Low-frequency Rotational Motion Research Articles

Theoretical Study of the Photocyclization Reaction-Induced Dual Aggregation-Induced Emission Phenomenon.

Photochromic molecules with aggregation-induced emission (AIE) effects are of great value and prospective in various practical applications. To explore its inherent mechanism, the open isomer ap-BBTE and the closed isomer c-BBTE were chosen to perform the theoretical calculation using the quantum mechanics/molecular mechanics model combined with thermal vibration correlation function formalism. The calculations show that the photocyclization (PC) reaction from ap-BBTE to c-BBTE facilitates an improvement in the AIE effect. It is found that the fluorescence quantum yield (ΦF) enhancement of ap-BBTE is attributed to the restriction of the low-frequency rotational motion of the benzothiophene moiety and the high-frequency stretching vibrations of the C-C bond between the benzothiophene and benzylbis(thiadiazole) vinyl groups after aggregation. For c-BBTE, the increase in ΦF upon aggregation is mainly due to the suppression of the high-frequency stretching vibration of the C-C bond between the benzothiophene and the benzobis(thiadiazole) vinyl groups. In addition, the AIE effect was also enhanced from ap-BBTE to c-BBTE, which is consistent with the experimental phenomenon. The corresponding emission spectrum red-shifted from ap-BBTE to c-BBTE in both dilute solution and the crystalline state due to the improved intramolecular conjugation of c-BBTE. Moreover, the PC reaction from ap-BBTE to c-BBTE easily occurs in an excited state with a low energy barrier transition state by forming a C-C bond between benzothiophene groups effectively in dilute solution. Our calculations provide theoretical guidance for the further rational design of efficient AIE luminogens.

Photon Energy-Dependent Ultrafast Exciton Transfer in Chlorosomes of Chlorobium tepidum and the Role of Supramolecular Dynamics.

The antenna complex of green sulfur bacteria, the chlorosome, is one of the most efficient supramolecular systems for efficient long-range exciton transfer in nature. Femtosecond transient absorption experiments provide new insight into how vibrationally induced quantum overlap between exciton states supports highly efficient long-range exciton transfer in the chlorosome of Chlorobium tepidum. Our work shows that excitation energy is delocalized over the chlorosome in <1 ps at room temperature. The following exciton transfer to the baseplate occurs in ∼3 to 5 ps, in line with earlier work also performed at room temperature, but significantly faster than at the cryogenic temperatures used in previous studies. This difference can be attributed to the increased vibrational motion at room temperature. We observe a so far unknown impact of the excitation photon energy on the efficiency of this process. This dependency can be assigned to distinct optical domains due to structural disorder, combined with an exciton trapping channel competing with exciton transfer toward the baseplate. An oscillatory transient signal damped in <1 ps has the highest intensity in the case of the most efficient exciton transfer to the baseplate. These results agree well with an earlier computational finding of exciton transfer driven by low-frequency rotational motion of molecules in the chlorosome. Such an exciton transfer process belongs to the quantum coherent regime, for which the Förster theory for intermolecular exciton transfer does not apply. Our work hence strongly indicates that structural flexibility is important for efficient long-range exciton transfer in chlorosomes.

Theoretical and Experimental Investigation of a Rotational Magnetic Couple Piezoelectric Energy Harvester.

With the rapid development of Internet of Things (IoT) and the popularity of wireless sensors, using internal permanent or rechargeable batteries as a power source will face a higher maintenance workload. Therefore, self-powered wireless sensors through environmental energy harvesting are becoming an important development trend. Among the many studies of energy harvesting, the research on rotational energy harvesting still has many shortcomings, such as rarely working effectively under low-frequency rotational motion or working in a narrow frequency band. In this article, a rotational magnetic couple piezoelectric energy harvester is proposed. Under the low-frequency excitation (<10 Hz) condition, the harvester can convert low-frequency rotational into high-frequency vibrational of the piezoelectric beam by frequency up-conversion, effectively increasing the working bandwidth (0.5–16 Hz) and improving the efficiency of low-speed rotational energy harvesting. In addition, when the excitation frequency is too high (>16 Hz), it can solve the condition that the piezoelectric beam cannot respond in time by frequency down-conversion. Therefore, the energy harvester still has a certain degree of energy harvesting ability (18–22 Hz and 29–31 Hz) under high-frequency conditions. Meanwhile, corresponding theoretical analyses and experimental verifications were carried out to investigate the dynamic characteristics of the harvester with different excitation and installation directions. The experimental results illustrate that the proposed energy harvester has a wider working bandwidth benefiting from the frequency up-conversion mechanism and frequency down-conversion mechanism. In addition, the forward beam will have a wider bandwidth than the inverse beam due to the softening effect. In addition, the maximum powers of the forward and inverse beams at 310 rpm (15.5 Hz) are 93.8 μW and 58.5 μW, respectively. The maximum powers of the two beams at 420 rpm (21 Hz) reached 177 μW and 85.2 μW, respectively. The self-powered requirement of micromechanical systems can be achieved. Furthermore, this study provides the theoretical and experimental basis for rotational energy harvesting.

Magnetic plucked meso-scale piezoelectric energy harvester for low-frequency rotational motion

Wind energy is one of the most attractive renewable energy sources; however, wind turbine blades suffer from erosion caused by sand particles carried by strong winds. The surface condition monitoring system installed on a wind turbine blade requires power, but directly connecting a cable from a stationary generator to rotating blade is difficult. In this paper, a well-packaged piezoelectric energy harvester (PEH) is proposed for installation on the blade to convert the low-frequency rotational motion into electric energy. The package, 52 mm in diameter and 14 mm thick, consists of a small clamp with a bearing and a meso-scale PEH, repelling magnets, and an outer cover with a pendulum. The PEH, redesigned in a tapered shape, has improved output performance and long-term reliability owing to the greater flexibility and low stress concentration. Because the typical rotational speed of a wind turbine varies from 5 rpm (0.08 Hz) to 30 rpm (0.5 Hz), a frequency up-conversion method is introduced to accommodate the PEH working under its off-resonant-frequency rotation. The output performance of the PEH was thoroughly tested on a lab-scale wind turbine blade. Additionally, an equivalent circuit model was established to described the nonlinear electromechanical behavior of the PEH during the magnetic plucking. Finally, the PEH was tested under practical rotation of a wind turbine for 24 h. The resulting daily energy of 1.2 J indicates the proposed rotational PEH is a promising solution for the autonomous monitoring of wind turbines.

Rotational coherence of encapsulated ortho and para water in fullerene-C60 revealed by time-domain terahertz spectroscopy

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.

Design and analysis of a broadband three-beam impact piezoelectric energy harvester for low-frequency rotational motion

This paper proposes a three-beam impact energy harvester (TIEH) to enhance the energy harveting efficiency for low-frequency rotational motion. The design includes three beams, including the excitation beam, the harvesting beam and the protection beam. The excitation is generated by the impact of the excitation beam and the harvesting beam, and the harvesting beam breaks through the resonance state limitation to obtain more excellent performance in the low-frequency band. Through the design of centrifugal force, the soften or harden effect can be obtained to adapt to different application scenarios. In this paper, the TIEH electromechanical coupling model is established, and the concept of design factor is proposed for the centrifugal force scheme to simplify the design and analysis. The tested TIEH can obtain 52.1 μW at 4.2 Hz with 1.9 Hz bandwidth over 2 V. The Soften TIEH has a 23.0% power increase and 15.8% bandwidth reduction and the Harden TIEH has a 11.9% power reduction and 52.6% bandwidth increase. The results show that the TIEH has the characteristics of low frequency band, wide frequency band, and parameter insensitivity, and has the potential to be applied to a low-frequency rotational structure with limited volume.

Enhancing energy harvesting in low-frequency rotational motion by a quad-stable energy harvester with time-varying potential wells

In recent years, various nonlinear energy harvesters have been designed and investigated for efficiently harvesting energy in rotational motions, and they were aimed to provide suitable power supply for wireless sensors in Internet of Things (IoT). However, few of them could effectively work in low-frequency rotational motion (less than 120 rpm). In this paper, combining with the advantages of lower potential barriers and time-varying potential wells, a quad-stable piezoelectric energy harvester (PEH) is proposed to enhance the energy harvesting performance especially in low-frequency rotational motion. Additionally, a corresponding theoretical model is derived, and the related calculation model of the magnetic force is established to analyze the influence of potential barriers and nonlinear characteristics. What’s more, the influence of external magnets and configuration parameters on the potential wells for different PEHs are numerically and experimentally investigated. Furthermore, the corresponding experiments are conducted and verify that the quad-stable PEH with time-varying potential wells exhibits a wider operational frequency range (1–7 Hz) in rotational motion, compared with that of the bi-stable PEH (3–7.3 Hz) and the tri-stable PEH (4–7.3 Hz), respectively. The parametric studies are carried out to explore the influence of the load resistance and the rotational radius on the energy harvesting performance, providing valuable insights into energy harvesting in rotational motion. Overall, the proposed quad-stable PEH is numerically and experimentally verified to be effective for harvesting energy in low-frequency rotational environment.

Energy harvesting for autonomous thermal sensing using a linked E-shape multi-beam piezoelectric device in a low frequency rotational motion

Abstract This paper presents a multi-beam energy harvester (MBT2ML) capable to convert kinetic energy from rotational motion at low frequency (

The benefits of an asymmetric tri-stable energy harvester in low-frequency rotational motion

This letter investigates the benefits of an asymmetric tri-stable energy harvester in low-frequency rotational motion. A theoretical model framework is presented, which considers the effect of the rotational motion, to describe the dynamic characteristics and output voltage of the harvester. More importantly, the asymmetric tri-stable energy harvester is experimentally verified to be better than the symmetric one under various rotational speeds. The former exhibits a wide working rotational speed range of 140–460 rpm. The weight component of tip mass produces a periodic harmonic exciting force due to rotational motion, which can be designed to enhance energy harvesting.

Canal and otolith contributions to compensatory tilt responses in pigeons.

Gaze-stabilizing eye and head responses compensate more effectively for low-frequency rotational motion when such motion stimulates the otolith organs, as during earth-horizontal axis rotations. However, the nature of the otolith signal responsible for this improvement in performance has not been previously determined. In this study, we used combinations of earth-horizontal axis rotational and translational motion to manipulate the magnitude of net linear acceleration experienced by pigeons, under both head-fixed and head-free conditions. We show that phase enhancement of eye and head responses to low-frequency rotational motion was causally related to the magnitude of dynamic net linear acceleration and not the gravitational acceleration component. We also show that canal-driven and otolith-driven eye responses were both spatially and temporally appropriate to combine linearly, and that a simple linear model combining canal- and otolith-driven components predicted eye responses to complex motion that were consistent with our experimental observations. However, the same model did not predict the observed head responses, which were spatially but not temporally appropriate to combine according to the same linear scheme. These results suggest that distinct vestibular processing substrates exist for eye and head responses in pigeons and that these are likely different from the vestibular processing substrates observed in primates.

Rotation of a droplet in a viscous fluid

Unique capabilities for modeling the bulk motion of one liquid in another arise from the use of droplets made of a magnetic liquid. In this paper the low-frequency rotational motion of a magnetic droplet suspended in a viscous liquid is investigated. In this frequency range, the shape of the droplet does not depend on the field frequency and is determined only by its amplitude. An analytic solution has been found in the Stokes approximation to the problem, which generalizes the classic problem of Jeffrey to the case of a liquid ellipsoidal particle. This solution makes it possible to determine the velocity field inside and outside the liquid particle, the moments of the viscous forces acting on the droplet, its coefficient of rotational mobility.