An apparatus and method are disclosed for modifying the position of particles distributed in a fluid flow in a channel, comprising a channel formed by two substrates, each of the two substrates being on opposite sides of the channel, each substrate having a preselected periodic profile pattern along a length of the channel, and a transducer, wherein one of the substrates is between the transducer and the channel, the transducer to generate an acoustic standing wave within the channel with at least one node or antinode positioned within the channel.