![]() ![]() Increases in free drop properties are larger: drop volumes ~10 18-fold, capillary response times ~10 9-fold, and viscous times ~10 12-fold. In microgravity, wall-bound drop volumes increase ~10 9-fold, capillary response times increase ~10 4-fold, and viscous settling times increase ~10 6-fold. From such relationships it is easy to see how significant reductions in gravity level dramatically increase liquid volumes that might remain categorized as ‘droplets.’ For example, in the nearly weightless environment of orbiting or coast spacecraft, local body force accelerations are indeed low, with g ~ 10 −8 g o reported for free fliers 2, and with ‘microgravity’ conditions common for crewed vehicles where g ~ 10 −6 g o. In either case, capillary oscillation and viscous settling times are characterized by τ cap ~ ( ρ l V/ σ) 1/2 and τ visc ~ V 2/3/ ν, respectively, where ν is the kinematic viscosity of the liquid. Thus, in terrestrial environments, wall-bound drop volumes are approximately limited to V ~ g −3/2 and free drops to V ~ g −3. In terrestrial environments, U is often dependent on g i.e., for falling drops of characteristic radius R, U ~ (8 ρ l Rg/3 ρ g) 1/2, and V \(\lesssim\) (27/2)( σ/ρ l Rg) 3 larger drops break up until the latter condition is satisfied. For free drops, inertial forces such as the drag from the surrounding gas of density ρ g and characteristic velocity difference U can also quickly overwhelm those of surface tension such that we expect drop volumes limited to V \(\lesssim\) 4 4( σ/ ρ g U 2) 3. Such large drops are better described as puddles 1, with drops better identified for small volumes V \(\lesssim\) ( σ/ ρg) 3 /2, where σ is the liquid surface tension, ρ is the density difference across the liquid interface (≈ ρ l, the density of the liquid), and g is the acceleration field strength, i.e., gravity with g o = 9.8 m/s 2. For poorly or nonwetting wall-bound liquid drops in the air, the force of gravity quickly overwhelms that of surface tension as drop volumes increase. In general, provided buoyancy and inertia are sufficiently low, wall-bound drops and free drops assume constant curvature minimal surface energy states attributed to capillary forces due to surface tension. Liquid drop dynamics is a large field of research within the fluid mechanics discipline. ![]() The method is ideal for hand-held non-oscillatory ‘droplet’ generation in low-gravity environments. We then demonstrate how such geometries may be employed as passive no-moving-parts droplet generators for very large drop dynamics investigations. We control ejection velocities as a function of drop volume, substrate tilt angle, initial confinement, and fluid properties. In our experiments, the brief nearly weightless environment of a 2.1 s drop tower allows for the study of such capillary dominated behavior for up to 10 mL water drops with migration velocities up to 12 cm/s. Our study focuses on the migration and ejection of large inertial-capillary drops confined between tilted planar hydrophobic substrates (a.k.a., wedges). Such capillary phenomena are exploited for passive phase separation operations in micro-fluidic devices on earth and macro-fluidic devices aboard spacecraft. When confined within containers or conduits, drops and bubbles migrate to regions of minimum energy by the combined effects of surface tension, surface wetting, system geometry, and initial conditions.
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