Static Stability of Helically Supported Fluid Interfaces at Zero Bond Number

When gravitational effects are negligible with respect to capillary effects, it is possible to stabilize an infinite channel of liquid with a helical wire. Capillary-driven flow in such minimal support structures may have applications for use in heat- or mass-transfer processes under microgravity conditions or at small scales in micro- and nano-fluidic applications. Stability issues limit the initial penetration of the meniscus into the structure as well as steady flow. The static stability of infinite-length helical interfaces is theoretically determined at zero Bond number as a function of the contact angle and two dimensionless geometric parameters. The theory predicts a minimum and maximum stable pressure and corresponding volumes at which respectively breakup and blowout of the interface occurs. An approximate theory for the equilibrium of finite-length, free-ended segments is also presented which predicts a critical value of the pitch beyond which no stable free-ended interfaces exist. Predictions of stability limits for infinite and free-ended equilibria are confirmed experimentally in a Plateau tank with satisfactory agreement. Observations of the bath fluid flow in the vicinity of the free interface suggest a screw-like flow in the creeping flow regime. High-speed imaging was used to capture the instability mechanisms.