Submicron Particle Concentration and Patterning with Ultralow Frequency Acoustic Vibration.

Acoustofluidics have been widely used for particle and cell manipulations. Given the scaling of acoustic radiation forces and acoustic streaming flow velocities with increasing frequency, existing acoustofluidic manipulation of submicron particles require actuation at MHz and even GHz frequencies. In this work, we explore a novel acoustofluidic phenomenon, where an ultralow frequency (800 Hz) acoustic vibration is capable of concentrating and patterning submicron particles at two poles of each pillar in an array embedded in a microfluidic device. This unprecedented phenomenon is attributed to a collective effect of acoustic streaming induced drag force and non-Newtonian fluid induced elastic lift force, arising from symmetric acoustic microstreaming flows around each pillar uniformly across the entire pillar array. To our knowledge, this is the first demonstration that particles can be manipulated by an acoustic wave with a wavelength that is 6 orders of magnitude larger than the particle size. This ultralow frequency acoustofluidics will enable a simple and cost-effective solution to effective and uniform manipulation of submicron biological particles in large scales, which has the potential to be widely exploited in clinical and biomedical fields.