Omniphobic surfaces, which repel polar and non-polar liquids alike, have proven of value in a myriad of applications ranging from piping networks, textiles, food and electronics packaging, and underwater drag reduction. A limitation of currently employed omniphobic surfaces is their reliance on perfluorinated coatings/chemicals, increasing cost and environmental impact and preventing applications in harsh environments. Thus, there is a keen interest in rendering conventional materials, such as hydrocarbon-based plastics, omniphobic by micro/ nanotexturing rather than via chemical makeup, with notable success having been achieved for silica surfaces with doubly reentrant pillars (DRPs). We discovered a critical limitation of DRPs – they catastrophically lose superomniphobicity in the presence of localized physical damages/defects or on immersion in wetting liquids. In response, we pioneered bio-inspired gas-entrapping microtextured surfaces (GEMS) architecture composed of doubly reentrant cavities (DRCs). DRCs are capable of robustly entrapping air when brought into contact with liquid droplets or on immersion, which prevents catastrophic wetting transitions even in the presence of localized structural damage/defects. This dissertation presents our multifaceted research on DRCs via custom-built pressure cells, confocal laser scanning microscopy, environmental scanning electron microscopy, contact angle goniometry, high-speed imaging, and upright optical microscopy. Specific accomplishments detailed in this thesis include: (i) the microfabrication protocols for silica GEMS developed at KAUST; (ii) the characterization of GEMS’ omniphobicity via apparent contact angles and immersion; (iii) the demonstration of ~ 1000,000,000% delays in wetting transitions in DRCs compared to those in simple cavities (SCs) under hexadecane; (iv) a proposal for immersion of surfaces as a criterion for assessing their omniphobicity in addition to apparent contact angles; (v) effects of surface chemistry, hydrostatic pressure, and cavity dimensions on Cassie-to-Wenzel transitions in DRCs and SCs; (vi) the demonstration of “breathing” (liquid-vapor) interfaces in GEMS under fluctuating hydrostatic pressures; and (vii) the demonstration of directional wetting transitions in DRCs (or cavities in general) arranged in one- and two-dimensional lattices. The last chapter in the thesis presents future research directions such as breathing surfaces capable of preempting vapor condensation and gas replenishment.

<p>  </p> <p>Water scarcity is a global problem impacting a majority of the world population. A significant proportion of the global population is deprived of clean drinking water, an impact felt by the rural as well as urban population. Saltwater desalination provides an attractive option to produce clean water. Some technologies to generate potable water include reverse osmosis (RO), multi-stage flash distillation (MSF), vapor compression distillation and multi-effect distillation (MED). Distillation plants such as those in MED often have falling-film evaporators operating at low energy conversion efficiency and hence distillation is performed over multiple stages (or effects). Porous materials can be utilized as evaporators in such plants with the objective of leveraging their superior efficiency. This can potentially decrease the number of effects over which distillation occurs. However, evaporation of high-salinity salt solution eventually results in salt precipitation which can cause fouling and induce structural damages, especially if the precipitates appear within the porous medium. Crystallization-induced structural damages are also of significant concern to building materials and for their role in weathering of historical monuments. It is thus crucial to understand the mechanisms governing salt precipitation in a porous medium.</p> <p>Transport of solute in such a medium is either driven by flow of the solution (advection) or by concentration gradients (diffusion). The dynamics of solute transport is further complicated due to the involvement of a reaction term accounting for any salt precipitation. The relative strengths of these driving forces determine the solute transport behavior during an evaporation-driven process. The wide-scale applications of solute transport and its complicated nature warrant investigation, both experimental and theoretical, of the dependence of solute transport and the subsequent precipitation on the operating conditions and the properties of the porous medium.</p> <p>This dissertation first focuses on developing a novel modeling framework for evaluating the transient behavior of the solute mass fraction profile within the domain of a one-dimensional porous medium, and extending its capability to predict the formation of salt precipitate in the medium.  Experimental investigations are then performed to study the formation of precipitate on sintered porous copper wicks of different particle-size compositions, and developing a mechanistic understanding of the governing principles.</p> <p>A numerical modeling framework is developed to analyze evaporation-driven solute transport. Transient advection-diffusion equations govern the salt mass fraction profile of the solution inside the porous medium. These governing equations are solved to obtain the solute mass fraction profile within the porous medium as well as the effloresced salt crust. Further accounting for precipitation allows a study of the formation and growth of efflorescence and subflorescence. Crystallization experiments are performed by allowing a NaCl solution to evaporate from a porous medium of copper particles and the subflorescence trends predicted by the model are validated. The modeling framework offers a comprehensive tool for predicting the spatio-temporal solute mass fraction profiles and subsequent precipitation in a porous medium.</p> <p>The dependence of efflorescence pattern on the properties of a porous medium is also investigated. Efflorescence patterns are visually observed and characterized on sintered copper particle wicks with spatially unimodal and bimodal compositions of different particle sizes. Efflorescence is found to form earlier and spread readily over a wick made from smaller particles, owing to their lower porosity, while it is limited to certain areas of the surface for wicks composed of the larger particles. A scaling analysis explains the observed efflorescence patterns in the bimodal wicks caused by particle size-induced non-uniform porosity and permeability. The non-uniformity reduces the advective flux in a high-permeability region by diverting flow towards a low-permeability region. This reduction in advective flux manifests as an exclusion distance surrounding a crystallization site where efflorescence is not expected to occur. The dependence of this exclusion distance on the porosity and permeability of the porous medium and the operating conditions is investigated. A large exclusion distance associated with the regions with bigger particles in the bimodal wicks explains preferential efflorescence over the regions with smaller particles. This novel scaling analysis coupled with the introduction of the exclusion distance provides guidelines for designing heterogeneous porous media that can localize efflorescence.</p> <p>Additionally, droplet interactions with microstructured superhydrophobic surfaces as well as soft surfaces were investigated during the course of this dissertation, separate from the above investigations. These investigations involve the interplay of surface energies with electrical or elastic energies and are studied both experimentally and through theoretical models, and therefore are retained as additional chapters in the thesis as being of relevant interest.  Electrowetting experiments are performed on superhydrophobic surfaces with re-entrant structures to study their resilience to the Cassie-to-Wenzel transition. The deformation of soft surfaces caused by forces exerted by microscale droplets is studied and the resulting interaction between multiple droplets is explored. </p>