can be seen that each of the 28 sites was host to nucleation of at least two of the compounds, and many of them were effective sites for all four, indicating that sites which favor nucleation of one compound tend to favor nucleation of others. However, the correlation is far from perfect, and there is some variation in the sites which dominate nucleation between the compounds.

Figure 8. Counts of observed nucleation sites of each type across all substrates for each compound. Note that sites on the two substrates common to all compounds are counted once for each compound seen to nucleate there.

where Ts is the temperature of the substrate, Tr is the temperature of the crystal reservoir, ΔHsub is the molar enthalpy of sublimation, and R is the gas constant. The saturation at first nucleation was compared between the different substrates for each compound, but with a few 1921



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peeled apart (delaminated), this must form an almost infinitely acute wedge at the line where the two surfaces meet. From classical nucleation theory, we might expect that nucleation would occur at lines of delamination in preference to step edges, and at step edges in preference to the flat plane. The results presented here strongly support this since nearly all of the favorable site geometries observed can be assumed to contain lines of delamination. Hanging flakes of mica must have a straight line of delamination where they join the surface. The cave-like openings are expected to feature semicircular or Ushaped lines of delamination along their edges. The curvature of these lines is large compared to the length scale of nucleation, and so hanging flakes and cave-like openings may be expected to be similarly effective at promoting nucleation. Loose fragments of mica need not show delamination, but their contact with the surface likely exhibits a very similar acute wedge geometry. Although we cannot here show conclusively that nucleation occurs in the acute wedge in such features, the correlation between wedge geometries and effective nucleation sites is strong. The edges of such features are unlikely to be responsible for nucleation as these should be similar to those found on step edges, which are not effective at promoting nucleation. Figure 10 shows reflected light optical micrographs of several feature types. The presence of interference fringes evidences an acute wedge geometry in each case, including frequently below surface splits, where a wedge geometry might not be intuitively expected. The wedge angles can be estimated from consideration of the fringe spacing nearest to the contact line: for Figure 10a we obtain an angle of (1.8 ± 0.1)°; for Figure 10b (6.3 ± 0.3)°; for Figure 10c (using the rightmost fringes, although where the contact point is is not clear), we find (2.3 ± 0.1)°; for Figure 10d (1.6 ± 0.2)°. These angles are likely to be considerably wider than at the line of contact.

Figure 9. Comparison of nucleation of all four compounds at 14 sites on each of two mica substrates: ◊ (sites a−n, top) and ◆ (sites A−N, bottom). Site labeling is arbitrary and site A bears no connection to site a, etc. For each site the bar shows the number of times nucleation was observed there for each compound, out of eight experimental runs per compound. Note that there is no result for site A with camphor as it was not within view during these runs.

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DISCUSSION From consideration of Figure 1, it may be expected that crystals will preferentially nucleate in the most acute feature available to them. This figure deals only with a planar wedge geometry, and in the more confined environment of an acute pit the nucleation energy barrier may be expected to be reduced yet further. However, while it is difficult to imagine how an acute pit geometry could form on a mica surface, the layered structure of the material lends itself to the formation of a planar wedge. Step edges must form a wedge geometry at their base, probably close to 90°, and where two layers of material have

Figure 10. Optical micrographs of four feature types using reflected green-filtered light. The clear fringes in each case evidence an acute wedge geometry: (a) hanging flake; (b) cave-like opening; (c) loose fragment; (d) splits in the surface. In each image the white bar measures 50 μm. 1922



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where Tm is the melting point, ΔT is the temperature depression, ρ is the density of the solid phase, and ΔHfus is the enthalpy of fusion per unit mass. This can be compared to the radius of the critical nucleus straight from vapor, which is given by2

It must be considered that the mica surfaces are for the most part smooth, so that if nucleation was occurring randomly across the surface, we would expect the vast majority of sites to lie on the flat surface, with a small proportion on step edges and a much smaller proportion again chancing to fall on any of the other classes of feature. That the distribution seen in Figure 8 is exactly the inverse of this for all four compounds is thus a highly significant indication that such sites are enhancing nucleation. In considering the ability of a particular site to promote nucleation over multiple experimental runs, the possibility that a crystal embryo may remain in the base of a wedge so as to seed the next growth must be considered. Such an embryo may be stable below the bulk saturation.43 However, this seems highly unlikely in the current experiments. A retained crystal embryo would allow the crystal to grow without a nucleation barrier, which was not observed, and no reduction was seen in the nucleation saturation between the prerun and subsequent runs on each substrate, as would be expected if the prerun left behind crystal germs. Finally, the flushing of the cell after each run was continued significantly longer than required to evaporate all visible crystals. The possibility of epitaxial ordering dominating the nucleation process must be considered. A study of the nucleation of several organic compounds on mica and other inorganic substrates has found the degree of lattice match to be of little importance in determining nucleation parameters.44 Also, none of the four compounds used here were seen to exhibit alignment with the 6-fold symmetry of the mica. Therefore, it seems unlikely that epitaxy is a significant factor. A more challenging question relates to the pathway of crystallization: are we observing direct nucleation of crystals, or the freezing of an intermediate liquid phase? It might be expected that an acute, liquid-filled wedge may solidify if the mica−mica separation at the liquid−vapor interface is larger than 2r* (at which point the condensate can contain a critical nucleus) or perhaps 3r* (at which point the condensate can contain a nucleus of the new phase large enough for the free energy change of the transition to be negative), where r* is the critical radius of the compound crystallizing from the melt. Assuming that the liquid has a 0° contact angle, these conditions are equivalent to γlv > 2γls and γlv > 3γls respectively at saturation, where γlv is the surface energy of the liquid−vapor interface and γls that of the liquid−solid interface.45 If these conditions are met, nucleation from a capillary condensate may be expected prior to any supersaturation having been obtained. This is not observed in the experiments described here, which can be explained as the volumes of the liquid condensates at saturation will be very small (probably not much larger than a critical nucleus). Nucleation is therefore kinetically unlikely until a significant supersaturation has allowed the condensates to grow. Alternatively, if the constraints on the surface energies are not met, then no nucleation would be possible at saturation, although it would become possible after supersaturation has increased the size of the condensate. Across the four compounds studied, nucleation occurred at 65−167 °C below the melting point, such that r* should be small. For each material r* of a solid-in-liquid nucleus can be estimated using the expression2 r* =

r* =

2γsvVm

RT ln S where γsv is the solid−vapor interfacial energy and Vm is the molar volume. The results of applying these expressions to each compound are shown in Table 2, where estimated values of 10 Table 2. Estimated Critical Radius (r*) in Nanometers and Nucleation Free Energy Barrier (Δμ*) in Units of 10−20 J of a Solid Nucleus Forming from a Liquid and from a Vapor of Each Compound at 22 °C, Alongside the Saturation (S) Used in the Calculations for Each Compound compound

S

r*liq

r*vap

Δμ*liq

Δμ*vap

carbon tetrabromide camphor norbornane hexachloroethane

1.6 1.5 1.3 1.5

3.1 1.3 2.2 0.65

5.8 9.3 8.2 6.8

40 7.1 21 1.8

430 1100 840 590

mJ m−2 for γls and 30 mJ m−2 for γsv were used in each case. Saturations were chosen to be representative of those at which nucleation was observed for each compound. Values of the critical radius between 0.65 and 3.1 nm are obtained for nucleation from liquid, and between 5.8 and 9.3 nm for nucleation from vapor. Note that the lowest of these values are likely to be too small for the validity of the classical theory; however, they serve to illustrate that the nucleation barrier for these cases is vanishingly small. Table 2 also shows the estimated homogeneous nucleation free energy barrier Δμ*, calculated from the critical radius using the expression2 4π *2 Δμ* = γr 3 Values of Δμ* are significantly higher for nucleation from vapor than from liquid, in all cases by an order of magnitude or more. Although the nucleation free energy barrier from vapor will be reduced when forming heterogeneously, this is unlikely to lower it sufficiently to compete with nucleation from liquid. Therefore, nucleation from the liquid phase will dominate if the capillary condensates are sufficiently large. Theoretically, the dominant pathway of nucleation in an acute wedge would therefore depend solely on the volume of the liquid condensate. Experimentally, it is difficult to test as any condensates are likely to solidify long before they are large enough to become optically visible. However, the appearance of some of the crystals formed are suggestive of them having formed via a capillary condensate. Taking the example of camphor, Figure 11 shows an elongated crystal which has grown along a step edge. A capillary condensate could easily form all along a wedge and then freeze and grow; it is much harder to imagine how such a crystal might arise from a direct crystal nucleation event. Similar crystals were also seen with carbon tetrabromide. In the case of norbornane and hexachloroethane, the growth of these from a saturated environment in an annular wedge formed between crossed mica cylinders has been studied experimentally.8−10 In both cases the crystals grew too quickly

2γlsTm ΔTρΔHfus 1923



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Figure 11. Time series of optical micrographs showing growth of camphor along a step. The number in the top right of each image is the number of seconds since the first image.

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for any liquid precursor to be observed; however, as the vapor was not supersaturated in these experiments, direct nucleation would not have been thermodynamically viable, and we must assume a two-step nucleation mechanism.

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ASSOCIATED CONTENT

* Supporting Information S

Optical micrographs of crystals growing at all sites shown in Figures 3−6; saturations at first nucleation for each substrate with description; scanning electron micrographs of all sites referenced in Figure 9 alongside site type designations. This information is available free of charge via the Internet at http:// pubs.acs.org/.

CONCLUSIONS

Four compounds have been nucleated repeatedly on a number of mica substrates. All four were seen to nucleate preferentially on the same types of sites: those characterized by an acute wedge geometry. An acute wedge could ease nucleation by reducing the free energy barrier directly or by allowing the formation of a supercooled liquid condensate which then freezes. We cannot conclusively show which mechanism underpins these results, but the latter has been shown to be a strong possibility. Close comparison of the nucleation of all four compounds on the same two mica substrates revealed that the effective sites were somewhat compound-specific, but the same types of sites were generally favored by all four crystals. This shared behavior, despite the wide range of chemistries, vapor pressures, and melting points of the crystallizing compounds, suggests that the principal driving force behind the choice of sites is a thermodynamic easing of nucleation provided by the surface, rather than specific chemical effects. Likewise, the mica, being flat and chemically fairly inert, is not expected to exercise any influence on the process except through the topography of its features. As such, these results are expected to be quite general across many compounds and substrates, suggesting that design of surface topography could provide an effective route to controlling crystal nucleation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.K.C. acknowledges support from the Leverhulme Trust, and J.M.C. a DTG award from the EPSRC. F.C.M. is also grateful to the EPSRC for funding (Grant Number EP/H005374/1).

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REFERENCES

(1) Crystals do not, of course, strictly have a contact angle. Here we use the term as a shorthand; it should be interpreted as the angle which an isotropic phase, having the same interfacial energies, would make with the substrate. (2) Kashchiev, D. Nucleation: Basic Theory with Applications; Butterworth-Heinemann: Oxford, 2000. (3) Fukuta, N. J. Atmos. Sci. 1966, 23, 741−750. (4) Fletcher, N. H. J. Atmos. Sci. 1969, 26, 1266−1271. (5) Buseck, P. R.; Pósfai, M. Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 3372−3379.

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