Superhydrophobic Particle Ejections from Liquid Surfaces in Microgravity Environments

Abstract

This paper explores a completely new avenue of microgravity fluidics that has not been systematically studied before, exploring superhydrophobic particle ejections from liquid surfaces in microgravity environments and quantifying the particle velocity varying particle. Superhydrophobic surfaces greatly reduce liquid-substrate contact. This allows for spontaneous ejection of floating particles in drop tower experiments. To quantify such phenomena, a drop tower experiment is constructed and tested. The Dryden Tower (DDT) is a laboratory facility at Portland State University that allows for the investigation of short duration exploration and research on Earth of micro-gravity phenomena similar to that aboard orbiting spacecraft. Perhaps surprisingly, this short 2.1 second period of weightlessness during free fall provides ample time for many fluids, combustion, and materials science investigations. This study employs large superhydrophobic spherical particles of varying masses of the spheres to determine ejection velocities. Simple energy analysis is shown to provide fair agreement with the experimental results. Introduction In a recent article by Wollman et al. (2016), observations of “floating spheres” (a model of macrosurfactants), macro-scale particle injections, capture, and ejections were reported from drop tower tests. Additionally, puddle jumping from superhydrophobic substrates was demonstrated by Attari et el. (2016), where puddle jump limits, times, and velocities were reported as functions of fluid properties, wetting conditions, and relatively enormous puddle volumes. Such studies provide insight into the fundamental behaviors of large length scale capillary fluidic behavior. Applications of such phenomenon may be made to fluid systems aboard spacecraft (i.e. fuel tanks, coolants, and water processing). This study aims to demonstrate superhydrophobic particle ejection from liquid surfaces in the microgravity environment and quantify ejection velocity as a function of particle mass. The experiments are simple, where a Fig. 1 ping pong ball (particle) is coated with superhydrophobic treatment establishing contact angle \uf071 ≈ 150°. Such superhydrophobic surfaces repel water with low coefficients of friction. Masses are added to the ping pong ball by drilling in a hole in its top and gluing the mass to the inside bottom surface of the ball. The ball is then floated on the surface of the water bath, balanced, and released in the drop tower, the events are recorded via high-speed video camera. The short period of weightlessness during free fall provides ample time to observe the superhydrophobic ‘particle’ ejections from the liquid surface. The Dryden Tower (DDT) at Portland State University allows for the exploration for fluids, combustion, and materials science investigations (Wollman, 2013). Methodology The experiment drop tower rig is shown in Fig. 1. The camera is secured to mounts and bolted to the aluminum rig. The water bath is secured, levelled, and backlit by a diffuse light panel. To vary the mass of the ping pong ball, a hole was drilled into its top, and a weight glued inside. This method lowers the 20mm OD the particle’s center of gravity allowing for easy alignment of the sphere during testing. The spheres were coated with Cytonix aerosol spray resulting in a superhydrophobic substrate with contact angle \uf071 ≈ 150°. The superhydrophobic sphere with radius R of 20mm was placed and partially submerged in a tank of water as shown in Fig. 2. With the camera recording, the drop capsule consisting of drag shield with the rig and experiment inside was released, retrieved, and video footage stored. Three spherical masses are tested, the results of which will be presented, following a brief review of a simplified analysis of the process. Fig. 1. Drop Tower experiment rig setup where the superhydrophobic particle (1) is centered in a liquid bath (2) is placed on the experiment rig (3) between the camera (4) and backlight (5). Fig. 2. a) Schematic of experiment geometry. The solid of radius R is the superhydrophobic particle, submerged to depth h, with unsubmerged portion of radius ĥ, submerged area a, internal angle φ, and contact angle \uf071. b) Idealized schematic of experiment geometry. Energy Analysis Surface structure has a controlling influence at the three-phase contact line region where the interfacial parameters depend on the surface energies of the solid, liquid, and gas phases (Attari et al, 2016). A surface energy balance at initial and final state allows us to estimate particle ejection velocity. We follow conventional notation for: liquid-solid ls, gas-solid gs, liquid-gas lg, mass m and particle radius R. Applying a simple surface energy balance, the spherical particle’s velocity U is quickly predicted by equating the total surface energy of the initial state E1 (static floating particle) with the total surface energy of the final energy state E2 (particle ejected at constant velocity U). The submerged height h is a spherical cap depending on of the particle’s radius R, volume of the particle V, mass m, and density of the water in the bath. Submerged heights for the experiments performed herein are listed in Table 1. From an energy standpoint, the energy before exiting the water must equal the energy as the sphere leaves the water; namely, E1 = E2. Table 1. Theoretical submerged depth h of the superhydrophobic particle, average velocity Uavg, and standard deviation. Such idealized initial and final states are depicted schematically in Fig. 4. The ejection velocity is the maximum velocity of the particle achieved the moment the particle detaches from the bath. The water in the bath is assumed ideal, with negligible dissipation, meeting the particle on a flat, infinite plane satisfying the contact angle condition \uf071. Thus, we have initial state E1 = (σA)gs1 + (σA)ls1 + (σA)lg1 + mU1 2 2 (1) and final state E2 = (σA)gs2 + (σA)ls2 + (σA)lg2 + mU2 2 2 . (2) Setting E1 = E2 and solving for U yields U = [− ( 2 m ) (1 − cosθ + 2h R cos θ)] 1 2 ⁄ (3)