Intricate Dynamics of Droplet-Substrate Interactions Beyond Conventional Limitations

Droplet dynamics, encompassing relatively static processes such as evaporation to more vigorous phenomena like self-propelled jumping, are of considerable interest due to their significance in both natural phenomena and practical applications. These behaviors are pivotal in facilitating mass, momentum, and energy transfer between droplets and their surroundings, with applications spanning phase-change heat transfer, material transport, surface engineering, and energy optimization. While droplet dynamics have been extensively studied over the past several decades, advancements in surface engineering, such as the development of functional surface materials, have introduced novel mechanisms governing droplet behavior. These complex droplet-substrate interactions exhibit intricate dynamics that transcend conventional understanding and remain inadequately explored. This dissertation investigates the intricate dynamics of droplet-substrate interactions, spanning processes from evaporation to out-of-surface jumping, offering insights into the interplay of thermal, capillary, and inertial forces that govern these phenomena. The evaporation of sessile water droplets on heated microstructured superhydrophobic surfaces is experimentally and theoretically explored across a temperature range of 20 °C to 120 °C. A thermal circuit model is developed to decouple heat and mass transfer contributions from the droplet cap and base. The findings reveal that substrate roughness and temperature significantly influence evaporation behavior, with suppressed boiling observed due to evaporative cooling. The results elucidate the role of substrate microstructures in modulating heat transfer pathways, advancing the understanding of evaporation dynamics on non-wetting surfaces. As the substrate temperature increases, vapor bubbles form at the droplet base, transitioning the droplet into the nucleate boiling regime. At relatively low temperatures, droplets exhibit versatile jumping behaviors similar to the high-temperature Leidenfrost effect. Unlike the traditional Leidenfrost effect, which occurs above 230 °C, fin-array-like micropillars enable water microdroplets to levitate and jump off the surface within milliseconds at just 130 °C, triggered by the inertia-controlled growth of individual vapor bubbles at the droplet base. The droplet jumping, driven by momentum interactions between the expanding vapor bubble and the droplet, can be modulated by adjusting the thermal boundary layer thickness through pillar height. This allows for precise control over bubble expansion, switching between inertia-controlled and heat-transfer-limited modes. These two modes lead to distinct droplet jumping behaviors: one characterized by constant velocity and the other by constant energy. Bubble expansion provides an effective method for achieving droplet out-of-surface jumping. To better understand the gas-liquid-substrate three-phase interactions, we inject an air bubble into a sessile droplet to explore the bubble burst-induced droplet jumping process. Upon bubble bursting, the surface energy released from both the inner and outer surfaces of the bubble drives the droplet jumping. Specifically, the bursting bubble generates capillary waves that propagate nearly vertically towards the substrate, causing the droplet to retract with minimal spreading upon impact with the capillary waves. When sufficient surface energy is released, this bubble burst-based strategy facilitates efficient momentum transfer through direct and localized capillary wave-solid surface interactions, enabling the lifting of large puddle droplets on the centimeter scale.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Mechanical Engineering

Department dc:contributor.department

Mechanical Engineering

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2025

Author dc:creator

• Huang, Wenge

Chair dc:contributor.committeechair

• Cheng, Jiangtao

Committee members dc:contributor.committeemember

• Zhou, Wei
• Boreyko, Jonathan B.
• Paul, Mark R.
• Liu, Yang

• droplet bubble capillary wave evaporation boiling jumping

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Dc Identifier Other

vt_gsexam:42435

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/123910