Spontaneous Elastocapillary Winding of Thin Elastic Fibers in Contact with Bubbles

We study the elastocapillary interaction between flexible microfibers in contact with bubbles trapped at the surface of a liquid bath. Microfibers placed on top of bubbles are found to migrate to and wrap into a coil around the perimeter of the bubble for certain bubble-fiber size combinations. The wrapping process is spontaneous: the coil spins atop the bubble, thereby drawing in excess fiber floating on the bath. A two-dimensional microfiber coil emerges which increases the lifetime of the bubbles. A simple model incorporating surface and bending energies captures the spontaneous winding process.

Figure 1

An elastic fiber (SIS) with a radius $r=5\mu \mathrm{m}$ winds around the liquid film at the top of an air bubble at the interface of a glycerol bath before (a), and after (b), the bubble bursts [in this case a small air bubble remained trapped below the coils on the right side of (b)]. (c) A time sequence of the winding process. The fiber can coil spontaneously, forming structures like these in no more than 2–3 minutes. For scale, the diameter of the coil in (a) and (c) is $\sim 500\mu \mathrm{m}$.

Figure 2

(a) A schematic of an air bubble which deforms the surface of a liquid bath via buoyancy. (b)–(g) A SIS fiber spontaneously winding around an air bubble in a bath of glycerol over approximately 30 seconds. The scale bar is $200\mu \mathrm{m}$.

Figure 3

(a),(b) Top-down, monochromatic optical images of a fiber-bubble system before and after migrating to the perimeter of the bubble cap. Red circles are numbered 1–3 and correspond to the different states of a segment of fiber shown in (c). The scale bar is $250\mu \mathrm{m}$. Inset: interference fringes from the meniscus surrounding the bridged fiber. (c) Schematic of the fiber-bubble system side-on. The fiber is shown in light blue in position 1, 2, and 3, representing the unbridged, prewinding and postwinding positions of the fiber. (d),(e) Schematics of a fiber cross section floating on a fluid bath (unbridged), and bridged across a fluid membrane respectively. The schematic in (e) omits the bridging meniscus for clarity.

Figure 4

Phase diagram of the winding criterion for Elastollan (circles), SIS (diamonds), and PS (triangles) fibers in glycerol. The slope defining the transition from Eq. (4) is $m=1$. Green (light gray) points designate fiber-bubble combinations which wind, and red (dark gray) points designate those that do not wind.

Figure 5

A macroscopic demonstration of how the fiber coil can collapse to a flat surface, forming a unique, noncircular shape.