“For the first time we can controllably harness the very large, nanoscale electric field of molecular dipoles to reversibly generate solid-liquid transitions within 0.01 s,” Ping Sheng told nanotechweb.org. “This is achieved by going to nanoscale systems because only there can one find large electrical forces.”
To demonstrate this giant electrorheological effect, the researchers used nanoparticles of barium titanyl oxalate coated with a 3-10 nm thick layer of urea, a material that has a large molecular dipole moment. Sheng, Weijia Wen, and colleagues suspended the particles, which had an average size of 50-70 nm, in silicone oil and applied an electric field. This polarized the nanoparticles, causing them to form columns aligned with the direction of the electric field.
The presence of the columns meant that the material acted as a solid when shear forces were applied perpendicular to the columns. The suspension’s static yield stress increased roughly linearly with increasing electric field. Under a field of 5 kV/mm, for example, a nanoparticle suspension containing roughly 30% nanoparticles by volume had a static yield stress of 130 kPa - roughly 20 times larger than the theoretical value calculated from the electric properties of the materials making up the suspension.
“Our electrorheological (ER) fluids can realize many of the applications thought of before but never made practical due to the insufficient yield stress of conventional ER fluids,” said Sheng. “In the 1980s, detailed engineering studies were carried out to see what kind of yield stress is needed for widespread applications. The consensus at that time was a value of 20-50 kPa.”
The applications could include active vibration dampers, electrically controllable clutches and electrically controlled mechanical valves. “In general, our material can serve as an effective interface that translates electrical signals, which are the easiest to make, into mechanical ones,” explained Sheng. “So when ER fluids are coupled with sensors that generate electrical signals according to environmental variations, one can achieve ‘smart’ dampers, ‘smart’ clutches and ‘smart’ valves.”
The scientists believe that short-range interactions between the urea-coating molecules and the core nanoparticle and/or the oil are crucial to the giant electrorheological effect. Now they plan to examine the interaction of the coating with the nanoparticle core in more detail and are also aiming to develop prototype ER devices for industrial environments.
The researchers reported their work in Nature Materials.