The Grand Challenges in Smart Materials Research

The current generation of smart materials has diverse dynamic features that enable them to adapt to the environment and make them the materials of the future. There are four of these aspects of which at least one is incorporated in a functional smart-material based device: smart materials can be sensors or actuators, they can be controlled or they can have biomimetic characteristics. Sensors are either bonded to the surface of a structural material or are embedded within a smart material to produce an electric signal thanks to the static or dynamic changes in of the structural material. Actuators are typically excited by an external stimulus, such as electricity, in order to change the stiffness and damping properties in a controlled manner. The control capability permits the dynamic behavior of the material to respond to an external stimulus according to a prescribed control algorithm associated to microprocessors. Biomimetic characteristics are inspired by biological patterns in order to equip materials with the possibility of self-diagnosis, self-repair, and self-degradation of a broad range of structural materials. Among materials with the above features, electrorheological (ER) fluids, magnetorheological (MR) fluids, magnetorheological (MR) elastomers, and electroactive polymers (EAPs) are in the spotlight of both the most active research activity and of the most commercial interest. For this reason, I expand and highlight grand challenges of these four smart material classes. Electrorheological fluids (ER fluids) are smart suspensions of soft conductive particles in viscous liquid medium. ERs can be reversibly solidified from and to a free-flowing liquid state in the presence of an external electric field. This transition is associated to significant changes in the rheological characteristics of the fluid including the generation of yield stress, increased shear viscosity and elastic properties. Despite the abundant research performed on ER fluid technology over last two decades, there is no commercial ER fluid device available up to now. In order to overcome this, the community needs to resolve to create an ER fluid that can produce high yield stress. A so-called giant electrorheological (GER) fluid, producing 50–60 times higher yield stress than conventional ER, is under development (Wen et al., 2003, 2004) involving barium titanyl oxalate nanoparticles coated with a thin urea layer (nanoparticles: 50–70 nm, urea layer dielectric constant 60 at 10 Hz, 3–10 nm thick, can produce yield stress up to 130 kPa at 5 kV/mm field strength). Yet, unmet market needs of for ER fluids consist of the need to be non-toxic to both humans and the environment, to be chemically durable and physically stable without particle settling issues, to have low power consumption and manufacturing cost, to have a wide working temperature range covering −40 to 200°C, to be compatible with sealing materials, to be non-abrasive and non-corrosive to the device, and to have rapid on/off responses. Furthermore, significant research effort is invested in commercializing the ER fluid with surfactants and additives, which can potentially broaden the applicability of ER fluids while maintaining a user-friendly point of view. Magnetorheological fluids are smart suspensions of soft magnetizable particles in non-magnetic liquid medium. Analogously to ER fluids, their phase and their rheological properties can be altered by an external magnetic field. However, contrarily to ER fluids, they are commercially applied in a broad spectrum of areas due to their field-tunability. Shock absorbers, brakes, clutches, seismic vibration dampers, control valves, and precision polishing are just a few examples to illustrate their use. Derived from the thermal decomposition of iron pentacarbonyl, carbonyl iron (CI) particles are widely employed as a dispersed phase for MR fluids because of their outstanding magnetic properties and suitable particle size. Nevertheless, further engineering development of CI particles is impeded by sedimentation and stability problems caused by the large density difference between CI particles and the medium. Various strategies have been proposed to this issue: adding stabilizers or additives, and modifying the magnetic particles with coating technology. Dispersion stability can be improved by occupying the interspaces between magnetic particles and to prevent the physical contact of CI particles. Prevailing methods exploiting these ideas involve introducing nano/sub-micron sized fillers such as carbon nanotubes, graphite nanotubes, graphene oxide, fumed silica, glass bead, and organoclay (Powell et al., 2012). Coating techniques provide an effective way to reduce particle density and prevent chemical oxidation of magnetic particles. Introducing suspension polymerization in the core-shell coating procedure, with CI particles as a core material, one can obtain a 2 ~ 10 μm coating thickness. This is much thicker than that of the composites prepared by a dispersion polymerization, which yields about submicron coating thickness. Dual-step coating on the CI particle surface with carbon nanotubes as the final layer would not only reduce the density of the CI particles but also provide rough surfaces. The resulting composites are expected to be for improving dispersion stability while retaining MR characteristics (Park et al., 2010). The rheological properties of MR fluids, including yield stress analysis as a