Electrorheological (ER) fluid is a high-tech smart material that can be converted from fluid to solid within milliseconds by the effect of the electric field. Also, the transformation process is reversible, continuous, and consumes low energy [
1]. The ER finishing technique utilizes these performances of the ER fluid, that is, the ER finishing technique is based on the ER effect. When an electric field is applied, ER particles form an array of stable chains of particles along the electric field, gather at the polishing tool, and polish the workpiece. Kuriyagawa et al. first utilized ER fluid to finish an optical micro-aspherical lens in 1999 [
2]. Kim used the diamond-mixed ER fluid and obtained average surface roughness of 2.8 nm after polishing a borosilicate glass surface, whose initial surface roughness was 28 nm [
3]. Yan and Zhang et al. [
4-
8] also studied ER finishing in China, and achieved some results. Today, ER finishing is a cutting-edge technology; however, some key practical problems remain to be solved for ER to meet practical applications. The ER fluid for optical finishing is the foundation of ER finishing; thus, it is important for us to study the ER polishing slurry.