ISS Microgravity Experiments on Flat and Perforated Capillary Plates

377 pagesTo inform the development of numerical simulations of complex capillary phenomena, we observed the microgravity spreading and imbibition of relatively large water drops on planar surfaces that were either flat, or perforated by hydrophilic, unconnected, parallel cylindrical capillaries of 1 mm diameter covering a fraction ? of the surface. Apart from two unintentional tests, the surfaces were initially dry. When a drop touches a porous medium, it spreads as if laid on a composite surface. The medium first behaves as a hydrophobic material, since liquid must penetrate pores filled with air. When contact is established, some of the liquid is drawn into pores through a capillarity that is resisted by viscous forces growing with depth of the imbibed region. The spreading process always begins with an inertial regime that depends on drop size, liquid density, and liquid-gas interfacial tension. Because time and distance must be shrunk to mitigate gravity-induced distortion, it is difficult to observe details of this rapid initial regime on Earth. In contrast, microgravity on the International Space Station allowed us to stage slower experiments involving relatively large water spheres of 12 mm diameter. These drops spread on, - and with ?=0.25 and ?=0.50 imbibed in -, nine kinds of perforated plates made of electroless gold-coated brass treated with Self-Assembled Monolayers (SAM) of known advancing and receding contact angles. In each test, astronaut Luca Parmitano slowly extruded a water drop from a pipette until it touched the plate. The drop had a large enough diameter for available high-speed video cameras to visualize its curvature, spreading rate, dynamic contact angle, and imbibition rate at 127 frames per second. The data confirmed numerical results of Frank and Perré (Phys. Fluids 24, 2012) showing that the contact patch radius spreads as a power-law of time, even for strongly imbibing plates. The imbibition rate data also pointed to limitations of the ab initio spherical cap model of Louge and Sahoo (AIChE J 63, 2017). The dynamic contact angle behavior was surprising. It did not conform to the models of Cox (J. Fluid Mech. 168, 1986) or Kistler (1993) on flat surfaces. On the most hydrophilic perforated plates, the angle transitioned from the dry Cassie-Baxter effective angle to its wet counterpart (also respectively called non-hysteretic regimes VI and IV in the statistical mechanics of Louge (Phys Rev. E 95, 2017), as the imbibing drop receded before disappearing altogether. However, on plates coated with the least hydrophilic SAM, the dynamic contact angle remained near or above the dry Cassie-Baxter angle. The serendipitous repetition of an experiment on a flat plate covered with a thin water film from a previous test confirmed the observation of Biance, et al (Phys. Rev. E 69, 2004) that the inertial phase of spreading is unaffected by presence of such film. However, a similar accidental repetition on a plate with ?=0.50 at the most hydrophobic SAM revealed substantially different behavior than on the same initially dry plate, namely that the dynamic contact angle freely oscillated between the dry and wet Cassie-Baxter angles. Lastly, we monitored the time-history of curvature at the neck above the contact patch to test the conjecture of Biance, et al that it is inversely proportional to the square of the patch radius. We could only confirm this conjecture for ?=0.25 and a dry Cassie-Baxter angle of 67, and for a numerical simulation on a flat plate with a similar angle. For these conditions, both experiment and simulation exhibited a contact patch power-law exponent near one half, as predicted by the model of Biance, et al. However, in other cases where we could determine curvature, the latter did not uphold the conjecture