224 research outputs found
Virtual Frame Technique: Ultrafast Imaging with Any Camera
Many phenomena of interest in nature and industry occur rapidly and are
difficult and cost-prohibitive to visualize properly without specialized
cameras. Here we describe in detail the Virtual Frame Technique (VFT), a
simple, useful, and accessible form of compressed sensing that increases the
frame acquisition rate of any camera by several orders of magnitude by
leveraging its dynamic range. VFT is a powerful tool for capturing rapid
phenomenon where the dynamics facilitate a transition between two states, and
are thus binary. The advantages of VFT are demonstrated by examining such
dynamics in five physical processes at unprecedented rates and spatial
resolution: fracture of an elastic solid, wetting of a solid surface, rapid
fingerprint reading, peeling of adhesive tape, and impact of an elastic
hemisphere on a hard surface. We show that the performance of the VFT exceeds
that of any commercial high speed camera not only in rate of imaging but also
in field of view, achieving a 65MHz frame rate at 4MPx resolution. Finally, we
discuss the performance of the VFT with several commercially available
conventional and high-speed cameras. In principle, modern cell phones can
achieve imaging rates of over a million frames per second using the VFT.Comment: 7 Pages, 4 Figures, 1 Supplementary Vide
Superspreading events suggest aerosol transmission of SARS-CoV-2 by accumulation in enclosed spaces
Viral transmission pathways have profound implications for public safety; it
is thus imperative to establish a complete understanding of viable infectious
avenues. Mounting evidence suggests SARS-CoV-2 can be transmitted via the air;
however, this has not yet been demonstrated. Here we quantitatively analyze
virion accumulation by accounting for aerosolized virion emission and
destabilization. Reported superspreading events analyzed within this framework
point towards aerosol mediated transmission of SARS-CoV-2. Virion exposure
calculated for these events is found to trace out a single value, suggesting a
universal minimum infective dose (MID) via aerosol that is comparable to the
MIDs measured for other respiratory viruses; thus, the consistent infectious
exposure levels and their commensurability to known aerosol-MIDs establishes
the plausibility of aerosol transmission of SARS-CoV-2. Using filtration at a
rate exceeding the destabilization rate of aerosolized SARS-CoV-2 can reduce
exposure below this infective dose.Comment: 6 pages, 4 figure
Skating on a Film of Air: Drops Impacting on a Surface
Drops impacting on a surface are ubiquitous in our everyday experience. This
impact is understood within a commonly accepted hydrodynamic picture: it is
initiated by a rapid shock and a subsequent ejection of a sheet leading to
beautiful splashing patterns. However, this picture ignores the essential role
of the air that is trapped between the impacting drop and the surface. Here we
describe a new imaging modality that is sensitive to the behavior right at the
surface. We show that a very thin film of air, only a few tens of nanometers
thick, remains trapped between the falling drop and the surface as the drop
spreads. The thin film of air serves to lubricate the drop enabling the fluid
to skate on the air film laterally outward at surprisingly high velocities,
consistent with theoretical predictions. Eventually this thin film of air must
break down as the fluid wets the surface. We suggest that this occurs in a
spinodal-like fashion, and causes a very rapid spreading of a wetting front
outwards; simultaneously the wetting fluid spreads inward much more slowly,
trapping a bubble of air within the drop. Our results show that the dynamics of
impacting drops are much more complex than previously thought and exhibit a
rich array of unexpected phenomena that require rethinking classical paradigms.Comment: 4 pages, 4 figure
Chirality and Protein Folding
There are several simple criteria of folding to a native state in model
proteins. One of them involves crossing of a threshold value of the RMSD
distance away from the native state. Another checks whether all native contacts
are established, i.e. whether the interacting amino acids come closer than some
characteristic distance. We use Go-like models of proteins and show that such
simple criteria may prompt one to declare folding even though fragments of the
resulting conformations have a wrong sense of chirality. We propose that a
better condition of folding should augment the simple criteria with the
requirement that most of the local values of the chirality should be nearly
native. The kinetic discrepancy between the simple and compound criteria can be
substantially reduced in the Go-like models by providing the Hamiltonian with a
term which favors native values of the local chirality. We study the effects of
this term as a function of its amplitude and compare it to other models such as
with the side groups and with the angle-dependent potentials.Comment: To be published in a special issue of J. Phys.: Cond. Mat. (Bedlewo
Workshop
Topological effects in ring polymers: A computer simulation study
Unconcatenated, unknotted polymer rings in the melt are subject to strong
interactions with neighboring chains due to the presence of topological
constraints. We study this by computer simulation using the bond-fluctuation
algorithm for chains with up to N=512 statistical segments at a volume fraction
\Phi=0.5 and show that rings in the melt are more compact than gaussian chains.
A careful finite size analysis of the average ring size R \propto N^{\nu}
yields an exponent \nu \approx 0.39 \pm 0.03 in agreement with a Flory-like
argument for the topologica interactions. We show (using the same algorithm)
that the dynamics of molten rings is similar to that of linear chains of the
same mass, confirming recent experimental findings. The diffusion constant
varies effectively as D_{N} \propto N^{-1.22(3) and is slightly higher than
that of corresponding linear chains. For the ring sizes considered (up to 256
statistical segments) we find only one characteristic time scale \tau_{ee}
\propto N^{2.0(2); this is shown by the collapse of several mean-square
displacements and correlation functions onto corresponding master curves.
Because of the shrunken state of the chain, this scaling is not compatible with
simple Rouse motion. It applies for all sizes of ring studied and no sign of a
crossover to any entangled regime is found.Comment: 20 Pages,11 eps figures, Late
How super-tough gels break
Fracture of highly stretched materials challenges our view of how things
break. We directly visualize rupture of tough double-network (DN) gels at >50\%
strain. During fracture, crack tip shapes obey a power-law, in
contrast to the parabolic profile observed in low-strain cracks. A new
length-scale emerges from the power-law; we show that scales
directly with the stored elastic energy, and diverges when the crack velocity
approaches the shear wave speed. Our results show that DN gels undergo brittle
fracture, and provide a testing ground for large-strain fracture mechanics
Phase Transitions of Single Semi-stiff Polymer Chains
We study numerically a lattice model of semiflexible homopolymers with
nearest neighbor attraction and energetic preference for straight joints
between bonded monomers. For this we use a new algorithm, the "Pruned-Enriched
Rosenbluth Method" (PERM). It is very efficient both for relatively open
configurations at high temperatures and for compact and frozen-in low-T states.
This allows us to study in detail the phase diagram as a function of
nn-attraction epsilon and stiffness x. It shows a theta-collapse line with a
transition from open coils to molten compact globules (large epsilon) and a
freezing transition toward a state with orientational global order (large
stiffness x). Qualitatively this is similar to a recently studied mean field
theory (Doniach et al. (1996), J. Chem. Phys. 105, 1601), but there are
important differences. In contrast to the mean field theory, the
theta-temperature increases with stiffness x. The freezing temperature
increases even faster, and reaches the theta-line at a finite value of x. For
even stiffer chains, the freezing transition takes place directly without the
formation of an intermediate globule state. Although being in contrast with
mean filed theory, the latter has been conjectured already by Doniach et al. on
the basis of low statistics Monte Carlo simulations. Finally, we discuss the
relevance of the present model as a very crude model for protein folding.Comment: 11 pages, Latex, 8 figure
Cooperative Dynamics in Unentangled Polymer Fluids
We present a Generalized Langevin Equation for the dynamics of interacting
semiflexible polymer chains, undergoing slow cooperative dynamics. The
calculated Gaussian intermolecular center-of-mass and monomer potentials, wich
enter the GLE, are in quantitative agreement with computer simulation data. The
experimentally observed, short-time subdiffusive regime of the polymer
mean-square displacements, emerges here from the competition between the
intramolecular and the intermolecular mean-force potentials.Comment: 9 pages, latex, 3 figure
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