A new thin-reflector mode for ultrasonic particle manipulation in layered resonators

Previous literature has described two major classes of sub-wavelength planar acoustic particle manipulation devices: (a) those where the dominant resonance is in the fluid layer, leading to agglomeration at one or more pressure nodes within the fluid layer; and (b) those where a resonant reflector layer provides a pressure release boundary condition, causing the agglomeration position to occur at a pressure node close to the fluid/ reflector interface (Quarter-wave devices). We describe here a new arrangement which operates at the first thickness resonance of a layered structure. This leads to pressure nodes at the air boundaries of a device. By designing with only a thin reflector layer (significantly less than ¿/4) particles at all positions within the channel are forced to the reflector/fluid layer boundary. We model and experimentally characterize a device, and show that it can produces forces of order 55pN on a 10¿m diameter polystyrene bead with transducer excitation of 25Vpp. We also explore the parameter space to find optimum designs, and present a particle concentration device using this mode. We demonstrate that this configuration will work efficiently with lossy polymer reflector layers, making possible cheap, disposable devices.