In this paper we have used both analytical models and finite element simulations to analyze the role of the particle-wall dipole interaction in field-flow fractionation dielectrophoretic (FFF-DEP) devices. We identify the existence of "anomalous" regions where the dielectrophoretic response is altered, independently of the complex dielectric permittivity of the particles and suspending medium. In these regions the interaction between the particle and the conductive (isolating) walls induces cohesive (repulsive) forces, independently of the Clausius-Mossotti term. We quantify the impact of such an effect, which can critically decrease the specificity and sensitivity of both continuous-and batch-mode FFF-DEP. We find a scale invariant relation correlating the particles radius (R-p) and the electrodes width (W-el), which permits the design of dielectrophoretic schema capable of avoiding the generation of such regions. Specifically, to avoid the generation of the anomalous DEP regions, W-el should be chosen smaller than similar to 5.2 R-p. For this reason, interdigitate schema with electrode widths of 14 mu m and gaps of 50 mu m could improve the separation efficiency of FFF-DEP devices in the case of rare cells separation in blood samples.