10. Adhesion of flat-ended pillars with non-circular contacts
Fibrillar adhesives composed of fibers with non-circular cross-sections and contacts, including squares and rectangles, offer advantages that include larger real contact area when arranged in arrays and simplicity in fabrication. However, they typically have lower adhesion strength compared to circular pillars due a stress concentration at the corner of the non-circular contact. We investigate the adhesion of composite pillars with circular, square and rectangular crosssections each consisting of a stiff pillar terminated by a thin compliant layer at the tip. Finite element mechanics modeling is used to assess differences in the stress distribution at the interface for the different geometries and the adhesion strength of different shape pillars is measured in experiments. The composite fibrillar structure results in a favorable stress distribution on the adhered interface that shifts the crack initiation site away from the edge for all of the cross-sectional contact shapes studied. The highest adhesion strength achieved among the square and rectangular composite pillars with various tip layer thicknesses is approximately 65 kPa. This is comparable to the highest strength measured for circular composite pillars and is about 6.5× higher than the adhesion strength of a homogenous square or rectangular pillar. The results suggest that a composite fibrillar adhesive structure with a local stress concentration at a corner can achieve comparable adhesion strength to a fibrillar structure without such local stress concentrations if the magnitude of the corner stress concentrations are sufficiently small such that failure does not initiate near the corners, and the magnitude of the peak interface stress away from the edge and the tip layer thickness are comparable.
9. Uniform Conductivity in Stretchable Silicones via Multiphase Inclusions
Many soft robotic components require highly stretchable, electrically conductive materials for proper operation. Often these conductive materials are used as sensors or as heaters for thermally responsive materials. However, there is a scarcity of stretchable materials that can withstand the high strains typically experienced by soft robots, while maintaining the electrical properties necessary for Joule heating (e.g., uniform conductivity). In this project, we present a silicone composite containing both liquid and solid inclusions that can maintain a uniform conductivity while experiencing 200% linear strains. This composite can be cast in thin sheets enabling it to be wrapped around thermally responsive soft materials that increase their volume or stretchability when heated. We show how this material opens up possibilities for electrically controllable shape changing soft robotic actuators, as well as all-silicone actuation systems powered only by electrical stimulus. Additionally, we show that this stretchable composite can be used as an electrode material in other applications, including a strain sensor with a linear response up to 200% strain and near-zero signal noise.
Multiphase composite being simultaneously twisted and stretched, while maintaining a consistent, uniform Joule heating profile.
8. Switchable Adhesion via Subsurface Pressure Modulation
Materials and devices with tunable dry adhesion have many applications, including transfer printing, climbing robots, and gripping in pick-and-place processes. In this work, a novel soft device to achieve dynamically tunable dry adhesion via modulation of sub-surface pneumatic pressure is introduced. Specifically, a cylindrical elastomer pillar with a mushroom-shaped cap and annular chamber that can be pressurized to tune the adhesion is investigated. Finite element-based mechanics models and experiments are used to design, understand, and demonstrate the adhesion of the device. Specifically, the device is designed using mechanics modelling such that the pressure applied inside the annular chamber significantly alters the stress distribution at the adhered interface and thus changes the effective adhesion strength. Devices made of polydimethylsiloxane (PDMS) with different elastic moduli were tested against glass, silicon, and aluminum substrates. Adhesion strengths (σ0) ranging from ~37 kPa (between PDMS and glass) to ~67 kPa (between PDMS and polished aluminum) are achieved for the unpressurized state. For all cases, regardless of the material and roughness of the substrates, the adhesion strength dropped to 40% of the strength of the unpressurized state (equivalent to a 2.5× adhesion switching ratio) by increasing the chamber pressure from 0.3σ0 to 0.6σ0. Furthermore, the strength drops to 20% of the unpressurized strength (equivalent to a 5× adhesion switching ratio) when the chamber pressure is increased to σ0.
Principles of operation of the device.
By using compliant lightweight actuators with shape memory alloy, we created untethered soft robots that are capable of dynamic locomotion at biologically relevant speeds.
Untethered Soft Robots with Shape Memory Alloy Actuators.
Shape memory alloys (SMA) are popular as actuators for soft bio-inspired robots because they are naturally compliant, have high work density, and can be operated using miniaturized on-board electronics for power and control. However, SMA actuators typically exhibit limited bandwidth due to the long duration of time required for the alloy to cool down and return to its natural shape and compliance following electrical actuation. We address this by constructing SMA-based actuators out of thermally conductive elastomers and examining the influence of electrical current and actuation frequency on blocking force, bending amplitude, and operating temperature. The actuator is composed of a U-shape SMA wire that is sandwiched between layers of stretched and unstretched thermal elastomer. Based on our studies, we demonstrate the ability to create a highly dynamic soft actuator that weighs 3.7g, generates a force of ~0.2N, bends with curvature change of ~60 m–1 in 0.15 second, can be activated with a frequency above 0.3Hz, and requires a pair of miniature 3.7V lithium-polymer (LiPo) batteries for actuation. Together, these properties allow the actuator to be used as an “artificial muscle” for a variety of tethered and untethered soft robots capable of dynamic locomotion.
a) Schematic layout of the experimental setup for measuring the blocking force (Fb). The actuator is cyclically activated with electrical current and then allowed to cool. b) Plot of blocking force versus prestretch for various applied currents. C) Plot of temperature of the SMA wire measured at the end of each cycle for 150 cycles with various cooling times.
Like their natural counterparts, soft bioinspired robots capable of actively tuning their mechanical rigidity can rapidly transition between a broad range of motor tasks—from lifting heavy loads to dexterous manipulation of delicate objects. Reversible rigidity tuning also enables soft robot actuators to reroute their internal loading and alter their mode of deformation in response to intrinsic activation. In this study, we demonstrate this principle with a three-fingered pneumatic gripper that contains “programmable” ligaments that change stiffness when activated with electrical current. The ligaments are composed of a conductive, thermoplastic elastomer composite that reversibly softens under resistive heating. Depending on which ligaments are activated, the gripper will bend inward to pick up an object, bend laterally to twist it, and bend outward to release it. All of the gripper motions are generated with a single pneumatic source of pressure. An activation–deactivation cycle can be completed within 15 s. The ability to incorporate electrically programmable ligaments in a pneumatic or hydraulic actuator has the potential to enhance versatility and reduce dependency on tubing and valves.
Grasping, releasing, and twisting manipulations by two soft grippers, one with ‘‘shorter’’ distal phalanges and the other with ‘‘longer’’ distal phalanges.
4. Buckling shape transition of an embedded thin elastic rod after failure of surrounding elastic medium
When the compressive load to a thin elastic rod embedded in an elastic medium exceeds a threshold, the thin rod buckles into an exponentially decaying short wavelength profile to minimize the total energy of the system. As the compressive load continues to increase, the buckling amplitude increases correspondingly, until the rod/medium interface fractures and the short wavelength buckling profile morphs into a different shape as fracture propagates into the surrounding medium. In this study, such shape transition in the presence of surrounding medium failure is investigated using a combined experimental and theoretical approach. We identify the ansatz that can be used to describe the post-fracture buckling profile, and then develop a forward scheme using the energy principle to predict the buckling profile of the thin rod when fracture happens in the medium. We also develop a backward scheme where we use the post-fracture buckling profile to estimate the buckling profile before fracture of the surrounding medium. Comparison of experimental and theoretical results indicates that the modeling framework can be used to characterize the buckling profile transition of a thin elastic rod embedded in a fractured elastic medium.
(a) Buckling shape of a compressed wire embedded in gelatin before fracture. (b) Buckling shape of a compressed wire embedded in gelatin after fracture. (c) The deformation profile of the compressed wire in panel (a) and a fitted curve by Matlab. (d) The deformation profile of the compressed wire in panel (b) and a fitted curve by Matlab.
Tunable dry adhesion has a range of applications, including transfer printing, climbing robots, and gripping in automated manufacturing processes. Here, a novel concept to achieve dynamically tunable dry adhesion via modulation of the stiffness of subsurface mechanical elements is introduced and demonstrated. A composite post structure, consisting of an elastomer shell and a core with a stiffness that can be tuned via application of electrical voltage, is fabricated. In the nonactivated state, the core is stiff and the effective adhesion strength between the composite post and contact surface is high. Activation of the core via application of electrical voltage reduces the stiffness of the core, resulting in a change in the stress distribution and driving force for delamination at the interface and, thus a reduction in the effective adhesion strength. The adhesion of composite posts with a range of dimensions is characterized and activation of the core is shown to reduce the adhesion by as much as a factor of 6. The experimentally observed reduction in adhesion is primarily due to the change in stiffness of the core. However, the activation of the core also results in heating of the interface and this plays a secondary role in the adhesion change.
a) Principle of operation of the tunable adhesive. The composite post can (1–3) pick up a part in the high adhesion (nonactivated state), but (4) once activated, the stiffness of the core is reduced, and consequently, the adhesion is reduced, and (5) the part is released; red is activated with the CPBE strip in a compliant state, green is PDMS, and black is the nonactivated CPBE strip. b) Dimetric, front, and side views of the composite post. c) Schematic of electrical connection and photograph of a composite post sample, consisting of PDMS with an embedded U‐shaped CPBE core and copper wires for electrical connection.
Many biological and engineered systems can be modeled as buckled thin rods with constraints. Examples include microtubules in cytoskeleton, plant roots in soil, and oil pipes within a wellbore. However, most previous studies focused on the buckling of a rod in a homogeneous environment, an idealization which is often not realistic. Here, we study the buckling behaviors of an elastic rod embedded in a bilayer elastic matrix using a combined experimental, theoretical, and computational method. Our experiments showed, for the first time, that the buckling amplitude can increase from the end where the compressive load is applied. To interpret this new phenomenon, we built a theoretical model and identified an ansatz for the transverse displacement. Our numerical results showed that material inhomogeneity, geometry, and loading all have significant influences on the post-buckling behaviors of the rod. Moreover, our study indicated that the stiffer layer of the elastic medium can be treated as a clamped boundary. These results could find applications ranging from the penetration of needles through biological tissues to the development of underground structures.
Image of the experimental setup showing a compressed wire embedded in (a) 40 g/L (top) - 60 g/L (bottom) bilayer, (c) 60 g/L (top) - 40 g/L (bottom) bilayer, (e) 40 g/L single layer and (f) 60 g/L single layer . (b)/(d) The theoretical results (red), finite element simulation (blue) and experimental profile (black) of the wire in (a)/(c). (g) Schematic illustration of an elatic rod embedded within a bilayer elastic medium.
The pressure in the main natural gas transport pipelines should be reduced for proper consumption in vicinity of cities. A common procedure of reducing pressure in natural gas station (city gate station, CGS) is using expansion valves, which causes the waste of large amount of exergy (availability). In this paper a combined heat and power (CHP) system was used instead which included an expander, gas engines, boilers, a pump and a preheater. A new and relatively quick method for selecting the required number of gas engine/boiler, and determining their nominal power/heating capacity, as well as the expander efficiency are also presented. An objective function named actual annual benefit ($) was defined as the sum of income (from selling electricity) and expenses (such as investment cost, operation and maintenance costs). Subsequently different parts of the objective function were expressed in terms of 9 decision variables. The optimum values of decision variables were obtained by maximizing the objective function using genetic algorithm optimization technique. By applying the above procedure for our case study, it was obtained that two 5.48 (MW) gas engines and one 5.94 (MW) boiler was needed while the payback period was found to be 1.23 (year).
The schematic diagram of the studied pressure reduction station.