Abstract

Water is often found adhering to surfaces in droplet form. Droplets can develop as a result of precipitation, condensation, or chemical reactions. Drops are retained or held on a surface due to a retention force exerted on the drop by the surface. This retention force is a function of the surface tension of the liquid, drop geometry, and the contact angle between the drop and the surface. When external or body forces exceed this retention force the drop begins to move. This thesis seeks to understand and quantify the forces present at the point of droplet departure once it begins to move under an applied force. The conditions of drop departure in shear flow are explored for five microstructured superhydrophobic surfaces, one smooth hydrophobic surface, and one carbon nanotube (CNT) surface. Surface solid fractions range from 0.05 - 1.00, and static contact angles range from 121.4 - 160.6°. Droplet volumes of 5, 10, 20, 30, 40 and 50 µL are tested on each surface. For each experiment, a shear airflow is applied to a droplet placed on a surface. Shear air velocities start at zero and are increased at a consistent speed until the droplet departs. Highspeed imaging is used to track the droplets shape and position with respect to time, and a mass flowmeter is used to track bulk air velocity with respect to time. Particle imaging velocimetry (PIV) measurements were carried out to characterize the velocity profile of the shear airflow, and an appropriate velocity profile is calculated for any given flowrate by interpolating/extrapolating within PIV measurements. The condition of drop departure is defined as the point at which a droplet's acceleration no longer oscillates about zero and instead increases monotonically. As the applied shear force increases until drop departure occur, measurements of droplet base length, height, and cross section area (as viewed from the side) indicate that these quantities remain approximately constant throughout the length of the test. Measurements of advancing and receding contact angles are reported at the point of departure, with increasing contact angle hysteresis (difference between advancing and receding contact angles) observed until departure occurs. Contact angle hysteresis is shown to be a good indicator of droplet mobility. Measurements of bulk air velocity upstream of the droplet and measurements of the average air velocity over the height of the droplet are both reported at the point of departure. Bulk air velocity at the point of departure is shown to vary with droplet volume, while the average air velocity over the droplet height at the point of departure is shown to be independent of droplet volume. Both measures of air velocity decrease with decreasing surface solid fraction, and decrease most rapidly at low solid fractions. Retention forces are calculated using a retention force factor and vary from 10 - 64 µN. Retention force is shown to be roughly constant for a given droplet base length. An appropriate coefficient of drag is calculated in the range of 0.55 to 2.66 for droplet Reynolds numbers of 225 - 908. The coefficient of drag is shown to be independent of droplet Reynolds number, and to be approximately constant for a given droplet Bond number.

Degree

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

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