Wettability Patterns Research Articles

Does biochar affect soil wettability and flow pattern?

Despite the known benefits of biochar application to soils, little is known about its effect on soil wettability and the consequences on soil water movement across the soil profile. Focusing on the latter, three types of commercial biochar derived from wheat, corn, and rice straw produced under low temperature (350–450 ℃) were mixed with a wettable sandy soil at rates of 0%, 2%, 5%, and 10% w/w. Soil water repellency (SWR) of the biochars and biochar–soil mixtures was determined by water drop penetration time (WDPT) test and sessile-drop contact angle (CA) measurement. The WDPT results (< 5 s) showed complete wettability for the biochar–soil mixtures, whereas CA results indicated some water repellency. The mean initial CA of the three biochars varied between 105.3° and 113.9°, with ∼ 30° for the pure sand, and between 56.8° and 75.7° for the biochar–soil mixtures. SWR increased with biochar-application rate, whereas biochar particle size (< 1 mm and > 1 mm) had no significant effects on SWR. A 2-D flow chamber experiment with point source water application at the surface was used to continuously monitor the flow pattern and soil water content (SWC) distribution across the soil profile during infiltration and redistribution periods. Biochar–soil mixtures (2% and 5% w/w) with the three biochar types were studied. Flow chamber results showed substantial differences in plumes shape and internal SWC distribution in the biochar–soil mixtures compared to the untreated sand during the wetting and drainage. The plumes' shape and nonmonotonic spatial SWC distribution suggested unstable flow in the biochar–soil mixtures. The internal fingers developed in the biochar treated sand indicate that SWR induced by biochar addition seemed to be physically induced via the blending of the biochar and soil particles, rather than chemically induced, via coating of the soil particles with amphiphilic molecules. Under field conditions, the primary and inner finger-like plumes may eventually form preferential flow paths that will affect the spatial distribution of water and fertilizer in soil profiles and their availability to plant roots.

Effects of non-wetting fraction and pitch distance in flow boiling heat transfer in a wettability-patterned microchannel

Flow boiling in microchannels offers a promising and attractive solution for thermal management of electronic devices and power systems. In this paper, microchannels composed of a hydrophilic surface with hydrophobic dots were studied to characterize the effects of non-wetting fraction and pitch distance of adjacent dots on flow boiling heat transfer and pressure drop. The pitch distances ranging from 122 µm to 172 µm were studied. Using deionized water as the working fluid, highly subcooled flow boiling experiments were conducted at different mass fluxes ranging from 41.1 to 246.6 kg/m2s over a heat flux up to 146.2 W/cm 2. Bubble dynamics and flow patterns were visualized using a high-speed camera. It was found that bubbles coalesced more easily, and flow patterns transited faster in the microchannel with smaller pitch distance. Heat transfer coefficient (HTC), critical heat flux (CHF) and pressure drop were found to significantly rely on the pitch distance of dots and the mass flux. Furthermore, based on a force-balance model, bubble detached diameters were predicted in hydrophilic, hydrophobic and wettability-patterned microchannels, respectively. This provides a useful insight to optimize the wettability pattern design and then improve flow boiling heat transfer in a microchannel.

Multi-Bioinspired Janus Copper Mesh for Improved Gravity-Irrelevant Directional Water Droplet and Flow Transport.

A conceptually novel multi-bioinspired strategy based on structures and functions derived from the Namib desert beetle and lotus leaf is proposed in this paper. The proposed scheme synergistically combines the features of alternating wettability patterns and asymmetric wettability for improved directional water transport. Consequently, a Janus copper mesh, which substantially outperforms other single-bioinspired synthetic materials, is produced. The Janus copper mesh achieves directional self-transportation of tiny water droplets and continuous water flow in a gravity-irrelevant or an anti-gravity manner without energy consumption. This depends on the asymmetric wettability and alternating hydrophobic-hydrophilic wettability patterns on the hydrophobic surface of the mesh. In particular, Janus copper shows remarkable selective directional water transport in a water-oil system, rendering it a promising candidate for practical applications.

Aspirin/PLGA coated 3D-printed Ti-6Al-4V alloy modulate macrophage polarization to enhance osteoblast differentiation and osseointegration

Although titanium (Ti) and Ti-based alloy have been widely used as dental and orthopedic implant materials, its bioinertness hindered the rapid osseointegration. Therefore, it is recommended to acquire ideal topographic and chemical characteristics through surface modification methods. 3D printing is a delicate manufacture technique which possesses superior controllability and reproducibility. While aspirin serve as a well-established non-steroidal anti-inflammatory agent. Recently, the importance of immune system in regulating bone dynamics has attracted increasing attention. We herein superimposed the aspirin/poly (lactic–co–glycolic acid) (ASP/PLGA) coating on the 3D-printed Ti-6Al-4V surface with uniform micro-structure to establish the Ti64-M-ASP/PLGA substrate. Scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS) and contact angle test confirmed the successful fabrication of the Ti64-M-ASP/PLGA substrate, with increased wettability and sustained release pattern of ASP. Compared with the Ti64 base material, the Ti64-M-ASP/PLGA substrate showed enhanced M2 and depressed M1 genes and proteins expressions in macrophages. The novel Ti64-M-ASP/PLGA substrate also displayed enhanced osteoblast proliferation, adhesion, extracellular mineralization ability and osteogenic gene expressions when cultured with macrophage conditioned medium in vitro. Furthermore, rat femora implantation model was used for in vivo evaluation. After 4 weeks of implantation, push out test, micro-computed tomography (micro-CT) and histological analyses all confirmed the superior osseointegration capabilities of the Ti64-M-ASP/PLGA implant than the other groups. Our study revealed the synergistic role played by 3D-printed micro topography and immunoregulatory drug aspirin in promoting osteogenesis in vitro and accelerating osseointegration in vivo, thus providing a promising method for better modifying the implant surface.Graphical abstract

Wick-free paradigm for high-performance vapor-chamber heat spreaders

In this study, a novel vapor chamber featuring wickless wettability-patterned components is designed, fabricated, and tested. This is the first phase-change heat transfer apparatus comprised entirely of wick-free elements that transport liquid by way of wetting forces generated by precise surface-wettability patterning. Our approach takes advantage of the phase-changing properties of water within a closed loop composed of two opposing, wickless wettability-patterned plates, one acting as an evaporator, and the other designed as a condenser. The plates feature different geometric wettability patterns, each especially designed/engineered to transform a flat rectangular copper plate to a functional component of a low-profile vapor chamber. The wettability pattern of the condenser facilitates spatially controlled dropwise and filmwise condensation and offers an optimal way to move the condensate through specifically built wedge tracks utilizing capillary forces. The wettability pattern laid on the evaporator enables the collection/accumulation and transport of the returning condensate fluid to the heat-input portion where evaporation must be strongest for maximum heat removal. Using this system, we effectively cooled a 0.9 cm2 heated area by spreading the heating power (∼200 W) over a 30× larger condenser area. The best performance metrics were thermal resistance 0.18 K/W at 10 W, in-plane thermal conductivity 1.17 kW/(m·K) at 22 W, and maximum heat flux ∼ 2.2 MW/m2. The present wick-free vapor-chamber heat-spreader apparatus is the first of its type and the simplicity of its design along with its moderate dimensions (making it light and thin) are important advantages that make this system well-suited for a wide range of thermal management applications.

On-demand evaporation mode of sessile droplet by designing substrate surface wettability patterns

A conceptual strategy of controlling the evaporation mode of sessile droplet that is demonstrated by the variation of its contact angle and contact radius through designing the surface wettability pattern of substrate is proposed. A theoretical model is established based on the geometry shape of the droplet and minimization of the total surface free energy of the droplet-substrate system. The model is implemented to predict the variation of the surface free energy during evaporation and is used to determine the time evolutions of the contact angle and contact radius. With this theory, a relationship between surface wettability patterns and droplet evaporation modes is established. Comprehensive molecular dynamics simulations are employed to validate the theoretical predictions, and the evolutions of the contact angle and contact radius during evaporation agree well with the theoretical predictions. The proposed technique to control the evaporation mode of sessile droplets is further implemented to control the deposition pattern of nanoparticles by liquid evaporation, which manifests that the theoretical model established herein could explain the underlying mechanism of controlling evaporation mode of sessile droplets and such method of designing substrate surface wettability pattern has potential applications in inkjet printing, nano-assembly, and nano-device manufacturing.

Breaking the symmetry to suppress the Plateau\u2013Rayleigh instability and optimize hydropower utilization

Droplet impact on solid surfaces is essential for natural and industrial processes. Particularly, controlling the instability after droplet impact, and avoiding the satellite drops generation, have aroused great interest for its significance in inkjet printing, pesticide spraying, and hydroelectric power collection. Herein, we found that breaking the symmetry of the droplet impact dynamics using patterned-wettability surfaces can suppress the Plateau–Rayleigh instability during the droplet rebounding and improve the energy collection efficiency. Systematic experimental investigation, together with mechanical modeling and numerical simulation, revealed that the asymmetric wettability patterns can regulate the internal liquid flow and reduce the vertical velocity gradient inside the droplet, thus suppressing the instability during droplet rebounding and eliminating the satellite drops. Accordingly, the droplet energy utilization was promoted, as demonstrated by the improved hydroelectric power generation efficiency by 36.5%. These findings deepen the understanding of the wettability-induced asymmetrical droplet dynamics during the liquid–solid interactions, and facilitate related applications such as hydroelectric power generation and materials transportation.

Cactus‐Spine‐Inspired Sweat‐Collecting Patch for Fast and Continuous Monitoring of Sweat (Adv. Mater. 40/2021)

Noninvasive Healthcare Monitoring In article number 2102740, Kilwon Cho and co-workers report a sweat-collecting patch with directional sweat transportation, enabling fast and continuous monitoring of sweat by a sensor. The wedge-shaped wettability patterns of the patch are inspired by spines of cacti; the channel shape, in combination with the superhydrophobic/superhydrophilic surface materials, induces a unidirectional Laplace pressure that transports the sweat to the sensing-area fast and spontaneously.

High-Performance Planar Thermal Diode With Wickless Components

Abstract The characterization “thermal diode” (TD) has been used to portray systems that spread heat very efficiently in a specific direction but obstruct it from flowing in the opposite direction. In this study, a planar vapor chamber (VC) with a wickless, wettability-patterned side and an opposing wick-lined side is fabricated and tested as a thermal diode. When the chamber operates in the forward mode, heat is naturally driven away from the heat source; in the reverse mode, the system blocks heat flow, thus acting as a thermal diode. The low-profile assembly takes advantage of the phase-changing properties of water inside a sealed chamber. The wettability-patterned plate—when on the cooled side, e.g., forward operation mode—enables spatially controlled dropwise condensation (high heat transfer rate) and filmwise condensation (high drainage rate), thus facilitating an efficient transport mechanism of the condensed medium on superhydrophilic wedge tracks by way of Laplace pressure-driven capillary forces. The same chamber acts as a thermal blocker when the wick-covered plate is on the cooled side (reverse operating mode), trapping the condensate in the wick pores and blocking heat flow to the opposite side. The system's thermal behavior is similar to the theoretical electrical diode. This work explores the effect of the condenser's wettability pattern design and the chamber's fluid charging ratio (CR). With this system, thermal diodicities exceeding 20 have been achieved, and are tunable by altering the wettability pattern. The thermal rectification concept and its proper quantification in terms of possible definitions are discussed. The present vapor chamber—thermal diode design could be well-suited for an extensive range of thermal-management applications, ranging from aerospace, spacecraft, and smart-building construction materials, to electronics protection, electronics packaging, refrigeration, thermal control during energy harvesting, thermal isolation, etc.

Understanding the role of wettability distribution on pore-filling and displacement patterns in a homogeneous structure via quasi 3D pore-scale modelling

Most numerical simulation studies have focused on the effect of homogenous wettability on fluid flow dynamics; however, most rocks display spatially heterogeneous wettability. Therefore, we have used direct numerical simulations (DNS) to investigate wettability heterogeneity at pore-scale. We have built a quasi-3D pore-scale model and simulated two-phase flow in a homogenous porous media with homogenous and heterogeneous wettability distributions. Five different heterogeneous wettability patterns were used in this study. We observed that heterogenous wettability significantly affects the evolution of fluid interface, trapped saturation, and displacement patterns. Wettability heterogeneity results in fingering and specific trapping patterns which do not follow the flow behaviour characteristic of a porous medium with homogenous wettability. This flow behaviour indicates a different flow regime that cannot be estimated using homogenous wettability distributions represented by an average contact angle. Moreover, our simulation results show that certain spatial configurations of wettability heterogeneity at the microscale, e.g. being perpendicular to the flow direction, may assist the stability of the displacement and delay the breakthrough time. In contrast, other configurations such as being parallel to the flow direction promote flow instability for the same pore-scale geometry.

Limited Enhancement of Subatmospheric Boiling on Treated Structured Surfaces With Biphilic Pattern

Abstract Boiling heat transfer suffers deteriorations under subatmospheric conditions, which can be attributed to a shortage of viable nucleation sites at declining pressures. In this work, the possibility of enhancing low-pressure saturated boiling of water using a combination of wettability patterning and structural modifications was experimentally explored. The copper test surface, comprised of an array of circular “dimples” (0.3 mm in depth, 0.5 mm in diameter, and 3.0 mm in pitch), was spray-coated by polytetrafluoroethylene (PTFE) coatings so as to form a matching biphilic pattern with the surface cavities. The resulting dimpled biphilic surface showed appreciable heat transfer enhancement—with a maximum 60% increase of the average heat transfer coefficient of nucleate boiling compared with a flat biphilic surface—down to about 9.5 kPa. Further lowering the pressure to 7.8 kPa, however, was found to lead to diminished performance gains. The visualization study of the bubble departure dynamics revealed signs of additional vapor trapping of the hydrophobic-coated cavities, which can induce uninterrupted bubble regeneration with zero waiting time and explain the qualified enhancement of subatmospheric boiling. Thanks to a potential secondary pinning of contact line inside the hydrophobic cavities, incomplete bubble detachment could prevail at somewhat lower pressures than was otherwise possible without the dimple structure, leaving behind significantly more vapor residues. However, the vapor-trapping capacity was found to decrease with pressure, which provided clues with regard to the reduced efficacy of the surface at even lower pressures.

Large-area surface wettability patterning of metal alloys via a maskless laser-assisted functionalization method

An innovative laser-assisted functionalization (LAF) method is presented in this work to fabricate a superhydrophobic-superhydrophilic patterned surface on a large-area metal alloy with a high process throughput. The LAF method comprises four steps, including two laser processing steps and two surface chemistry modification steps. Two strategies are demonstrated to fabricate extreme wettability patterns on aluminum alloy without using protective masking. Strategy I fabricates contrasting topography and chemistry where superhydrophobic regions are nanostructured with fluorocarbon chemistry, and superhydrophilic regions contain microchannels with nitrile chemistry. Conversely, strategy II fabricates similar microchannel topography for both wettability regions with contrasting chemistry. Surface topography patterning, from low-roughness, isotropic, random nanostructured texture to highly structured texture, is achieved by adjusting the laser parameters during LAF. LAF constitutes the first known laser-based surface engineering method that decouples extreme wettability from surface topography. In addition, anti-reflection surface property, ranging from moderately reduced reflectivity of about 50% to ultralow reflectivity of less than 10%, is achieved using this process. The processing mechanism, surface chemistry, and surface topography are analyzed to demonstrate the simplicity, robustness, and feasibility of the process.

Rapid, Simple, and Inexpensive Spatial Patterning of Wettability in Microfluidic Devices for Double Emulsion Generation.

Water-in-oil-in-water (w/o/w) double emulsion (DE) encapsulation has been widely used as a promising platform technology for various applications in the fields of food, cosmetics, pharmacy, chemical engineering, materials science, and synthetic biology. Unfortunately, DEs formed by conventional emulsion generation approaches in most cases are highly polydisperse, making them less desirable for quantitative assays, controlled biomaterial synthesis, and entrapped ingredient release. Microfluidic devices can generate monodisperse DEs with controllable size, morphology, and production rate, but these generally require multistep fabrication processes and use of different solvents or bulky external instrumentation to pattern channel wettability. To overcome these limitations, we propose a rapid, simple, and inexpensive method to spatially pattern wettability in microfluidic devices for the continuous generation of monodisperse DEs. This is achieved by applying corona-plasma treatment to a select zone of the microchannel surface aided by a custom-designed corona resistance microchannel to strictly confine the plasma-treatment zone in a single polydimethylsiloxane (PDMS) microfluidic device. The properties of PDMS channel surfaces and key microchannel regions for DE generation are characterized under different levels of treatment. The size, shell thickness, and number of inner cores of generated DEs are shown to be highly controllable by tuning the phase flow rate ratios. Using DEs as templates, we successfully achieve a one-step generation and collection of gelatin microgels. Additionally, we demonstrate the biological capability of generated DEs by flow cytometric screening of the encapsulation and growth of yeast cells within DEs. We expect that the proposed approach will be widely used to create microfluidic devices with more complex wettability patterns.

The study of nucleation site interactions on the mixed wettability rough surface

In order to achieve the best heat transfer performance on a mixed wettability surface, engineering of the wettability pattern is often used as heat transfer performance is significantly dependent on pitch distance between hydrophobic spots, which is maybe linked to the nucleation site interaction effects. In this work, the effects of nucleation site interactions between two hydrophobic cavities on a hydrophilic surface (a mixed wettability rough surface) are investigated, using a two-dimensional pseudopotential phase-change lattice Boltzmann method. The results of the mixed wettability rough surface are compared with completely hydrophilic rough surface and mixed wettability smooth surface to understand the effects of wettability and surface roughness. The focus is placed on the hydrodynamic interaction (which has promotive effect on the bubble formation activity) and thermal interaction (which has inhibitive effect). There exists an optimum pitch distance at which the heat flux and heat transfer coefficient are highest for mixed wettability surface, which are due to larger intensity of hydrodynamic interaction and nearly zero intensity of thermal interaction. Moreover, heat flux is found to be larger for the mixed wettability rough surface than the mixed wettability smooth surface. The bubble nucleation mechanism and time are found to be different for all surfaces.

Tunable Wettability Pattern Transfer Photothermally Achieved on Zinc with Microholes Fabricated by Femtosecond Laser.

A quickly tunable wettability pattern plays an important role in regulating the surface behavior of liquids. Light irradiation can effectively control the pattern to achieve a specific wettability pattern on the photoresponsive material. However, metal oxide materials based on light adjustable wettability have a low regulation efficiency. In this paper, zinc (Zn) superhydrophobic surfaces can be obtained by femtosecond-laser-ablated microholes. Owing to ultraviolet (UV) irradiation increasing the surface energy of Zn and heating water temperature decreasing the surface energy of water, the wettability of Zn can be quickly tuned photothermally. Then, the Zn superhydrophobic surfaces can be restored by heating in the dark. Moreover, by tuning the pattern of UV irradiation, a specific wettability pattern can be transferred by the Zn microholes, which has a potential application value in the field of new location-controlled micro-/nanofluidic devices, such as microreactors and lab-on-chip devices.

Droplet Dynamics on a Wettability Patterned Surface during Spray Impact

Wettability patterning of a surface is a passive method to manipulate the flow and heat transport mechanism in many physical processes and industrial applications. This paper proposes a rational wettability pattern comprised of multiple superhydrophilic wedges on a superhydrophobic background, which can continuously remove the impacted spray droplets from the horizontal surface. We observed that the spray droplets falling on the superhydrophilic wedge region spread and form a thin liquid film, which is passively transported away from the surface. However, most of the droplets falling on the superhydrophobic region move towards the wedge without any flooding. The physics of the passive transport of the liquid film on a wedge is also delved into using numerical modelling. In particular, we elucidate the different modes of droplet transport in the superhydrophobic region and the interaction of multiple droplets. The observed droplet dynamics could have profound implications in spray cooling systems and passive removal of liquid from a horizontal surface. This study’s findings will be beneficial for the optimization of efficient wettability patterned surfaces for spray cooling application.

Control of droplet evaporation on smooth chemical patterns

We investigate droplets evaporating on flat pinning-free substrates with smooth wettability patterns. Symmetric patterns lead to a hierarchy of bifurcations in the three-dimensional parameter space represented by the droplet cross sectional area $A$, midpoint $\ensuremath{\ell}$, and footprint $R$. In asymmetrical patterns, the presence of disconnected stable branches forces the droplet to move towards one direction, hence showing that a droplet's motion can be controlled upon evaporation.

Microstructured wettability pattern for enhancing thermal performance in an ultrathin vapor chamber

We investigated the thermal performance of a novel wettability patterned evaporator for an ultrathin vapor chamber. Because the evaporator integrates a wettability patterned substrate underneath the nanostructured mesh wick which can pin the three-phase contact lines on the hydrophilic/hydrophobic boundaries, it enlarges the area of thin-film evaporation. Microstructured wettability pattern is fabricated on the evaporator surface and the wick is pressed onto the evaporator by a micropillar array to make an intimate contact with each other. The micropillar array electroplated on the inner side of the condenser also supports a vapor core as a vapor flow path. The thermal resistance of the ultrathin vapor chamber is experimentally evaluated, and the measurement results show that the wettability pattern underneath the nanostructured mesh wick can greatly reduce the horizontal thermal resistance, giving a better temperature uniformity across the condenser side, though the vertical thermal resistance may be slightly larger than that without a wettability pattern. The highest in-plane effective thermal conductivity of a 200 μm-thick vapor chamber can reach 11914.9 W/(m·K) at 23.91 W/cm2 heat flux, which shows a 210.7% further improvement in comparison with that of the ultrathin vapor chamber with the nanostructured mesh wick only.

Facile fabrication of binary wettability patterned microstructure for microfluidics

Diverse wetting contrasts of binary wettability patterns in nature have inspired a versatile platform for microfluidics manipulation. To date, nevertheless, building a binary wettability patterned micro/nano structure with highly ordered configuration has inevitably brought wettability contamination challenges in interface control. Herein, this work studied a facile approach to fabricate permanent and contamination-free microstructure with high contrast wettability by means of adding a small amount of surfactant to the photoresist, rather than through any surface modification of the substrate. Accordingly, we focused on a systematic research of the wettability contamination prevention, binary wettability interfacial pattern control, and the dynamic electrowetting performance. The results demonstrated the proposed fabrication of binary wettability patterned microstructure without reactive-ion etching would be advantageous for contamination-free, permanent, simple, reliable and precise process, which could enable the microfluidic electrowetting display highly stable and controllable, thus it is expected to suit fields all the way from microfluidic-based fundamental studies to materials processing engineering and device applications.

Investigation of Coalescence-Induced Droplet Jumping on Mixed-Wettability Superhydrophobic Surfaces

Coalescence-induced droplet jumping has received more attention recently, because of its potential applications in condensation heat transfer enhancement, anti-icing and self-cleaning, etc. In this paper, the molecular dynamics simulation method is applied to study the coalescence-induced jumping of two nanodroplets with equal size on the surfaces of periodic strip-like wettability patterns. The results show that the strip width, contact angle and relative position of the center of two droplets are all related to the jumping velocity, and the jumping velocity on the mixed-wettability superhydrophobic surfaces can exceed the one on the perfect surface with a 180° contact angle on appropriately designed surfaces. Moreover, the larger both the strip width and the difference of wettability are, the higher the jumping velocity is, and when the width of the hydrophilic strip is fixed, the jumping velocity becomes larger with the increase of the width of the hydrophobic strip, which is contrary to the trend of fixing the width of the hydrophobic strip and altering the other strip width.