Harmful algae blooms (HAB) in eutrophic lakes and rivers have become serious water quality problems that are difficult to eliminate using common methods. Previous research has demonstrated that powerful ultrasound can somewhat control cyanobacteria in HABs; however, effective and energy-efficient settings for ultrasonic parameters have not yet been rigorously determined. The results of this study showed that the effect of cyanobacteria removal was enhanced with ultrasonic frequencies, densities, and radiation durations of 20–90 kHz, 0.0005–0.1 W/mL and 0.5–10 min, respectively. Our analyses further demonstrated that the effective distance of ultrasound decreased with increasing frequency, and that damaged algae cells were able to repair themselves at low ultrasonic densities. To address the high energy consumption and small effective distance of conventional ultrasonic radiation treatments, we proposed a new cyanobacteria removal method based on two applications of low-frequency, low-density and short-duration ultrasonic radiation. We defined the energy effectiveness factors of ultrasonic radiation for algae removal as the algae removal rate divided by ultrasonic dosage. This method yielded an 87.6% cyanobacteria removal and the highest energy effectiveness factor, suggesting that two cycles of treatment provide a low-energy method for enhancing existing algae-removing technologies used in large bodies of water.

The locomotive ability of a water strider that generates an ellipse-like spatial stroke trajectory when moving is proved to improve its moving efficiency and is seldom imitated by water strider robots. A hydrodynamic analysis can predict the performance of the robots, but research in this area is not enough. A buoyancy-supported water strider robot with the ability of producing an ellipse-like spatial stroke trajectory was designed and analyzed in this study. A semiempirical hydrodynamic model suitable for the buoyancy-supported robot was proposed to study the hydrodynamic characteristics, based on the Newton’s second law of motion. The speed, power and efficiency of the robot were analyzed on the basis of the model which confirmed that the robot performing an ellipse-like spatial stroke trajectory was conducive to its moving efficiency. The experimental results verified that the stroke mechanism produced an ellipse-like spatial trajectory in a practical application and that the robot had good mobility. The proposed hydrodynamic model was validated by comparing the theoretical speed of the robot to the experimental results.

The bionic pectoral fin serves as the primary propulsion component of ray-inspired robots. In our previous research, a motion equation was proposed for the real pectoral fin, which can be modeled as a series of NACA airfoil-shaped cross-sections distributed along the spanwise direction. Each cross-section undergoes two coupled rotational motions about its chord line and spanwise rotational axis. To achieve this type of motion, this article introduces a novel bionic pectoral fin mechanism driven by a series of differential gear units. The differential unit generates two coupled rotational motions corresponding to the cross-section of the pectoral fin in motion. A series of interconnected differential units provides a unique topology for the bionic mechanism and can generate a diverse range of motions. Through kinematic analysis, the motion equation was mapped onto the rotational angles of motors in the differential units. The proposed bionic mechanism was then fabricated and subjected to experimental test, demonstrating its effectiveness with a maximum thrust of 0.71 N. The distinctive structure of this bionic mechanism differentiates it from conventional designs and is expected to provide some inspiration for bionic pectoral fins and ray-inspired robots.