MechE PhD Final Oral Defense: Garam Lee

TITLE: IMPACT, SPREADING, AND DRYING OF COMPLEX FLUID DROPS

ABSTRACT: A liquid drop impacting a surface leaves behind a footprint that encodes the history of its deformation, transport, and drying. Yet the final stain is not determined by impact alone. It emerges from a sequence of coupled processes—rapid inertial spreading, capillary-driven relaxation, contact-line motion, imbibition into porous substrates, evaporation, and, in complex fluids, compositional redistribution. This thesis investigates how these mechanisms collectively shape the observable footprint and how that footprint can be interpreted in terms of underlying physics. Experiments with human blood on non-porous surfaces show that the maximum spreading reached during impact does not necessarily fix the final stain size or shape. Contact-line mobility during post-impact relaxation can reduce or modify the footprint, challenging assumptions that the dried diameter directly reflects impact conditions. When impacts occur obliquely, asymmetric features such as narrow terminal tails arise from transient momentum redistribution. These features provide additional geometric constraints that allow impact velocity, angle, and drop size to be disentangled from a single dried stain. On porous substrates, the surface actively transports liquid through capillary suction while evaporation simultaneously depletes the finite droplet volume. By systematically varying droplet size and liquid volatility, a universal description of coupled wicking and drying dynamics is identified, and a model is developed that predicts both the maximum wetted radius and the characteristic drying time. Extending this framework to binary mixtures reveals how differential mobility and substrate interactions generate spatial segregation and alter deposition patterns beyond single-component expectations. Together, these results establish a unified physical picture in which droplet footprints are understood as the cumulative outcome of impact, transport, and evaporation across multiple timescales. This perspective provides a mechanistic foundation for interpreting stains and for predicting wetting outcomes in applications ranging from forensics to engineered porous materials.

COMMITTEE: ADVISOR Professor Jacy Bird, ME/MESE; CHAIR Professor Sean Andersson, ME/SE; Professor Douglas Holmes, ME/MSE; Professor Abigaiil Plummer, ME/MSE; Professor Daniel Attinger, ME