Microfluidic devices can provide means to handle the transport of (bio) particles within a fluid flow. The great advantage of microscale devices is that different components can be integrated in a single chip at low costs, with a negligible power consumption, compared to alternative solutions. In this work, a numerical investigation is provided on the exploitation of standing Surface Acoustic Waves (SAW) generated in a microfluidic channel to manipulate micro-particles. Far-field waves are generated via several InterDigital Transducers (IDT), travel on the surface of a piezoelectric substrate and finally interfere in the channel giving rise to a standing solution in terms of acoustic pressure. Results are reported for different geometries of the channel, to define the sensitivity of the acoustic pressure field to the relevant geometric features of the channel. This investigation shows how the acoustic radiation and drag forces interact with each other to move and focus the particles, possibly leading to a separation of heterogeneous ones, and generally provide a way to manipulate them at a small scale.