A novel cylindrical overlap-and-fling mechanism used by sea butterflies

A bioinspired multimotion modality underwater microrobot

Exploring narrow underwater environments presents notable challenges due to the need for flexible movement and robust transition between different motion modalities. Inspired by the pteropod, a small aquatic organism known for its ability to change direction by adjusting the attack angle of its wings, we developed a biomimetic robotic platform called RoboPteropod. This micro-underwater robot, equipped with flexible flapping wings that mimic the propulsive mechanisms of the pteropod, reaches a float velocity of 1.88 body height per second and a forward velocity of 1.2 body length per second, while maintaining a power consumption of merely 580 milliwatts. The ability to dynamically adjust the attack angle of the wings enables smooth transition among various modes (float, move straight, yaw, and pitch) of underwater locomotion, allowing for agile, three-dimensional maneuvering in complex aquatic environments. RoboPteropod offers meaningful potential for detailed exploration of confined and otherwise inaccessible underwater spaces.

An inverse problems approach to micro-PIV for measuring flow around freely flying tiny insects

Brightfield micro-particle image velocimetry (micro-PIV) has traditionally been limited to aqueous media and by a poor signal to noise ratio. Here, we introduce a brightfield micro-PIV system suitable for measuring the 2D flows generated by freely flying sub-millimeter insects while simultaneously measuring the 3D wing and body kinematics. Our methodology couples a novel aerosolization system and an inverse problems approach to image preprocessing to alleviate these limitations. Using optimization, the inverse problems approach obtains each particle's position relative to the focal plane and generates a synthetic image comprising the in-focus and nearly in-focus particles and excluding noise from out-of-focus particles. We find that a 0.85 mm tobacco whitefly (Bemisia tabaci) utilizes a deep U-shaped wingtip trajectory to generate a 0.5 m s-1 downward jet as the wings clap together. Our technique can validate numerical simulations of tiny insect flight and measure the aerodynamics of various insect species exhibiting high morphological diversity.

Spatiotemporal Asymmetry in Metachronal Rowing at Intermediate Reynolds Numbers

In drag-based swimming, individual propulsors operating at low Reynolds numbers (where viscous forces dominate over inertial forces) must execute a spatially asymmetric stroke to produce net fluid displacement. Temporal asymmetry (that is, differing duration between the power vs. recovery stroke) does not affect the overall generated thrust in this time-reversible regime. Metachronal rowing, in which multiple appendages beat sequentially, is used by a wide variety of organisms from low to intermediate Reynolds numbers. At the upper end of this range, inertia becomes important, and increasing temporal asymmetry can be an effective way to increase thrust. However, the combined effects of spatial and temporal asymmetry are not fully understood in the context of metachronal rowing. To explore the role of spatiotemporal asymmetry in metachronal rowing, we combine laboratory experiments and reduced-order analytical modeling. We measure beat kinematics and generated flows in two species of lobate ctenophores across a range of body sizes, from 7 mm to 40 mm in length. We observe characteristically different flows in ctenophores of differing body size and Reynolds number, and a general decrease in spatial asymmetry and increase in temporal asymmetry with increasing Reynolds number. We also construct a one-dimensional mathematical model consisting of a row of oscillating flat plates whose flow-normal areas change with time, and use it to explore the propulsive forces generated across a range of Reynolds numbers and kinematic parameters. The model results show that while both types of asymmetry increase force production, they have different effects in different regions of the parameter space. These results may have strong biological implications, as temporal asymmetry can be actively controlled while spatial asymmetry is likely to be partially or entirely driven by passive fluid-structure interaction.