Effects of additive manufacturing on capillary-driven fluid flow for provision of water and nutrients to free floating plants
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An autonomous environmentally controlled floating plant cultivation system needs robust water and nutrient delivery for use in microgravity. Passive fluid control in the microgravity environment can be accomplished through the exploitation of capillary forces in various geometries, such as a network of isolated interior corners. Capillary driven flow largely depends on the surface properties of the material. Due to the Concus-Finn condition, capillary flow designs using interior corners are limited to a range of contact angles and interior corner half-angles which meet this condition. Additive manufacturing – particularly through the fused deposition method – introduces a new dimension to this design space in that the surface structure of the additively manufactured system is axially dependent. The resulting anisotropy in surface properties due to the layer-by-layer deposition of material means that the contact between a fluid and a particular surface depends upon the surface's orientation with respect to the print axis. This study develops an empirically derived relationship between controllable variables in the additive manufacturing process (particularly, layer height and axial orientation) and their effects on capillary flow rates of water through given channel geometries constructed from plastic and metal materials. These rates are compared to the rate of evaporation and the uptake requirements for sustenance of duckweed, as well as those from established and validated modeling methods that simulate fluid surface interactions between the fluid and material for a diverse set of geometries in order to identify whether or not the empirical relationship is material agnostic.