This study seeks to provide physical insight into the friction-driven crawling locomotion of systems with radially symmetric bodies. Laboratory experiments with a tripedal robot show that both translation and rotation can be achieved with just three independently actuated rigid limbs, i.e., with 3 degrees-of-freedom. These observations are rationalized using a simple mathematical model, which assumes that the friction at each limb is linearly proportional to the normal force at the contact point, and opposes the direction of motion. This dynamic model reproduces experimental observations across an extensive parametric sweep involving sinusoidal rotation of the limbs with varying amplitudes and phase shifts. Model predictions highlight the role played by time-varying normal forces at the contact points. These predictions are confirmed using embedded force transducers in the limbs. We present a further simplified analysis explaining that a geometric nonlinearity is induced in the dynamics from the radial symmetry and that this nonlinearity is essential to the generation of pure translation. We also show that this nonlinearity can be amplified by a cyclic time-varying limb length variation. These results provide a framework for further study of radially symmetric movers.

Many existing rigid underwater robots use propellers to create thrust. These traditional robots are widely used because they are capable of moving large payloads at high speeds through open water. However, the movement patterns exhibited by soft underwater organisms suggest that there are alternative locomotion methods that robots can use to be more efficient, less disruptive to the environment, and to navigate confined spaces more easily than current rigid robots. Biological strategies that have survived through the evolutionary process can be used as inspiration to create robust locomotion of robots using soft materials. This chapter reviews how soft, underwater organisms change their shapes to enhance motion, catalogues how researchers have used soft active materials to mimic these abilities, and discusses how recent work uses optimization to produce more effective underwater robotic locomotion. We categorize underwater locomotion into crawling and swimming, where crawling uses contact with a solid substrate for moving through the fluid, and swimming relies purely on hydrodynamic forces for propulsion. We summarize the main categories of actuators for soft robotics with a focus on how these actuators can be used to create shape-changing mechanisms. By further categorizing the state-of-the-art in bioinspired robotic locomotion for different types of crawling and swimming motions, we identify future avenues for research. We also detail how optimization can be used to design robot components and behaviors, and to explore how active online optimization of design parameters can improve robot performance. The biological inspirations and engineering applications described in this chapter form the basis for the design of soft underwater robots that locomote effectively by creating favorable interactions with fluid environments.

This paper presents a novel omnidirectional gait design and feedback control of a radially symmetric tripedal friction-driven robot. The robot features 3 servo motors mounted on a 3-D printed chassis 7 cm from the center of mass and separated 120 degrees. These motors drive limbs, which impart frictional reactive forces on the body. We first introduce a mathematical model for the robot motion, then show experimental observations performed on a uniform friction surface, which validated the accuracy of the model. This model was then used to create an omnidirectional gait that allows the robot to translate in any direction. Based on this gait, we also introduce a Proportional-Integral (PI) feedback control framework that enables the robot to closely follow a desired path. Contrasting with feedforward motion generation, the proposed feedback controller reduced the tracking error by approximately 46%. We have successfully demonstrated the approach in our robot hardware for the problem of line following using live feedback from an overhead tracking camera. Our controller is also able to correct for aerodynamic disturbances generated by a high-volume industrial fan with a mean flow speed of 5.5ms −1 , reducing path error by 65% relative to the basic position update procedure.

Intertidal sea stars often function in environments with extreme hydrodynamic loads that can compromise their ability to remain attached to surfaces. While behavioral responses such as burrowing into sand or sheltering in rock crevices can help minimize hydrodynamic loads, previous work shows that sea stars also alter body shape in response to flow conditions. This morphological plasticity suggests that sea star body shape may play an important hydrodynamic role. In this study, we measured the fluid forces acting on surface-mounted sea star and spherical dome models in water channel tests. All sea star models created downforce, i.e., the fluid pushed the body towards the surface. In contrast, the spherical dome generated lift. We also used Particle Image Velocimetry (PIV) to measure the midplane flow field around the models. Control volume analyses based on the PIV data show that downforce arises because the sea star bodies serve as ramps that divert fluid away from the surface. These observations are further rationalized using force predictions and flow visualizations from numerical simulations. The discovery of downforce generation could explain why sea stars are shaped as they are: the pentaradial geometry aids attachment to surfaces in the presence of high hydrodynamic loads.

Two light beams with different colors indicate there are two lasers of different wavelengths in the 3D printer to improve printing efficiency, as presented in article number 1800638 by Wei Wu and co-workers. The denticle array being printed by the lasers is the structure printed in this work and was inspired by shark skin texture with fluid drag reduction function.

Existing platforms for underwater exploration and inspection are often limited to traversing open water and must expend large amounts of energy to maintain a position in flow for long periods of time. Many benthic animals overcome these limitations using legged locomotion and have different hydrodynamic profiles dictated by different body morphologies. This work presents an underwater legged robot with soft legs and a soft inflatable morphing body that can change shape to influence its hydrodynamic characteristics. Flow over the morphing body separates behind the trailing edge of the inflated shape, so whether the protrusion is at the front, center, or back of the robot influences the amount of drag and lift. When the legged robot (2.87 N underwater weight) needs to remain stationary in flow, an asymmetrically inflated body resists sliding by reducing lift on the body by 40% (from 0.52 N to 0.31 N) at the highest flow rate tested while only increasing drag by 5.5% (from 1.75 N to 1.85 N). When the legged robot needs to walk with flow, a large inflated body is pushed along by the flow, causing the robot to walk 16% faster than it would with an uninflated body. The body shape significantly affects the ability of the robot to walk against flow as it is able to walk against 0.09 m/s flow with the uninflated body, but is pushed backwards with a large inflated body. We demonstrate that the robot can detect changes in flow velocity with a commercial flow sensor and respond by morphing into a hydrodynamically preferable shape.

Additive manufacturing has many advantages in creating highly complex customized structures. In this study, a low‐cost multiscale stereolithography technology that can print a macroscale object with microscale surface structures with high throughput is demonstrated. The developed multiscale stereolithography is realized by dynamic switching of laser spot size and adaptively sliced layer thickness. An optical filter based on subwavelength resonance grating is used to modify laser spot size for lasers with different wavelengths and achieves a maximum resolution of 37 µm. The multiscale stereolithography process has 4.4× throughput improvement compared with the traditional stereolithography process with a single laser spot. For proof‐of‐concept testing, artificial shark skins with microriblet features are designed and 3D printed. In pipe flow experiments, the 3D printed shark skin demonstrates almost 10% average fluid drag reduction. Artificial lotus leaf surfaces are also 3D printed to demonstrate superhydrophobic property. This new process has the potential to serve as a powerful tool that can bring bioinspired structures into real‐life applications.

Boiling influences many industrial processes like quenching, desalination and steam generation. Boiling heat transfer at high temperatures is limited by the formation of a vapor layer between the solid and fluid. Low thermal conductivity of this vapor layer inhibits heat transfer. Electrowetting (EW) fields can breakdown this vapor layer to promote wetting, and this concept works for many quenching media including water and organic solvents. This work studies the suppression of this vapor layer and measures the resulting heat transfer enhancement during quenching of metals. We image the fluid-surface interactions and boiling patterns in the presence of an electrical voltage. EW fields replace film boiling with periodic wetting-rewetting cycles and thus fundamentally change the heat transfer mode. The increased wettability substantially reduces the cool down time. The cooling rate can by increased by as much as 3X. The results show that electric fields can dynamically tune the classical quenching curve. This study opens up new avenues to control the metallurgy of metals via electrical control of the cooling rate.

In our work, the new stereolithography technology has been used to construct 3D shark skin teeth with both high resolution and high throughput. In this way complex structures more closely mimicking real shark skin textures have been fabricated (Figure 3-4). Cylinder pipe (at the same size) with and without interior shark skin textures were printed and used to study the effects of the texture in pipe flow. The pressure gradient across shark skin pipe section are measured by existing wet-wet differential pressure transducers. Then this pressure drop measurement can be translated into a friction coefficient by using standard Darcy-Weisbach law. A schematic of the pressure drop measurement setup is shown in Figure 5 and future detailed data analysis will be presented.

Boiling heat transfer impacts the performance of various industrial processes like quenching, desalination and steam generation. At high temperatures, boiling heat transfer is limited by the formation of a vapor layer at the solid-liquid interface (Leidenfrost effect), where the low thermal conductivity of the vapor layer inhibits heat transfer. Interfacial electrowetting (EW) fields can disrupt this vapor layer to promote liquid-surface wetting. This concept works for a variety of quenching media including water and organic solvents. We experimentally analyze EW-induced disruption of the vapor layer, and measure the resulting enhanced cooling during quenching. Imaging is employed to visualize the fluid-surface interactions and understand boiling patterns in the presence of an electrical voltage. It is seen that EW fundamentally changes the boiling pattern, wherein, a stable vapor layer is replaced by intermittent wetting of the surface. This switch in the heat transfer mode substantially reduces the cool down time. An order of magnitude increase in the cooling rate is observed. An analytical model is developed to extract instantaneous voltage dependent heat transfer rates from the cooling curve. The results show that electric fields can alter and tune the traditional cooling curve. Overall, this study presents a new concept to control the mechanical properties and metallurgy, by electrical control of the quench rate.

Heat transfer associated with boiling degrades at elevated temperatures due to the formation of an insulating vapor layer at the solid-liquid interface (Leidenfrost effect). Interfacial electrowetting (EW) fields can disrupt this vapor layer to promote liquid-surface wetting. We experimentally analyze EW-induced disruption of the vapor layer and measure the resulting enhanced cooling during the process of quenching. Imaging is employed to visualize the fluid-surface interactions and understand boiling patterns in the presence of an electrical voltage. It is seen that EW fields fundamentally change the boiling pattern, wherein a stable vapor layer is replaced by intermittent wetting of the surface. Heat conduction across the vapor gap is thus replaced with transient convection. This fundamental switch in the heat transfer mode significantly accelerates cooling during quenching. An order of magnitude increase in the cooling rate is observed, with the heat transfer seen approaching saturation at higher voltages. An analytical model is developed to extract voltage dependent heat transfer rates from the measured cooling curve. The results show that electric fields can alter and tune the traditional cooling curve. Overall, this study presents an ultralow power consumption concept to control the mechanical properties and metallurgy, by electrically tuning the cooling rate during quenching.