Electrorheological effects in lecithin organogels with water and glycerol

Abstract

The effects of an external electric field on lecithin organogels formed by the addition of trace amounts of water or glycerol, inducing a formation of polymer-like micelles, have been studied by oscillating rheology. Field application brought about an increase in the organogel viscosity and shear moduli when the threshold field strength, E _min, was exceeded. A second critical field strength, E _max, determines a boundary between stable and unstable electrohelogical (ER) effects. The dependencies of E _max, on the molar ratio of polar additive to lecithin differ for water- and glycerol-containing systems. In the former, E _max, increases over the whole concentration range for the polar additive whereas for systems including glycerol E _max, passes through a maximum. Frequency-sweep measurements on the lecithin organogels under an electric field revealed a new rheological regime at intermediate frequencies; both the dynamic moduli depend on f ^1/2. The same behavior has only previously been observed with polymer-containing systems. This finding — together with observations of the formation of filaments oriented in the direction of the electric field — has enabled us to suggest that the ER effects in lecithin organogels are caused by an alignment of parts or whole polymer-like micelles along the electric field lines.