1,510 research outputs found
Snow crystals: a case study in spontaneous structure formation/ Kenneth G. Libbrecht.
No full textIncludes bibliographical references and index."Despite substantial, cross-disciplinary interest in the subject as a scientific case study, surprisingly little has been written on the science of snowflakes and their formation. For materials scientists, snowflakes constitute archetypal examples of crystal growth; for chemists, the site of complex molecular dynamics at the ice surface. Physicists can learn from snowflake symmetry and self-assembly; geologists study snow as mineral crystals; and biologists can even gain insight into the creation of shape and order in organisms. In the humble snowflake are condensed many of the processes-many of them still not fully understood-that govern the organization of classical systems at all levels of the natural world. This book by Kenneth Libbrecht-inarguably the world's foremost expert on the subject-will be the authoritative text on the science of snow crystals. It will cover all of the physical processes that govern the life of a snowflake, including how snowflakes grow and why they have the shapes they do. It will also outline techniques for creating and experimenting with snow crystals, both with computer models and in the lab. Featuring hundreds of color illustrations, the book will be comprehensive and is sure to become definitive resource for researchers for years, if not decades, to come"--1 online resource (ix, 440 pages)
Interview with Jaroslav Pelikan, theologian
No full textJaroslav Pelikan was professor of religious studies at Yale University at the time of this interview. Author of From Luther to Kierkegaard, Obedient Rebels, The Christian Intellectual and Sprit Versus Structure, Pelikan discusses the problem of Christianity as a viable institution in twentieth-century American society. Interviewed by Kenneth G. Hagen, Ted Guzie, S.J., and Meredith Watts.GrayscaleSoun
Physical Dynamics of Ice Crystal Growth
No full textWe examine ice crystallization from liquid water and from water vapor, focusing on the underlying physical processes that determine growth rates and structure formation. Ice crystal growth is largely controlled by a combination of molecular attachment kinetics on faceted surfaces and large-scale diffusion processes, yielding a remarkably rich phenomenology of solidification behaviors under different conditions. Layer nucleation plays an especially important role, with nucleation rates determined primarily by step energies on faceted ice/water and ice/vapor interfaces. The measured step energies depend strongly on temperature and other factors, and it appears promising that molecular dynamics simulations could soon be used in conjunction with experiments to better understand the energetics of these terrace steps. On larger scales, computational techniques have recently demonstrated the ability to accurately model the diffusion-limited growth of complex structures that are both faceted and branched. Together with proper boundary conditions determined by surface attachment kinetics, this opens a path to fully reproducing the variety of complex structures that commonly arise during ice crystal growth
Snow Crystal Structure
No full textSnow crystals, also called snowflakes, are single crystals of ice that grow from water vapor. They form in copious numbers in the atmosphere and are well known for their elaborate, symmetrical patterns. Figure 1 shows several examples of natural snow crystals
The physics of snow crystals
No full textWe examine the physical mechanisms governing the formation of snow crystals, treating this problem as a case study of the dynamics of crystal growth from the vapour phase. Particular attention is given to the basic theoretical underpinnings of the subject, especially the interplay of particle diffusion, heat diffusion and surface attachment kinetics during crystal growth, as well as growth instabilities that have important effects on snow crystal development. The first part of this review focuses on understanding the dramatic variations seen in snow crystal morphology as a function of temperature, a mystery that has remained largely unsolved since its discovery 75 years ago. To this end we examine the growth of simple hexagonal ice prisms in considerable detail, comparing crystal growth theory with laboratory measurements of growth rates under a broad range of conditions. This turns out to be a surprisingly rich problem, which ultimately originates from the unusual molecular structure of the ice surface and its sensitive dependence on temperature. With new clues from precision measurements of attachment kinetics, we are now just beginning to understand these structural changes at the ice surface and how they affect the crystal growth process. We also touch upon the mostl
A Dual Diffusion Chamber for Observing Ice Crystal Growth on c-Axis Ice Needles
No full textWe describe a dual diffusion chamber for observing ice crystal growth from water vapor in air as a function of temperature and supersaturation. In the first diffusion chamber, thin c-axis ice needles with tip radii ~100 nm are grown to lengths of ~2 mm. The needle crystals are then transported to a second diffusion chamber where the temperature and supersaturation can be independently controlled. By creating a linear temperature gradient in the second chamber, convection currents are suppressed and the supersaturation can be modeled with high accuracy. The c-axis needle crystals provide a unique starting geometry compared with other experiments, and the dual diffusion chamber allows rapid quantitative observations of ice growth behavior over a wide range of environmental conditions
Toward a Comprehensive Model of Snow Crystal Growth: 6. Ice Attachment Kinetics near -5 C
No full textI examine a variety of snow crystal growth measurements taken at a temperature of -5 C, as a function of supersaturation, background gas pressure, and crystal morphology. Both plate-like and columnar prismatic forms are observed under different conditions at this temperature, along with a diverse collection of complex dendritic structures. The observations can all be reasonably understood using a single comprehensive physical model for the basal and prism attachment kinetics, together with particle diffusion of water vapor through the surrounding medium and other well-understood physical processes. A critical model feature is structure-dependent attachment kinetics (SDAK), for which the molecular attachment kinetics on a faceted surface depend strongly on the nearby mesoscopic structure of the crystal
Toward a Comprehensive Model of Snow Crystal Growth: 7. Ice Attachment Kinetics near -2 C
No full textI examine a variety snow crystal growth experiments performed at temperatures near -2 C, as a function of supersaturation, background gas pressure, and crystal morphology. Although the different experimental data were obtained using quite diverse experimental techniques, the resulting measurements can all be reasonably understood using a single comprehensive physical model for the basal and prism attachment kinetics, together with particle diffusion of water vapor through the surrounding medium and other well-understood physical processes. As with the previous paper in this series, comparing and reconciling different data sets at a single temperature yields significant insights into the underlying physical processes that govern snow crystal growth dynamics
A Taxonomy of Snow Crystal Growth Behaviors: 2. Quantifying the Nakaya Diagram
No full textThis paper presents a matrix of 206 snow crystal growth observations as a
function of temperature and water vapor supersaturation in air, each
illustrating the morphology and size of a crystal forming on the tip of an
isolated c-axis ice needle after a known growth time. Because each complex
structure emerged from a simple, well-defined seed crystal under known
environmental conditions, this data set is well suited for making comparisons
with three-dimensional computational models. These observations thus provide a
needed extension of the well-known Nakaya diagram, as they allow a quantitative
evaluation of model predictions over a broad range of growth conditions. I also
briefly discuss computational methods along with an initial model of the most
relevant microphysical processes governing snow crystal growth. My overarching
goal with this new data set is to facilitate the development of quantitative
computational growth models that can eventually reproduce the remarkable
diversity of morphological structures seen in snow crystal formation
Cylindrically symmetric Green’s function approach for modeling the crystal growth morphology of ice
No full textWe describe a front-tracking Green’s function approach to modeling cylindrically symmetric crystal growth. This method is simple to implement, and with little computer power can adequately model a wide range of physical situations. We apply the method to modeling the hexagonal prism growth of ice crystals, which is governed primarily by diffusion along with anisotropic surface kinetic processes. From ice crystal growth observations in air, we derive measurements of the kinetic growth coefficients for the basal and prism faces as a function of temperature, for supersaturations near the water saturation level. These measurements are interpreted in the context of a model for the nucleation and growth of ice, in which the growth dynamics are dominated by the structure of a disordered layer on the ice surfaces
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