Tiefeng Li

Harnessing snap-through instability in soft dielectrics to achieve giant voltage-triggered deformation

A soft dielectric membrane is prone to snap-through instability. We present theory and experiment to show that the instability can be harnessed to achieve giant voltage-triggered deformation. We mount a membrane on a chamber of a suitable volume, pressurize the membrane into a state near the verge of the instability, and apply a voltage to trigger the snap without causing electrical breakdown. For an acrylic membrane we demonstrate voltage-triggered expansion of area by 1692%, far beyond the largest value reported in the literature. The large expansion can even be retained after the voltage is switched off.

Dielectric Elastomer Generators: How Much Energy Can Be Converted?

Dielectric elastomers are being developed as generators to harvest energy from renewable sources, such as human movements and ocean waves. We model a generator as a system of two degrees of freedom, represented on either the stress-stretch plane or the voltage-charge plane. A point in such a plane represents a state of the generator, a curve represents a path of operation, a contour represents a cycle of operation, and the area enclosed by the contour represents the energy of conversion per cycle. Each mechanism of failure is represented by a curve in the plane. The curves of all the known mechanics of failure enclose the region of allowable states. The area of this region defines the maximum energy of conversion. This study includes the following mechanisms of failure: material rupture, loss of tension, electrical breakdown, and electromechanical instability. It is found that natural rubber outperforms VHB elastomer as a generator at strains less than 15%. Furthermore, by varying material parameters, energy of conversion can be increased above 1.0 J/g.

Giant, voltage-actuated deformation of a dielectric elastomer under dead load

Far greater voltage-actuated deformation is achievable for a dielectric elastomer under equal-biaxial dead load than under rigid constraint usually employed. Areal strains of 488% are demonstrated. The dead load suppresses electric breakdown, enabling the elastomer to survive the snap-through electromechanical instability. The breakdown voltage is found to increase with the voltage ramp rate. A nonlinear model for viscoelastic dielectric elastomers is developed and shown to be consistent with the experimental observations.

Method for measuring energy generation and efficiency of dielectric elastomer generators

Dielectric elastomer generators convert mechanical into electrical energy at high energy density, showing promise for large and small scale energy harvesting. We present an experiment to monitor electrical and mechanical energy flows separately and show the cycle of energy conversion in work-conjugate planes. A specific electrical energy generated per cycle of 102mJ/g, at a specific average power of 17mW/g, is demonstrated with an acrylic elastomer in a showcase generation cycle. The measured mechanical to electrical energy conversion efficiency is 7.5%. The experiment may be used to assess the aptitude of specifically designed elastomers for energy harvesting.

Supramolecular Lego Assembly Towards Three‐Dimensional Multi‐Responsive Hydrogels

Inspired by the assembly of Lego toys, hydrogel building blocks with heterogeneous responsiveness are assembled utilizing macroscopic supramolecular recognition as the adhesion force. The Lego hydrogel provides 3D transformation upon pH variation. After disassembly of the building blocks by changing the oxidation state, they can be re-assembled into a completely new shape.

Energy harvesting of dielectric elastomer generators concerning inhomogeneous fields and viscoelastic deformation

Dielectric elastomer generators convert mechanical work into electrical energy. Previous tests on membrane inflation elastomer generators, however, indicated rather low efficiency on energy harvesting. To characterize this phenomenon, an analytical model for viscoelastic dielectric elastomer generators is presented to maximize the energy conversion. The analysis is intended for inhomogeneous fields. The result indicates that viscoelasticity and instabilities during inflation and deflation degrade the efficiency of energy conversion and the specific electrical energy generated per cycle. Rapid loading and unloading, as well as appropriate pre-stretches, are found to upgrade the performances of the dielectric elastomer generators. The analysis may guide the design of dielectric elastomer generators.

Fast-moving soft electronic fish

A soft robotic fish can quickly swim and turn with a fully integrated onboard system for power and remote control. Soft robots driven by stimuli-responsive materials have unique advantages over conventional rigid robots, especially in their high adaptability for field exploration and seamless interaction with humans. The grand challenge lies in achieving self-powered soft robots with high mobility, environmental tolerance, and long endurance. We are able to advance a soft electronic fish with a fully integrated onboard system for power and remote control. Without any motor, the fish is driven solely by a soft electroactive structure made of dielectric elastomer and ionically conductive hydrogel. The electronic fish can swim at a speed of 6.4 cm/s (0.69 body length per second), which is much faster than previously reported untethered soft robotic fish driven by soft responsive materials. The fish shows consistent performance in a wide temperature range and permits stealth sailing due to its nearly transparent nature. Furthermore, the fish is robust, as it uses the surrounding water as the electric ground and can operate for 3 hours with one single charge. The design principle can be potentially extended to a variety of flexible devices and soft robots.

Snap-through Expansion of a Gas Bubble in an Elastomer

When a gas is injected into a bubble in an elastomer, the bubble may first expand gradually, and then snap suddenly to a large size. This snap-through instability is analyzed here using a model that accounts for both the surface tension and the limiting stretch of the elastomer. In a state of equilibrium, the pressure in the bubble counteracts three contributions: the ambient pressure due to the environment outside the elastomer, the Laplace pressure due to the surface tension of the elastomer, and the additional pressure due to the elasticity of the elastomer. The Laplace pressure is large for a small bubble, but falls as the bubble expands. The additional pressure due to elasticity increases as the bubble expands, and rises steeply when the surface of the bubble approaches the limiting stretch of the elastomer. We show that the bubble snaps only if a sufficient amount of gas can rush into the bubble at the onset of instability.

A bioinspired reversible snapping hydrogel assembly

Controlling the response for stimuli-responsive shape changing polymers is critically important for their device applications. The snapping transformation of the Venus Flytrap has inspired the design of shape changing devices with a unique controlling mechanism in mechanical instability, yet their practical potential has been quite limited due to the irreversible nature. Herein, we report an approach to achieve an unprecedented reversible snapping. The material system is a hydrogel assembly that can be mechanically programmed to exhibit instability based bi-stable states. Taking advantages of the multi-responsiveness of the hydrogels allows reversible switching between the two stable states in an abrupt non-continuous (snap) fashion, with unique benefits in precise time-delayed deployment, accelerated deployment speed, and enhanced output force. The mechanism behind our design can be readily extended beyond hydrogels to enhance the performances of diverse multifunctional smart devices.

Active Shape Control and Phase Coexistence of Dielectric Elastomer Membrane With Patterned Electrodes

Various applications of dielectric elastomers (DEs) have been realized in recent years due to their lightweight, low cost, large actuation and fast response. In this paper, experiments and simulations are performed on the active shape control of DE structures with various two-dimensional patterned electrodes by applying voltage. A DE membrane with a pattern of electrodes is mounted on an air chamber. It is first inflated by air pressure and then further deformed by applying voltage, which actively controls the membrane shape. Under higher voltage, an acrylic membrane with larger actuation can induce shape instability and demonstrate multiphase coexistence behavior. In the framework of electromechanical theory, finite element simulations are carried out and the results are in good agreement with those obtained by experiments.