The interaction of spatially modulated vortex pairs with free surfaces

Spatially modulated vortex pairs were generated below a free surface by two counter-rotating flaps whose edges approximate a sinusoid. The surface interactions of the vertically approaching vortex pairs were visualized by the shadowgraph technique. Two limiting cases were investigated in detail: the interaction with a surfactant-rich (contaminated) surface and with a surfactant-poor (‘clean’) surface. In the latter case shadowgraph images showed that the underlying vortex core formed a line of circular surface depressions. Subsequent measurements of the temporally evolving velocity fields using digital particle image velocimetry (DPIV) of the vortex pair cross-sections and the subsurface plane confirmed the connection process of the main vortex core with the surface. As a result of the connection the initially modulated vortex tube was broken into a line of U-vortices. In the presence of surfactants this connection could not be observed; rather a Reynolds ridge (or stagnation line) was formed and a very weak connection of the secondary separation vortex could be seen in the shadowgraphs as well as measured with the time-resolved DPIV technique. A prerequisite for connection of the vortex with the surface is that the flow's kinematics force the vortex core, that is, regions of concentrated vorticity, toward the surface. The ensuing locally concentrated viscous flux of surface-parallel vorticity through the surface is balanced by a local surface deceleration. Surface-normal vorticity appears on each side of the decelerated region whose gradually increasing circulation is directly balanced by the loss of circulation of the surface-parallel vortex. However, the shear forces caused by small amounts of surface contamination and its associated subsurface boundary layer inhibit the connection process by preventing the essential viscous flux of parallel vorticity through the surface. Instead, the subsurface boundary layer is associated with a flux of parallel vorticity into the surface which then concentrates into the observable secondary separation vortex

Event-based imaging velocimetry using pulsed illumination

The paper addresses the shortcoming of current event-based vision (EBV) sensors in the context of particle imaging. Latency is introduced both on the pixel level as well as during read-out from the array and results in systemic timing errors when processing the recorded event data. Using pulsed illumination, the overall latency can be quantified and indicates an upper bound on the frequency response on the order of 10-20 kHz for the specific EBV sensor. In particle-based flow measurement applications, particles scattering the light from a pulsed light source operating below this upper frequency can be reliably tracked in time. Through the combination of event-based vision and pulsed illumination, flow field measurements are demonstrated at light pulsing rates up to 10 kHz in both water and air flows by providing turbulence statistics and velocity spectra. The described EBV-based velocimetry system consists of only an EBV camera and a (low-cost) laser that can be directly modulated by the camera, making the system compact, portable and cost effective

Utilization and Dissipation of Absorbed Light Energy in the Epiphytic Crassulacean Acid Metabolism Bromeliad Tillandsia Ionantha

This is the publisher's official version, also available electronically from: http://www.jstor.org/stable/10.1086/314130.Past studies of the ability of epiphytic Crassulacean acid metabolism bromeliads to acclimate to different light levels yield conflicting findings; some indicate that these plants are similar to shade plants whereas others stress their similarity to sun plants. This study investigates the ability of individuals of Tillandsia ionantha to acclimate to low or high irradiance. Plants were exposed to 100 and 800 jitmol m~2 s"1 photosynthetic photon flux density under controlled conditions for 4 wk. Individuals exposed to the lower light level exhibited higher chlorophyll concentrations and higher photosynthetic rates at low light relative to plants exposed to high light. Low-light plants also exhibited a greater efficiency in the photochemical utilization of absorbed light energy and a lower ability to dissipate excess energy nonphotochemically, relative to the plants exposed to the higher light level. Photosynthetic rates at high light were similar in both sets of plants, reflecting the higher efficiency of energy conversion in the low-light plants and an apparent saturation of photosynthetic capacity in the high-light plants. The latter may have resulted from high-light-induced damage to the photosynthetic apparatus in addition to an increase in nonphotochemical dissipation of excess light energy. The higher capacity for harmless dissipation of excess light energy in the high-light plants should prove beneficial in plants growing in exposed locations and subject to drought and nutrient stresses. Thus, the results support and expand those of previous studies: T. ionantha can acclimate to both low and high light but does so in different ways. Such flexibility in adjusting the photosynthetic apparatus to varying light levels constitutes a valuable adaptation to growing throughout the canopy of a host tree

Gap width modification on fully screen-printed coplanar Zn|MnO2 batteries

Fully printed primary zinc-manganese dioxide (Zn|MnO2) batteries in coplanar configuration were fabricated by sequential screen printing. While electrode dimensions and transferred active masses were kept at constant levels, electrode separating gaps were incrementally enlarged from 1 mm to 5 mm. Calendering of solely zinc anodes increased interparticle contact of active material within the electrodes while the porosity of manganese dioxide based electrodes was maintained by non-calendering. Chronopotentiometry revealed areal capacities for coplanar batteries up to 2.8 mAh cm−2. Galvanostatic electrochemical impedance spectroscopy measurements and short circuit measurements were used to comprehensively characterise the effect of gap width extension on bulk electrolyte resistance and charge transfer resistance values. Linear relationships between nominal gap widths, short circuit currents and internal resistances were evidenced, but showed only minor impact on actual discharge capacities. The findings contradict previous assumptions to minimise gap widths of printed coplanar batteries to a sub-millimetre range in order to retain useful discharge capacities. The results presented in this study may facilitate process transfer of printed batteries to an industrial environment

Dynamics of intracellular mannan and cell wall folding in the drought responses of succulent <i>Aloe</i> species

Plants have evolved a multitude of adaptations to survive extreme conditions. Succulent plants have the capacity to tolerate periodically dry environments, due to their ability to retain water in a specialized tissue, termed hydrenchyma. Cell wall polysaccharides are important components of water storage in hydrenchyma cells. However, the role of the cell wall and its polysaccharide composition in relation to drought resistance of succulent plants are unknown. We investigate the drought response of leaf-succulent Aloe (Asphodelaceae) species using a combination of histological microscopy, quantification of water content, and comprehensive microarray polymer profiling. We observed a previously unreported mode of polysaccharide and cell wall structural dynamics triggered by water shortage. Microscopical analysis of the hydrenchyma cell walls revealed highly regular folding patterns indicative of predetermined cell wall mechanics in the remobilization of stored water and the possible role of homogalacturonan in this process. The in situ distribution of mannans in distinct intracellular compartments during drought, for storage, and apparent upregulation of pectins, imparting flexibility to the cell wall, facilitate elaborate cell wall folding during drought stress. We conclude that cell wall polysaccharide composition plays an important role in water storage and drought response in Aloe

Self-assembled nanoscale photomimetic models: structure and related dynamics

Using static and time-resolved measurements, dynamics of non-radiative relaxation processes have been studied in self-assembled porphyrin triads of various geometry, containing the main biomimetic components, Zn–porphyrin dimers, free-base extra-ligands (porphyrin, chlorin or tetrahydroporphyrin), and electron acceptors A (quinone or pyromellitimide). The strong quenching of the dimer fluorescence is due to energy and sequential electron transfer (ET) processes to the extra-ligand (~0.9–1.7 ps), which are faster than a slower ET (34–135 ps) from the dimer to covalently linked A in toluene at 293 K. The extra-ligand S₁-state decay (τₛ = 940–2670 ps) is governed by competing processes: a bridge (dimer) mediated long-range (r_DA = 18–24 Å) superexchange ET to an acceptor, and photoinduced hole transfer from the excited extra-ligand to the dimer followed by possible superexchange ET steps to low-lying charge transfer states of the triads. The subsequent ET steps dimer → monomer → A taking place in the triads, mimic the sequence of primary ET reactions in photosynthetic reaction centers in vivo. © 2002 Elsevier Science B.V. All rights reserved

Electron transfer in porphyrin multimolecular self-organized nanostructures

On the base of of covalent and non-covalent bonds nanoscale self-assembling multiporphyrin arrays with well-defined geometry, the controllable number of interacting components and their spectral and photophysical properties were formed. The deactivation of excited singlet and triplet states was studied using steady-state, time-resolved picosecond fluorescence (∆½≈30 ps) and femtosecond pump-probe (∆½≈280 fs) spectroscopy in solvents of various polarity at 77-300 K. It has been found that the competition between the non-radiative energy transfer (within ≤10 ps) and charge transfer (within 300 fs - 700 ps) processes in the systems depends on the structure, spectral and redox properties of interacting subunits and may be driven by the distance, temperature and solvent polarity. The possible pathways and mechanisms of the electron transfer in the systems of various types are discussed (Marcus theory for the “normal” region and the non-adiabatic case, the “superexchange” mechanism)

Competition between electron transfer and energy migration in self-assembled porphyrin triads

The photoinduced electron transfer (ET) and the energy migration (EM) processes have been studied in liquid solutions and polymeric (PMMA) films for the triads consisting of the Zn-octaethylporphyrin chemical dimer (the energy and electron donor, D) and dipyridyl substituted tetrapyrrole extra-ligands (porphyrins, chlorin, tetrahydroporphyrin) as the acceptors, A. On the basis of the time correlated single photon counting technique and femtosecond pump-probe spectroscopy, it has been shown that D fluorescence quenching with time constant ranging from 1.7 to 10 ps is due to competing EM and ET processes from the dimer to A's. In addition, the fluorescence decay time shortening (by ∼1.3–1.6 times in toluene at 293 K) is observed for electron accepting extra-ligands in the triads. The acceptor fluorescence quenching is hard dependent on the mutual spatial arrangement of the triad subunits, but becomes stronger upon the solvent polarity increase (addition of acetone to toluene solutions) as well as the temperature lowering (from 278 to 221 K). The possible reasons and mechanisms of the non-radiative deactivation of locally excited S₁-states in the triads are discussed taking into account a close lying charge-separated state. The obtained experimental data are analyzed using the reduced density matrix formalism in the frame of Haken–Strobl–Reineker approach. This model includes EM and ET processes as well as the dephasing of coherence between the excited electronic states of the triad. © 2001 Elsevier Science B.V. All rights reserved

Photoinduced electron transfer dynamics for self-assembled porphyrin arrays in solutions and films

Electronic excitation energy deactivation in self-assembled porphyrin triads has been studied by the time correlated single photon counting technique as a function of the solvent polarity (toluene-acetone mixtures), temperature (77-350 K), and mutual spatial arrangement of the donor and acceptor subunits. The donor (Zn-octaethylporphyrin chemical dimer) fluorescence quenching with time constant of 1.7÷10 ps is due to competing energy migration and electron transfer processes to the acceptor (dipyridyl substituted tetrapyrrole extra-ligand). The quenching of the acceptor fluorescence (by ~ 1.3–1.6 times) does not significantly depend on the mutual spatial arrangement of the triad subunits and increases with the solvent polarity rising and the decrease of the temperature. The obtained experimental data are analyzed using the reduced density matrix formalism in the frame of Haken-Strobl-Reineker approach taking into account the energy transfer, charge separation, and the dephasing of coherence between the excited electronic states of the triad