44 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
How Material Heterogeneity Creates Rough Fractures
Fractures are a critical process in how materials wear, weaken, and fail
whose unpredictable behavior can have dire consequences. While the behavior of
smooth cracks in ideal materials is well understood, it is assumed that for
real, heterogeneous systems, fracture propagation is complex, generating rough
fracture surfaces that are highly sensitive to specific details of the medium.
Here we show how fracture roughness and material heterogeneity are inextricably
connected via a simple framework. Studying hydraulic fractures in brittle
hydrogels that have been supplemented with microbeads or glycerol to create
controlled material heterogeneity, we show that the morphology of the crack
surface depends solely on one parameter: the probability to perturb the front
above a critical size to produce a step-like instability. This probability
scales linearly with the number density, and as heterogeneity size to the
power. The ensuing behavior is universal and is captured by the 1D ballistic
propagation and annihilation of steps along the singular fracture front
A model for the fragmentation kinetics of crumpled thin sheets
As a confined thin sheet crumples, it spontaneously segments into flat facets
delimited by a network of ridges. Despite the apparent disorder of this
process, statistical properties of crumpled sheets exhibit striking
reproducibility. Experiments have shown that the total crease length accrues
logarithmically when repeatedly compacting and unfolding a sheet of paper.
Here, we offer insight to this unexpected result by exploring the
correspondence between crumpling and fragmentation processes. We identify a
physical model for the evolution of facet area and ridge length distributions
of crumpled sheets, and propose a mechanism for re-fragmentation driven by
geometric frustration. This mechanism establishes a feedback loop in which the
facet size distribution informs the subsequent rate of fragmentation under
repeated confinement, thereby producing a new size distribution. We then
demonstrate the capacity of this model to reproduce the characteristic
logarithmic scaling of total crease length, thereby supplying a missing
physical basis for the observed phenomenon.Comment: 11 pages, 7 figures (+ Supplemental Materials: 15 pages, 9 figures);
introduced a simpler approximation to model, key results unchanged; added
references, expanded supplementary information, corrected Fig. 2 and revised
Figs. 4 and 7 for clearer presentation of result
A state variable for crumpled thin sheets
Despite the apparent ease with which a sheet of paper is crumpled and tossed
away, crumpling dynamics are often considered a paradigm of complexity. This
complexity arises from the infinite number of configurations a disordered
crumpled sheet can take. Here we experimentally show that key aspects of
crumpling have a very simple description; the evolution of the damage in
crumpling dynamics can largely be described by a single global quantity, the
total length of all creases. We follow the evolution of the damage network in
repetitively crumpled elastoplastic sheets, and show that the dynamics of this
quantity are deterministic, and depend only on the instantaneous state of the
crease network and not at all on the crumpling history. We also show that this
global quantity captures the crumpling dynamics of a sheet crumpled for the
first time. This leads to a remarkable reduction in complexity, allowing a
description of a highly disordered system by a single state parameter. Similar
strategies may also be useful in analyzing other systems that evolve under
geometric and mechanical constraints, from faulting of tectonic plates to the
evolution of proteins
A Sequence of Developmental Events Occurs Underneath Growing Bacillus subtilis Pellicles
Biofilms are structured communities of bacteria that exhibit complex spatio-temporal dynamics. In liquid media, Bacillus subtilis produces an opaque floating biofilm, or a pellicle. Biofilms are generally associated with an interface, but the ability of Bacillus subtilis to swim means the bacteria are additionally able to reside within the liquid phase. However, due to imaging complications associated with the opacity of pellicles, the extent to which bacteria coexist within the liquid bulk as well as their behavior in the liquid is not well studied. We therefore develop a high-throughput imaging system to image underneath developing pellicles. Here we report a well-defined sequence of developmental events that occurs underneath a growing pellicle. Comparison with bacteria deficient in swimming and chemotaxis suggest that these properties enable collective bacterial swimming within the liquid phase which facilitate faster surface colonization. Furthermore, comparison to bacteria deficient in exopolymeric substances (EPS) suggest that the lack of a surface pellicle prevents further developmental steps from occurring within the liquid phase. Our results reveal a sequence of developmental events during pellicle growth, encompassing adhesion, conversion, growth, maturity, and detachment on the interface, which are synchronized with the bacteria in the liquid bulk increasing in density until the formation of a mature surface pellicle, after which the density of bacteria in the liquid drops