Dynamic sub-wavelength microparticle patterning via phase-modulated pulsing coherent surface acoustic wave tweezers

Acoustic tweezers capable of manipulating microparticles and other objects in a non-contact manner have attracted great interest in the fields of cell culture, targeted drug delivery, and tissue engineering. However, the patterning resolution of surface acoustic wave (SAW)-based tweezers is generally determined by the finger width of interdigital transducers (IDTs). This inherent constraint severely limits the versatility and maneuverability of SAW tweezers. Here, we introduce a phase-modulated pulsing coherent SAW method that sequentially modulates the phase difference between two counter-propagating traveling SAWs. This enables dynamic microparticle patterning with adjustable, sub-wavelength spacing, while using uniform-finger-width IDTs. A highly reflective interlayer in the chamber ceiling provides a distinct acoustic pressure field, suppressing the travelling-wave component of standing SAWs, and thereby allowing particles to respond sequentially to the phase-shifted standing waves. Pattern spacings of one-half and one-third of the original period, along with variable particle density distributions are readily realized through phase modulation between pulsations, greatly extending the flexibility of acoustic tweezers for precise manipulation.