Sushmitha Vinikumar

• Interfaces play a crucial role in various physical and chemical phenomena, such as catalysis, molecular recognition, charge transfer, adsorption, and diffusion. While the role of external electric fields in controlling interfacial behavior, such as flow control and droplet manipulation has been well explored, the influence of surface charge on these processes is less understood. Surface charge can arise naturally, as in ferroelectric materials, or be artificially induced through the triboelectric effect in polymers. This research aims to bridge the gap in understanding how surface charge affects key interfacial processes like wetting, spreading, and evaporation. The present study investigates the dynamic interactions at the interface between fluids and charged surfaces, focusing on the interaction between water and lithium niobate (LN), an intrinsically charged ferroelectric material. LN, a ferroelectric material with an exceptionally high surface charge density (theoretically 0.7 C/$\mathrm{m^2}$), was chosen as the primary material for this study. The study begins by investigating classical interface phenomena, including electroosmosis, specifically on a $z$-cut LN surface. Electroosmotic flow in LN-based microchannels is compared with that in glass channels, highlighting the role of spontaneous polarization and surface charge in modulating flow velocity. The study then examines the wetting dynamics of a water droplet on a $z$-cut LN surface, focusing on how polarization direction and surface charge influence its spreading behavior. Additionally, it explores the effects of adsorption and ferroelectric aging on wetting characteristics. The study also investigates the evaporation behavior of water droplets on $z$-cut LN surfaces, examining how factors such as relative humidity and surface polarization direction influence evaporation rates. The evaporation rate on the $z$-cut LN surface is then compared to that of other materials with similar contact angles to assess the unique role of surface charge in evaporation. The final part of the investigation focuses on the fabrication of hydrophobic, photocatalytically active antibacterial surfaces using PDMS and LN. The hydrophobic surface aims to minimize bacterial adhesion, while the photocatalytic activity targets bacterial disinfection. Different fabrication parameters, such as LN concentration, illumination conditions, and surface roughness, are optimized to enhance antibacterial performance. The antibacterial properties of these surfaces were tested using two bacterial strains: \textit{Escherichia coli} DH5$\alpha$ and \textit{Pseudomonas aeruginosa} PA14. This study investigated the often-overlooked impact of surface charge on interfacial processes such as wetting and evaporation. The findings of the study reveal that, despite LN's significantly large surface charge, it does not translate into an exceptionally high zeta potential due to charge neutralization in the Stern layer and a minimal diffuse layer, which limits its potential for electroosmotic flow. The droplet spreading dynamics of LN deviate from classical Tanner's law, with an exponential spreading phase dominating. In addition, the presence of an adsorbed layer substantially reduces the contact angle, accelerating the spreading process. These findings emphasize the importance of cleaning procedures and storage conditions for wetting studies and suggest broader relevance for understanding charge-driven spreading processes, such as electrowetting. The evaporation process on LN involves three stages: constant contact radius, mixed, and stick-slip phases. The stick-slip behavior on the smooth LN surface, even with pure water, indicates strong pinning caused by surface charge and high water affinity. The droplet evaporation studies show that polarization direction significantly affects the initial contact angle, evaporation rate, and total evaporation time, while relative humidity impacts the wettability of the +$z$-cut surface but not the -$z$-cut surface. Antibacterial studies demonstrate that LN-based surfaces can be used as light-stimuli-sensitive, requiring UV illumination for activation, making them effective for reducing bacterial transmission on frequently touched surfaces. Furthermore, the potential for LN's polarization-specific surface chemistry and wettability alterations could be leveraged in fabricating nanoscale devices, sensors, and biocompatible materials for tissue engineering. LN-based materials also hold promise for applications in energy harvesting from droplet motion and stimuli-responsive antibacterial therapeutics, with further research needed to optimize these effects.