How to assign different values to dependent variables

Hi,
I am just wondering that how it is possible to assign optional values to the dependent variable in some part of the domain ? I always get "Duplicate variable error".
Actually I am trying to put electrostatic B.C on the interface of the droplet in the Level-set method using the level-set variable (phils), I am trying to put the voltage (V=0) on the surface , so
V V(if phils==0.5,0,V)

But I get error.