361 research outputs found
Absolute diffusion measurements of active enzyme solutions by NMR
The diffusion of enzymes is of fundamental importance for many biochemical
processes. Enhanced or directed enzyme diffusion can alter the accessibility of
substrates and the organization of enzymes within cells. Several studies based
on fluorescence correlation spectroscopy (FCS) report enhanced diffusion of
enzymes upon interaction with their substrate or inhibitor. In this context,
major importance is given to the enzyme fructose-bisphosphate aldolase, for
which enhanced diffusion has been reported even though the catalysed reaction
is endothermic. Additionally, enhanced diffusion of tracer particles
surrounding the active aldolase enzymes has been reported. These studies
suggest that active enzymes can act as chemical motors that self-propel and
give rise to enhanced diffusion. However, fluorescence studies of enzymes can,
despite several advantages, suffer from artefacts. Here we show that the
absolute diffusion coefficients of active enzyme solutions can be determined
with Pulsed Field Gradient Nuclear Magnetic Resonance (PFG-NMR). The advantage
of PFG-NMR is that the motion of the molecule of interest is directly observed
in its native state without the need for any labelling. Further, PFG-NMR is
model-free and thus yields absolute diffusion constants. Our PFG-NMR
experiments of solutions containing active fructose bisphosphate aldolase from
rabbit muscle do not show any diffusion enhancement for the active enzymes nor
the surrounding molecules. Additionally, we do not observe any diffusion
enhancement of aldolase in the presence of its inhibitor pyrophosphate
Gait learning for soft microrobots controlled by light fields
Soft microrobots based on photoresponsive materials and controlled by light
fields can generate a variety of different gaits. This inherent flexibility can
be exploited to maximize their locomotion performance in a given environment
and used to adapt them to changing conditions. Albeit, because of the lack of
accurate locomotion models, and given the intrinsic variability among
microrobots, analytical control design is not possible. Common data-driven
approaches, on the other hand, require running prohibitive numbers of
experiments and lead to very sample-specific results. Here we propose a
probabilistic learning approach for light-controlled soft microrobots based on
Bayesian Optimization (BO) and Gaussian Processes (GPs). The proposed approach
results in a learning scheme that is data-efficient, enabling gait optimization
with a limited experimental budget, and robust against differences among
microrobot samples. These features are obtained by designing the learning
scheme through the comparison of different GP priors and BO settings on a
semi-synthetic data set. The developed learning scheme is validated in
microrobot experiments, resulting in a 115% improvement in a microrobot's
locomotion performance with an experimental budget of only 20 tests. These
encouraging results lead the way toward self-adaptive microrobotic systems
based on light-controlled soft microrobots and probabilistic learning control.Comment: 8 pages, 7 figures, to appear in the proceedings of the IEEE/RSJ
International Conference on Intelligent Robots and Systems 201
The role of symmetry in driven propulsion at low Reynolds number
We theoretically and experimentally investigate low-Reynolds-number
propulsion of geometrically achiral planar objects that possess a dipole moment
and that are driven by a rotating magnetic field. Symmetry considerations
(involving parity, , and charge conjugation, )
establish correspondence between propulsive states depending on orientation of
the dipolar moment. Although basic symmetry arguments do not forbid individual
symmetric objects to efficiently propel due to spontaneous symmetry breaking,
they suggest that the average ensemble velocity vanishes. Some additional
arguments show, however, that highly symmetrical (-even) objects
exhibit no net propulsion while individual less symmetrical
(-even) propellers do propel. Particular magnetization
orientation, rendering the shape -odd, yields
unidirectional motion typically associated with chiral structures, such as
helices. If instead of a structure with a permanent dipole we consider a
polarizable object, some of the arguments have to be modified. For instance, we
demonstrate a truly achiral (- and -even)
planar shape with an induced electric dipole that can propel by
electro-rotation. We thereby show that chirality is not essential for
propulsion due to rotation-translation coupling at low Reynolds number.Comment: 5 pages, 5 figure
Observation of the Faraday effect via beam deflection in a longitudinal magnetic field
We report the observation of the magnetic field induced circular differential
deflection of light at the interface of a Faraday medium. The difference in the
angles of refraction or reflection between the two circular polarization
components is a function of the magnetic field strength and the Verdet
constant. The reported phenomena permit the observation of the Faraday effect
not via polarization rotation in transmission, but via changes in the
propagation direction in refraction or in reflection. An unpolarized light beam
is predicted to split into its two circular polarization components. The light
deflection arises within a few wavelengths at the interface and is therefore
independent of pathlength
Fernlesen mit Foucault? Ăśberlegungen zur Praxis des distant reading und zur Operationalisierung von Foucaults Diskursanalyse
Our paper offers a critical examination of the concept and practice of 'Distant Reading', as coined by Franco Moretti in 2000. We consider several definitions of the term and look for possible operationalizations. It becomes clear that 'Distant Reading' has largely been a theoretical vehicle or mere buzzword in the past one and a half decades that adapts only slowly to the practices and technological standards of the Digital Humanities. In the light of these findings, we conclude with an examination of the operational potential of Foucauldian discourse analysis
Visual Place Recognition: A Tutorial
Localization is an essential capability for mobile robots. A rapidly growing
field of research in this area is Visual Place Recognition (VPR), which is the
ability to recognize previously seen places in the world based solely on
images. This present work is the first tutorial paper on visual place
recognition. It unifies the terminology of VPR and complements prior research
in two important directions: 1) It provides a systematic introduction for
newcomers to the field, covering topics such as the formulation of the VPR
problem, a general-purpose algorithmic pipeline, an evaluation methodology for
VPR approaches, and the major challenges for VPR and how they may be addressed.
2) As a contribution for researchers acquainted with the VPR problem, it
examines the intricacies of different VPR problem types regarding input, data
processing, and output. The tutorial also discusses the subtleties behind the
evaluation of VPR algorithms, e.g., the evaluation of a VPR system that has to
find all matching database images per query, as opposed to just a single match.
Practical code examples in Python illustrate to prospective practitioners and
researchers how VPR is implemented and evaluated.Comment: IEEE Robotics & Automation Magazine (RAM
Acoustic Hologram Enhanced Phased Arrays for Ultrasonic Particle Manipulation
The ability to shape ultrasound fields is important for particle manipulation, medical therapeutics, and imaging applications. If the amplitude and/or phase is spatially varied across the wave front, then it is possible to project “acoustic images.” When attempting to form an arbitrary desired static sound field, acoustic holograms are superior to phased arrays due to their significantly higher phase fidelity. However, they lack the dynamic flexibility of phased arrays. Here, we demonstrate how to combine the high-fidelity advantages of acoustic holograms with the dynamic control of phased arrays in the ultrasonic frequency range. Holograms are used with a 64-element phased array, driven with continuous excitation. Movement of the position of the projected hologram via phase delays that steer the output beam is demonstrated experimentally. This allows the creation of a much more tightly focused point than with the phased array alone, while still being reconfigurable. It also allows the complex movement at a water-air interface of a “phase surfer” along a phase track or the manipulation of a more arbitrarily shaped particle via amplitude traps. Furthermore, a particle manipulation device with two emitters and a single split hologram is demonstrated that allows the positioning of a “phase surfer” along a one-dimensional axis. This paper opens the door for new applications with complex manipulation of ultrasound while minimizing the complexity and cost of the apparatus
Antibubbles enable tunable payload release with low-intensity ultrasound
The benefits of ultrasound are its ease-of-use and its ability to precisely
deliver energy in opaque and complex media. However, most materials responsive
to ultrasound show a weak response, requiring the use of high powers, which are
associated with undesirable streaming, cavitation, or temperature rise. These
effects hinder response control and may even cause damage to the medium where
the ultrasound is applied. Moreover, materials that are currently in use rely
on all-or-nothing effects, limiting the ability to fine-tune the response of
the material on the fly. For these reasons, there is a need for materials that
can respond to low intensity ultrasound with programmable responses. Here it is
demonstrated that antibubbles are a low-intensity-ultrasound-responsive
material system that can controllably release a payload using acoustic
pressures in the kPa range. Varying their size and composition tunes the
release pressure, and the response can be switched between a single release and
stepwise release across multiple ultrasound pulses. Observations using confocal
and high-speed microscopy revealed different ways that can lead to release.
These findings lay the groundwork to design antibubbles that controllably
respond to low-intensity ultrasound, opening a wide range of applications
ranging from ultrasound-responsive material systems to carriers for targeted
delivery.Comment: Main Text: 14 pages, 4 figures. Embedded SI: 4 pages, 5 figure
A right-handed isotropic medium with a negative refractive index
The sign of the refractive index of any medium is soley determined by the
requirement that the propagation of an electromagnetic wave obeys Einstein
causality. Our analysis shows that this requirement predicts that the real part
of the refractive index may be negative in an isotropic medium even if the
electric permittivity and the magnetic permeability are both positive. Such a
system may be a route to negative index media at optical frequencies. We also
demonstrate that the refractive index may be positive in left-handed media that
contain two molecular species where one is in its excited state.Comment: 4.1 pages, 4 figures, submitted to Physical Review Letter
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