Toward a Comprehensive Model of Snow Crystal Growth: 4. Measurements of Diffusion-limited Growth at -15 C

We present measurements of the diffusion-limited growth of ice crystals from water vapor at different supersaturation levels in air at a temperature of -15 C. Starting with thin, c-axis ice needle crystals, the subsequent growth morphologies ranged from blocky structures on the needle tips (at low supersaturation) to thin faceted plates on the needle tips (at high supersaturation). We successfully modeled the experimental data, reproducing both growth rates and growth morphologies, using a cellular-automata method that yields faceted crystalline structures in diffusion-limited growth. From this quantitative analysis of well-controlled experimental measurements, we were able to extract information about the attachment coefficients governing ice growth under different circumstances. The results strongly support previous work indicating that the attachment coefficient on the prism surface is a function of the width of the prism facet. Including this behavior, we created a comprehensive model at -15 C that explains all the experimental data. To our knowledge, this is the first demonstration of a kinetic model that reproduces a range of diffusion-limited ice growth behaviors as a function of supersaturation

Modeling ice crystal growth using the lattice Boltzmann method

Given the multitude of growth habits, pronounced sensitivity to ambient conditions and wide range of scales involved, snowflake crystals are one of the most challenging systems to model. The present work focuses on the development and validation of a coupled flow/species/phase solver based on the lattice Boltzmann method. It is first shown that the model is able to correctly capture species and phase growth coupling. Furthermore, through a study of crystal growth subject to ventilation effects, it is shown that the model correctly captures hydrodynamics-induced asymmetrical growth. The validated solver is then used to model snowflake growth under different ambient conditions with respect to humidity and temperature in the plate-growth regime section of the Nakaya diagram. The resulting crystal habits are compared to both numerical and experimental reference data available in the literature. The overall agreement with experimental data shows that the proposed algorithm correctly captures both the crystal shape and the onset of primary and secondary branching instabilities. As a final part of the study the effects of forced convection on snowflake growth are studied. It is shown, in agreement with observations in the literature, that under such condition the crystal exhibits non-symmetrical growth. The non-uniform humidity around the crystal due to forced convection can even result in the coexistence of different growth modes on different sides of the same crystal

A Taxonomy of Snow Crystal Growth Behaviors: 2. Quantifying the Nakaya Diagram

This 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

Characterizing porous protein crystal materials for applications in nanomedicine and nanobiotechnology

2018 Summer.Includes bibliographical references.Protein crystals are biologically derived, self-assembling, porous structures that have been used for decades in structure determination via X-ray diffraction. Recently, however, there has been increased interest in utilizing protein crystals for their unique material properties—most notably, their highly ordered porous structure, innate biocompatibility, and chemical plasticity. The diverse topologies of protein crystals and the relative ease with which their chemical properties can be altered via genetic mutation or chemical modification offers a wider and more dynamic design palette than existing chemically-synthesized nanoporous frameworks. These traits make protein crystals an attractive new material for applications in nanomedicine and nanobiotechnology. The intent of this project is to demonstrate the application potential of porous protein crystal materials for use in nanostructured devices. This work highlights our efforts to: experimentally and computationally investigate macromolecular transport and interaction energies within a large-pore protein crystal environment using time-lapse confocal microscopy, bulk equilibrium adsorption, and hindered diffusion simulation; assess the cytocompatibility of various cross-linking chemistries for the production of biostable protein crystal materials for use in biologically sensitive environments; and create multifunctional textiles by covalently attaching various cross-linked protein crystals to cellulose fibers in woven cotton fabrics. By pursuing this research, we hope to better understand porous protein crystal materials and leverage that knowledge to design advanced nanostructured devices for applications in medicine and biotechnology

Toward a Comprehensive Model of Snow Crystal Growth Dynamics: 1. Overarching Features and Physical Origins

We describe a comprehensive model for the formation and morphological development of atmospheric ice crystals growing from water vapor, also known as snow crystals. Our model derives in part from empirical measurements of the intrinsic ice growth rates as a function of temperature and supersaturation, along with additional observations and analyses of diffusion-driven growth instabilities. We find that temperature-dependent conformational changes associated with surface melting strongly affect layer nucleation dynamics, which in turn determines many snow-crystal characteristics. A key feature in our model is the substantial role played by structure-dependent attachment kinetics, producing a growth instability that is largely responsible for the formation of thin plates and hollow columnar forms. Putting these elements together, we are able to explain the overall growth behavior of atmospheric ice crystals over a broad range of conditions. Although our model is complex and still incomplete, we believe it provides a useful framework for directing further investigations into the physics underlying snow crystal growth. Additional targeted experimental investigations should better characterize the model, or suggest modifications, and we plan to pursue these investigations in future publications in this series. Our model also suggests new avenues for the continued exploration of ice surface structure and dynamics using molecular dynamics simulations

Sludge Production and Disposal for Small Cold Climate Bio-Treatment Plants

Ultimate disposal of wastewater sludge has long been a problem which to a large degree has been ignored. Haney (1971) stated that: "Until process sludge can be handled with minimum environmental impact, we cannot claim to have a viable wastewater treatment process". The relationship of sludge disposal to total treatment processes is emphasized by the fact that sludge handling and disposal represents up to 50 percent of the total treatment capital and operating costs (Burd, 1968). Processing of wastewater sludge will, no doubt, receive increased attention in the future because of environmental concerns for our air, land and water. The present technology for processing wastewater treatment plant sludge is well established and includes conditioning, dewatering, and disposal. Many of these processes are highly sophisticated and relatively expensive. Most of the more advanced processes are unsuitable for small wastewater treatment facilities in Alaska.The work upon which this report is based was supported in part by funds (Proj. A-033-ALAS) provided by the United States Department of the Interior, Office of Water Resources Research, as authorized under the Water Resources Act of 1964, as amended