Effects of Nonlinearity of Restoring Springs on Propulsion Performance of Wave Glider

Wave glider is an unmanned surface vehicle which can directly convert wave energy into forward propulsion and fulfill long-term marine monitoring. Previous study suggested that the wave motion and stiffness of restoring springs mounted on the hydrofoil are main factors affecting the propulsion performance of wave glider. In this paper, the dynamic responses and nonlinear characteristics of underwater propulsion mechanism considering the nonlinear stiffness of restoring springs are investigated based on a fluid-rigid body coupled model. Firstly, the models of propulsion mechanism with different kind of restoring spring are proposed, and the linear and nonlinear characteristics of restoring spring are considered. Then, a fluid-rigid body coupled model of wave glider is developed by coupling the rigid body dynamics model and hydrodynamic model. Dynamic responses are simulated by numerical analysis method and the nonlinear characteristics with different restoring springs are illustrated by time/frequency domain motion response and phase diagram analysis. The effects of wave excitation frequency and wave heights on the propulsion performance of wave glider are analyzed. The results show that, multi-frequency responses occurred in propulsion system. And the study suggests that the nonlinear restoring spring on the hydrofoil can be suitable for different sea condition and better propulsion performance can obtained than linear stiffness spring, which provides a reference for developing propulsion mechanism with high performance in complex marine environment.

A Novel Push-pull Central-lever Mechanism Reduces Peak Forces and Energy-cost Compared to Hand-rim Wheelchair Propulsion During a Controlled Lab-based Experiment

Background: Hand-rim wheelchair propulsion is straining and mechanically inefficient, often leading to upper limb complaints. Previous push-pull lever propulsion mechanisms have shown to perform better or equal in efficiency and physiological strain. Propulsion biomechanics have not been evaluated thus far. A novel push-pull central-lever propulsion mechanism is compared to conventional hand-rim wheelchair propulsion, using both physiological and biomechanical outcomes under low-intensity steady-state conditions on a motor driven treadmill. Methods: In this 5-day (distributed over a maximum of 21 days) between-group experiment, 30 able-bodied novices performed 60 minutes (5x3x4 min) of practice in either the push-pull central lever wheelchair (n=15) or the hand-rim wheelchair (n=15). At the first and final sessions cardiopulmonary strain, propulsion kinematics and force production were determined in both instrumented propulsion mechanisms. Repeated measures ANOVA evaluated between (propulsion mechanism type), within (over practice) and interaction effects. Results: Over practice, both groups significantly improved on all outcome measures. After practice the peak forces during the push and pull phase of lever propulsion were considerably lower compared to those in the handrim push phase (42±10 & 46±10 vs 63±21 N). Concomitantly, energy expenditure was found to be lower as well (263±45 vs 298±59 W), on the other hand gross mechanical efficiency (6.4±1.5 vs 5.9±1.3 %), heart-rate (97±10 vs 98±10 bpm) and perceived exertion (9±2 vs 10±1) were not significantly different between modes.Conclusion: The current study shows the potential benefits of the newly designed push-pull central-lever propulsion mechanism over regular hand rim wheelchair propulsion. The much lower forces and energy expenditure might help to reduce the strain on the upper extremities and thus prevent the development of overuse injury. This proof of concept in a controlled laboratory experiment warrants continued experimental research in wheelchair-users during daily life.

Self-adaptive and efficient propulsion of Ray sperms at different viscosities enabled by heterogeneous dual helixes

We disclose a peculiar rotational propulsion mechanism of Ray sperms enabled by its unusual heterogeneous dual helixes with a rigid spiral head and a soft tail, named Heterogeneous Dual Helixes (HDH) model for short. Different from the conventional beating propulsion of sperm, the propulsion of Ray sperms is from both the rotational motion of the soft helical tail and the rigid spiral head. Such heterogeneous dual helical propulsion style provides the Ray sperm with high adaptability in viscous solutions along with advantages in linearity, straightness, and bidirectional motion. This HDH model is further corroborated by a miniature swimming robot actuated via a rigid spiral head and a soft tail, which demonstrates similar superiorities over conventional ones in terms of adaptability and efficiency under the same power input. Such findings expand our knowledge on microorganisms’ motion, motivate further studies on natural fertilization, and inspire engineering designs.

Leidenfrost droplet trampolining

A liquid droplet dispensed over a sufficiently hot surface does not make contact but instead hovers on a cushion of its own self-generated vapor. Since its discovery in 1756, this so-called Leidenfrost effect has been intensively studied. Here we report a remarkable self-propulsion mechanism of Leidenfrost droplets against gravity, that we term Leidenfrost droplet trampolining. Leidenfrost droplets gently deposited on fully rigid surfaces experience self-induced spontaneous oscillations and start to gradually bounce from an initial resting altitude to increasing heights, thereby violating the traditionally accepted Leidenfrost equilibrium. We found that the continuously draining vapor cushion initiates and fuels Leidenfrost trampolining by inducing ripples on the droplet bottom surface, which translate into pressure oscillations and induce self-sustained periodic vertical droplet bouncing over a broad range of experimental conditions.

Impact of surface charge on the motion of light-activated Janus micromotors

Control over micromotors’ motion is of high relevance for lab-on-a-chip and biomedical engineering, wherein such particles encounter complex microenvironments. Here, we introduce an efficient way to influence Janus micromotors’ direction of motion and speed by modifying their surface properties and those of their immediate surroundings. We fabricated light-responsive Janus micromotors with positive and negative surface charge, both driven by ionic self-diffusiophoresis. These were used to observe direction-of-motion reversal in proximity to glass substrates for which we varied the surface charge. Quantitative analysis allowed us to extract the dependence of the particle velocity on the surface charge density of the substrate. This constitutes the first quantitative demonstration of the substrate’s surface charge on the motility of the light-activated diffusiophoretic motors in water. We provide qualitative understanding of these observations in terms of osmotic flow along the substrate generated through the ions released by the propulsion mechanism. Our results constitute a crucial step in moving toward practical application of self-phoretic artificial micromotors. Graphic abstract