Fundamentals and Applications of Surface Wetting.

In an era defined by an insatiable thirst for sustainable energy solutions, responsible water management, and cutting-edge lab-on-a-chip diagnostics, surface wettability plays a pivotal role in these fields. The seamless integration of fundamental research and the following demonstration of applications on these groundbreaking technologies hinges on manipulating fluid through surface wettability, significantly optimizing performance, enhancing efficiency, and advancing overall sustainability. This Review explores the behavior of liquids when they engage with engineered surfaces, delving into the far-reaching implications of these interactions in various applications. Specifically, we explore surface wetting, dissecting it into three distinctive facets. First, we delve into the fundamental principles that underpin surface wetting. Next, we navigate the intricate liquid-surface interactions, unraveling the complex interplay of various fluid dynamics, as well as heat- and mass-transport mechanisms. Finally, we report on the practical realm, where we scrutinize the myriad applications of these principles in everyday processes and real-world scenarios.

Estimation of the proximal temperature rise of an excited upconversion particle by detecting the wavefront of emission.

Monitoring the temperature distribution within a local environment at the micro and nanoscale is vital as many processes are solely thermal. Various thermometric techniques have been explored in the community, and out of these, fluorescent nano/micro particle-based mechanisms are accepted widely (fluorescence intensity ratio (FIR) techniques, where the ratio of populations in two consecutive energy levels is compared with Boltzmann distribution). We describe a new technique to account for the temperature rise near an illuminated upconverting particle (UCP) using wavefront imaging, which is more sensitive than the conventional thermometric techniques on the microscale. We rely on a thermo-optical phase microscopic technique by reconstructing the wavefront of emission from an upconverting particle using a Shack-Hartmann wavefront sensor. The wavefront maps the local phase distribution, which is an indicator of the surroundings' optical parameters, particularly the suspended medium's temperature-induced refractive index in the presence of convection currents. We describe how these extracted phase values can provide information about the optical heating due to the particle and hence its local environment along the direction of the emission. Our findings demonstrate the detection of a minimum temperature rise of 0.23 K, while the FIR methods indicate a minimum of 0.3 K rise. This technique is used to study the temperature increase in the backscattered direction for an upconverting particle illuminated on pump resonance. We also estimate the Soret coefficient for an upconverting particle optically trapped on pump resonance and experiencing anisotropic heating across the body.

Preferred Mode of Atmospheric Water Vapor Condensation on Nanoengineered Surfaces: Dropwise or Filmwise?

Condensing atmospheric water vapor on surfaces is a sustainable approach to addressing the potable water crisis. However, despite extensive research, a key question remains: what is the optimal combination of the mode and mechanism of condensation as well as the surface wettability for the best possible water harvesting efficacy? Here, we show how various modes of condensation fare differently in a humid air environment. During condensation from humid air, it is important to note that the thermal resistance across the condensate is nondominant, and the energy transfer is controlled by vapor diffusion across the boundary layer and condensate drainage from the condenser surface. This implies that, unlike condensation from pure steam, filmwise condensation from humid air would exhibit the highest water collection efficiency on superhydrophilic surfaces. To demonstrate this, we measured the condensation rates on different sets of superhydrophilic and superhydrophobic surfaces that were cooled below the dew points using a Peltier cooler. Experiments were performed over a wide range of degrees of subcooling (10-26 °C) and humidity-ratio differences (5-45 g/kg of dry air). Depending upon the thermodynamic parameters, the condensation rate is found to be 57-333% higher on the superhydrophilic surfaces compared to the superhydrophobic ones. The findings of the study dispel ambiguity about the preferred mode of vapor condensation from humid air on wettability-engineered surfaces and lead to the design of efficient atmospheric water harvesting systems.

Accelerated self assembly of particles at the air-water interface with optically assisted heating due to an upconverting particle.

Particles can be assembled at the air-water interface due to optically induced local heating. This induces convection currents in the water which brings particles to the surface. We improve the technique by employing an upconverting particle (UCP), which, when illuminated with 975 nm light, not only emits visible emission but also generates heat owing to the poor efficiency of the upconversion process. This induces strong convection currents which makes particles dispersed in the suspension assemble at the interface and immediately under the UCP. We show assembly of polystyrene particles of 1 µm diameter and diamonds of 500 nm diameter bearing Nitrogen-Vacancy (NV) centers around the UCP. We also show, for the first time, that the microdiamonds are assembled within about 30 nm at the bottom of the UCP by utilizing non-radiative energy transfer that reduces the lifetime of the 550 nm emission from about 90 µs to about 50 µs.

Patterning Wettability for Open-Surface Fluidic Manipulation: Fundamentals and Applications.

Effective manipulation of liquids on open surfaces without external energy input is indispensable for the advancement of point-of-care diagnostic devices. Open-surface microfluidics has the potential to benefit health care, especially in the developing world. This review highlights the prospects for harnessing capillary forces on surface-microfluidic platforms, chiefly by inducing smooth gradients or sharp steps of wettability on substrates, to elicit passive liquid transport and higher-order fluidic manipulations without off-the-chip energy sources. A broad spectrum of the recent progress in the emerging field of passive surface microfluidics is highlighted, and its promise for developing facile, low-cost, easy-to-operate microfluidic devices is discussed in light of recent applications, not only in the domain of biomedical microfluidics but also in the general areas of energy and water conservation.

Imbibition of Liquids through a Paper Substrate in a Controlled Environment.

Liquid spreading on open surfaces is a widely observed phenomenon. The physics of liquid spreading has become more complex when the surface is porous like paper or fabrics due to the evaporation of the liquid and swelling of the fibers. In this study, we have performed liquid imbibition experiments on paper strips in a controlled environment with and without using hydrophobic boundaries. The experimental results are compared to the existing analytical models that account for each effect separately. The existing models were found to be inaccurate in predicting the experimental results. We developed new analytical models by modifying existing models to predict the capillary rise of the liquid through the paper substrate accurately. Different effects, such as the barrier (hydrophobic boundary), evaporation, and swelling, are considered simultaneously while developing the modified models to mimic the exact practical situation for the first time. We discovered that the modified models predict the experimental results more accurately than the existing models. For cases with and without barriers, the final models considering several effects simultaneously predict the data with a maximum error range of 7 and 10%, respectively. Finally, we conducted capillary rise experiments with volatile (water) and non-volatile (silicon oil) liquids at various temperatures and under various relative humidity conditions to validate the analytical results.

Condensation of Humid Air on Superhydrophobic Surfaces: Effect of Nanocoatings on a Hierarchical Interface.

Vapor condensation is a well-known phase-change phenomenon observed in nature as well as in different industrial applications. Superhydrophobic surfaces (SHSs) with low hysteresis can efficiently drain off the condensate and rejuvenate the nucleation sites further. In this work, three distinct SHSs were fabricated by nanocoating three hydrophobic agents, viz., perfluoro-octyl-triethoxy-silane (PFOTS), perfluoro-octanoic-acid (PFOA), and commercial Glaco solution on a hierarchical aluminum surface. The surface morphology of all surfaces was investigated, and its effects on the wetting, droplet departure, and overall heat-transfer coefficient (HTC) during condensation phenomena in the humid air (>95% noncondensable gases) were analyzed. The contact angle hysteresis of all three surfaces was very low (∼5°); however, different wetting behaviors were observed during the condensation, depending on the adhesion of the condensate drop with nanoscale textures in the microcavities. Dropwise condensation (DWC) was observed in silane and Glaco-coated surfaces. A gravity-assisted sweeping mechanism removed the condensate from the silane-coated surface. In contrast, the condensate was ejected out of the plane of the Glaco-coated surface by droplet jumping. The PFOA-coated surface has shown DWC initially and floods in the later stages due to highly pinned condensed droplets. This study reports an enhancement of ∼35 to ∼110% in the HTC for the SHS-exhibiting gravity-assisted sweeping mechanism compared to the droplet-jumping mechanism. The present work will provide substantial insights into the fabrication of efficient hierarchical interfaces for water-energy nexus applications.

Precise Liquid Transport on and through Thin Porous Materials.

Porous substrates have the ability to transport liquids not only laterally on their open surfaces but also transversally through their thickness. Directionality of the fluid transport can be achieved through spatial wettability patterning of these substrates. Different designs of wettability patterns are implemented herein to attain different schemes (modes) of three-dimensional transport in a high-density paper towel, which acts as a thin porous matrix directing the fluid. All schemes facilitate precise transport of metered liquid microvolumes (dispensed as droplets) on the surface and through the substrate. One selected mode features lateral fluid transport along the bottom surface of the substrate, with the top surface remaining dry, except at the initial droplet dispension point. This configuration is investigated in further detail, and an analytical model is developed to predict the temporal variation of the penetrating drop shape. The analysis and respective measurements agree within the experimental error limits, thus confirming the model's ability to account for the main transport mechanisms.

Surface-Wettability Patterning for Distributing High-Momentum Water Jets on Porous Polymeric Substrates.

Liquid jet impingement on porous materials is particularly important in many applications of heat transfer, filtration, or in incontinence products. Generally, it is desired that the liquid not penetrate the substrate at or near the point of jet impact, but rather be distributed over a wider area before reaching the back side. A facile wettability-patterning technique is presented, whereby a water jet impinging orthogonally on a wettability-patterned nonwoven substrate is distributed on the top surface and through the porous matrix, and ultimately dispensed from prespecified points underneath the sample. A systematic approach is adopted to identify the optimum design that allows for a uniform distribution of the liquid on horizontally mounted substrates of ∼50 cm2 area, with minimal or no spilling over the sample edges at jet flow rates exceeding 1 L/min. The effect of the location of jet impingement on liquid distribution is also studied, and the design is observed to perform well even under offset jet impact conditions.