Experimental study of liquid-infused surfaces in turbulent and laminar flow regimes

QC250122

Abstract

In natural and industrial fluid dynamics, wall-bounded turbulent flow over and along surfaces are the norm. Engineering surfaces to modulate these flows, for example with the purpose to reduce drag, can play a significant role in enhancing operations efficiency and performance. Liquid-infused surfaces (LISs) are a recent surface modification approach utilizing surface structures infused with a fluid immiscible with the overlying fluid. LISs are thereby lubricated, offering the potential of reducing drag and surface fouling. However, the practical implementation of LISs faces challenges due to lubricant drainage under high shear stresses and the detrimental effects of contaminants, such as surfactants, which compromise their performance. Numerical studies dominate the field, but there remains a pressing need for experimental insight and data to validate these findings and guide future research directions. This thesis addresses these challenges by using experimental techniques to investigate LISs performance in turbulent and laminar flow regimes.  

A major contributions of this thesis are the setup and validation of the experimental facility for turbulent flows over larger surfaces, named F-SHARC, and the adoption of Doppler-optical coherence tomography (D-OCT) for detail studies of the fluid motion. In this thesis, the F-SHARC facility has been used in a parametric study focusing on the lubricant retention.In turbulent regimes, the thesis demonstrates that contact-angle hysteresis, rather than chemical compatibility, can serve as a powerful retention mechanism, extending the lifespan of LISs under shear. A theoretical model predicting the maximum retention length of lubricant droplets, based on interfacial forces and flow dynamics, has been developed and validated. Fluorescence imaging and numerical simulations complement each other in the understanding of the physical mechanisms that regulate the droplets' formation and shape.

In the laminar regime, slip velocity measurements with D-OCT demonstrated the substantial impact of surfactants, which rigidify the liquid-liquid interface and drastically reduce the slip length. Experimental results suggest that even minor contamination can impair LISs performance by inducing Marangoni stresses. These add to the overlying fluid interfacial stress and effectively oppose the flow, therefore increasing the drag. A comprehensive study of slip length definitions further refines the understanding of interfacial dynamics, reconciling discrepancies between experimental and numerical approaches.  

This thesis highlights the potential of partially wetting LIS designs to overcome conventional limitations and contributes to advancements for scalable, durable applications in harsh environments. By combining insights from experimental studies with understanding from numerical studies and theory, this work contributes to the understanding of LIS dynamics, offering practical design principles for their broader implementation. These findings can be extended to multi-phase flows and to the exploration of bio-compatible, sustainable LIS materials for use in marine and biomedical applications. 

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