Offset AC signals are also useful for particle manipulation, since net EOF is nonzero in that case. However, this net EOF is small, so a smaller DEP force, requiring smaller applied potentials, may be enough for particle manipulation. This was demonstrated with 1 and 2 μm diameter polystyrene particles. Microfluidic channels with cylindrical insulating posts were filled with a mixture of 1 and 2 μm diameter particles, whose concentrations were 1.21 × 108 and 6.06 × 107 particles/ml, respectively. A sinusoidal, 20 Hz AC signal was applied across the channel. Figure 7(a) shows the representation of the applied signal. Initially, the amplitude oscillated between −500 and +500 V, followed by changes of 100 V steps to produce increasing offset, until the signal oscillated between 0 and +500 V. Figure 7 shows the behavior of 1 μm (green) and 2 μm (red) particles under these conditions. With a symmetrical signal (Fig. 7(b)), particles oscillated in place with EOF. With a −300/+500 V signal (Fig. 7(c)), particles were retained at constrictions thanks to iDEP. The offset signal caused net EOF that brought the particles to the constrictions. In Fig. 7(c), particles are seen separated into two bands in the trapping regions because of the difference in DEP force they experience due to their size difference. Also, some green 1 μm particles are not trapped but pass through the constrictions, while virtually all red 2 μm particles are trapped. Figure 7(d) shows the particles under a more significantly offset signal of −100/+500 V, where more particles escape the trapping regions and bands are less clear. It was observed that 1 μm particles flowed through the channel at a high rate, and some 2 μm particles also escaped. This sequence of offset steps facilitates size based separation consisting of these stages: (i) All particles are trapped in the constrictions, (ii) A relatively small offset releases the smaller particles from the constrictions, (iii) A larger offset releases the larger particles from the constrictions, preferably once the smaller particles have all been eluted. This separation scheme has the potential to be applied to complex mixtures of particles, including particles in the sub-micron size range.