Electrorheological fluid compositions

An electrorheological fluid composition consisting essentially of an electrically nonconducting liquid and 10 to 50 vol. % silica particles dispersed therein,

• wherein the nonconducting liquid being at least one member selected from the group consisting of petroleum-derived lubricants, transformer oils, silicone oils, dibutyl sebacate, chlorinated paraffins, alkyl bromides, aromatic polycarboxylic acid alkyl esters, aromatic polycarboxylic acid halophenylalkyl esters, halophenyl alkyl ethers, and fluorine-containing oils and
• the silica particles being spherical particles formed by hydrolyzing a silicon alkoxide in a reaction mother liquor, the silicon alkoxide being of the following general formula (1)
• Si(OR).sub.4 ( 1)
• in which R is an alkyl group in the presence of an alkali catalyst, and drying at or below 500.degree. C. and
• the silica particles:
• have an average diameter of 0.1-2.0 .mu.m,
• have a BET nitrogen specific absorption area of .ltoreq.100 m.sup.2 /g,
• have a carbon content of .ltoreq.3% and
• have a weight lose of 2-12% when heated up to 200.degree. C. in thermogravimetric analysis and a weight lose of 2-10% when heated to 200.degree. C. to 1000.degree. C. in thermogravimetric analysis.