Sliding-cavity fluid contactors in low-gravity fluids, materials, and biotechnology research

The well-known method of sliding-cavity fluid contactors used by Gosting for diffusion measurements and by Tiselius in electrophoresis has found considerable use in low-gravity research. To date, sliding-cavity contactors have been used in liquid diffusion experiments, interfacial transport experiments, biomolecular crystal growth, biphasic extraction, multistage extraction, microencapsulation, seed germination, invertebrate development, and thin-film casting. Sliding-cavity technology has several advantages for spaceflight: it is simple, it accommodates small samples, samples can be fully enclosed, phases can be combined, multiple samples can be processed at high sample density, real-time observations can be made, and mixed and diffused samples can be compared. An analysis of the transport phenomena that govern the sliding-cavity method is offered. During sliding of one liquid over another flow rates between 0.001 and 0.1m/sec are developed, giving Reynolds numbers in the range 0.1-100. Assuming no slip at liquid-solid boundaries shear rates are of the order 1sec(-1). The measured consequence is the transfer of 2-5% of the content of a cavity to the opposite cavity. In the absence of gravity, buoyancy-driven transport is assumed absent. Transport processes are limited to (1) molecular diffusion, in which reactants diffuse toward one another at rates that depend on their diffusion coefficient and concentration gradient (Fick's second law), (2) solutocapillary (Marangoni) flow driven by surface-tension gradients, (3) capillary flow (drop spreading) at liquid-solid three-phase lines leading to immiscible phase demixing, and (4) vapor-phase diffusive mass transfer in evaporative processes. Quantitative treatment of these phenomena has been accomplished over the past few years in low-gravity research in space and on aircraft.