Node formation mechanisms in acoustofluidic capillary bridges

Using acoustofluidic channels formed by capillary bridges two models are developed to describe nodes formed from leaky and evanescent waves. The capillary bridge, formed between a microscope slide (waveguide) and a strip of polystyrene film (fluid guide) excludes solid-sidewall interactions in the channel. With this simplification, our experimental and numerical study showed that waves emitted from a single plane surface, interfere and form the nodes without any resonance in the fluid. Both models pay particular attention to the elements of the tensors normal to the solid-liquid interfaces, they find that the nodes form initially in the solid and then, antinodes in the stress emit waves into the fluid, replicating the pattern. In fluids with depths near half an acoustic wavelength most nodes are formed by leaky waves. In the glass, normal stress tensors reveal that water-loading reduces node-node separation and forms an overlay type waveguide which aligns the nodes predominantly along the channel. One new practical insight is that node separation can be controlled by water depth. In 0.2 mm deep channels (which are smaller than a ¼ wavelength) nodes from evanescent waves were realized. Here a suspension of yeast cells formed a pattern of small dot-like clumps of cells on the surface of the polystyrene film. We found the same pattern in the normal component of sound intensity in water near the polystyrene. The capillary bridge channel developed for this study is simple, low-cost, and could be developed for filtration, separation, or patterning of biological species in rapid immuno-sensing applications.