Two-step crystal nucleation via capillary condensation

Christenson, Hugo K.

doi:10.1039/c3ce26887j

Cited by 77 publications

(79 citation statements)

References 81 publications

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“…However, the correlation between lattice match and nucleation is in general not strong (29), and there is a wide scatter in the reported nucleating abilities of atmospheric aerosols (30). The mechanism of ice nucleation in capillary-condensed water that was proposed 50 y ago by Fukuta has only recently been revisited (31,32), but it has already been suggested that it contributes to the ice nucleation capacity of kaolinite (33,34) and leads to enhanced ice nucleation by porous aerosol particles (35,36). It is also noteworthy that alkali feldspars, which have been shown to be particularly efficient ice h is the predicted size of a liquid condensate at saturation with respect to a solid.…”

Section: Discussionmentioning

confidence: 99%

Observing the formation of ice and organic crystals in active sites

Campbell

Meldrum

Christenson

2016

Proc. Natl. Acad. Sci. U.S.A.

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112

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Heterogeneous nucleation is vital to a wide range of areas as diverse as ice nucleation on atmospheric aerosols and the fabrication of high-performance thin films. There is excellent evidence that surface topography is a key factor in directing crystallization in real systems; however, the mechanisms by which nanoscale pits and pores promote nucleation remain unclear. Here, we use natural cleavage defects on Muscovite mica to investigate the activity of topographical features in the nucleation from vapor of ice and various organic crystals. Direct observation of crystallization within surface pockets using optical microscopy and also interferometry demonstrates that these sharply acute features provide extremely effective nucleation sites and allows us to determine the mechanism by which this occurs. A confined phase is first seen to form along the apex of the wedge and then grows out of the pocket opening to generate a bulk crystal after a threshold saturation has been achieved. Ice nucleation proceeds in a comparable manner, although our resolution is insufficient to directly observe a condensate before the growth of a bulk crystal. These results provide insight into the mechanism of crystal deposition from vapor on real surfaces, where this will ultimately enable us to use topography to control crystal deposition on surfaces. They are also particularly relevant to our understanding of processes such as cirrus cloud formation, where such topographical features are likely candidates for the "active sites" that make clay particles effective nucleants for ice in the atmosphere.nucleation | confinement | topography | pores | active sites T he growth of a new phase is almost always dependent on a nucleation event. Nucleation is therefore fundamental to a number of processes including crystallization, freezing, condensation, and bubble formation and is typically described in terms of classical nucleation theory. However, because this theory was developed to describe the nucleation of liquid droplets in vapor it cannot give a complete understanding of all nucleation processes, and in particular the formation of crystalline materials. Nucleation in the real world is also usually heterogeneous, occurring on seeds, impurities, or container surfaces. Although simple models consider nucleation to occur on perfectly flat, uniform surfaces, it is clear that real surfaces inevitably vary in chemistry and topography. We focus here on the effects of surface topography. Classical nucleation theory predicts a lower free energy barrier to nucleation in surface cracks or pores on the length scale of a critical nucleus (1). The extent of the reduction is contact-angle-dependent, such that nuclei with a low contact angle experience a more significant reduction from topography.Topography is known to promote crystallization directly from a vapor (2-5), because these systems typically exhibit low contact angles. Crystallization from the melt, in contrast, is associated with very high contact angles such that topography is usually ineff...

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Section: Discussionmentioning

confidence: 99%

Observing the formation of ice and organic crystals in active sites

Campbell

Meldrum

Christenson

2016

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

112

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“…Ice nucleation in clay minerals and mineral dusts is taken as the prime example, because the database for these materials is best. The idea that capillary condensation of water is essential for the ice nucleation ability of clay minerals was already advanced in 1966 by Fukuta (1966) and taken up again very recently by Christenson (2013).…”

Section: Marcolli: Deposition Nucleationmentioning

confidence: 99%

Deposition nucleation viewed as homogeneous or immersion freezing in pores and cavities

2014

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Abstract. Heterogeneous ice nucleation is an important mechanism for the glaciation of mixed phase clouds and may also be relevant for cloud formation and dehydration at the cirrus cloud level. It is thought to proceed through different mechanisms, namely contact, condensation, immersion and deposition nucleation. Conceptually, deposition nucleation is the only pathway that does not involve liquid water, but occurs by direct water vapor deposition onto a surface. This study challenges this classical view by putting forward the hypothesis that what is called deposition nucleation is in fact pore condensation and freezing (PCF) occurring in voids and cavities that may form between aggregated primary particles and host water at relative humidity RH w < 100 % because of the inverse Kelvin effect. Homogeneous ice nucleation is expected to occur below 235 K when at least one pore is filled with water. Ice nucleation in pores may also happen in immersion mode but with a lower probability because it requires at least one active site in a water filled pore. Therefore a significant enhancement in ice nucleation efficiency is expected when temperature falls below 235 K. For a deposition nucleation process from water vapor no discontinuous change in ice nucleation efficiency should occur at T = 235 K because no liquid water is involved in this process. Studies on freezing in confinement carried out on mesoporous silica materials such as SBA-15, SBA-16, MCM-41, zeolites and KIT have shown that homogeneous ice nucleation occurs abruptly at T = 230-235 K in pores with diameters (D) of 3.5-4 nm or larger but only gradually at T = 210-230 K in pores with D = 2.5-3.5 nm. Pore analysis of clay minerals shows that kaolinites exhibit pore structures with pore diameters (D p ) of 20-50 nm. The mesoporosity of illites and montmorillonites is characterized by pores with D p = 2-5 nm. The number and size of pores is distinctly increased in acid treated montmorillonites like K10. Water adsorption isotherms of MCM-41 show that pores with D p = 3.5-4 nm fill with water at RH w = 56-60 % in accordance with an inverse Kelvin effect. Water in such pores should freeze homogeneously for T < 235 K even before relative humidity with respect to ice (RH i ) reaches ice saturation. Ice crystal growth by water vapor deposition from the gas phase is therefore expected to set in as soon as RH i > 100 %. Pores with D > 7.5 nm fill with water at RH i > 100 % for T < 235 K and are likely to freeze homogeneously as soon as they are filled with water. Given the pore structure of clay minerals, PCF should be highly efficient for T < 235 K and may occur at T > 235 K in particles that exhibit active sites for immersion freezing within pores. Most ice nucleation studies on clay minerals and mineral dusts indeed show a strong increase in ice nucleation efficiency when temperature is decreased below 235 K in accordance with PCF and are not explicable by the classical view of deposition nucleation. PCF is probably also the prevailing ice nucleation mechanism below...

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“…The subsequent enhanced IN capability might be due to ice nucleation in pores taking place at lower supersaturations (45) and the availability of a larger surface area (46). Because HULIS in the form of solid organic particles were shown to nucleate ice in the deposition and immersion modes (20), HPA would likely be even more efficient IN.…”

Section: Atmospheric Conditions For Occurrence Of Highly Porous Aerosmentioning

confidence: 99%

Formation of highly porous aerosol particles by atmospheric freeze-drying in ice clouds

Adler

Koop

Haspel

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

149

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The cycling of atmospheric aerosols through clouds can change their chemical and physical properties and thus modify how aerosols affect cloud microphysics and, subsequently, precipitation and climate. Current knowledge about aerosol processing by clouds is rather limited to chemical reactions within water droplets in warm low-altitude clouds. However, in cold high-altitude cirrus clouds and anvils of high convective clouds in the tropics and midlatitudes, humidified aerosols freeze to form ice, which upon exposure to subsaturation conditions with respect to ice can sublimate, leaving behind residual modified aerosols. This freezedrying process can occur in various types of clouds. Here we simulate an atmospheric freeze-drying cycle of aerosols in laboratory experiments using proxies for atmospheric aerosols. We find that aerosols that contain organic material that undergo such a process can form highly porous aerosol particles with a larger diameter and a lower density than the initial homogeneous aerosol. We attribute this morphology change to phase separation upon freezing followed by a glass transition of the organic material that can preserve a porous structure after ice sublimation. A porous structure may explain the previously observed enhancement in ice nucleation efficiency of glassy organic particles. We find that highly porous aerosol particles scatter solar light less efficiently than nonporous aerosol particles. Using a combination of satellite and radiosonde data, we show that highly porous aerosol formation can readily occur in highly convective clouds, which are widespread in the tropics and midlatitudes. These observations may have implications for subsequent cloud formation cycles and aerosol albedo near cloud edges.glassy aerosols | size distribution shift | aerosol extinction

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Two-step crystal nucleation via capillary condensation

Cited by 77 publications

References 81 publications

Observing the formation of ice and organic crystals in active sites

Observing the formation of ice and organic crystals in active sites

Deposition nucleation viewed as homogeneous or immersion freezing in pores and cavities

Formation of highly porous aerosol particles by atmospheric freeze-drying in ice clouds

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