The ability to tune the mechanical stiffness between a soft state and a rigid state is essential for various living systems to navigate nature. Examples for this range from muscle-powered motor tasks and sexual reproduction, to spontaneous change in shape for predator evasion. [1] Similar to their natural counterparts, engineered materials with tunable properties including mechanical stiffness have the potential to be used in a broad range of engineering applications. [1,2] Structures made with these materials can change their mechanical rigidity in static or dynamic systems to extend their workspace. [1-4] Multiple strategies have been pursued recently to achieve stiffness tunability, including pneumatic jamming, [3,5,6] chemical interactions, [7] opposing actuator structures, [8-10] magnetorheological fluids, [11,12] external/internal heating of materials with phase change [13-24] or glass transition, [25-28] or through combinations of these techniques. [13,29] Among phase-changing materials, low melting point alloys (LMPA) have been used widely as they are highly electrically conductive, rigid as metal at room temperature, and their melting point can be as low as 47.2 C [21] or 62.0 C. [20] LMPA layers, channels, foams, lattices, and particles have been incorporated as fillers into soft elastomers and shape memory polymers to create engineering materials with stiffness tunability. [13,17,18,20-23] In addition to tuning mechanical stiffness, LMPA fillers can also enhance the thermal and electrical properties of the composites. [18,30,31] Another example of smart composites with tunable stiffness containing phase-changing components is conductive propylene-based elastomers (CPBE), [24] which have a propylene-ethylene copolymer elastomer matrix and dispersed
Materials and devices with tunable dry adhesion have many applications, including transfer printing, climbing robots, and gripping in pick-and-place processes. In this paper, a novel soft device to achieve dynamically tunable dry adhesion via modulation of subsurface 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 modeling 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 nonpressurized state. For all cases, regardless of the material and roughness of the substrates, the adhesion strength dropped to 40% of the strength of the nonpressurized 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.
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