Nanomaterials - 21

Abstract

The star-nosed moves its 22 tentacles underwater to control the shape of bubbles exhaled from its nose. These bubbles capture odor molecules, and by inhaling the bubbles back in, the star-nosed mole perceives odor underwater.[1] While the bubble growth is regulated by the dynamic motion of the tentacles, the effect of the tentacles' dynamic structure on the bubble growth mechanism remains unclear. Here, we emulated the tentacles topology and surface wettability to study the bubble growth dynamics. We designed the three-dimensional model structures whose surface is modified with a superaerophobic layer, and observed the bubble growth behavior. Figure 1 shows the time-lapsed stuck image of the underwater bubble growth within the model structure. We defined the tentacle opening angle as α, the tentacle inclination angle as β, and quantified the bubble growth with advancing length X_Front. We found the bubble growth process can be classified into three stages. First, the bubble grows isotropically while maintaining a spherical shape (stage I). Then, the bubble starts deforming (stage II). Finally, the bubble is detached from the model structure (stage III). These transitions during the bubble growth can be explained by a change in the working direction of buoyancy and capillary forces, as illustrated in Figure 2. In that, the threshold determining whether the direction of buoyancy and the parallel component of the bubble growth direction are in the same direction or opposite direction is for α+β = 90 °. We experimentally confirmed X_Front as functions of α and β, indicating that X_Front reaches maximum for α+β = 90 °. This indicates that the balance between bubble retention and forward motion is optimal for α+β = 90 °. Therefore, we conclude the most effective topology to grow bubble as far as possible without detachment is the topology fulfilling α+β = 90 °.