172 research outputs found
Diffusive evaporation dynamics in polymer solutions is ubiquitous
Recent theory and experiments have shown how the buildup of a
high-concentration polymer layer at a one-dimensional solvent-air interface can
lead to an evaporation rate that scales with time as and that is
insensitive to the ambient humidity. Using phase field modelling we show that
this scaling law constitutes a naturally emerging robust regime,
Diffusion-Limited Evaporation (DLE). This regime dominates the dynamical state
diagram of the system, which also contains regions of constant and arrested
evaporation, confirming and extending understanding of recent experimental
observations and theoretical predictions. We provide a theoretical argument to
show that the scaling observed in the DLE regime occurs for a wide range of
parameters, and our simulations predict that it can occur in two-dimensional
geometries as well. Finally, we discuss possible extensions to more complex
systems.Comment: 6 pages, 4 figure
In search of colloidal hard spheres
We recently reviewed the experimental determination of the volume fraction,
, of hard-sphere colloids, and concluded that the absolute value of
was unlikely to be known to better than -6%. Here, in a second
part to that review, we survey effects due to softness in the interparticle
potential, which necessitates the use of an {\em effective} volume fraction. We
review current experimental systems, and conclude that the one that most
closely approximates hard spheres remains polymethylmethacrylate spheres
sterically stabilised by polyhydroxystearic acid `hairs'. For these particles
their effective hard sphere diameter is around 1-10% larger than the core
diameter, depending on the particle size. We argue that for larger colloids
suitable for confocal microscopy, the effect of electrostatic charge cannot be
neglected, so that mapping to hard spheres must be treated with caution.Comment: 11 page
Multi-layer silicon nitride-on-silicon polarization-independent grating couplers
A polarization-independent grating coupler is proposed and demonstrated in a 3-layer silicon nitride-on-silicon photonic platform. Polarization independent coupling was made possible by the supermodes and added degrees of geometric freedom unique to the 3-layer photonic platform. The grating was designed via optimization algorithms, and the simulated peak coupling efficiency was −2.1 dB with a 1 dB polarization dependent loss (PDL) bandwidth of 69 nm. The fabricated grating couplers had a peak coupling efficiency of −4.8 dB with 1 dB PDL bandwidth of over 100 nm
Monolithically Integrated Multilayer Silicon Nitride-on-Silicon Waveguide Platforms for 3-D Photonic Circuits and Devices
In this paper, we review and provide additional details about our progress on multilayer silicon nitride (SiN)-on-silicon (Si) integrated photonic platforms. In these platforms, one or more SiN waveguide layers are monolithically integrated onto a Si photonic layer. This paper focuses on the development of three-layer platforms for the O- and SCL-bands for very large-scale photonic integrated circuits requiring hundreds or thousands of waveguide crossings. Low-loss interlayer transitions and ultralow-loss waveguide crossings have been demonstrated, along with bilevel and trilevel grating couplers for fiber-to-chip coupling. The SiN and Si passive devices have been monolithically integrated with high-efficiency optical modulators, photodetectors, and thermal tuners in a single photonic platform
On measuring colloidal volume fractions
Hard-sphere colloids are popular as models for testing fundamental theories
in condensed matter and statistical physics, from crystal nucleation to the
glass transition. A single parameter, the volume fraction (phi), characterizes
an ideal, monodisperse hard-sphere suspension. In comparing experiments with
theories and simulation, researchers to date have paid little attention to
likely uncertainties in experimentally-quoted phi values. We critically review
the experimental measurement of phi in hard-sphere colloids, and show that
while statistical uncertainties in comparing relative values of phi can be as
low as 0.0001, systematic errors of 3-6% are probably unavoidable. The
consequences of this are illustrated by way of a case study comparing
literature data sets on hard-sphere viscosity and diffusion.Comment: 11 page
Temperature as an external field for colloid-polymer mixtures : "quenching" by heating and "melting" by cooling
We investigate the response to temperature of a well-known colloid-polymer
mixture. At room temperature, the critical value of the second virial
coefficient of the effective interaction for the Asakura-Oosawa model predicts
the onset of gelation with remarkable accuracy. Upon cooling the system, the
effective attractions between colloids induced by polymer depletion are
reduced, because the polymer radius of gyration is decreases as the
theta-temperature is approached. Paradoxically, this raises the effective
temperature, leading to "melting" of colloidal gels. We find the Asakura-Oosawa
model of effective colloid interactions with a simple description of the
polymer temperature response provides a quantitative description of the
fluid-gel transition. Further we present evidence for enhancement of
crystallisation rates near the metastable critical point.Comment: 13 page
Radio Astronomy
Contains reports on three research project.National Science Foundation (Grant GP-21348A#2)California Institute of Technology (Contract 952568)National Aeronautics and Space Administration (Grant NGR 22-009-421)U. S. Air Force - Electronic Systems Division (Contract F19628-73-C-0196
Multi-layer silicon nitride-on-silicon polarization-independent grating couplers
A polarization-independent grating coupler is proposed and demonstrated in a 3-layer silicon nitride-on-silicon photonic platform. Polarization independent coupling was made possible by the supermodes and added degrees of geometric freedom unique to the 3-layer photonic platform. The grating was designed via optimization algorithms, and the simulated peak coupling efficiency was −2.1 dB with a 1 dB polarization dependent loss (PDL) bandwidth of 69 nm. The fabricated grating couplers had a peak coupling efficiency of −4.8 dB with 1 dB PDL bandwidth of over 100 nm
Five-year follow-up of underexpanded and overexpanded bioresorbable scaffolds: Self-correction and impact on shear stress
Underexpansion and overexpansion have been incriminated as causative factors of adverse cardiac events. However, dynamic biological interaction between vessel wall and scaffold may attenuate the adverse haemodynamic impact of overexpansion or underexpansion
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