A combination of experiment and theory shows that dielectric elastomers exhibit complex interplay of nonlinear processes. Membranes of a dielectric elastomer are prepared in various states of prestretches by using rigid clamps and mechanical forces. Upon actuation by voltage, some membranes form wrinkles followed by snap-through instability, others form wrinkles without the snap-through instability, and still others fail by local instability without formingwrinkles.Membranes surviving these nonlinear processes are found to attain a constant dielectric strength, independent of the state of prestretches. Giant voltage-induced stretch of 3.6 is attained.

Choon Chiang Foo, Soo Jin Adrian Koh, Christoph Keplinger, Rainer Kaltseis, Siegfried Bauer, Zhigang Suo.

Performance of dissipative dielectric elastomer generators.
Journal of Applied Physics 111, 094107 (2012)

A membrane of a dielectric elastomer deforms when a voltage is applied through its thickness. The achievable voltage-induced deformation is strongly affected by how mechanical loads are applied. Large voltage-induced deformation has been demonstrated for a membrane under equal-biaxial forces, but only small voltage-induced deformation has been observed for a membrane under a uniaxial force. This difference is interpreted here theoretically. The theory also predicts that, when the deformation of a membrane is constrained in one direction, a voltage applied through the thickness of the membrane can cause it to deform substantially in the other direction. Experiments are performed on membranes under equal-biaxial forces and uniaxial forces, as well as on fiber-constrained membranes of two types: a dielectric elastomer membrane with carbon fibers on both faces, and two dielectric elastomer membranes sandwiching nylon fibers. The experimental observations are compared with the theory.

Sigurd Wagner and Siegfried Bauer have just edited an exciting issue of the MRS Bulletin: Materials for Stretchable Electronics.   All articles contain phenomena related to mechanics.  Here is the abstract of my contribution:  Mechanics of Stretchable Electronics and Soft Machines.  In the emerging field of soft machines, large deformation of soft materials is harnessed to provide functions such as regulating flow in microfluidics, shaping light in adaptive optics, harvesting energy from ocean waves, and stretching electronics to interface with living tissues. Soft materials, however, do not provide all of the requisite functions; rather, soft machines are mostly hybrids of soft and hard materials. In addition to requiring stretchable electronics, soft machines often use soft materials that can deform in response to stimuli other than mechanical forces. Dielectric elastomers deform under a voltage. Hydrogels swell in response to changes in humidity, pH, temperature, and salt concentration. How does mechanics meet geometry, chemistry, and electrostatics to generate large deformation? How do molecular processes affect the functions of transducers? How efficiently can materials convert energy from one form to another?  These questions are stimulating intriguing and useful advances in mechanics. This review highlights the mechanics that enables the creation of soft machines.

Choon Chiang Foo, Shengqiang Cai, Soo Jin Adrian Koh, Siegfried Bauer, Zhigang Suo.

Model of dissipative dielectric elastomers.
Journal of Applied Physics 111, 034102 (2012).

Abstract

Submerged in a solvent-containing environment and subject to applied forces, a covalent polymer network absorbs the solvent and deforms, forming an elastomeric gel.  The equations of state are derived under two assumptions.  First, the amount of the solvent in the gel varies when the gel changes volume, but remains constant when the gel changes shape.  Second, the Helmholtz free energy of the gel is separable into the contribution due to stretching the network and that due to mixing the polymer and the solvent.  We demonstrate that these equations of state fit several sets of experimental data in the literature remarkably well.

The paper will be published in EPL and can be downloaded from

The creasing instability of elastomer films under compression is studied by a combination of experiment and numerical simulation.  Experimentally, we attach a stress-free film on a much thicker and stiffer pre-stretched substrate.  When the substrate is partially released, the film is uniaxially compressed, leading to formation of an array of creases beyond a critical strain.  The profile of the folded surface is extracted using confocal fluorescence microscopy, yielding the depths, spacings, and shapes of creases.  Numerically, the onset and development of creases are simulated by introducing appropriately sized defects into a finite-element mesh and allowing the surface of the film to self-contact.  The measurements and simulations are found to be in excellent agreement.

The 26th issue of the WW-EAP Newsletter is now available at: 

http://ndeaa.jpl.nasa.gov/nasa-nde/newsltr/WW-EAP_Newsletter13-2.pdf

This newsletter on the field of electroactive polymers, edited by Dr. Yoseph Bar-Cohen, addresses the need for rapid communication of progress in the field and state-of-the-art capabilities.

For a dielectric elastomer membrane we show giant voltage-triggered expansion of area by 1692%, far beyond the largest values reported in the literature.

For a dielectric elastomer membrane we show giant voltage-triggered expansion of area by 1692%, far beyond the largest values reported in the literature.

Abstract:A hydrostatically coupled dielectric elastomer (HCDE) actuator consists of two membranes of a dielectric elastomer, clamped with rigid circular rings.  Confined between the membranes is a fixed volume of a fluid, which couples the movements of the two membranes when a voltage or a force is applied.  This paper presents a computational model of the actuator, assuming that the membranes are neo-Hookean, capable of large and axisymmetric deformation.  The voltage-induced deformation is described by the model of ideal dielectric elastomer.&nb

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 102 mJ/g , at a specific average power of 17 mW/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.

This paper is in press at APL.

We develop a method of poroelastic relaxation indentation (PRI) to characterize thin layers of gels.  The solution to the time-dependent boundary-value problem is obtained in a remarkably simple form, so that the force-relaxation curve obtained by indenting a gel readily determines all the poroelastic constants of the gel—the shear modulus, Poisson’s ratio, and the effective diffusivity.  The method is demonstrated with a layer of polydimethylsiloxane immersed in heptane.

The paper is accepted for publication by J. Appl. Phys, and can be downloaded from: http://www.seas.harvard.edu/suo/papers/254.pdf

I gave a seminar at Xian Jiaotong University on 27 October 2009.  I recently found the video of the seminar online.  The seminar was in Chinese, but the slides were in English.

• Video of the seminar
• Slides of the seminar  

If the subject interests you, the following papers will lead you to the literature.

We use instrumented indentation to characterize the mechanical and transport behavior of a pH-sensitive hydrogel in various aqueous buffer solutions. In the measurement an indenter is pressed to a fixed depth into a hydrogel disk and the load on the indenter is recorded as a function of time. By analyzing the load-relaxation curve using the theory of poroelasticity, the elastic constants of the hydrogel and the diffusivity of water in the gel can be evaluated. We investigate how the pH and ionic strength of the buffer solution, the hydrogel cross-link density, and the density of functional groups on the polymer backbone affect the properties of the hydrogel. This work demonstrates the utility of indentation techniques in the characterization of pH-sensitive hydrogels.

This paper uses the thermodynamic data of aqueous solutions of uncrosslinked poly(N-isopropylacrylamide) (PNIPAM) to study the phase transition of PNIPAM hydrogels.  At a low temperature, uncrosslinked PNIPAM  can be dissolved in water and form a homogenous liquid  solution.  When the temperature is increased, the solution separates into two liquid phases with different concentrations of the polymer.   Covalently crosslinked PNIPAM, however, does not dissolve in water, but can imbibe water and form a hydrogel.  When the temperature is changed, the hydrogel undergoes a phase transition:  the amount of water in the hydrogel in equilibrium changes with temperature discontinuously. While the aqueous solution is a liquid and cannot sustain any nonhydrostatic stress in equilibrium, the hydrogel is a solid and can sustain nonhydrostatic stressin equilibrium.  The nonhydrostatic stress can markedly affect various aspects of the phase transition in the hydrogel. 

Silicon can host a large amount of lithium, making it a promising electrode for high-capacity lithium-ion batteries.  Recent experiments indicate that silicon experiences large plastic deformation upon Li absorption, which can significantly decrease the stresses induced by lithiation and thus mitigate fracture failure of electrodes. These issues become especially relevant in nanostructured electrodes with confined geometries. Based on first-principles calculations, we present a study of the microscopic deformation mechanism of lithiated silicon at relatively low Li concentration, which captures the onset of plasticity induced by lithiation.