Temporal evolution and instability in a viscoelastic dielectric elastomer

Dielectric elastomer transducers are being developed for applications in stretchable electronics, tunable optics, biomedical devices, and soft machines. These transducers exhibit highly nonlinear electromechanical behavior: a dielectric membrane under voltage can form wrinkles, undergo snap-through instability, and suffer electrical breakdown. We investigate temporal evolution and instability by conducting a large set of experiments under various prestretches and loading rates, and by developing a model that allows viscoelastic instability. We use the model to classify types of instability, and map the experimental observations according to prestreches and loading rates. The model describes the entire set of experimental observations. A new type of instability is discovered, which we call wrinkle-to-wrinkle transition. A flat membrane at a critical voltage forms wrinkles and then, at a second critical voltage, snaps into another state of winkles of a shorter wavelength. This study demonstrates that viscoelasticity is essential to the understanding of temporal evolution and instability of dielectric elastomers.Engineering and Applied Science

Robust Flexible Capacitive Surface Sensor for Structural Health Monitoring Applications

Early detection of possible defects in civil infrastructure is vital to ensuring timely maintenance and extending structure life expectancy. The authors recently proposed a novel method for structural health monitoring based on soft capacitors. The sensor consisted of an off-the-shelf flexible capacitor that could be easily deployed over large surfaces, the main advantages being cost-effectiveness, easy installation, and allowing simple signal processing. In this paper, a capacitive sensor with tailored mechanical and electrical properties is presented, resulting in greatly improved robustness while retaining measurement sensitivity. The sensor is fabricated from a thermoplastic elastomer mixed with titanium dioxide and sandwiched between conductive composite electrodes. Experimental verifications conducted on wood and concrete specimens demonstrate the improved robustness, as well as the ability of the sensing method to diagnose and locate strain

Exponentially fast Thinning of Nanoscale Films by Turbulent Mixing

Films are nanoscopic elements of foams, emulsions and suspensions, and form a paradigm for nanochannel transport that eventually tests the limits of hydrodynamic descriptions. Here, we study the collapse of a freestanding film to its equilibrium. The generation of nanoscale films usually is a slow linear process; using thermal forcing we find unprecedented dynamics with exponentially fast thinning. The complex interplay of thermal convection, interface and gravitational forces yields optimal turbulent mixing and transport. Domains of collapsed film are generated, elongated and convected in a beautiful display of chaotic mixing. With a timescale analysis we identify mixing as the dominant dynamical process responsible for exponential thinning.Comment: accepted for publication in Phys. Rev. Let

Large-scale surface strain gauge for health monitoring of civil structures

Health monitoring of civil structures is a process that aims at diagnosing and localizing structural damages. It is typically conducted by visual inspections, therefore relying vastly on the monitoring frequency and individual judgement of the inspectors. The automation of the monitoring process would be greatly beneficial by increasing life expectancy of civil structures via timely maintenance, thus improving their sustainability. In this paper, we present a sensing method for automatically localizing strain over large surfaces. The sensor consists of several soft capacitors arranged in a matrix form, which can be applied over large areas. Local strains are converted into changes in capacitance among a soft capacitors matrix, permitting damage localization. The proposed sensing method has the fundamental advantage of being inexpensive to apply over large-scale surfaces. which allows local monitoring over large regions, analogous to a biological skin. In addition, its installation is simple, necessitating only limited surface preparation and deployable utilizing off-the-shelf epoxy. Here, we demonstrate the performance of the sensor at measuring static and dynamic strain, and discuss preliminary results from an application on a bridge located in Ames, IA. Results show that the proposed sensor is a promising health monitoring method for diagnosing and localizing strain on a large-scale surface

Spray-coated Cu2ZnSnS4 thin films for large-scale photovoltaic applications

The kesterite material, Cu2ZnSnS4 (CZTS), has in the preceding ten years been investigated and developed as a new Earth-abundant material for solar cells. The interest in this inorganic semiconductor originates in its optimal energy band gap of approx. 1.5 eV, high absorption coefficient, and the high material abundance and low toxicity of all elements included. The current challenges are related to unavoidable antisite disordering stemming from the chemical similarity of the cations, which causes bulk defects and lowers the open-circuit voltage detrimentally. This, however, did not restrict the “cousin”-material, CuInGaSe2 (CIGS), which is currently one of the main thin-film photovoltaic (PV) technologies on the market. In this work, CZTS thin films have been fabricated by solution-processing, which allows relatively fast and inexpensive deposition when compared to vacuum-processed films. The nanoparticles are synthesized by the hot-injection method by mixing targeted ratios of metal salts with sulfur in diethylene glycol, resulting in a phase-pure CZTS material [1]. Inks are formulated by dispersing the particles in ethanol and water using a suitable dispersing agent. The solvents used allow that alkali metal chloride salts can also be dissolved in controllable amounts, which we have found enhances grain growth in the films during the subsequent annealing step. A Sono-tek spray-coating system with ultrasonic atomization is used. We investigate the effect of ink concentration, and spray-coating conditions, including spray power, flow rate from syringe pump, and time between consecutive spray layers. The films are annealed in a tube furnace, and to avoid decomposing the material into secondary phases, a graphite box is used to enable an overpressure of sulfur and tin-sulfide. The annealed, spray-coated films are characterized by scanning electron microscopy (SEM), optical microscopy, and Dektak profilometry