Spreading, Splashing and Sparkling: Drop Impingement Phenomena on Porous Media
Speaker: Cullen Buie, Massachusetts Institute of Technology
Department: Mechanical & Aerospace Engineering
Location: Bowen Hall Auditorium 222
Date/Time: Friday, September 20, 2013, 3:30 p.m. - 4:30 p.m.
This study investigates drop impingement on porous media including thin films, paper, and soil. Experiments reveal previously unexplored impingement modes on porous surfaces designated as necking, spreading, and jetting. Dimensional analysis yields a new non-dimensional parameter, denoted the Washburn-Reynolds number, relating droplet kinetic energy and surface energy. The impingement modes correlate with Washburn-Reynolds number variations spanning four orders of magnitude and a corresponding energy conservation analysis for droplet spreading shows good agreement with the experimental results. The simple scaling laws presented will inform the investigation of dynamic interactions between porous surfaces and liquid drops for applications ranging from droplet microfluidics to inkjet printing. In addition, high-speed imaging has revealed evidence of aerosol generation during drop impingement on dry porous media including soils. After impact, tiny gas bubbles form inside the droplet, fed by air escaping the porous media. The tiny bubbles break resulting in microscale jets that quickly break up into droplets. Within a specific range of impact velocity, we observe furious ejection of tiny droplets, producing aerosol clouds above the surface. Aerosol generation can be predicted with knowledge of the surface properties and impact conditions. This work illustrates that aerosols can easily be generated on porous surfaces, with intriguing environmental and engineering implications.