The research progress of electrorheological fluids

Electrorheological (ER) fluids are such smart materials whose rheological properties (yield stress, viscosity, etc.) can be reversibly and continuously controlled using an external electric field. They are colloids composing of dielectric particles and insulating liquids. They switch from a liquid-like state to a solid-like state within a millisecond with the aid of an electric field, which is called the ER effect. ER fluids can therefore be used as electrical and mechanical interfaces in various industries, including the fast acting valves, clutches, brakes, shock absorbers, accurate polishing, robotics and tactile displays. Since the ER effect was first described by Winslow in 1949, ER fluids show a promising prospect in the application in various industries and a great deal of research interest in ER fluids and ER devices has been stimulated. A large body of literature on ER fluids, the mechanism of the ER effect, and the design of industrial applicable ER devices has been published. In the meantime, after the invention of ERF, a number of theories are put forward, such as Fibrillation Theory, "Water Bridge" Theory, Double layer Theory, Particle Polarization Theory and so on. Ma et al. calculated that the theoretical upper bound on conventional ER static yield stress is 10 kPa based on first-principles calculations. The highest yield strength of the dielectric electrorheological fluid obtained by Lu et al. in the experiment is 5 kPa. However, electrorheological fluids are not industrialized practically because of the low yield strength. In recent years, Wen developed giant electrorheological fluid that can reach a yield strength of 130 kPa, breaking the theoretical upper bound of traditional ER static yield stress. Later, Lu invented polar-molecule-dominated electrorheological fluids whose solid state can reach yield strength of 200 kPa. With the inventions of the giant electrorheological fluid and polar-molecule-dominated electrorheological fluids with high yield stress under low electric field, there is a new opportunity for ER fluids to apply to the industrial application because their yield strength of the new kind of ER fluids is more than 40 kPa, the lowest practical limit. However, the problems about sedimentation and redispersibility of electrorheological fluids still restrict its wide application. With the research of surfactant and the development of the hollow multilayer porous nano particles, the stability of electrorheological fluids has greatly improved. This paper focuses on the study of the giant electrorheological fluid and its application in smart microfluidics. Specifically, it gives a detailed introduction of micropumps and micro-valves and smart electroresponsive droplets in microfluidics based on giant electrorheological fluids. This paper also offers a review of the composition, macroscopic properties, microscopic mechanism and applications of electrorheological fluids, as well as a summary of its current research status and its future development. Research of the stability and service life of electrorheological fluids will definitely be the most popular topic in the future, thus accelerating the progress of industrialization with the settlement of the issues discussed above. © 2017, Science Press. All right reserved.