43 research outputs found
Dynamics of a Monolayer of Microspheres on an Elastic Substrate
We present a model for wave propagation in a monolayer of spheres on an
elastic substrate. The model, which considers sagittally polarized waves,
includes: horizontal, vertical, and rotational degrees of freedom; normal and
shear coupling between the spheres and substrate, as well as between adjacent
spheres; and the effects of wave propagation in the elastic substrate. For a
monolayer of interacting spheres, we find three contact resonances, whose
frequencies are given by simple closed-form expressions. For a monolayer of
isolated spheres, only two resonances are present. The contact resonances
couple to surface acoustic waves in the substrate, leading to mode
hybridization and "avoided crossing" phenomena. We present dispersion curves
for a monolayer of silica microspheres on a silica substrate, assuming
adhesive, Hertzian interactions, and compare calculations using an effective
medium approximation to a discrete model of a monolayer on a rigid substrate.
While the effective medium model does not account for discrete lattice effects
at short wavelengths, we find that it is well suited for describing the
interaction between the monolayer and substrate in the long wavelength limit.
We suggest that a complete picture of the dynamics of a discrete monolayer
adhered to an elastic substrate can be found using a combination of the results
presented for the discrete and effective medium descriptions. This model is
potentially scalable for use with both micro- and macroscale systems, and
offers the prospect of experimentally extracting contact stiffnesses from
measurements of acoustic dispersion
A Variational Approach to Extracting the Phonon Mean Free Path Distribution from the Spectral Boltzmann Transport Equation
The phonon Boltzmann transport equation (BTE) is a powerful tool for studying
non-diffusive thermal transport. Here, we develop a new universal variational
approach to solving the BTE that enables extraction of phonon mean free path
(MFP) distributions from experiments exploring non-diffusive transport. By
utilizing the known Fourier solution as a trial function, we present a direct
approach to calculating the effective thermal conductivity from the BTE. We
demonstrate this technique on the transient thermal grating (TTG) experiment,
which is a useful tool for studying non-diffusive thermal transport and probing
the mean free path (MFP) distribution of materials. We obtain a closed form
expression for a suppression function that is materials dependent, successfully
addressing the non-universality of the suppression function used in the past,
while providing a general approach to studying thermal properties in the
non-diffusive regime.Comment: 17 pages, 2 figure
Non-Contact Measurement of Thermal Diffusivity in Ion-Implanted Nuclear Materials
Knowledge of mechanical and physical property evolution due to irradiation
damage is essential for the development of future fission and fusion reactors.
Ion-irradiation provides an excellent proxy for studying irradiation damage,
allowing high damage doses without sample activation. Limited
ion-penetration-depth means that only few-micron-thick damaged layers are
produced. Substantial effort has been devoted to probing the mechanical
properties of these thin implanted layers. Yet, whilst key to reactor design,
their thermal transport properties remain largely unexplored due to a lack of
suitable measurement techniques. Here we demonstrate non-contact thermal
diffusivity measurements in ion-implanted tungsten for nuclear fusion armour.
Alloying with transmutation elements and the interaction of retained gas with
implantation-induced defects both lead to dramatic reductions in thermal
diffusivity. These changes are well captured by our modelling approaches. Our
observations have important implications for the design of future fusion power
plants.Comment: 15 pages, 3 figure
Unifying first principle theoretical predictions and experimental measurements of size effects on thermal transport in SiGe alloys
In this work, we demonstrate the correspondence between first principle
calculations and experimental measurements of size effects on thermal transport
in SiGe alloys. Transient thermal grating (TTG) is used to measure the
effective thermal conductivity. The virtual crystal approximation under the
density functional theory (DFT) framework combined with impurity scattering is
used to determine the phonon properties for the exact alloy composition of the
measured samples. With these properties, classical size effects are calculated
for the experimental geometry of reflection mode TTG using the
recently-developed variational solution to the phonon Boltzmann transport
equation (BTE), which is verified against established Monte Carlo simulations.
We find agreement between theoretical predictions and experimental measurements
in the reduction of thermal conductivity (as much as 25\% of the bulk
value) across grating periods spanning one order of magnitude. This work
provides a framework for the tabletop study of size effects on thermal
transport
Longitudinal Eigenvibration of Multilayer Colloidal Crystals and the Effect of Nanoscale Contact Bridges
Longitudinal contact-based vibrations of colloidal crystals with a controlled
layer thickness are studied. These crystals consist of 390 nm diameter
polystyrene spheres arranged into close packed, ordered lattices with a
thickness of one to twelve layers. Using laser ultrasonics, eigenmodes of the
crystals that have out-of-plane motion are excited. The particle-substrate and
effective interlayer contact stiffnesses in the colloidal crystals are
extracted using a discrete, coupled oscillator model. Extracted stiffnesses are
correlated with scanning electron microscope images of the contacts and atomic
force microscope characterization of the substrate surface topography after
removal of the spheres. Solid bridges of nanometric thickness are found to
drastically alter the stiffness of the contacts, and their presence is found to
be dependent on the self-assembly process. Measurements of the eigenmode
quality factors suggest that energy leakage into the substrate plays a role for
low frequency modes but is overcome by disorder- or material-induced losses at
higher frequencies. These findings help further the understanding of the
contact mechanics, and the effects of disorder in three-dimensional micro- and
nano-particulate systems, and open new avenues to engineer new types of micro-
and nanostructured materials with wave tailoring functionalities via control of
the adhesive contact properties
Complex Contact-Based Dynamics of Microsphere Monolayers Revealed by Resonant Attenuation of Surface Acoustic Waves
Contact-based vibrations play an essential role in the dynamics of granular materials. Significant insights into vibrational granular dynamics have previously been obtained with reduced-dimensional systems containing macroscale particles. We study contact-based vibrations of a two-dimensional monolayer of micron-sized spheres on a solid substrate that forms a microscale granular crystal. Measurements of the resonant attenuation of laser-generated surface acoustic waves reveal three collective vibrational modes that involve displacements and rotations of the microspheres, as well as interparticle and particle-substrate interactions. To identify the modes, we tune the interparticle stiffness, which shifts the frequency of the horizontal-rotational resonances while leaving the vertical resonance unaffected. From the measured contact resonance frequencies we determine both particle-substrate and interparticle contact stiffnesses and find that the former is an order of magnitude larger than the latter. This study paves the way for investigating complex contact-based dynamics of microscale granular crystals and yields a new approach to studying micro- to nanoscale contact mechanics in multiparticle networks.National Science Foundation (U.S.) (Grant CMMI-1333858)United States. Army Research Office (Grant W911NF-15-1-0030)University of Washington. Royalty Research FoundationNational Science Foundation (U.S.) (Grant CHE-1111557
Multi-frame Interferometric Imaging with a Femtosecond Stroboscopic Pulse Train for Observing Irreversible Phenomena
We describe a high-speed single-shot multi-frame interferometric imaging
technique enabling multiple interferometric images with femtosecond exposure
time over a 50 ns event window to be recorded following a single laser-induced
excitation event. The stroboscopic illumination of a framing camera is made
possible through the use of a doubling cavity which produces a femtosecond
pulse train that is synchronized to the gated exposure windows of the
individual frames of the camera. The imaging system utilizes a Michelson
interferometer to extract phase and ultimately displacement information. We
demonstrate the method by monitoring laser-induced deformation and the
propagation of high-amplitude acoustic waves in a silicon nitride membrane. The
method is applicable to a wide range of fast irreversible phenomena such as
crack branching, shock-induced material damage, cavitation and dielectric
breakdown