Abstract
Compliant structures, particularly soft thin films and deformable material systems, are increasingly important in flexible electronics, soft robotics, adaptive devices, and environmental sensing. However, their fabrication still relies largely on rigid-substrate-based processes that often require multistep transfer, induce residual stress, and limit compatibility with deformable or fluidic environments. This dissertation develops a mechanics-guided framework for the liquid-mediated fabrication of compliant structures and applications, in which liquids are treated as active media for material assembly, direct printing, structural deformation, and functional operation.
The first part of the dissertation investigates the mechanics of liquid-mediated assembly on solid surfaces through droplet drying of graphene sheets. An energy-based rotational spring–mechanical slider model is developed to capture the coupled effects of sheet deformation, van der Waals adhesion, evaporation-induced flow, and droplet drying mode on final assembly patterns. Coarse-grained molecular dynamics simulations are used to validate the theoretical predictions and to analyze the effects of sheet size, concentration, initial contact angle, and substrate interaction strength. This study provides a mechanics-based understanding of how drying-mediated liquid processes govern the formation of compliant nanostructures on solids.
The second part advances from assembly on solids to direct printing on liquid surfaces. A liquid-surface fabrication strategy, HydroSpread, is introduced for creating ultrathin polymeric soft films directly on water. A theoretical framework is established for the spreading dynamics of ink droplets on liquid surfaces, identifying the roles of interfacial tension imbalance, spreading coefficient, viscous dissipation, and droplet geometry in film formation. Experiments demonstrate the fabrication of highly uniform films with ultralow surface roughness, together with direct laser engraving of structures and patterns on liquid-supported films. These results establish water as a mechanically unconstrained platform for direct fabrication of compliant floating structures.
The third part examines the mechanics of compliant structures at liquid interfaces, focusing on thermally induced deformation in bilayer composite strips floating on water. Upon heating, these structures exhibit two distinct deformation modes, bending and buckling. Theoretical models are developed to differentiate the two modes and to quantitatively characterize them using the bending angle and the amplitude-to-buckled-length ratio. Comparison with experiments shows good agreement and clarifies how material mismatch, strip geometry, and liquid-interface support govern mode selection and deformation magnitude.
The final part extends this framework to compliant structures under surface perturbation and their application in floating sensors. An integrated floating sensor is developed for localized monitoring of carbon dioxide at the seawater surface, together with colocated measurements of temperature and salinity. The ultrathin compliant platform integrates a central CO2 sensing chamber, a bioinspired inlet–microfluidic collection module, an impedance-based salinity sensor, and a resistance-based temperature sensor into a single floating architecture that remains conformal to the dynamic water surface. Supporting theoretical and experimental studies establish the separation criterion under wave disturbance, the interfacial water collection mechanism, the robustness of the sensing modules under motion and perturbation, and a temperature- and salinity-coupled compensation framework for reconstructing local CO2 concentration.
Overall, this dissertation establishes liquid-mediated mechanics as a general framework for the assembly, fabrication, deformation, and application of compliant structures. The results provide a mechanics-guided foundation for designing compliant interfacial systems for soft robotics, flexible devices, and marine environmental monitoring.
