Abstract
Using acoustofluidic channels formed by capillary bridges two models are developed to describe nodes formed by leaky and by evanescent waves. The liquid channel held between a microscope slide (waveguide) and a strip of polystyrene film (fluid guide) avoids solid-sidewall interactions. 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 tensor elements normal to the solid–liquid interfaces they find that; initially nodes form in the solid and the node pattern is replicated by waves emitted into the fluid from antinodes in the stress. At fluids depths near half an acoustic wavelength, most nodes are formed by leaky waves. In the glass, 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. At 0.2 mm water depths (which are smaller than a ¼ wavelength) nodes form from evanescent waves. 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 sound intensity normal, and close, to the water-polystyrene interface. 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.
Highlights
We summarise the current descriptions of node formation in the bulk fluid of soft-walled channels with the following five explanations i) The nodes are not produced by a resonance in the fluid [7,38]. ii) Waves emitted from the solid substrate interfere to form the nodes [38]. iii) The node separation is governed by the acoustic wavelength of the substrate [5,38,47,48]. iv) Waves leaking from a solid substrate are refracted (Snell’s law)
We have shown that at the glass-water interface σy stress nodes in the glass join to the pressure nodes in the fluid
We have presented a model (NTM1) where the water-loading reduces the distance between nodes in the solid and the node pattern first forms in the glass
Summary
NTM-1 applies to leaky waves and introduces the concept of fluid pressure nodes aligned with nodes of y–component stress, σy in the solid. These stress nodes are controlled by mass-loading [10,11]. A capillary bridge was used to form an acoustofluidic channel, (Fig. 1a) which has water–air sidewalls This simplified our analysis of node formation by removing the sound paths and mass interactions contributed by solid sidewalls. Since the polymers used for soft-walled channels absorb sound their walls do not pro vide sound paths from the substrate to the top of the channel Due to this similarity between no-wall and soft-wall channels, insights into node creation gained in capillary bridge channels may be transferable to softwalled channels
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