Figure 10. On-demand patterning with media exchange and particle co-location.

(a) RBCs in a continuous flow can be (b) individually patterned and held for an arbitrary length of time through the imposition of an externally applied acoustic field (229 MHz at 0.25 W). The ability to hold particles and cells in place can further be used for applications such as media exchange, where c and d show the same trapped 6.1-μm particle grid suspended in water with a dye solution injected from a separate inlet, although some reconfiguring occurs due to changing local flow conditions (171 MHz at 0.52 W). Arrows between panels (a/b) and (c/d) denote interchangeability between random/patterned cells and suspension in different media, respectively. Using a patterned array of 10-μm particles, smaller particles can be trapped and co-located in the same locations, useful for cell-pair interaction studies where there is a large size difference between cells. (e) The limits to this activity, as defined by the difference between the size of the larger and smaller particle. When the smaller particle is larger than ≈50% of the larger particle's size, particles can either bridge between trapping locations (double-lined arrow) or cluster in a nodal location independently (solid arrow). Below ≈30%, particles can access trapping locations near the chamber roof offset from the primary trapping locations (dashed arrows). (a–e) Scale bars, 50 μm.