Wettability Contrast Research Articles

Drop impact onto wettability-patterned solid surfaces: A phase field approach

Drop impact onto wettability-patterned surfaces is of great significance in industries. Self-propulsion, self-splitting, and directional rebounding can be realized when drops impact on such surfaces. This paper established a diffuse interface/phase field model to delve into drop impact onto wettability-patterned surfaces, with two typical surfaces considered, one having a step change in the contact angle and the other having a smooth change in it. The diffuse interface model used the phase field to track the liquid–gas interface, was discretized on a half-staggered grid, and was run in a parallel manner. The model was validated first against an impact onto a uniform surface and then against an impact onto a hydrophilic surface coated with a superhydrophobic strip. A mesh independence study was conducted for the phase field modeling. Grid independence was achieved while the phase field mobility was kept fixed in meshes of varied resolutions. The major findings are as follows. The spreading of a spherical drop on gradient wettability surfaces resembles that of an ellipsoidal drop on a uniform surface, and axis-switching was observed. On the other hand, directional rebounding on multi-region wettability surfaces is enhanced with increased wettability contrast.

Surface-Templated Polymer Microparticle Synthesis Based on Droplet Microarrays.

Polymer microparticle synthesis based on the surface-templated method is a simple and environmentally friendly method to produce various microparticles. Unique particles with different compositions can be fabricated by simply annealing a polymer on a liquid-repellent surface. However, there are hurdles to producing particles of homogeneous sizes with large quantities and varying the shape of particles. Here, a new approach to synthesizing multiple polymer microparticles using micropatterns with wettability contrast is presented. Polymer microparticles are formed in two steps. First, a layer of poly(sodium-4-styrenesulfonate) is deposited on the hydrophilic regions by dipping and withdrawing this micropattern from a polymer solution, and an array of microdroplets is formed. A dewetting-inducing layer on the pattern is introduced, and then target polymer patches are sequentially generated on it. By annealing over Tg, the contact line of the target polymer patch is freely receded, creating a particle form. The size and shape of the microparticle can be controlled by varying the micropatterns. In addition, it is demonstrated that microparticles made of polymer blends or polymer/nanoparticle composite are easily produced. This versatile method offers the potential of surface-templated synthesis to tailor polymer microparticles with different sizes, shapes, and functionalities in various research and applications.

Enhancing Water Condensation on Hybrid Surfaces by Optimizing Wettability Contrast

Enhancing Water Condensation on Hybrid Surfaces by Optimizing Wettability Contrast

A Wettability Contrast SERS Droplet Assay for Multiplexed Analyte Detection.

Droplet assay platforms have emerged as a significant methodology, providing distinct advantages such as sample compartmentalization, high throughput, and minimal analyte consumption. However, inherent complexities, especially in multiplexed detection, remain a challenge. We demonstrate a novel strategy to fabricate a plasmonic droplet assay platform (PDAP) for multiplexed analyte detection, enabling surface-enhanced Raman spectroscopy (SERS). PDAP efficiently splits a microliter droplet into submicroliter to nanoliter droplets under gravity-driven flow by wettability contrast between two distinct regions. The desired hydrophobicity and adhesive contrast between the silicone oil-grafted nonadhesive hydrophilic zone with gold nanoparticles is attained through (3-aminopropyl) triethoxysilane (APTES) functionalization of gold nanoparticles (AuNPs) using a scotch-tape mask. The wettability contrast surface facilitates the splitting of aqueous droplets with various surface tensions (ranging from 39.08 to 72 mN/m) into ultralow volumes of nanoliters. The developed PDAP was used for the multiplexed detection of Rhodamine 6G (Rh6G) and Crystal Violet (CV) dyes. The limit of detection for 120 nL droplet using PDAP was found to be 134 pM and 10.1 nM for Rh6G and CV, respectively. These results align with those from previously reported platforms, highlighting the comparable sensitivity of the developed PDAP. We have also demonstrated the competence of PDAP by testing adulterant spiked milk and obtained very good sensitivity. Thus, PDAP has the potential to be used for the multiplexed screening of food adulterants.

Controlled wettability of biphilic patterned surfaces for enhanced atmospheric water harvesting

Water is a vital component for all living organisms, yet persistent water scarcity remains a global challenge. One potential solution lies in replicating the atmospheric water collection mechanism observed in the Stenocara beetle, characterized by a dorsal surface featuring alternating hydrophilic and hydrophobic regions. In this study, we have designed and examined two distinct biphilic patterned surface configurations, integrating various technologies, to mimic the beetle's water collection strategy. Our investigation evaluates the efficiency of these surfaces in both capturing water from fog and condensing water from dew. For fog collection two parameters were the most impactful: the roughness and the wettability contrast between hydrophilic and hydrophobic zones. In contrast, dew condensation was influenced by additional parameters notably the patterns' size and density that directly affect the water contact angle. It is worth noting, however, that the optimal surface for fog collection may not necessarily coincide with the most effective surface for dew condensation. Furthermore, our research includes a comparative analysis between the theoretically predicted volume of water droplet departure and the empirically observed results.

Application of Surface Wettability to Control Spreading of an Impacting Droplet.

While the simplest outcome of a normal impact on a flat stationary solid surface is radially symmetric spreading, it is important to note that asymmetric spreading can intrinsically occur with a tangential velocity along the surface. However, no previous attempt has been made to restore the symmetry of a lamella that intrinsically spreads asymmetrically. Adjusting the lamella's asymmetric shape to a symmetric one is achieved in this work by varying wettability to affect the receding velocity of the contact line, according to the Taylor-Culick theory. Here we theoretically and practically show how restoring the symmetry can be achieved. Theoretically we built a framework to map the needed receding velocity at every given point of the contact line to allow for symmetry to be restored, and then this framework was applied to generate a wetting map that shows how at each local the wettability of the surface needs to be defined. Simulated results confirmed the effectiveness of our framework and identified the envelope of its applicability. Next, to apply the idea experimentally, the wetting map was transformed to a single wettability contrast area dubbed the "patch". Experimental results showed the effectiveness of the patch design in correcting the asymmetric spreading lamella for water droplets impacting a surface for the following Weber number conditions: Wen ≤ 300, Wet ≤ 300, and 0.51 ≤ Wen/Wet ≤ 2.04.

Droplet splitting on chemically heterogeneous surface: A 3D lattice Boltzmann study

The uniform splitting of droplets on heterogeneous surfaces without external stimuli has gained increasing attention due to its wide and flexible applications. To precisely control and predict the droplet splitting, the 3D multiphase lattice Boltzmann model (LBM) is used to study the mechanism of the impact splitting on a hydrophilic surface with a superhydrophobic strip. A parallel implementation of the 3D multiphase LBM is presented to improve the performance by using an effective kernel fusion to reduce the off-chip memory data traffic. Two splitting modes with single or double liquid bridge are contrasted and analyzed. The results show that the size of the air bubble trapped during the impacting process leads to various splitting modes. By calculating the unbalanced Young's force in the three-phase contact lines, unbalanced surface tension at the superhydrophobic/hydrophilic surface is observed. In addition, we find that the increase in the initial velocity of the drop can not only shorten the splitting time but also lead to a change in splitting mode. The increase in the wettability contrast of the surface can stably shorten the splitting time. However the splitting time did not linearly shorten with the increase in strip width. Our findings would advance the understanding of drop impact splitting and possibly inspire new guidance for the design of functional interfaces. The present study also provides an efficient parallel implementation to simulate the 3D droplet motion on heterogeneous surfaces.

Micro "Hyper-Channels" on Laser-Refined Cellulose Structures.

Controlled liquid transportation is widely applied in both academia and industry. However, liquid transport applications are limited by parameters such as driving forces, precision, and velocity. Herein, a simple laser-refining technology is presented to produce micro "hyper-channels". A cellulose substrate is rendered hydrophobic through silanization and refined with a laser to produce both hierarchical nanostructures and a wettability contrast simultaneously. Such a method enables faster ("hyper"-channel) aqueous liquid transportation (≈25X, 50mms-1 ) compared to conventional methods. Complex patterns can be readily produced at different scales with spatial resolution as low as 50µm. This technique also controls the refining depth on the thin paper substrate. Shallow channels can be fabricated on thin paper substrates that enable fluidic channel-crossover without liquid mixing. With certain parameters, the technique creates "portals" through the substrate, allowing trans-dimensional liquid transportation between two layers of a single sheet of substrate. The fluid throughput can be increased, while also permitting fluidic channel crossover without liquid mixing. By introducing multiple portals, the controlled fluid can transfer trans-dimensionally several times, enabling further fluidic complexity. The real-life utility of the method is demonstrated by creating a trans-dimensional microfluidic device for colorimetric detection.

Interplay of Drop Shedding Mechanisms on High Wettability Contrast Biphilic Stripe-Patterned Surfaces.

To improve the rate of DWC, numerous studies have adjusted the distribution of drops through biphilic surface patterning and wettability gradients to control the nucleation and drop shedding rates on the condensing surface, yet the connection between drop shedding mechanisms and surface wettability patterning remains unclear. Moreover, wettability patterning places geometric bounds on the governing forces (i.e., gravity, capillary, and inertia), which drive the droplet shedding mechanisms. Thus, the subsequent influence of droplet distribution along the DWC regions on the shedding mechanisms may not be known a priori. In this study, the area fraction, ADWC, of the DWC and also the DWC region width, LN, were varied between 10 and 50% and 0.5-1.5 mm, respectively, to probe the dominant droplet shedding mechanisms on a high wettability contrast surface (i.e., the contact angle on the DWC was 159 ± 3.4° and the hysteresis 9 ± 3.6°, whereas the FWC was nearly perfectly wetting). Humid air was introduced inside a custom-built chamber with the upright steady-state condensation imaged by both real-time and high-speed imaging techniques. We found that the droplet shedding mechanisms changed with increasing LN where the sliding drop radii are reduced with LN while the jumping drop radii remained unchanged with LN. The maximum drop size for shedding also decreased by 13%, which we attribute to the secondary droplet inertia, which helps gravity overcome the capillary retention force. Lastly, although many studies have probed DWC enhancements via surface wettability patterning, an optimal combination of ADWC and LN provided in this study significantly aids in the improvement of future DWC-based condensers and water collector applications.

Directional droplet transport behavior on gradient wettability wedge track with extreme wettability contrast

To enhance transport capacity of open-surfaces, a gradient wettability wedge track laid on superhydrophobic background (wedge track No. 2) was fabricated by using masking tape and alkaline-assisted oxidation. The effects of gradient wettability and extreme wettability contrast are emphasized and investigated. The maximum instant velocity of a 10 μL droplet on track No. 2 is ∼ 883.53 mm/s with final transport displacement x = 46.79 mm, which was 64.15 % faster and 11.67 % farther than that on a superhydrophilic wedge track (wedge track No. 1) respectively. Oscillation happens at the wide end of wedge tracks when kinetic energy remains. Oscillation time on wedge track No. 2 is longer, which means gradient wettability can provide an extra driving force. This novel wedge track highlights the necessity of both gradient wettability and extreme wettability contrast and will have broad prospects in directional transport on open-surfaces, biological research, energy conversion and microfluidic mixers.

Efficient Collection of Oil Microdroplets by Hyperbranched, Space‐Filling Open Microfluidic Channels

AbstractOpen microfluidic channels, which capture and transport liquid on flat substrate surfaces through wettability contrast, offer a promising platform for the analysis, mixing, separation, etc. of small‐volume liquid samples. This study focuses on the development of open microfluidic channels featuring oleophilic/oleophobic micropatterning in a hyperbranched, space‐filling structure, thereby creating high‐density channels. These newly designed channels allow for the swift collection of microdroplets with low surface tension within less than 10 s through hierarchical transport and merging of the microdroplets within the channel. The collection efficiency reaches 66% in the channel with the optimized channel geometry. These results underscore the effectiveness of this method for the collection of low surface tension microdroplets, paving the way for the advancement in the domain of open microfluidics of oil, a field that has yet to be fully explored.

Different Roles of Surface Chemistry and Roughness of Laser-Induced Graphene: Implications for Tunable Wettability.

The control of surface wettability is a technological key aspect and usually poses considerable challenges connected to high cost, nanostructure, and durability, especially when aiming at surface patterning with high and extreme wettability contrast. This work shows a simple and scalable approach by using laser-induced graphene (LIG) and a locally inert atmosphere to continuously tune the wettability of a polyimide/LIG surface from hydrophilic to superhydrophobic (Φ ∼ 160°). This is related to the reduced amount of oxygen on the LIG surface, influenced by the local atmosphere. Furthermore, the influence of the roughness pattern of LIG on the wettability is investigated. Both approaches are combined, and the influence of surface chemistry and roughness is discussed. Measurements of the roll-off angle show that LIG scribed in an inert atmosphere with a low roughness has the highest droplet mobility with a roll-off angle of ΦRO = (1.7 ± 0.3)°. The superhydrophobic properties of the samples were maintained for over a year and showed no degradation after multiple uses. Applications of surfaces with extreme wettability contrast in millifluidics and fog basking are demonstrated. Overall, the proposed processing allows for the continuous tuning and patterning of the surface properties of LIG in a very accessible fashion useful for "lab-on-chip" applications.

Wettability contrast on the surface of CuO nanostructures for highly efficient fog harvesting

Fog collection can provide a sustainable and efficient method for solving the shortage of water resources. In this article, CuO nanostructures were designed and synthesized on the Cu mesh by a one-step oxidation method and used to collect water from fog. Their morphology and surface wettability are characterized through scanning electron microscopy and dynamic contact angle measuring device, while the process of fog collection is investigated by a high-speed camera. The fog collection efficiency depends on the microstructure of the sample surface, which could be well controlled by adjusting the reaction temperature. Compared with CuO nanoneedles and CuO nanowire clusters, CuO nanosheets promote the rapid falling of liquid droplets, improving the fog harvesting efficiency, which can achieve 3168 mg/cm2 within 120 s. CuO nanosheets have ultra-low adhesion due to high roughness and high air restraint ability, accelerating the transport of water droplets and reducing the re-evaporation of water droplets.

Droplet impact on a wettability‐patterned woven mesh

AbstractDroplet impact and breakup on meshes are relevant to a number of applications involving filters, textiles, and other spatially inhomogeneous media encountering gas‐dispersed liquids. This study presents high‐resolution simulation results of mm‐size droplets striking wettability‐patterned meshes with the goal of (a) replicating prior physical experiments, (b) identifying sensitivities to the initial conditions and wettability of the mesh wires, and (c) studying the fluid‐field dynamics when droplets strike such meshes. The insights from the present model may help to advance understanding of droplet atomization on meshes, which depends on a number of parameters that are nontrivial to control in an experimental setting. The analysis is carried out by benchmarking the numerical methods used in a commercial software package for orthogonal droplet impact on a flat smooth surface, followed by a convergence analysis, and finally, simulation of specific experiments and case studies involving wettability‐patterned mesh targets. We show that the wettability contrast between the hydrophilic and hydrophobic domains on the mesh as well as the contact angle hysteresis on each side play a critical role in determining whether liquid pinch‐off occurs. The three‐dimensional computational framework constructed in this work is a step toward predicting the postimpact behavior of droplets that strike woven meshes and other porous inhomogeneous media consisting of materials with different wetting properties.

Mold-free self-assembled scalable microlens arrays with ultrasmooth surface and record-high resolution

Microlens arrays (MLAs) based on the selective wetting have opened new avenues for developing compact and miniaturized imaging and display techniques with ultrahigh resolution beyond the traditional bulky and volumetric optics. However, the selective wetting lenses explored so far have been constrained by the lack of precisely defined pattern for highly controllable wettability contrast, thus limiting the available droplet curvature and numerical aperture, which is a major challenge towards the practical high-performance MLAs. Here we report a mold-free and self-assembly approach of mass-production of scalable MLAs, which can also have ultrasmooth surface, ultrahigh resolution, and the large tuning range of the curvatures. The selective surface modification based on tunable oxygen plasma can facilitate the precise pattern with adjusted chemical contrast, thus creating large-scale microdroplets array with controlled curvature. The numerical aperture of the MLAs can be up to 0.26 and precisely tuned by adjusting the modification intensity or the droplet dose. The fabricated MLAs have high-quality surface with subnanometer roughness and allow for record-high resolution imaging up to equivalently 10,328 ppi, as we demonstrated. This study shows a cost-effective roadmap for mass-production of high-performance MLAs, which may find applications in the rapid proliferating integral imaging industry and high-resolution display.

Facile fabrication of extreme-wettability contrast surfaces for efficient water harvesting using hydrophilic and hydrophobic silica nanoparticles

Freshwater shortages threaten ecosystems and human communities around the world. Inspired by the ability of cacti and desert beetles to collect water from the atmosphere, we successfully fabricated a wettability contrast surface with efficient water harvesting using a simple and low-cost process. A solution of isopropyl alcohol and a mixture of hydrophobic and hydrophilic silica nanoparticles was sprayed on a laminating film of ethylene vinyl acetate and polyethylene terephthalate, after which sandpaper was laid on the dry-coated film for hot-press lamination. After peeling the film from the sandpaper, the fabricated surface exhibited random wettability contrast patterns with nano-microstructures formed by nanoparticles and the sandpaper surface. The highest water-harvesting efficiency of the prepared surface was approximately 446.7 mg/cm2/h when the mass ratio of the hydrophobic to hydrophilic silica nanoparticles in the spraying solution was 75/25 due to the optimal performance between the generation of water nucleation, coalescence, and the movement of water droplets. An analysis of the effect of ultraviolet irradiation and adhesive damage on the surface revealed that the efficiency of water collection was nearly unchanged in the face of the ultraviolet irradiation and decreased slightly after the tape test. The proposed fabrication method can make flexible and wettable contrast surfaces at large scales and low costs. In addition, superhydrophobic, or nearly superhydrophilic surfaces can be fabricated with the same process by using solely hydrophobic or hydrophilic silica nanoparticles. Therefore, this process is versatile and can be applied for creating surfaces with a range of wettability properties.

Electric field inspired alternating surface wettability for enhancing pool boiling heat transfer performance: A lattice Boltzmann method study

Electric field and surfactants have been proven to potentially alter surface wettability on demand. In this work, alternating wettability inspired by an electric field has been used to enhance boiling heat transfer performance in terms of critical heat flux (CHF) over a rough surface by using two-dimensional lattice Boltzmann simulations. In alternating wettability, the surface wettability changes with time between hydrophobic and hydrophilic. The effects of the parameters, including alternating wettability time-period and wettability contrast on the boiling process have been revealed. The simulation results show that for the alternating wettability: when surface is hydrophobic bubble expansion is faster, and then wettability transition to hydrophilic quickly retracts three-phase contact line leading to faster bubble shrinking as compared to constantly hydrophobic and constantly hydrophilic surfaces. Furthermore, the rapid shrinking of the bubble tends to disintegrate vapor film on the surface at high wall superheats, resulting in a 53% enhancement in CHF compared to a constantly hydrophilic surface. The bubble departure takes much longer time for too small/large time-periods of alternating wettability. The CHF increases with time-period, becomes maximum at a certain value and then reduces. The CHF also increases with wettability contrast, becomes largest for hydrophilic and hydrophobic contact angles 55o and 107o, respectively, and decreases with further increase in wettability contrast due to high potential for vapor film formation.

Controlled stretching and splitting behaviors of nanodroplets by designing surface wettability patterns

The stretching and splitting behaviors of nanodroplets can be achieved and further controlled through designing two hydrophilic fan-shaped zones symmetrically distributed on the hydrophobic substrate. Comprehensive molecular dynamics (MD) simulations are implemented to validate this idea. When a nanodroplet is wetted on such surfaces, it will be stretched, and its shape is not a commonly recognized spherical cap. It is verified that the wettability contrast between the hydrophobic substrate and the hydrophilic zones plays an important role in the stretching and splitting behaviors. Greater wettability contrasts lead to more stretched nanodroplets. While the wettability contrast has a critical value, after which the droplet will split into two smaller droplets. This critical value is quantified and the boundary between the stretching and splitting behaviors is illustrated. This proposed idea is further implemented to control deposition patterns of nanoparticles by liquid evaporation, which manifests that abundant deposition patterns can be achieved.

Defect-Density-Controlled Phase-Change Phenomena.

Juxtaposing hydrophilicity and hydrophobicity on the same surface, known as hybrid surface engineering, can enhance phase-change heat transfer. However, controlling hydrophilicity on hybrid surfaces in a scalable fashion is a challenge, limiting their application. Here, using widely available metal meshes with variable dimensions and by controlling the patterning pressure, we scalably fabricate hybrid surfaces having spot and gridlike patterns using stamping. Using fog harvesting in a controlled chamber, we show that optimized hybrid surfaces have a ∼37% higher fog harvesting rate when compared to homogeneous superhydrophobic surfaces. Furthermore, condensation frosting experiments reveal that, on grid-patterned hybrid surfaces, frost propagates at ∼160% higher velocity and provides ∼20% less frost coverage when compared to homogeneous superhydrophobic surfaces. During defrost, our hybrid surfaces retain more water when compared to superhydrophobic surfaces due to the presence of hydrophilic patterns and melt water pinning. We adapt our fabrication technique to roll-to-roll patterning, demonstrating wettability contrast on round metallic geometries via atmospheric water vapor condensation. This work provides guidelines for the rapid, substrate-independent, and scalable fabrication of hybrid wettability surfaces for a wide variety of applications.

Oil-Grafted Track-Assisted Directional Transport of Water Droplets and Submerged Air Bubbles on Solid Surfaces

Wettability-tailored tracks are emerging as an efficient approach to collecting and transporting underwater air bubbles as well as water from the mist. However, tailoring the surface wettability by modifying the surface structural features via physiochemical methods to create superhydrophilic-superhydrophobic contrast tracks suffers from long-term durability issues, while the emerging liquid-infused slippery surface has inherent design engineering limitations and issues from infused oil depletion. Herein, we demonstrate that by selective silicone oil grafting onto the glass substrate, it is possible to create a wettability contrast of ∼ 43°. Further, we illustrate the application of such tracks for underwater air bubble capturing and transportation in an aqueous medium with surface tension ranging from 72 to 43.5 mN/m. In addition, the potential of these nonadhesive and adhesive tracks for water collection from the mist is shown and the critical effect of the track dimension and intertrack spacing on the water harvesting rate is investigated in detail. The study illustrates that the nonadhesive nature of the oil-grafted region enables the easy transport of underwater air bubbles as well as water from the flow medium and thus offers an easy and facile approach to creating substrates for underwater air bubble collection and water harvesting.