Effects of ionic impurities on EHD motion and conduction in nonpolar liquids

This paper reports the influence of an anionic surfactant, dodecyl benzenesulfonic acid on charge injection and convection assisted conduction in a naphthenic transformer oil, and three paraffinic liquids subjected to a step voltage. When impurities are present in a liquid they scavenge the surfactant and reduce its effective conduction; hence the surfactant has less influence on conduction in naphtenic oil than in the paraffinic liquids. The transient currents in test liquids containing surfactant were similar to injection currents with electrohydrodynamic (EHD) motion rather than to currents due to ionic dissociation. Injection in nonpolar liquids containing an anionic surfactant was controlled by the charge density at the interface due to adsorption of surfactant molecules and not by true charge injection. Based on the relation between the delay time t_d and field E a simple expression of the form C_n=kE^-(2+β) has been derived to describe the injection strength in nonpolar liquids with weak injection. The results show that the β values for nonpolar liquids with weak injection range between -0.90 and -1.32 which are significantly different from the β=-2 value for polar liquids with strong injection. Also, the steady state current I_ss varied as I_ss=k´E^α with α<2. Hence conduction in naphtenic and paraffinic solvents is considered electrode limited and not space charge limited