Water and complex fluid drop dynamics on vibrating surfaces

Abstract

This PhD thesis explores the wetting dynamics and behavior of water and complex fluid droplets on vibrated surfaces, focusing on how substrate vibrations can be applied to control droplet dynamics, spreading, evaporation, and particle self-assembly. Central to this investigation is the design and construction of an experimental setup to study droplet-based systems. The constructed experimental setup is capable of performing comprehensive studies on drop shape analysis, center of mass tracking, drop vibration spectroscopy, Particle Image Velocimetry (PIV), and laser speckle analysis (LSA). The methodology employed in this research involved the meticulous design of experimental setups to control and measure droplet behavior under various conditions. High-speed imaging was used for drop shape analysis, while advanced tracking algorithms were implemented for center of mass movements. Precision instruments for vibration spectroscopy allowed for detailed analysis of droplet oscillations. PIV provided comprehensive flow visualizations within the droplets, capturing the complex internal dynamics induced by vibrations. LSA offered high-resolution insights into the micro and nanoscale dynamics of nanoparticles during evaporation, enabling a detailed understanding of the underlying processes. The findings reveal that surface properties significantly affect the motion dynamics of pure water droplets. For instance, elastic substrates showed reduced asymmetric deformation and facilitated more regular internal flow, leading to less bulk friction compared to more traditional rigid substrates. Detailed experiments demonstrated that on superhydrophobic surfaces, droplets exhibit a terminal speed, allowing for consistent droplet motion with minimal influence from droplet volume or vibration parameters. Internal flow dynamics was explored to provide a deeper understanding of the interaction between droplet and substrate. The research indicates that these factors are crucial in determining the efficiency of droplet transport and stability across different surfaces.For complex fluids, two primary topics were investigated: drop spreading and evaporation-induced self-assembly. Experimental investigations revealed that nanofluid drops exhibit unique spreading behaviors under vibrational forces, which are not observed with pure water drops. Studies on the effect of hydrophilic and hydrophobized nanofluid drops showed that nanoparticles significantly alter the drop’s energy landscape, influenced by NPs-surface interactions that control the spreading process. Vibration amplitude and particle concentration effects were meticulously analyzed to build comprehensive drop spreading diagrams based on different spreading regimes. These diagrams highlighted the critical parameters that influence spreading dynamics, offering new insights into the behavior of complex fluids under vibrational forces. Furthermore, the impact of vibration on the evaporation dynamics and NPs deposition on superhydrophobic surfaces was rigorously explored. LSA was successfully employed to study NPs dynamics with improved temporal and spatial resolution, revealing the intricate patterns of particle movement and deposition. Detailed morphological and microstructural analyses via Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) confirmed that vibrations lead to different NPs arrangements inside the deposit. The use of vibrations resulted in smoother and more uniform deposits compared to static conditions, suggesting a method for controlling deposition patterns. Significant results include the observation that vibrations enhance the evaporation rate of droplets by increasing the effective surface area and inducing more efficient mixing within the droplet. This accelerated evaporation was particularly notable in nanofluid droplets, where the presence of nanoparticles further altered the thermal and flow dynamics. The experimental data showed that the combined effect of vibrations and nanoparticles leads to a distinct evaporation rate. In conclusion, the research presented in this thesis provides substantial contributions to the understanding of droplet dynamics under vibrational forces. The comprehensive experimental and analytical approach adopted in this study offers new perspectives on the behavior of both pure water and complex fluid droplets, emphasizing the critical role of substrate vibrations. By advancing the knowledge of droplet dynamics on vibrated surfaces, this thesis lays the groundwork for further research and development in this field, enhancing our understanding of fluid dynamics and particle deposition processes.