Polish Academy of Sciences

Triboelectric nanogenerators (TENGs) can effectively harvest mechanical energy from the environment, offering a promising solution for a sustainable power supply in wearable electronics. However, their widespread application is often hindered by expensive raw materials and complex fabrication processes. This study develops a simple and efficient integrated ultrasonic-shear process to exfoliate low-cost flake graphite (FG) into multi-layer flake graphite (MLFG), which is then embedded into polydimethylsiloxane (PDMS) to fabricate a novel composite triboelectric layer. The multi-layered structure of MLFG provides a larger specific surface area and more charge trapping sites, significantly enhancing capacitive behavior. The optimized 2 wt% MLFG-TENG achieved an open-circuit voltage of 90.3 V and a short-circuit current of 4.6 μA, which are 1.2 times and 1.6 times higher than those of the 3 wt% FG-TENG and 3.1 times and 4.2 times higher than those of the pure PDMS-TENG, respectively. This method delivers superior output performance with lower doping levels and maintains stable output after 20 000 cycles, demonstrating exceptional scalability. Furthermore, by integrating a rectifier circuit, the MLFG-TENGs successfully power small electronic devices such as LED arrays and electronic clocks. Concurrently, when integrated with machine learning, the MLFG-TENGs achieve 100% accurate recognition of five distinct hand motion patterns, highlighting their great potential in the fields of self-powered wearable devices and motion sensing.

The central Ionian Islands area, including Kefalonia and Lefkada Islands constitutes an area with the highest seismic hazard in Greece. Lefkada Island has experienced two strong (M > 6.0) earthquakes in 2003 (M W 6.2) associated with a fault segment in the northwestern part of the Island, and in 2015 (M W 6.5), associated with the adjacent southern fault segment, both aligned along the western coastline. The close temporal proximity along with the ongoing high seismic activity motivated our investigation. Microseismic data were relocated over a period of more than 4 years, following the 2015 mainshock resulting in a comprehensive seismic catalog and the determination of 110 focal mechanisms for the stronger events. A density-based clustering algorithm (DBSCAN) was applied to the relocated hypocenters and focal mechanisms, enabling improved association of seismicity patterns with the kinematic properties of the fault zone. The effects of the 2015 mainshock on regional seismicity were assessed through coseismic static stress changes, calculated using a variable slip model derived from a joint inversion of regional and teleseismic seismic data, combined with geodetic displacement estimates. Stress inversion of the calculated focal mechanisms identified faults prone to instability, which were used as receiver faults in the stress change analysis. The results reveal that post-2015 seismicity was strongly influenced by the mainshock, with clusters aligning along the primary fault zone as well as secondary structures within a rather narrow faulting zone. These findings enhance understanding of the seismic behavior of the Kefalonia Transform Fault Zone and its secondary faults, providing critical insights for seismic hazard assessment in this tectonically active region. • Relocated microseismicity reveals active fault structures in Lefkada Island. • Clustering of earthquakes and focal mechanisms aids structural interpretation. • Joint inversion yields a variable slip model for the 2015 Mw6.5 mainshock. • Static stress changes explain post-seismic clusters along fault structures.

This study deals with the problem of optimizing the geometry and motion of a two-segment articulated ichthyoid propulsor for autonomous underwater vehicles. The considered propulsor mimics the undulating body and caudal fin motion of a swimming fish; thus, the thrust and sideforce that are generated exhibit undesirable oscillations. The formulas for these hydrodynamic forces, which were derived in the author's previous work, are presented. For selected values of the mean thrust and the swimming speed, two problems of minimizing the thrust (1) and the sideforce (2) variance are solved by systematically searching the set of feasible solutions. Two considered objective functions lead to quite different results regarding the optimal geometry and motion of the propulsor. The propulsor minimizing the thrust variance should have the first segment shorter than the second one, and the propulsive fin should spread over almost the entire length of the second segment. When the objective is minimizing the sideforce variance, the first segment should be described by a length greater and the amplitude smaller than that of the second segment, and the propulsive fin should be small. For both objective functions, the optimal motion of the propulsor strongly depends on the swimming speed and generated thrust. Generating greater thrust at higher swimming speeds requires reducing the vibration period.

The aggregation behavior of dipyrrolonaphthyridinedione (DPND) chromophores in the solid state critically determines their optoelectronic properties. Here, we investigate how systematic variation in the side-chain geometry─specifically the branching point and steric profile─governs molecular packing and excitonic coupling. Using crystal structure prediction (CSP) combined with experimental GIWAXS and solid-state NMR, we obtain the packing geometry and crystal structure for three DPND derivatives (DPND-iPr, DPND-EtPr, and DPND-iBu). The results reveal that side-chain branching at the first carbon atom promotes herringbone packing and J-type behavior, while branching at the second carbon induces brick-wall stacking and H-type behavior in the solid state. Optical simulations based on the Holstein exciton-vibrational Hamiltonian reproduce experimental absorption and photoluminescence spectra, confirming the transition from J-like to H-like photophysics as the side-chain branching position shifts. This study demonstrates that fine-tuning alkyl side-chain geometry enables rational control of aggregation and excitonic behavior in cross-conjugated DPNDs, providing new design principles for functional organic semiconductors.