Acoustic Manipulation of Dense Nanorods in Microgravity

Because the absence of sedimentation in zero-gravity makes the culture and the manipulation of cells or particles challenging, an attractive alternative is to use the Acoustic Radiation Force (ARF) as an artificial “acoustic gravity.” To evaluate the potential of this approach we studied the behavior of dense gold nanorods under ARF during a parabolic flight campaign. Using dense objects enhances the effect of gravity on the axial position of the so-called “levitation plane,” which is the equilibrium position at which ARF balances gravity in the laboratory. Further, using elongated objects, instead of spherical particles provides information about their spatial orientations in addition to their propulsion observed in standard gravity conditions. Our experiments clearly show a different collective organization and individual behavior of the rods in micro-gravity conditions. First, the axial location of the levitation plane is different in microgravity than in hypergravity: it matches the nodal pressure plane in microgravity while it is much lower than the nodal plane in hypergravity. Our experiments also show a sharp transition from horizontal to axial orientation of the rods axis. The propulsion of the rods also stops when transitioning to micro-gravity. A possible explanation for the sudden change of orientation and stopping of propulsion is the modification of the equilibrium between the axial and transverse components of the ARF. While these experiments show that some phenomena, like the propulsion of nanorods by ARF, may not be applicable in microgravity, they do confirm that acoustic manipulation of particles or cells in microgravity is possible, which paves for the development of many useful techniques for particles or cells manipulation, like cell cultures, during long-term space travel.