46 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
Elastocapillary menisci mediate interaction of neighboring structures at the surface of a compliant solid
Surface stress drives long-range elastocapillary interactions at the surface
of compliant solids, where it has been observed to mediate interparticle
interactions and to alter the transport of liquid drops. We show that such an
elastocapillary interaction arises between neighboring structures that are
simply protrusions of the compliant solid. For compliant micropillars arranged
in a square lattice with spacing p less than an interaction distance p*, the
distance of a pillar to its neighbors determines how much it deforms due to
surface stress: pillars that are close together tend to be rounder and flatter
than those that are far apart. The interaction is mediated by the formation of
an elastocapillary meniscus at the base of each pillar, which sets the
interaction distance and causes neighboring structures to deform more than
those that are relatively isolated. Neighboring pillars also displace toward
each other to form clusters, leading to the emergence of pattern formation and
ordered domains
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
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The role of air in droplet impact on a smooth, solid surface
The impact of liquid drops on solid surfaces is a ubiquitous phenomenon in our everyday experience; nevertheless, a general understanding of the dynamics governing droplet impact remains elusive. The impact event is understood within a commonly accepted hydrodynamic picture: impact initiates with 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. 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. We directly visualize the rapid spreading dynamics succeeding the impact of a droplet of fluid on a solid, dry surface. We show that the approach of the spreading liquid toward the surface is unstable, and lift-off of the spreading front away from the surface occurs. Lift-off ensues well before the liquid contacts the surface, in contrast with prevailing paradigm where lift-off of the liquid is contingent on solid-liquid contact and the formation of a viscous boundary layer. We show that when a drop impacts an atomically smooth mica surface, a strikingly stable nanometer thin layer of air remains trapped between the liquid and the solid. This layer occludes the formation of contact, and ultimately causes the complete rebound of the drop.Engineering and Applied Science
Splashing or not
The splashing of a droplet when impacting a solid surface is common to our everyday experience as well as to industrial applications that require controlled deposition of liquid mass. Still the mechanism for splashing is not well understood. A recent study showed that a decrease in the ambient pressure inhibits splashing, motivating a hypothesis on the existence of a thin film of air trapped between the drop and the surface. The early dynamics of splashing could occur while the drop is still spreading on an air film. To gain insight into this early dynamics,
we supplement the side view with a synchronized bottom view, obtained using a novel Total Internal Reflection technique. I will discuss the existence of a transition regime between spreading and splashing. This regime appears by changing the impact velocity or the ambient pressure, while keeping the other fixed
Annular waves on the surface of impact-formed tektites
Tektites are naturally
occurring pieces of glass formed by melting of terrestrial rocks during a meteorite
impact. The most unusual tektites, known as Australites, were formed by an impact
at an unknown site in Austro-Asia, and are found in a large strewn field covering
Australia. These tektites solidified on ascent through the earth's atmosphere, and
partially remelted during re-entry. The thin remelted layer on the front surface
shows distinct flanges with annular wavy structures. We propose that the annular
wavy structure is a manifestation of surface waves on the flow of the thin layer
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
Crack tip kinematics reveal the cohesive zone structure in brittle hydrogel fracture
When brittle hydrogels fail, several mechanisms conspire to alter the state
of stress near the tip of a crack, and it is challenging to identify which
mechanism is dominant. In the fracture of brittle solids, a sufficient
far-field stress results in the complete loss of structural strength as the
material `unzips' at the tip of a crack, where stresses are concentrated.
Direct studies of the so-called small-scale yielding zone, where deformation is
large, are sparing. Using hydrogels as a model brittle solid, we probe the
small-scale yielding region with a combination of microscopy methods that
resolve the kinematics of the deformation. A zone over which most of the energy
is dissipated through the loss of cohesion is identified in the immediate
surroundings of the crack tip. With direct measurements, we determine the scale
and structure of this zone, and identify how the specific loss mechanisms in
this hydrogel material might generalize for brittle material failure.Comment: 29 pages, 9 figure