Micro-PIV analysis of the fluid velocity boundary layer over hydrophobic and superhydrophobic aerogels

Variation in fluid velocity due to velocity boundary layers generates internal friction drag via removing kinetic energy from flow in motion.  In order to maintain fluid flow, momentum must be added to the fluid via mechanical pumps. In an effort to decrease the energy necessary to move fluids, there have been experiments devised to generate surfaces capable of reducing the drag, by decreasing the volume of the boundary layer.  Several studies have reported slip velocities (i.e. fluid velocities along the solid-liquid interface) of up to 40% of the free stream velocity on textured, superhydrophobic surfaces due to drag reduction techniques. Furthermore, some of these surfaces were synthesized by adhering superhydrophobic aerogel particles to solid surfaces.  However, some particles are dislodged by the flow over time, resulting in decreased slip velocity. The purpose of this project was to design and test an experiment capable of measuring slip over monolithic superhydrophobic aerogels. Silica aerogels of varying hydrophobicity were generated using Union College’s patented rapid supercritical extraction technique.  The co-precursor method was utilized to control the hydrophobicity of the aerogels. The total mass of the two precursors, tetrmethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) was kept constant and the percent by mass of each precursor was varied. Increased concentrations of MTMS render aerogels more hydrophobic. It was determined that aerogels with 5% MTMS by mass had hydrophobic contact angles near 135˚, while aerogels with 7.5% and 10% MTMS by mass had contact angles near 155˚.  Additionally, a water channel was designed and constructed to allow the monolithic aerogel samples to be placed such that the surface of the aerogel is in plane with the bottom interior surface of the water channel. Micron-scale particle image velocimetry and 0.97-micron fluorescent microspheres were used to image the flow and extract velocity information.