Water Droplet Spreading and Wicking on Nanostructured Surfaces

Capillary Wicking on Heliamphora minor-Mimicking Mesoscopic Trichomes Array

Liquid spontaneously spreads on rough lyophilic surfaces, and this is driven by capillarity and defined as capillary wicking. Extensive studies on microtextured surfaces have been applied to microfluidics and their corresponding manufacturing. However, the imbibition at mesoscale roughness has seldom been studied due to lacking fabrication techniques. Inspired by the South American pitcher plant Heliamphora minor, which wicks water on its pubescent inside wall for lubrication and drainage, we implemented 3D printing to fabricate a mimetic mesoscopic trichomes array and investigated the high-flux capillary wicking process. Unlike a uniformly thick climbing film on a microtextured surface, the interval filling of millimeter-long and submillimeter-pitched trichomes creates a film of non-uniform thickness. Different from the viscous dissipation that dominated the spreading on microtextured surfaces, we unveiled an inertia-dominated transition regime with mesoscopic wicking dynamics and constructed a scaling law such that the height grows to 2/3 the power of time for various conditions. Finally, we examined the mass transportation inside the non-uniformly thick film, mimicking a plant nutrition supply method, and realized an open system siphon in the film, with the flux saturation condition experimentally determined. This work explores capillary wicking in mesoscopic structures and has potential applications in the design of low-cost high-flux open fluidics.

Robust Potentiometric Microelectrodes for In Situ Sensing of Ion Fluxes with High Sensitivity

Simple, reproducible, and reliable preparation of robust potentiometric microelectrodes is both challenging and of great importance for noninvasive real-time ion sensing. Herein, we report a simple strategy for the large-scale synthesis of nickel cobalt sulfide (NiCo2S4) nanowire arrays grown on carbon fibers for potentiometric microelectrodes. The highly uniform NiCo2S4 nanowire array serving as a transduction layer can provide a high capillary pressure and viscous resistance for loading the ion sensing membrane and exhibit a large redox capacitance for improving the stability. An all-solid-state lead-selective microelectrode, which presents a detection limit of 2.5 × 10-8 M in the simulated soil solution, was designed as a model for noninvasive, in situ, and real-time detection of ion fluxes near the rice root surface. Importantly, the microsensor enables sensitive detection of trace-level ion-fluxes. This work provides a simple yet general strategy for designing potentiometric microelectrodes.

Boosting water evaporation via continuous formation of a 3D thin film through triple-level super-wicking routes

Capillary-fed thin-film evaporation via micro/nanoscale structures has attracted increasing attention for its high evaporation flux and pumpless liquid replenishment. However, maximizing thin-film evaporation has been hindered by the intrinsic trade-off between the heat flux and liquid transport. Here, we designed and fabricated nanostructured micro-steam volcanoes on copper surfaces featuring triple-level super-wicking routes to overcome this trade-off and boost water evaporation. The triple-level super-wicking routes enable the continuous formation of a 3D thin film for highly efficient evaporation by continuous self-driven liquid replenishment and extending the thin-film region. The micro-steam volcanoes increased the surface area by 225%, improving the evaporation rate by 141%, with a rapid self-pumping water transport speed up to 80 mm s-1. A remarkable solar-driven water evaporation rate of 3.33 kg m-2 h-1 under one sun vertical incidence was achieved, which is among the highest reported values for metal-based evaporators. When attached to electric-heating plates, the evaporator realized an electrothermal evaporation rate of 12.13 kg m-2 h-1. Moreover, it can also be used for evaporative cooling with enhanced convective heat transfer, reaching a 36.2 °C temperature reduction on a heat source with a heat flux of 6 W cm-2. This study promises a general strategy for designing thin-film evaporators with high efficiencies, low costs, and multi-functional compatibilities.

An integrated immunochromatographic device for C-reactive protein detection using hierarchical dendritic gold nanostructure films

Immunochromatographic test strips typically consist of sample pad, conjugate pad, nitrocellulose membrane, and absorbent pad. Even minute variations in the assembly of these components can lead to inconsistent sample-reagent interactions, thereby reducing reproducibility. In addition, the nitrocellulose membrane is susceptible to damage during assembly and handling. To address this issue, we propose to replace the sample pad, conjugate pad, and nitrocellulose membrane with hierarchical dendritic gold nanostructure (HD-nanoAu) films to develop a compact integrated immunochromatographic strip. The strip uses quantum dots as a background fluorescence signal and employs fluorescence quenching to detect C-reactive protein (CRP) in human serum. A 5.9 μm thick HD-nanoAu film was electrodeposited on an ITO conductive glass by the constant potential method. The wicking kinetics of the HD-nanoAu film was thoroughly investigated, and the results indicated that the film exhibited favorable wicking properties, with a wicking coefficient of 0.72 μm ms^-0.5. The immunochromatographic device was fabricated by etching three interconnected rings on HD-nanoAu/ITO to designate sample/conjugate (S/C), test (T), and control (C) regions. The S/C region was immobilized with mouse anti-human CRP antibody (Ab1) labeled with gold nanoparticles (AuNPs), while the T region was preloaded with polystyrene microspheres decorated with CdSe@ZnS quantum dots (QDs) as background fluorescent material, followed by mouse anti-human CRP antibody (Ab2). The C region was immobilized with goat anti-mouse IgG antibody. After the samples were added to the S/C region, the excellent wicking properties of the HD-nanoAu film facilitated the lateral flow of the CRP-containing sample toward the T and C regions after binding to AuNPs labeled with CRP Ab1. In the T region, CRP-AuNPs-Ab1 formed sandwich immunocomplexes with Ab2, and the fluorescence of QDs was quenched by AuNPs. The ratio of fluorescence intensity in the T region to that in the C region was used to quantify CRP. The T/C fluorescence intensity ratio was negatively correlated with the CRP concentration in the range of 26.67-853.33 ng mL^-1 (corresponding to 300-fold diluted human serum), with a correlation coefficient (R²) of 0.98. The limit of detection was 15.0 ng mL^-1 (corresponding to 300-fold diluted human serum), and the range of relative standard deviation: 4.48-5.31%, with a recovery rate of 98.22-108.33%. Common interfering substances did not cause significant interference, and the range of relative standard deviation: 1.96-5.51%. This device integrates multiple components of conventional immunochromatographic strips onto a single HD-nanoAu film, resulting in a more compact structure that improves the reproducibility and robustness of detection, making it promising for point-of-care testing applications.

Dynamic wetting of various liquids: Theoretical models, experiments, simulations and applications

Dynamic wetting is a ubiquitous phenomenon and frequently observed in our daily life, as exemplified by the famous lotus effect. It is also an interfacial process of upmost importance involving many cutting-edge applications and has hence received significantly increasing academic and industrial attention for several decades. However, we are still far away to completely understand and predict wetting dynamics for a given system due to the complexity of this dynamic process. The physics of moving contact lines is mainly ascribed to the full coupling with the solid surface on which the liquids contact, the atmosphere surrounding the liquids, and the physico-chemical characteristics of the liquids involved (small-molecule liquids, metal liquids, polymer liquids, and simulated liquids). Therefore, to deepen the understanding and efficiently harness wetting dynamics, we propose to review the major advances in the available literature. After an introduction providing a concise and general background on dynamic wetting, the main theories are presented and critically compared. Next, the dynamic wetting of various liquids ranging from small-molecule liquids to simulated liquids are systematically summarized, in which the new physical concepts (such as surface segregation, contact line fluctuations, etc.) are particularly highlighted. Subsequently, the related emerging applications are briefly presented in this review. Finally, some tentative suggestions and challenges are proposed with the aim to guide future developments.

Anisotropic Hemiwicking Behavior on Laser Structured Prismatic Microgrooves

The wicking phenomenon, including wicking and hemiwicking, has attracted increasing attention for its critical importance to a wide range of engineering applications, such as thermal management, water harvesting, fuel cells, microfluidics, and biosciences. There exists a more urgent demand for anisotropic wicking behaviors since an increasing number of advanced applications are significantly complex. For example, special-shaped vapor chambers and heating atomizers in some electronic cigarettes need liquid replenishing with various velocities in different directions. Here, we report two-dimensional anisotropic hemiwicking behaviors with elliptical shapes on laser structured prismatic microgrooves. The prismatic microgrooves were fabricated via one-step femtosecond laser direct writing, and the anisotropic hemiwicking behaviors were observed when utilizing glycerol, glycol, and water as the test liquid. Specifically, the ratios of horizontal wicking distance in directions along short and long axes were tan 0°, tan 15°, tan 30°, and tan 45° for samples with cross-angles of 0°, 30°, 60°, and 90°, respectively. The vertical water wicking front displayed corresponding angles under the guidance of laser structured prismatic microgrooves. Theoretical analysis shows that the wicking distance is mainly dependent on the cross-angle θ and surface roughness, in which the wicking distance is proportional to cos(θ/2). Driven by the capillary pressure forming in the narrow microgrooves, the liquid initially filled the valleys of microgrooves and then surrounded and covered the prismatic ridges with laser-induced nanoparticles. The abundant nanoparticles increased the surface roughness, leading to the enhancement of wicking performance, which was further evidenced by the larger wicking speed of the sample with more nanoparticles. The mechanism of anisotropic hemiwicking behaviors revealed in this work paves the way for wicking control, and the proposed prismatic microgrooved surfaces with two-dimensional anisotropic hemiwicking performance and superhydrophilicity could serve in a broad range of applications, especially for the advanced thermal management with specific heat load configurations.

Droplet evaporation on porous fabric materials

Droplet evaporation on porous materials is a complex dynamic that occurs with spontaneous liquid imbibition through pores by capillary action. Here, we explore water dynamics on a porous fabric substrate with in-situ observations of X-ray and optical imaging techniques. We show how spreading and wicking lead to water imbibition through a porous substrate, enhancing the wetted surface area and consequently promoting evaporation. These sequential dynamics offer a framework to understand the alterations in the evaporation due to porosity for the particular case of fabric materials and a clue of how face masks interact with respiratory droplets.

Microscopic Observation of Preferential Capillary Pumping in Hollow Nanowire Bundles

Numerous studies have focused on designing micro/nanostructured surfaces to improve wicking capability for rapid liquid transport in many industrial applications. Although hierarchical surfaces have been demonstrated to enhance wicking capability, the underlying mechanism of liquid transport remains elusive. Here, we report the preferential capillary pumping on hollow hierarchical surfaces with internal nanostructures, which are different from the conventional solid hierarchical surfaces with external nanostructures. Specifically, capillary pumping preferentially occurs in the nanowire bundles instead of the interconnected V-groove on hollow hierarchical surfaces, observed by confocal laser scanning fluorescence microscopy. Theoretical analysis shows that capillary pumping capability is mainly dependent on the nanowire diameter and results in 15.5 times higher capillary climbing velocity in the nanowire bundles than that in the microscale V-groove. Driven by the Laplace pressure difference between nanowire bundles and V-grooves, the preferential capillary pumping is increased with the reduction of the nanowire diameter. Capillary pumping of the nanowire bundles provides a preferential path for rapid liquid flow, leading to 2 times higher wicking capability of the hollow hierarchical surface comparing with the conventional hierarchical surface. The unique mechanism of preferential capillary pumping revealed in this work paves the way for wicking enhancement and provides an insight into the design of wicking surfaces for high-performance capillary evaporation in a broad range of applications.

Enabling Superhydrophobicity-Guided Superwicking in Metal Alloys via a Nanosecond Laser-Based Surface Treatment Method

Enabling capillary wicking on bulk metal alloys is challenging due to processing complexity at different size scales. This work presents a laser-chemical surface treatment to fabricate superwicking patterns guided by a superhydrophobic region over a large-area metal alloy surface. The laser-chemical surface treatment generates surface micro/nanostructures and desirable surface chemistry simultaneously. The superhydrophobic surface was first fabricated over the whole surface by laser treatment under water confinement and fluorosilane treatment; subsequently, superwicking stripes were processed by a second laser treatment in air and cyanosilane treatment. The resultant surface shows superwicking regions surrounded by superhydrophobic regions. During the process, superwicking regions possess dual-scale structures and polar nitrile surface chemistry. In contrast, random nanoscale structures and fluorocarbon chemistry are generated on the superhydrophobic region of the aluminum alloy 6061 substrates. The resultant superwicking region demonstrates self-propelling anti-gravity liquid transport for methanol and water. The combination of the capillary effect of the dual-scale surface microgrooves and the water affinitive nitrile group contributes toward the self-propelling movement of water and methanol at the superwicking region. The initial phase of wicking followed Washburn dynamics, whereas it entered a non-linear regime in the later phase. The wicking height and rate are regulated by microgroove geometry and spacing.

Zinc oxide nanostructures and stearic acid as surface modifiers for flax fabrics in polylactic acid biocomposites

Different surface treatments including mercerization, stearic acid and growth of zinc oxide nanorods as well as their combinations were exploited to address their effects on the properties of green composites based on polylactic acid (PLA) and flax fabrics. The resulting fabrics were morphologically (SEM), crystallographically (XRD) and thermally (TGA) characterized, showing no significant changes with respect to the untreated samples. In contrast, tensile and flexural properties of composites produced by compression moulding were significantly influenced. A combination of mercerization and environmentally friendly stearic acid treatment turned the character of the flax fabric from hydrophilic to hydrophobic, and led to improved bending and tensile strengths by 20% and 12%, respectively, compared to untreated composites. The presence of ZnO nanorods promoted an increase in flexural and tensile stiffness by 58% and 31%, respectively, but at the expense of strength, with reductions ascribed to the degradation of polylactic acid under high-temperature conditions favoured by ZnO, as confirmed by a reduction in the initial thermal degradation temperature up to 26%. These latter composites can be suggested in those applications where a suitable combination of flexural properties and a shorter persistence in the environment is desired.

Laser-Induced Patterned Photonic Crystal Heterostructure for Multimetal Ion Recognition

In this work, a new method of direct laser writing patterned photonic crystal heterostructure on a glass surface is proposed. A multi-heterostructure photonic crystal (MHPC) is predeposited on the glass surface and then the laser spot is focused on it and moves according to the set program, leading to the formation of patterned MHPC. Scanning electron microscopy (SEM) and finite element simulation show that the patterning is caused by the local thermal annealing of the polymer colloidal spheres through the thermal conduction effect of the substrate on the laser energy. The patterned area presents a function of the water confinement effect and can be used as a high-performance droplet analysis chip. By integrating the patterned MHPC array and seven fluorescent dyes, nine metal ions can be successfully recognized and discriminated. This approach is quite facile and fast for designing colloidal photonic crystals with controllable patterns. Moreover, it is of considerable significance for the practical application of photonic crystal heterostructure in the detection, sensing, anti-counterfeiting, and display fields.

Droplet Evaporation on Porous Nanochannels for High Heat Flux Dissipation

Droplet wicking and evaporation in porous nanochannels is experimentally studied on a heated surface at temperatures ranging from 35 to 90 °C. The fabricated geometry consists of cross-connected nanochannels of height 728 nm with micropores of diameter 2 μm present at every channel intersection; the pores allow water from a droplet placed on the top surface to wick into the channels. Droplet volume is also varied, and a total of 16 experimental cases are conducted. Wicking characteristics such as wicked distance, capillary pressure, viscous resistance, and propagation coefficients are obtained at all surface temperatures. Evaporation flux from the nanochannels/micropores is estimated from the droplet experiments but is also independently confirmed via a new set of experiments where water is continuously fed to the sample through a microtube so that it matches the evaporation rate. Heat flux as high as ∼294 W/cm² is achieved from channels and pores. The experimental findings are applied to evaluate the use of porous nanochannel geometry in spray cooling application and is found to be capable of passively dissipating high heat fluxes upto ∼77 W/cm² at temperatures below nucleation, thus highlighting the thermal management potential of the fabricated geometry.

Evaporation Dynamics in Buried Nanochannels with Micropores

Cross-connected buried nanochannels of height ∼728 nm, with micropores of ∼2 μm diameter present at each intersection, are used in this work to numerically and experimentally study droplet-coupled evaporation dynamics at room temperature. The uniformly structured channels/pores, along with their well-defined porosity, allow for computational fluid dynamics simulations and experiments to be performed on the same geometry of samples. A water droplet is placed on top of the sample causing water to wick into the nanochannels through the micropores. After advancing, the meniscus front stabilizes when evaporation flux is balanced with the wicking flux, and it recedes once the water droplet is completely wicked in. Evaporation flux at the meniscus interface of channels/pores is estimated over time, while the flux at the water droplet interface is found to be negligible. When the meniscus recedes in the channels, local contact line regions are found to form underneath the pores, thus rapidly enhancing evaporation flux as a power-law function of time. Temporal variation of wicking flux velocity and pressure gradient in the nanochannels is also independently computed, from which the viscous resistance variation is estimated and compared to the theoretical prediction.

Superwicking on Nanoporous Micropillared Surfaces

Engineering surfaces with excellent wicking properties is of critical importance to a wide range of applications. Here, we report a facile method to create superhydrophilic nanoporous micropillared surfaces of silicon and their applicability to superwicking. Nanopores with a good control of the pore depth are realized over the entire surface of three-dimensional micropillar structures by electrochemical etching in hydrofluoric acid. After rinsing in hydrogen peroxide, the nanoporous micropillared surface shows superhydrophilicity with the superwicking effect. The entire spreading process of a water droplet on the superhydrophilic nanoporous micropillared surface is completed in less than 50 ms, with an average velocity of 91.2 mm/s, which is significantly faster than the other wicking surfaces reported. Owing to the presence of nanopores on the micropillar array, the wicking dynamics is distinct from the surfaces decorated only by micropillar arrays. The spreading dynamics of a water droplet shows two distinct processes simultaneously, including the capillary penetration between micropillars and the capillary imbibition into the nanopore's interior. The wicking dynamics can be described by the two stages separated by the time when the contact line starts to recede. The transition between the two wicking regimes is due to the increasing effect of the imbibition of the bulk droplet by the nanopores. While a similar transition of the wicking dynamics is shown on the surfaces with different pore depths, the nanopore structure with a greater depth causes a greater amount of imbibition to slow down the spreading and promote superwicking.

Nanoscale and Macroscale Effects of Mineral Deposition During Water Evaporation on Nanoporous Surfaces

Recent studies have indicated that droplet evaporation heat transfer can be substantially enhanced by fabricating a thin nanoporous superhydrophilic layer on a metal substrate. Such surfaces have immense potential to improve spray cooling processes, however, little durability testing of the surface has been performed. In spray cooling applications, as water evaporates any impurities in the water will be deposited onto the surface. Primarily, this investigation serves to demonstrate how minerals in hard water deposit on the surface and interact with the ZnO nanopillars of the superhydrophilic surface. Quantifying the effects of mineral scale on droplet spreading and vaporization heat transfer on the surface is important in determining implementation requirements to advance the surface into industry applications. Micrographs of the surface demonstrate minerals deposit nonuniformly and quickly fill the nanostructure. Despite a reduction in the extent of droplet spreading due to the mineral deposition, scaled surfaces still demonstrated improved thermal performance compared to an uncoated, smooth copper surface. Scale tended to build up on previously deposited scale, leaving largely uncoated areas where droplets chose to preferentially spread and resulting in a continued low contact angle. Maintaining these uncoated areas and reducing the contaminants present in the water will extend the life and performance of the nanostructured surface.

Large-scale efficient water harvesting using bioinspired micro-patterned copper oxide nanoneedle surfaces and guided droplet transport

As the Earth's atmosphere contains an abundant amount of water as vapors, a device which can capture a fraction of this water could be a cost-effective and practical way of solving the water crisis. There are many biological surfaces found in nature which display unique wettability due to the presence of hierarchical micro-nanostructures and play a major role in water deposition. Inspired by these biological microstructures, we present a large scale, facile and cost-effective method to fabricate water-harvesting functional surfaces consisting of high-density copper oxide nanoneedles. A controlled chemical oxidation approach on copper surfaces was employed to fabricate nanoneedles with controlled morphology, assisted by bisulfate ion adsorption on the surface. The fabricated surfaces with nanoneedles displayed high wettability and excellent fog harvesting capability. Furthermore, when the fabricated nanoneedles were subjected to hydrophobic coating, these were able to rapidly generate and shed coalesced droplets leading to further increase in fog harvesting efficiency. Overall, ∼99% and ∼150% increase in fog harvesting efficiency was achieved with non-coated and hydrophobic layer coated copper oxide nanoneedle surfaces respectively when compared to the control surfaces. As the transport of the harvested water is very important in any fog collection system, hydrophilic channels inspired by leaf veins were made on the surfaces via a milling technique which allowed an effective and sustainable way to transport the captured water and further enhanced the water collection efficiency by ∼9%. The system presented in this study can provide valuable insights towards the design and fabrication of fog harvesting systems, adaptable to arid or semi-arid environmental conditions.

A 3D-Structured Sustainable Solar-Driven Steam Generator Using Super-Black Nylon Flocking Materials

Solar-driven evaporation is a promising way of using abundant solar energy for desalinating polluted water or seawater, which addresses the challenge of global fresh water scarcity. Cost-effectiveness and durability are key factors for practical solar-driven evaporation technology. The present cutting-edge techniques mostly rely on costly and complex fabricated nanomaterials, such as metallic nanoparticles, nanotubes, nanoporous hydrogels, graphene, and graphene derivatives. Herein, a black nylon fiber (BNF) flocking board with a vertically aligned array prepared via a convenient electrostatic flocking technique is reported, presenting an extremely high solar absorbance (99.6%), a water self-supply capability, and a unique salt self-dissolution capability for seawater desalination. Through a carefully designed 3D structure, a plug-in-type BNF flocking board steam generator realizes a high evaporation rate of 2.09 kg m^-2 h^-1 under 1 kW m^-2 solar illumination, well beyond its corresponding upper limit of 1.50 kg m^-2 h^-1 (assuming 100% solar energy is being used for evaporation latent heat). With the advantages of high-efficiency fabrication, cost-effectiveness, high evaporation rate, and high endurance in seawater desalination, this 3D design provides a new strategy to build up an economic, sustainable, and rapid solar-driven steam generation system.

Micropattern-controlled wicking enhancement in hierarchical micro/nanostructures

Wicking in hierarchical micro/nanostructured surfaces has attracted significant attention due to its potential applications in thermal management, moisture capturing, drug delivery, and oil recovery. Although some studies have shown that hierarchical structures enhance wicking over micro-structured surfaces, others have found very limited wicking improvement. In this study, we demonstrate the importance of micropatterns in wicking enhancement in hierarchical surfaces using ZnO nanorods grown on silicon micropillars of varying spacings and heights. The wicking front over hierarchical surfaces is found to follow a two-stage motion, where wicking is faster around micropillars, but slower in between adjacent pillar rows and the latter stage dictates the wicking enhancement in hierarchical surfaces. The competition between the added capillary action and friction due to nanostructures in these two different wicking stages results in a strong dependence of wicking enhancement on the height and spacing of the micropillars. A scaling model for the propagation coefficient is developed for wicking in hierarchical surfaces considering nanostructures in both wicking stages and the model agrees well with the experiments. This microstructure-controlled two-stage wicking characteristic sheds light on a more effective design of hierarchical micro/nanostructured surfaces for wicking enhancement.

Can Wicking Control Droplet Cooling?

Wicking, defined as absorption and passive spreading of liquid into a porous medium, has been identified as a key mechanism to enhance the heat transfer and prevent the thermal crisis. Reducing the evaporation time and increasing the Leidenfrost point (LFP) are important for an efficient and safe design of thermal management applications, such as electronics, nuclear, and aeronautics industry. Here, we report the effect of the wicking of superhydrophilic nanowires (NWs) on the droplet vaporization from low temperatures to temperatures above the Leidenfrost transition. By tuning the wicking capability of the surface, we show that the most wickable NW results in the fastest evaporation time (reduction of 82, 76, and 68% compared with a bare surface at, respectively, 51, 69, and 92 °C) and in one of the highest shifts of the LFP of a water droplet (5 μL) in the literature (about 260 °C).

Direct Visualization of the Behavior and Shapes of the Nanoscale Menisci of an Evaporating Water Droplet on a Hydrophilic Nanotextured Surface via High-Resolution Synchrotron X-ray Imaging

Despite considerable research interest due to omnipresent and practical importance of interfacial phenomena (e.g., wetting and dewetting) on nanotextured surfaces in the academic and industrial fields, direct visualization of the behavior and shapes of liquid-vapor interfaces between nanoscale structures remains an arduous task because of the resolution limitations of visualization techniques. In this study, we succeeded in a first-hand visualization of the behavior and shapes of the liquid-vapor interfaces of a water droplet between nanometer-scale pillar during evaporation by introducing a synchrotron X-ray imaging technique with spatially high resolution (40 nm/a pixel). On the basis of the visualization data, we intensively analyzed and discussed the spreading and evaporation phenomena of a liquid droplet on hydrophilic nanotextured surfaces.

Numerical Simulation of Sessile Droplet Spreading and Penetration on Porous Substrates

Numerical simulation is performed for sessile droplet spreading and penetration on porous surfaces in this study. The volume of the fluid model is used to accurately track the droplet deformation, and the pressure implicit split operator algorithm is presented to calculate the coupling of the droplet pressure and velocity. The effects of droplet characteristics, porous media characteristics, and the wettability of liquid/porous media on sessile droplet spreading and permeation are investigated in detail. The studied problem can be characterized by four control parameters: the Bond number, Darcy number, static equilibrium contact angle, and ratio between the initial diameter of the droplet and the particle diameter in the porous substrate. The numerical simulations show that droplet spreading and penetration are competitive with each other and dependent on the above four dimensionless parameters. The results obtained in this work are of benefit to provide deep insights into the dynamic behavior of sessile droplet on porous substrates.