Abstract
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. Such capillary phenomena are exploited for passive phase separation operations in micro-fluidic devices on earth and macro-fluidic devices aboard spacecraft. Our study focuses on the migration and ejection of large inertial-capillary drops confined between tilted planar hydrophobic substrates (a.k.a., wedges). 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. We control ejection velocities as a function of drop volume, substrate tilt angle, initial confinement, and fluid properties. We then demonstrate how such geometries may be employed as passive no-moving-parts droplet generators for very large drop dynamics investigations. The method is ideal for hand-held non-oscillatory ‘droplet’ generation in low-gravity environments.
Highlights
Liquid drop dynamics is a large field of research within the fluid mechanics discipline
Such large drops are better described as puddles[1], with drops better identified for small volumes V t (σ/ρg)3/2, where σ is the liquid surface tension, ρ is the density difference across the liquid interface (≈ρl, the density of the liquid), and g is the acceleration field strength, i.e., gravity with go = 9.8 m/s2
The hydrophobic wedge geometry provides an attractive degree of control for use as a passive, large volume, low-g drop generator
Summary
Liquid drop dynamics is a large field of research within the fluid mechanics discipline. Capillary oscillation and viscous settling times are characterized by τcap ~ (ρlV/σ)1/2 and τvisc ~ V2/3/ν, respectively, where ν is the kinematic viscosity of the liquid 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 low, with g ~ 10−8go reported for free fliers[2], and with ‘microgravity’ conditions common for crewed vehicles where g ~ 10−6go. As the puddle begins to recoil following the step reduction of g-level during the drop tower test, a capillary pressure gradient is established in the partially confined liquid which drives it out of the wedge as a large drop, largely free of oscillations. We highlight design guides for large quiescent drop generators for follow-on research conducted in drop towers as well as in other reduced gravity facilities (i.e., parabolic aircraft, suborbital rockets, and spacecraft)
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