Microgravity/microscale double-helical fluid containment

Abstract

Double-helical containment is a novel approach to open containment in microgravity (or at microscale). In contrast to axisymmetric containers, there is no length restriction on properly designed double-helical containers. Use of a double helix permits drainage to zero volume, an uncommon feature in microgravity; near-complete drainage is a key feature for any practically useful container. The fact that double-helical containers are open and tubular makes possible a broad range of applications that rely on accessible fluids. The double-helical fluid containment and the stability of such volumes are examined in detail. Helical supports permit filamentary containers of infinite extent, but only double-helical containers are stable down to zero volume. The base case of symmetric supports (separation angle 180°) is considered in terms of symmetric volumes as well as asymmetric helicatenoid volumes. The distinct symmetric and asymmetric cases are then related by perturbing the angle of separation between the supports. The helicatenoid volumes may combine to form a dual helicatenoid volume. The dual helicatenoid is interesting in that it permits multiphase tube-like geometries. Double-helical behaviours such as drainability are found to vanish at a critical separation angle 246.48°. With larger separation angles, double-helical containers behave like single-helical containers (360°), due to the dominance of the longer-span interface. A second shift in behaviour occurs at the limit angle 209.12°, where the regions of stability including the cylinder and the helicatenoids become more connected. The common structures of DNA appear to coincide with the limiting geometry at 209.12°. The most robust double-helical containers are therefore those with separation angles between approximately 210° and 240°. Experimental results roughly verify the volume maximum for near-symmetry, and more importantly verify stability to zero volume.