Electrostatics is an old area of scientific research that over the years lagged behind most other areas of natural sciences. [1] Despite the great technological advances, even most basic questions like "what causes surfaces to charge?" or "how do dielectrics transfer charge?" are still matter of debate. [1c] This is also contemplated by the persistent controversy regarding the negative potential displayed by hydrophobic surfaces in contact with water, [2] which is revisited from time to time. [3] In fact, this widespread lack of understanding is responsible for triggering many industrial accidents, it costs billions of dollars to the electronic industry and restricts the appropriate development of important electrostatic technologies. [3e] For most systems, the outcome of contacting surfaces, in thermodynamic equilibrium or not, it is a net charge. In fact, electrified solid and liquid substances are easily formed either in natural or anthropic environments. Everyday activities or
This work describes the development of flexoelectric devices based on low-cost rubber parts with simple constructions. Flexoelectricity produces fast charging in a phase with rubber strain that can be used in both force sensing and energy harvesting technologies. The force transducer with high effectiveness and accuracy was built using only a flexible non-metal graphite-based electrode sandwiched by two vulcanized rubber parts, displaying a linear relationship between strain gradient and electric response. Also, mechanical-to-electrical energy transduction is benefited from synergy between flexo- and triboelectricity, where an energy harvesting device can be designed as simple as possible, requiring only natural latex to induce charge on an electrode. Moreover, elastomers are expected to play a key role in the next generation of soft electronics and wearable healthcare devices and these results may contribute to the employment of rubbers in many applications that are of great interest in flexoelectric technologies.
Rubber materials play an important role in robotics, due to their sensing and actuating abilities, that are exploited in soft smart materials endowed with shape-adaptive and electroadhesive properties. The application of an electric field produces non-linear deformation that has been extensively modelled, but is not understood at the molecular level. The symmetric effect (the production of an electric field due to rubber deformation) was recently discovered and explained as follows: rubber surface chemical composition and adsorptive properties change during rubber deformation, allowing the surface to exchange charge with the atmosphere. The present work describes the complex surface morphology and microchemistry of tubing made from vulcanized natural rubber, showing that it is rough and made from two domain types: stiffer elevations containing Br or Al (depending on the sample used) and O, that rise above an elastic base that is exempt of elements other than C and H. The surface area fraction occupied by the elastic base is higher in the strained rubber than when it is relaxed. Electrostatic potential on rubber surfaces was measured as a function of the stretching frequency, using Kelvin electrodes and showing frequency-dependent potential variation. This is explained considering charge exchange between the atmosphere and rubber surface, mediated by water vapor adsorbed in the stretched rubber and trapped when it relaxes.Colloids Interfaces 2018, 2, 55 2 of 11 of rubber performance in many important situations, but this does not benefit from knowledge on the molecular mechanisms of the observed phenomena.Recent work from this group described a new finding on the electrostatic behavior of elastomers: in short, rubber tube stretching, followed by relaxation, provokes the appearance of transient excess charge that is more pronounced under higher relative humidity [14]. This effect is not related to any existing electrostatic charging phenomenon (piezoelectricity, flexoelectricity, triboelectricity, and contact charging) and a new mechanism was then proposed to explain charge pick-up and dissipation by stretched elastomers, as the result of water adsorption and the partition of water ions (H + and OH − ) in the rubber-air interface, due to periodic rubber surface modification.Other unexpected findings on electrostatic phenomena in dielectrics have been described recently, leading to a revision of widespread ideas on electrostatic charging and its mechanisms [15]. Important aspects of electrostatic phenomena are not well understood [16][17][18], but tribo-and piezoelectricity are important sources of the electrostatic potentials detected in anthropic environments, often reaching many-thousand volts. They are currently used in nanotribogenerators, with great success. This is creating new opportunities for energy scavenging [19][20][21], and it is probably relevant to the prevention of harmful electrostatic discharge.Knowledge of the electrostatic behavior of elastomers is more limited than in the case of semicrystallin...
The recent discovery of electromechanical coupling in elastomers showed periodic electrification in phase with rubber stretching but following different electrostatic potential patterns.
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