Bubble stabilization by the star-nosed mole

Star-nosed moles sniff for prey underwater by rapidly exhaling and inhaling bubbles that in turn capture odors on their surface. While the sniff lasts only a tenth of a second, speed alone cannot explain how the star-nosed mole so reliably sucks the bubble back in before pinch-off occurs. In this combined experimental and theoretical study, we elucidate how the unique shape of the nose stabilizes underwater bubbles. The fleshy arms of the mole's star are separated by an average of ${16}^{\circ }\pm 9^{\circ }$ increments. We laser-cut plastic stars of various angles between the arms and tilt them by hand to find the angle at which a trapped sessile bubble is released. A bubble trapped beneath the star bulges through the gaps, enabling the plastic star to retain the bubble when tilted up to $7^{\circ }$, which is 40% greater than that of a flat plastic sheet. Using a semiempirical model, we show two regimes where a bubble escapes. If the gap width is wider than the capillary length, buoyancy forces pull the bubble up through the gap. If the gap width is too small, the bubble does not sufficiently anchor itself in place. We show order of magnitude agreement between biological measurements, plastic star experiments, and theory, suggesting we correctly identified the mechanism for the star retaining bubbles. This study may lead to new ways of stabilizing centimeter-scale bubbles for underwater chemical sensing.

Figure 1

Three semiaquatic mammals exhibit underwater sniffing: (a) the star-nosed mole blows a bubble of 0.1 ml on a timescale of 0.1 sec, (b) the American water shrew blows a bubble of 0.06 ml on a timescale of 0.08 sec, and (c) the Russian desman blows a bubble of 0.3 ml on a timescale of 0.07 sec. Photographs (a) and (b) courtesy of K. Catania. Photograph (c) courtesy of I. Shpilenok.

Figure 2

Geometry of the nose of the star-nosed mole. (a) The star-nosed mole, whose fleshy, star-shaped nose measures approximately 10 mm across. (b) Close-up photograph of the nose, with red lines showing how the gap angle $\theta$ is measured. The red line is drawn between the center of the nostril and the inside tip of the appendages. (c) Histogram showing the distribution of the gap angles $\theta$ for three star-nosed moles. Photographs (a) and (b) courtesy of K. Catania.

Figure 3

Design of plastic stars mimicking the star-nosed mole's nose. (a) Plastic stars laser-cut to mimic the star-nose. Like in the photos of star-nosed moles, the gap angle was measured along the inner edges of the arms with the vertex at the center of the nozzle. (b) Schematics of the five plastic stars used in this study. Angles indicate the gap angles for each of the stars. (c) Relationship between the gap angle $\theta$ and the number of gaps for each star. The dashed red curve represents the fabrication limitation that the minimum width of each fin is greater than 2 mm. The blue dots are the actual number of gaps used in the experimental stars. The solid black line is a linear approximation used to represent the relationship between the number of gaps and the gap angle.

Figure 4

(a) Schematic of the experimental setup in which a syringe with a plastic star is affixed to a protractor. As a measure of bubble stability, the syringe is tilted at an angle $\varphi$ before the bubble pinches off. Inset shows the contact angle, $\xi ={54}^{\circ }$, of the bubble below the star and the angle, $\psi ={127}^{\circ }$, of the bubble pushing up through the gap. (b)The relationship between the maximum tilt angle $\varphi$ and the gap angle $\theta$ of the star. Experimental data (black) suggests an optimal gap angle around $8^{\circ }–{15}^{\circ }$, and the theory predicts an optimum at ${11}^{\circ }\pm 3^{\circ }$. The theory described in Sec. 3 describes a large gap condition leading to pinch-off (red dotted line) and a small gap condition leading to pinch-off (red dashed line).

Figure 5

Position of the bubble with varying tilt angle $\varphi$. (a) Photograph shows portions of a bubble rising through the gaps between the arms of the star, forming ribs similar to a pumpkin. (b, c) Two photographs showing the position of the bubble before and after the star is tilted by an angle of $4^{\circ }$. The red dotted line shows the original position of the bubble. As the system is tilted, one side of the bubble advances, and the other recedes. (d) The shift in bubble position creates two lengths from the start of the gap to the edge of the bubble, $\mathrm{\Delta }{R}_a$ and $\mathrm{\Delta }{R}_r$ for the advancing and receding sides respectively. (e) The relationship between the the tilt angle $\varphi$ and the position $\mathrm{\Delta }R$ of the bubbles edge for a star of gap angle $\theta =8^{\circ }$. The advancing edge is shown as red dots, the receding edge as blue triangles. The solid lines are linear best fits. The bubble pinches off when the tilt angle $\varphi =8^{\circ }$.

Figure 6

Illustrations of conditions preventing pinch-off. (a) Schematic of the gap and geometric pinch-off condition for large gap angles. (b) Side view schematics of the bubble interacting with the star shape above it. The buoyancy forces acting on the red region denoted by $V$ are balanced by the buoyancy forces of the blue region of the bubble-star system for small gap angles. The portion of the bubble $V$ (dashed red hashed region) slides off the star, while the small portion in the gap, $v$ (solid blue hashed region), acts as a counterbalance, similar to an analogous mass-on-a-ramp system shown in panel (c). Inset in panel (b) shows how $v$ is split up into two regions $v_1$ and $v_2$ for volume calculations. (d) The parallel buoyant force must overcome the resistive surface tension force between the main bubble and air remaining in the syringe nozzle. Here the radius $R_{\mathrm{nozzle}}$ is labeled.

Figure 7

Schematic diagrams of the geometry of a bent plastic disk. (a) Photograph of the bubble held by the bent star and (b) schematic of the portions of the bubble showing counterbalance, for a tilt angle of $\varphi$ and a bend in the plastic of angle $\zeta$. The buoyancy force on the red hashed region causes the bubble to slide to the right. The bubble is held in place by the opposing buoyancy force on the blue hashed region.

Figure 8

Experiments demonstrating the ability of a bubble to maintain counterbalance to remain stable. (a) A flat disk can be tilted only to $\varphi =4.5^{\circ }\pm 1.5^{\circ }$ before the bubble escapes. (b) A disk with a $\zeta ={15}^{\circ }$ bend at a position off-center can hold onto a bubble up to $\varphi ={10}^{\circ }$ of tilt, demonstrating the importance of geometry in maintaining bubbles.