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Online Meetings in Times of Global Crisis: Toward Sustainable Conferencing
Get PDFInternational conferences are an essential component of the timely exchange of ideas and results in the scientific community, where scientists present their results, gain new insights, and meet colleagues and collaborators. In early 2020, a global pandemic caused by a coronavirus (SARS-CoV-2) and the corresponding disease (covid-19) resulted in a number of conferences being canceled or postponed.1 To foster scientific exchange despite the restrictions, a range of initiatives were launched across the globe to move international conferences onlin
Microtubule-based actin transport and localization in a spherical cell
Get PDFThe interaction between actin filaments and microtubules is crucial for many eukaryotic cellular processes, such as, among others, cell polarization, cell motility and cellular wound healing. The importance of this interaction has long been recognized, yet very little is understood about both the underlying mechanisms and the consequences for the spatial (re)organization of the cellular cytoskeleton. At the same time, understanding the causes and the consequences of the interaction between different biomolecular components are key questions for in vitro research involving reconstituted biomolecular systems, especially in the light of current interest in creating minimal synthetic cells. In this light, recent in vitro experiments have shown that the actin-microtubule interaction mediated by the cytolinker TipAct, which binds to actin lattice and microtubule tips, causes the directed transport of actin filaments. We develop an analytical theory of dynamically unstable microtubules, nucleated from the centre of a spherical cell, in interaction with actin filaments. We show that, depending on the balance between the diffusion of unbound actin filaments and propensity to bind microtubules, actin is either concentrated in the centre of the cell, where the density of microtubules is highest, or becomes localized to the cell cortex
Phase-retrieval Fourier microscopy of partially temporally coherent nanoantenna radiation patterns
Get PDFWe report an experimental technique for determining phase-resolved radiation patterns of single nanoantennas by phase-retrieval defocused imaging. A key property of nanoantennas is their ability to imprint spatial coherence, for instance, on fluorescent sources. Yet, measuring emitted wavefronts in absence of a reference field is difficult. We realize a defocused back focal plane microscope to measure phase even for partially temporally coherent light and benchmark the method using plasmonic bullseye antenna scattering. We outline the limitations of defocused imaging which are set by spectral bandwidth and antenna mode structure. This work is a first step to resolve wavefronts from fluorescence controlled by nanoantennas
Simultaneous Photonic and Excitonic Coupling in Spherical Quantum Dot Supercrystals
Get PDFSemiconductor nanocrystals, or quantum dots (QDs), simultaneously benefit from inexpensive low-temperature solution processing and exciting photophysics, making them the ideal candidates for next-generation solar cells and photodetectors. While the working principles of these devices rely on light absorption, QDs intrinsically belong to the Rayleigh regime and display optical behavior limited to electric dipole resonances, resulting in low absorption efficiencies. Increasing the absorption efficiency of QDs, together with their electronic and excitonic coupling to enhance charge carrier mobility, is therefore of critical importance to enable practical applications. Here, we demonstrate a general and scalable approach to increase both light absorption and excitonic coupling of QDs by fabricating hierarchical metamaterials. We assemble QDs into crystalline supraparticles using an emulsion template and demonstrate that these colloidal supercrystals (SCs) exhibit extended resonant optical behavior resulting in an enhancement in absorption efficiency in the visible range of more than 2 orders of magnitude with respect to the case of dispersed QDs. This successful light trapping strategy is complemented by the enhanced excitonic coupling observed in ligand-exchanged SCs, experimentally demonstrated through ultrafast transient absorption spectroscopy and leading to the formation of a free biexciton system on sub-picosecond time scales. These results introduce a colloidal metamaterial designed by self-assembly from the bottom up, simultaneously featuring a combination of nanoscale and mesoscale properties leading to simultaneous photonic and excitonic coupling, therefore presenting the nanocrystal analogue of supramolecular structures
Reaction-field model for the dielectric response of mixtures
Get PDFWe present a new effective medium theory for the dielectric response of mixtures of molecules with molecular polarizability and a permanent dipole moment. This model includes the interaction of each local dipole moment with the dipolar reaction fields of neighboring dipolar molecules. This interaction leads to an enhancement of the dielectric response of the mixture and constitutes an alternative method to describe the correlated motion of dipoles in liquids compared to the models of Fröhlich and Kirkwood. The model requires as input parameters the volume fractions of the components contained in the mixture and the dielectric parameters of the pure components. The results of the model are compared with experimental data and with the results of previous effective-medium theories
Resonant Metagratings for Spectral and Angular Control of Light for Colored Rooftop Photovoltaics
Get PDFWe designed semitransparent metagrating supercells that enable control over the spectrum and directivity of incident light for applications in photovoltaics with tailored angular appearance. The building block of the supercells is a 100–120 nm wide and 175 nm tall silicon nanowire that shows a strong Mie resonance around λ = 650 nm. By arranging the resonant Mie scatterers into metagratings of increasing pitch (675–1300 nm), we created a Lambertian-like scattering distribution over an angular range of choice. The millimeter-sized metasurfaces were fabricated using electron beam lithography and reactive ion etching. The fabricated metasurface nearly fully suppresses specular reflection on resonance while 10% of the incoming light around the resonance is scattered into the angular range between 30° and 75°, creating a bright red appearance over this specific range of angles. Off-resonant light in the blue, green, and near-infrared is efficiently transmitted through the metasurface and absorbed in the underlying photovoltaic cell. The implemented silicon heterojunction solar cells with integrated metagrating supercells shows a reduction in external quantum efficiency matching the resonant scattering spectral range. The short circuit current is reduced by 13% due to the combined effects of resonant scattering, reflection from the high-index substrate, and absorption in the Si nanowires. In addition to efficient colorful photovoltaics with tailored angular appearance, the metagrating concept can find application in many other light management designs for photovoltaics and other optoelectronic devices
Charge-dependent interactions of monomeric and filamentous actin with lipid bilayers
Get PDFThe cytoskeletal protein actin polymerizes into filaments that are essential for the mechanical stability of mammalian cells. In vitro experiments showed that direct interactions between actin filaments and lipid bilayers are possible and that the net charge of the bilayer as well as the presence of divalent ions in the buffer play an important role. In vivo, colocalization of actin filaments and divalent ions are suppressed, and cells rely on linker proteins to connect the plasma membrane to the actin network. Little is known, however, about why this is the case and what microscopic interactions are important. A deeper understanding is highly beneficial, first, to obtain understanding in the biological design of cells and, second, as a possible basis for the building of artificial cortices for the stabilization of synthetic cells. Here, we report the results of coarse-grained molecular dynamics simulations of monomeric and filamentous actin in the vicinity of differently charged lipid bilayers. We observe that charges on the lipid head groups strongly determine the ability of actin to adsorb to the bilayer. The inclusion of divalent ions leads to a reversal of the binding affinity. Our in silico results are validated experimentally by reconstitution assays with actin on lipid bilayer membranes and provide a molecular-level understanding of the actin-membrane interaction
Quantifying mobile ions and electronic defects in perovskite-based devices with temperature-dependent capacitance measurements: Frequency vs time domain
Get PDFPerovskites have proven to be a promising candidate for highly efficient solar cells, light-emitting diodes, and x-ray detectors, overcominglimitations of inorganic semiconductors. However, they are notoriously unstable. The main reason for this instability is the migration ofmobile ions through the device during operation as they are mixed ionic–electronic conductors. Here, we show how measuring the capacitancein both the frequency and the time domain can be used to study ionic dynamics within perovskite-based devices, quantifying activation energy,diffusion coefficient, sign of charge, concentration, and the length of the ionic double layer in the vicinity of the interfaces. Measuring thetransient of the capacitance furthermore allows for distinguishing between ionic and electronic effects
The mechanics of soft porous solids: from hydrogel dynamics to fibrin compression
Get PDFHydrogels generally consist of a solid network and of a fluid permeating the network.
Interactions between the solid and the fluid determine the response of a hydrogel when it is perturbed by, for example, the external mechanical pressure or the external osmotic pressure.
The resulting time-dependent dynamics of (de)swelling are characterized by the poromechanical properties of hydrogels: their (visco)elasticity, plasticity and permeability.
In this thesis we mainly focus on the characterization of the permeability and elasticity of hydrogels by modelling the dynamics of several (de)swelling processes for synthetic hydrogels and (bio)polymer gels.
Also, we delve into the broader problem of compression tests on solids where we investigated a novel compression testing geometry, and we perform a poromechanical study of an industrially relevant phenomenon: the spontaneous expulsion of fluid from low fat mayonnaise
Connecting the Stimuli-Responsive Rheology of Biopolymer Hydrogels to Underlying Hydrogen-Bonding Interactions
Get PDFMany biopolymer hydrogels are environmentally responsive because they are held together by physical associations that depend on pH and temperature. Here, we investigate how the pH and temperature responses of the rheology of hyaluronan hydrogels are connected to the underlying molecular interactions. Hyaluronan is an essential structural biopolymer in the human body with many applications in biomedicine. Using two-dimensional infrared spectroscopy, we show that hyaluronan chains become connected by hydrogen bonds when the pH is changed from 7.0 to 2.5 and that the bond density at pH 2.5 is independent of temperature. Temperature-dependent rheology measurements show that because of this hydrogen bonding the stress relaxation at pH 2.5 is strongly slowed down in comparison to pH 7.0, consistent with the sticky reptation model of associative polymers. From the flow activation energy, we conclude that each polymer is cross-linked by multiple (5-15) hydrogen bonds to others, causing slow macroscopic stress relaxation, despite the short time scale of breaking and reformation of each individual hydrogen bond. Our findings can aid the design of stimuli-responsive hydrogels with tailored viscoelastic properties for biomedical applications