Ultrasonic standing waves are utilised to manipulate and collect particles at distinct locations. Particles in suspension migrate to pressure nodes (provided they are stiffer and denser than the fluid medium) and gather at these stable locations indefinitely, provided the ultrasonic excitation is present. This work investigates a method which can separate particles of different diameters into distinct locations within the same system. The ability to sort particles of various sizes at different locations has applications in the drug testing industry, clinical diagnostics and as a biological and biochemical analysis tool. The method makes use of an open fluidic system; this means that there is a large water/air interface, rather than an enclosed system in which the fluid volume is enclosed within solid walls. The advantage of using such an open microfluidic chamber include the ability to perform rapid analysis and the ease of treating the particle arrays with antigens, proteins or chemicals to as well as ease the use of automated gripping procedures. The effects of particle size, amplitude dependencies, terminal orientations and fluid volume are examined. Firstly, different migrating patterns for different particle sizes are presented. Subsequently this phenomenon is used to separate particles of different sizes mixed in a homogenous solution. It is found that the co-existence of the acoustic radiation force and acoustic streaming induced drag forces facilitate the collection of larger particles at low pressure regions at the bottom surface as well as transporting smaller particles to the pressure nodes at the open surface (i.e. liquid-air interface). Finally, the effect amplitude of the excitation signal and fluid volume height which affects the streaming induced drag forces are explored, and seen to vary the migration patterns of the particles and to increase the robustness of the system in separating particles of various sizes.