A microfluidic device for separating, fractionating, or preconcentrating analytes contained in an electrolyte having at least two reservoirs separated by at least one microchannel and/or nanochannel. At least part of the wall of the microchannel is made of and/or coated interiorly with a conducting and polarizable material or group of materials constituting a polarizable interface or a network of polarizable interfaces. In that at least one electrode or at least one electrode network is connected at at least one point of the polarizable material or group of materials, the surface electrical conductance of said material being equal to at least 100 nS.