Linear concentration of microscale samples under an ultrasonically vibrating needle in water on a substrate surface

We show a method of concentrating microscale samples in a film of aqueous suspension on a stationary substrate (not in vibration), which employs the acoustic streaming generated by an ultrasonically vibrating needle parallel to and above the stationary substrate. Concentrated yeast particles with a diameter of 4–6 μm may form a series of lobed zones on the stationary substrate if the position of the vibrating needle has no change during the sonication, and form a continuous linear line if the vibrating needle is moved back and forth along a linear trajectory. Smaller objects such as AgNWs with a length of 30–40 μm and diameter of 300 nm, and ZnO particles with a diameter of about 1 μm, can also be concentrated by the method. For the yeast and ZnO microparticles, boundaries of the concentration zones are very distinct.