arXiv subjects

Kenneth G. Libbrecht

Publications and source records attributed to Kenneth G. Libbrecht.

At least 37 records · Page 2Linked to original sources

arXiv2016

Measurements of Cylindrical Ice Crystal Growth Limited by Combined Particle and Heat Diffusion

We present measurements of the growth of long columnar ice crystals from water vapor over a broad range of temperatures and supersaturation levels in air. Starting with thin, c-axis ice needle crystals, we observed their subsequent growth behavior in a vapor diffusion chamber, extracting the initial radial growth velocities of the needles under controlled conditions. Approximating the hexagonal needle crystals as infinitely long cylinders, we created an analytical growth model that includes effects from particle diffusion of water molecules through the surrounding air along with the diffusion of heat generated by solidification. With only minimal adjustment of model parameters, we obtained excellent agreement with our experimental data. To our knowledge, this is the first time that the combined effects from particle and heat diffusion have been measured in ice growth from water vapor. This analysis further provides an accurate method for calibration of the water-vapor supersaturation levels in experimental growth chambers.

cond-mat.mtrl-sci↗

arXiv2015

Incorporating Surface Diffusion into a Cellular Automata Model of Ice Growth from Water Vapor

We describe a numerical model of faceted crystal growth using a cellular automata method that incorporates admolecule diffusion on faceted surfaces in addition to bulk diffusion in the medium surrounding the crystal. The model was developed for investigating the diffusion-limited growth of ice crystals in air from water vapor, where the combination of bulk diffusion and strongly anisotropic molecular attachment kinetics yields complex faceted structures. We restricted the present model to cylindrically symmetric crystal growth with relatively simple growth morphologies, as this was sufficient for making quantitative comparisons between theoretical models and ice growth experiments. Overall this numerical model reproduces ice growth behavior with reasonable fidelity over a wide range of conditions, albeit with some limitations. The model could easily be adapted for other material systems, and the cellular automata technique appears well suited for investigating crystal growth dynamics when strongly anisotropic surface attachment kinetics cause faceted growth morphologies.

cond-mat.mtrl-sci↗

arXiv2015

The Surface Diffusion Length of Water Molecules on Faceted Ice: A Reanalysis of "Roles of Surface/Volume Diffusion in the Growth Kinetics of Elementary Spiral Steps on Ice Basal Faces Grown from Water Vapor", by Asakawa et al

We reanalyzed the measurements made by Asakawa et al. [1] of the growth velocities of single-molecule-high steps on basal ice surfaces, as we believe the authors made a number of incorrect assumptions regarding ice growth parameters and bulk diffusion in their experiments. Applying what we believe are more accurate assumptions, we used the data in [1] to derive a surface diffusion length of approximately 10 nm for water molecules on basal ice surfaces at -8.4 C, about 500 times lower than what was reported in [1]. Moreover, in our analysis we found that no information about the height of the Ehrlich-Schwoebel barrier could be obtained from these measurements.

cond-mat.mtrl-sci↗

arXiv2015

An experimental apparatus for observing deterministic structure formation in plate-on-pedestal ice crystal growth

We describe an experimental apparatus for making detailed morphological observations of the growth of isolated plate-like ice crystals from water vapor. Each crystal develops a plate-on-pedestal (POP) geometry, in which a large, thin, plate-like crystal grows out from the top edge of an initially prismatic seed crystal resting on a substrate. With the POP geometry, the substrate is not in contact with the growing plate (except at its center), so substrate interactions do not adversely affect the crystal growth. By controlling the temperature and supersaturation around the crystal, we can manipulate the resulting ice growth behavior in predictable ways, producing morphologies spanning the full range from simple faceted hexagonal plates to complex dendritic structures. We believe that the experimental apparatus described here will allow unprecedented investigations of ice crystal growth behaviors under controlled conditions, identifying and exploring robust morphological features in detail. Such investigations will provide valuable observational inputs for developing numerical modeling techniques that can accurately reproduce the faceted and branched structures that frequently emerge during diffusion-limited crystal growth.

cond-mat.mtrl-sci↗

arXiv2015

Material loss angles from direct measurements of broadband thermal noise

We estimate the loss angles of the materials currently used in the highly reflective test-mass coatings of interferometric detectors of gravitational waves, namely Silica, Tantala, and Ti-dop ed Tantala, from direct measurement of coating thermal noise in an optical interferometer testbench, the Caltech TNI. We also present a simple predictive theory for the material properties of amorphous glassy oxide mixtures, which gives results in good agreement with our measurements on Ti-doped Tantala. Alternative measure ment methods and results are reviewed, and some critical issues are discussed.

physics.optics↗

arXiv2014

Particle Dynamics in Damped Nonlinear Quadrupole Ion Traps

We examine the motions of particles in quadrupole ion traps as a function of damping and trapping forces, including cases where nonlinear damping or nonlinearities in the electric field geometry play significant roles. In the absence of nonlinearities, particles are either damped to the trap center or ejected, while their addition brings about a rich spectrum of stable closed particle trajectories. In three-dimensional (3D) quadrupole traps, the extended orbits are typically confined to the trap axis, and for this case we present a 1D analysis of the relevant equation of motion. We follow this with an analysis of 2D quadrupole traps that frequently show diamond-shaped closed orbits. For both the 1D and 2D cases we present experimental observations of the calculated trajectories in microparticle ion traps. We also report the discovery of a new collective behavior in damped 2D microparticle ion traps, where particles spontaneously assemble into a remarkable knot of overlapping, corotating diamond orbits, self-stabilized by air currents arising from the particle motion.

physics.atm-clus↗

arXiv2014

Toward a Comprehensive Model of Snow Crystal Growth: 3. The Correspondence Between Ice Growth from Water Vapor and Ice Growth from Liquid Water

We examine ice crystal growth from water vapor at temperatures near the melting point, when surface premelting creates a quasiliquid layer at the solid/vapor interface. Recent ice growth measurements as a function of vapor supersaturation have demonstrated a substantial nucleation barrier on the basal surface at these temperatures, from which a molecular step energy can be extracted using classical nucleation theory. Additional ice growth measurements from liquid water as a function of supercooling exhibit a similar nucleation barrier on the basal surface, yielding about the same molecular step energy. These data suggest that ice growth from water vapor and from liquid water are both well described by essentially the same underlying nucleation phenomenon over a substantial temperature range. A physical picture is emerging in which molecular step energies at the solid/liquid, solid/quasiliquid, and solid/vapor interfaces create nucleation barriers that dominate the growth behavior of ice over a broad range of conditions. Since the step energy is an equilibrium quantity, just as surface melting is an equilibrium phenomenon, there exists a considerable opportunity to use many-body simulations of the ice surface structure and energetics at equilibrium to better understand many dynamical aspects of ice crystal growth.

cond-mat.mtrl-sci↗

arXiv2014

A Laser Interferometer for the Undergraduate Teaching Laboratory Demonstrating Picometer Sesitivity

We describe a laser interferometer experiment for the undergraduate teaching laboratory that achieves picometer sensitivity in a hands-on table-top instrument. In addition to providing an introduction to interferometer physics and optical hardware, the experiment also focuses on precision measurement techniques including servo control, signal modulation, phase-sensitive detection, and different types of signal averaging. After students assemble, align, and characterize the interferometer, they then use it to measure nanoscale motions of a simple harmonic oscillator system, as a substantive example of how laser interferometry can be used as an effective tool in experimental science.

physics.ins-det↗

arXiv2014

A Dual Diffusion Chamber for Observing Ice Crystal Growth on c-Axis Ice Needles

We 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.

cond-mat.mtrl-sci↗

arXiv2013

Toward a Comprehensive Model of Snow Crystal Growth Dynamics: 2. Structure Dependent Attachment Kinetics near -5 C

We present experimental data demonstrating the presence of structure-dependent attachment kinetics (SDAK) in ice crystal growth from water vapor near -5 C. Specifically, we find that the nucleation barrier on the basal edge of a thin-walled hollow columnar crystal is approximately ten times smaller than the corresponding nucleation barrier on a large basal facet. These observations support the hypothesis that SDAK effects play an important role in determining the growth morphologies of atmospheric ice crystals as a function of temperature.

cond-mat.mtrl-sci↗

arXiv2012

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.

cond-mat.mtrl-sci↗

arXiv2012

An Edge-Enhancing Crystal Growth Instability Caused by Structure-Dependent Attachment Kinetics

We describe a novel crystal growth instability that enhances the development of thin edges, promoting the formation of plate-like or hollow columnar morphologies. This instability arises when diffusion-limited growth is coupled with structure-depdendent attachment kinetics, specifically when the nucleation barrier on a facet surface decreases substantially as the facet width approaches atomic dimensions. Experimental data are presented confirming the presence of this instability in the growth of ice from water vapor at -15 C. We believe this edge-enhancing effect plays an important role in determining the growth morphologies of atmospheric ice crystals as a function of temperature, a phenomenon that has been essentially unexplained for over 75 years. Our model of structure-dependent attachment kinetics appears to be related to surface melting, and thus may be present in other material systems, whenever crystal growth from the vapor phase occurs near the material melting point.

cond-mat.mtrl-sci↗

arXiv2012

Managing Systematic Errors in Ice Crystal Growth Experiments

We describe how surface interactions can affect the growth of ice crystal facets in contact with a substrate by lowering the normal nucleation barrier on the ice surface. We also describe how the resulting enhanced growth rates can produce systematic errors even when measuring the growth of facets not contacting the substrate. From an analysis of the diffusion dynamics we then develop a simple procedure for approximately correcting ice growth data, thus substantially reducing these systematic errors. We have found this technique to be quite useful for interpreting ice growth data and extracting the intrinsic attachment coefficients of ice surfaces.

cond-mat.mtrl-sci↗

arXiv2012

On the Equilibrium Shape of an Ice Crystal

We examine the shape of a an isolated, dislocation-free ice crystal when it is in equilibrium with the vapor phase in an isothermal closed environment, as a function of temperature. From our analysis we draw the following conclusions: 1) The equilibrium shape has not yet been definitively measured for ice crystals; 2) The surface energy anisotropy is likely cusp-like near the facet angles, and the size of the cusps can be estimated from crystal growth measurements; 3) The equilibrium shape is likely nearly spherical with only small faceted regions; 4) The time needed to reach equilibrium is likely prohibitively long, except under special circumstances; and 5) Surface energy effects likely play a relatively smaller role in ice crystal growth dynamics when compared to the role of attachment kinetics.

cond-mat.mtrl-sci↗

arXiv2011

Observations of an Edge-enhancing Instability in Snow Crystal Growth near -15 C

We present observations of the formation of plate-like snow crystals that provide evidence for an edge-enhancing crystal growth instability. This instability arises when the condensation coefficient describing the growth of an ice prism facet increases as the width of the facet becomes narrower. Coupled with the effects of particle diffusion, this phenomenon causes thin plate-like crystals to develop from thicker prisms, sharpening the edges of the plates to micron or sub-micron dimensions as they grow. This sharpening effect is largely responsible for the formation of thin plate-like ice crystals from water vapor near -15 C, which is a dominant feature in the snow crystal morphology diagram. Other faceted crystalline materials may exhibit similar morphological growth instabilities that promote the diffusion-limited growth of plate-like or needle-like structures.

cond-mat.mtrl-sci↗

arXiv2011

An Improved Apparatus For Measuring the Growth of Ice Crystals from Water Vapor

We describe an apparatus designed for obtaining precise measurements of the growth rates of ice crystals from water vapor over a range of experimental conditions. Our aim is to produce clean, high-quality test crystals in a well controlled environment for investigating the detailed molecular dynamics that controls the basic physics of ice crystal growth. In this paper we describe the nucleation and initial growth of test crystals, their transport and selection into a experimental chamber, the creation of a stable and controllable supersaturation, hardware and calibration issues, and the crystal measurement via direct imaging and broad-band interferometry.

cond-mat.mtrl-sci↗

arXiv2010

Chemical Influences on Ice Crystal Growth from Vapor

We present an investigation of chemical influences on the growth of ice crystals from water vapor. In one set of experiments, we grew ice crystals in a vapor diffusion chamber, observing crystal morphologies at temperatures from 0 C to -25 C in different background gases and with a variety of gaseous chemical additives. In a second set of experiments, we measured ice crystal growth rates at -5 C and -15 C in a free-fall flow chamber, using normal laboratory air and ultra-clean nitrogen gas, both with and without chemical additives. Conclusions from these experiments include:\ 1) In nitrogen gas at a pressure of one atmosphere, no tested chemical additives at concentrations below 10 ppm produced any observable changes in ice crystal growth morphologies; 2) Growth in ultra-clean nitrogen gas was not significantly different from growth in ordinary laboratory air; 3) Chemical additives affected plate-like dendritic growth at -15 C more readily than growth at higher temperatures; 4) Chemical additives tended to promote the growth of columnar crystals over plate-like crystals; 5) Ice growth in air, nitrogen, helium, argon, hydrogen, and methane gases at a pressure of one atmosphere yielded essentially the same temperature-dependent crystal morphologies.

cond-mat.mtrl-sci↗

arXiv2010

Measurement of Thermal Noise in Multilayer Coatings with Optimized Layer Thickness

A standard quarter-wavelength multilayer optical coating will produce the highest reflectivity for a given number of coating layers, but in general it will not yield the lowest thermal noise for a prescribed reflectivity. Coatings with the layer thicknesses optimized to minimize thermal noise could be useful in future generation interferometric gravitational wave detectors where coating thermal noise is expected to limit the sensitivity of the instrument. We present the results of direct measurements of the thermal noise of a standard quarter-wavelength coating and a low noise optimized coating. The measurements indicate a reduction in thermal noise in line with modeling predictions.

gr-qc↗