Electrorheological fluid and electric device

An electrorheological fluid comprising:

• a dispersion medium containing fluorine atoms in an amount of 38.0 wt % or greater and 41.0 wt % or smaller in which
• particles for electrorheological fluid containing a sulfonic acid group-containing polymer having a content of sulfonic acid groups of 30 wt % or greater and 70 wt % or smaller are contained in an amount of 10 vol % or greater and 50 vol % or smaller with respect to the total volume of the dispersion medium and the particles, wherein
• the dispersion medium is a mixed medium of
• a fluorine-based oil selected from a group consisting of a low polymer of ethylene trifluoride, a perfluoro polyether oil, and a fluorine-modified silicone oil;
• and
• a silicone oil,
• the sulfonic acid group-containing polymer has a sulfonated aromatic structure, and
• the particles for electrorheological fluid are selected from a group consisting of
• particles consisting of the sulfonic acid group-containing polymer;
• and
• particles comprised of cores of porous polymer particles or porous silica particles, and the sulfonic acid group-containing polymer coated on the surfaces of the cores, wherein
• the electrorheological fluid exhibits an electrorheological effect X expressed by the following formula (1):
• X=S1/S0 (1)
• in which formula (1) X is 140 or greater, S1 is the shear stress when applying a 3.0 kV/mm electric field, while S0 is the shear stress when not applying an electric field).