Scientists at the Hong Kong University of Science and Technology (HKUST) have developed a new class of electrorheological (ER) fluids that break the world record for solid strength.
The fluids are capable of exhibiting a high yield stress (the strength of the materials in solid state) of 130 kPa when electrically charged, ten times stronger than the previously achievable ER solid strength.
Such fluids and their unique properties have been recognised for more than 50 years, but practical applications had yet to be developed because, until now, researchers had been unable to break the 20 kPa barrier.
ER fluids are ‘smart’ materials consisting of dielectric microparticles dispersed in an insulating liquid, usually silicone oil. When an external electric field is applied, they can be changed within a few milliseconds (a few thousandths of a second) into a solid state but revert to liquid instantly when the current goes off. This process, called electrorheology, is reversible.
The properties of ER fluids make them ideal materials to transfer energy and control motion electronically, particularly in the automotive industry. When coupled with sensors, ER fluids become ‘smart’ by being able to respond nearly instantaneously to environmental variations.
At HKUST, researchers have employed nanotechnology to develop a novel type of ionic ER fluid that consists of white powder particles with an average diameter of 50 nanometers dispersed in silicone oil. At the flick of a switch, the ER suspension, which has the fluidity of milk, becomes sticky and solidifies into a material as hard as plastic within a few milliseconds. Researchers have used the fluids to create a new type of clutch.
‘Without the need for friction to transmit torque, there will be little wear and tear, or noise for our ER-based clutch, making it an eco-friendly product with a faster response time, and a longer lifespan than conventional designs,’ says Dr. Weijia Wen, Assistant Professor of Physics at HKUST.
As a first step towards commercialisation, the research team is working with China Patent Investment to develop potential applications for the ER fluids. A small ER-based clutch for use in motorised bicycles is expected to be ready in early 2004.