  We present an acoustofluidic method based on travelling surface acoustic
waves (TSAWs) for the induction of the self-assembly of microparticles inside a
microfluidic channel. The particles are trapped above an interdigitated
transducer, placed directly beneath the microchannel, by the TSAW-based direct
acoustic radiation force (ARF). This approach was applied to 10 {\mu}m
polystyrene particles, which were pushed towards the ceiling of the
microchannel by 72 MHz TSAWs to form single- and multiple-layer colloidal
structures. The repair of cracks and defects within the crystal lattice occurs
as part of the self-assembly process. The sample flow through the first inlet
can be switched with a buffer flow through a second inlet to control the number
of particles in the crystalline structure. The constant flow-induced Stokes
drag force on the parti-cles is balanced by the opposing TSAW-based ARF. This
force balance is essential for the acoustics-based self-assembly of
microparticles inside the microchannel. Moreover, we studied the effects of
varying the input voltage and fluid flow rate on the position and shape of the
colloidal structure. The active self-assembly of microparticles into crystals
with multiple layers can be used in the bottom-up fabrication of colloidal
structures with dimensions greater than 500 {\mu}m x 500 {\mu}m, which is
expected to have important applications in various fields.
