  Acoustics-based techniques are investigated to focus and bunch nanoparticle
beams. This allows to overcome the prominent problem of the longitudinal and
transverse mismatch of particle stream and x-ray beam in single-particle/single
molecule imaging at x-ray free-electron lasers (XFEL). It will also enable
synchronised injection of particle streams at kHz repetition rates. Transverse
focusing concentrates the particle flux to the (sub)micrometer x-ray focus. In
the longitudinal direction, focused acoustic waves can be used to bunch the
particle to the same repetition rate as the x-ray pulses. The acoustic
manipulation is based on simple mechanical recoil effects and could be
advantageous over light-pressure-based methods, which rely on absorption. The
acoustic equipment is easy to implement and can be conveniently inserted into
current XFEL endstations. With the proposed method, data collection times could
be reduced by a factor of $10^4$. This work does not just provide an efficient
method for acoustic manipulation of streams of arbitrary gas phase particles,
but also opens up wide avenues for acoustics-based particle optics.
