ABSTRACT. Grain growth observed in polar ice that is not deforming rapidly can be accounted for if concentrations and distributions of extrinsic materials (microparticles, bubbles, and dissolved impurities) are characterized fully. Dissolved impurities segregate to grain boundaries and slow grain growth in all cold glacial ice. The high concentration of soluble impurities in Wisconsinan ice from the Dome C (Antarctica) ice core (and perhaps other ice cores) probably causes the small grain-sizes observed in that ice. Microparticles have little effect on grain growth in ordinary ice . In ice layers that appear dirty owing to concentrations of volcanic tephra (such as in the Byrd Station (Antarctica) ice core) or of morainal material, microparticles reduce grain-growth rates significantly. The relatively high vapor pressure of ice allows rapid growth and high mobility of intergranular necks, so grain growth in firn is limited by boundary migration rather than by neck growth. Bubbles formed by pore close-off at the firn-ice transition are less mobile than grain boundaries, causing bubble-boundary separation whenever geometric constraints are satisfied; however, such separation reduces grain-growth rates by only about 10%. The observed linear increase of grain area with time is thus predicted by theory, but the growth rate depends on soluble-impurity concentrations as well as on temperature. RESUME. Croissance des grains dans la glace polaire: ll. Applications. On peut rendre compte de la croissance des grains observee dans la glace polaire a deformation lente par une bonne description des concentrations et repartitions des materiaux extrinseques (microparticules, bulles et impuretes dissoutes). Les impuretes dissoutes se rassemblent aux frontieres des grains pour en diminuer la croissance dans toute glace froide. La forte concentration des impuretes solubles dans la glace du Wisconsin au DOme C (Antarctique), et peut Mre d'autres carottages, est la cause probable de la faible taille des grains observee dans ce type de glace. Les microparticules n'ont qu'un faible effet sur la croissance des grains pour de la glace ordinaire. Dans des couches de glace qui apparaissent sales par suite de la presence de tephras volcaniques (comme dans la carotte de glace de la Byrd Station, Antarctique) ou bien chargees en materiau morainique, les microparticules reduisent notablement les tau x de croissance des grains. La relativement forte pression de vapeur de la glace permet une croissance rapide et une grande mobilite des appendices intergranulaires, de sorte que la croissance du grain dans le neve est limitee par les migrations a la frontiere plutOt que par la croissance des appendices. Les bulles formees par le fermeture des pores lors de la transition neve,lace sont moins mobiles que les frontieres des grains; en causant des frontieres de separation de bulles partout oil les conditions geometriques sont satisfaites; cependant une telle separation reduit la vitesse croissance des grains d'environ 10% seulement. La croissance lin...

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Nucleation during solidification is heterogeneous in nature in an overwhelmingly large fraction of all solidification events. Yet, most often the identity of the heterogeneous nucleants that initiate nucleation remains a matter of speculation. In fact, a series of dedicated experiments needs to be designed in order to verify if nucleation of the material under study is based on one type of heterogeneous nucleant and if the potency of that nucleant is constant, e.g. for a population of individual droplets, or stays constant over time, e.g. throughout repeated melting/solidification cycles. In this work it is demonstrated that one way to circumvent ambiguities and analyze nucleation kinetics under well-defined conditions experimentally is given by performing statistically significant numbers of repeated single-droplet experiments. The application of proper statistics analyses based upon a non-homogeneous Poisson process is shown to yield nucleation rates that are independent of a specific nucleation model. Based upon this approach nucleation undercooling measurements on pure Au, Cu and Ni as model materials have confirmed that the experimental strategy and analysis method are valid. The results are comparable to those obtained by classical nucleation theory applied to experimental data that has been verified to comply with the assertions that are necessary for applying this model framework. However, the results reveal also other complex nucleant-sample interactions such as an initial transient undercooling behavior and impurity removal during repeated cycling treatments. The transient undercooling behavior has been analyzed by a nucleant refining model to provide new insight on the operation of melt fluxing treatments.

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