Superhydrophobic Layer Research Articles

UV Resistance of Super-Hydrophobic Stainless Steel Surfaces Textured by Femtosecond Laser Pulses

The loss of spontaneous liquid repellency on the surface of AISI 304 stainless steel under UV irradiation has been investigated depending on the textures formed by femtosecond laser pulses using Owens–Wendt plot analysis. Laser-induced periodic surface structures (LIPSS) have shown less liquid repellency compared to microgrooves. The polarity of the super-hydrophobic non-polar layer increased under UV irradiation to a super-hydrophilic state. The rate of this transition is determined by the surface topography and was faster for LIPSS compared to the bihierarchical textures formed by LIPSS in combination with microgrooves. The applicability of the Owens–Wendt approach for the numerical comparison of the achievable liquid repellency of textured surfaces in the Cassie state and the degree of polarity reversal of the hydrophobic layer was shown.

Superhydrophobic Coating Based on Decorated Carbon Nanoparticles

A method is proposed for increasing the resistance of a superhydrophobic coating based on a CNT xerogel to frost deposition through the use of decorating nanoparticles. The effects of the addition of fullerenes, carbon nanoonions (CNOs), detonation nanodiamonds, silicon dioxide, and paraffin to the xerogel are tested. An increase in the resistance of the coating to the deposition of condensate in the form of frost is revealed. The addition of fullerene C60 leads to the best results. Increasing the resistance to icing allows us to spend less power on heating the surface during short cold snaps, bypassing the anti-icing properties of the protective superhydrophobic layer. However, the application of this approach shows a deterioration in the resistance of the coating to the penetration of the spray. This is given a qualitative explanation and measures to combat it are proposed. No effect of the additives on the mechanical properties of the coating or its resistance to damage is detected. In additon, decorating additives affect the formation of the coating relief. With this, it is possible to influence the stochastic processes of the formation of roughness during the drying of the xerogel.

Construction of superhydrophobic film on the titanium alloy welded joint and its corrosion resistance study

PurposeThis study aims to improve the corrosion resistance of TA2-welded joints by superhydrophobic surface modification using micro-arc oxidation technology and low surface energy substance modification.Design/methodology/approachThe microstructure and chemical state of the superhydrophobic film layer were analyzed using scanning electron microscopy, energy dispersive X-ray spectroscopy, three-dimensional morphology, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared absorption spectroscopy. The influence of the superhydrophobic film layer on the corrosion resistance of TA2-welded joints was investigated using classical electrochemical testing methods.FindingsThe characterization results showed that the super hydrophobic TiO2 ceramic membrane was successfully constructed on the surface of the TA2-welded joint, and the construction of the super hydrophobic film greatly improved the corrosion resistance of the TA2-welded joint.Originality/valueThe superhydrophobic TiO2 ceramic membrane has excellent corrosion resistance. The micro nanostructure in the superhydrophobic film can intercept air to form an air layer to prevent the corrosion medium from contacting the surface, thus, improving the corrosion resistance of the sample.

Light-Propelled Super-Hydrophobic Sponge Motor and its Application in Oil-Water Separation.

Self-propelled separation materials, that is, motor, are one of the keys to realizing smart oil-water separation. Although three-dimensional sponges such as commercial melamine sponge (MS) exhibit excellent oil-water separation ability, they cannot move by themselves on water. Aiming at solving this problem, a polydimethylsiloxane (PDMS) and molybdenum disulfide (MoS2) modified MS motor (PDMS@MS/MoS2) with an asymmetric multilayer structure was prepared, in which the photothermal layer MoS2 provided the propelling force for the motor under infrared light irradiation, and the middle layer PDMS was used as the superhydrophobic modified agent and adhesive agent between commercial MS and MoS2 powder. PDMS coated MS (PDMS@MS) as the superhydrophobic layer showed good superhydrophobic ability (153.1°) and oil-water separation capacity (52.33 g/g to liquid paraffin). Furthermore, the introduction of MoS2 made the speed of the sponge motor reach 8.27 mm s-1 with a removal quantity of 12.20 g/g for cyclohexane. After recycling 8 times, the contact angle, cyclohexane capturing amount, and average velocity of the motor were 150.3°, 11.40 g/g, and 8.41 mm/s, respectively. Meanwhile, PDMS@MS/MoS2 kept a similar light-propelling velocity (∼8 mm) at different pH values and in simulated seawater, demonstrating that the light-propelling motor possessed a good cycle and practical performance, which provides a possibility for the directional light propulsion of a sponge motor in oil-water separation.

Functional partition strategy in assistance by shear thinning/self-healing effect to prepare durable anti-corrosion coating

The application of self-healing polymers in anti-corrosion coatings is restricted by their nonautonomous healing ability and poor long-term durability. In this paper, we propose a functional partition strategy assisted by the shear thinning effect to overcome the above restriction. The shear thinning effect was achieved by preparing a branched ethyl acrylate-acrylonitrile copolymer (B-PEA). The B-PEA coating exhibits multiple autonomous self-healability at room temperature (1.5 h) and in a salt solution (2 h), with a high adhesion strength of 3.4 MPa. To further improve the performance, a functional partition architecture combining superhydrophobic barrier and self-healing multilayers was constructed. The introduction of the superhydrophobic barrier layer largely increases the low-frequency impedance modulus |Z|f-0.01Hz and salt solution resistance time to 3.9 × 1010 Ω⋅cm2 and 3600 h, respectively, compared to a pristine B-PEA coating. The introduction of the B-PEA layer provides the barrier layer with room-temperature self-healing ability (2 h), and increases the long-term |Z|f-0.01Hz and salt solution resistance time in the presence of scratches to 7.7 × 109 Ω⋅cm2 and > 3600 h, respectively. The as-prepared superhydrophobic anti-corrosion coating system combines long-term durability and room-temperature autonomous self-healability in harsh salt-solution environments, and thus has a high potential for anti-corrosion applications.

Airborne hydrocarbon induced multifunctional coating towards bouncing characteristics, icing delaying, hot-water repellency, and anti-corrosion on copper

Some studies have confirmed copper coating can realize superhydrophobicity after exposure to air. However, the comprehensive characteristics of the special wettable coating such as surface energy, bouncing behavior, hot water repellency, icing delaying, and corrosion resistance are still not well understood. Here, without further chemical processing, an electrodeposition technique creates a superhydrophobic layer on a copper substrate. The wettability versus different exposure routes is investigated. As a result, the slow oxidation of Cu and adsorption of airborne hydrocarbon contribute to a low rolling angle and enhanced water droplet bouncing ability. The coating displays strong 95 °C hot water repellency, icing delaying (∼78s), and great anti-corrosion ability, which also has a high protection efficiency (98.84%) and an improved coating resistance of 2.198 × 105 Ω cm2. The multifunctional properties are attributed to the airborne hydrocarbon and the constructed hierarchical structure. This preparation process avoids complex chemical modification and it can provide a new strategy for designing multifunctional coating for surface protection on metals.

Effect of modified MgAl-LDH coating on corrosion resistance and friction properties of aluminum alloy

The in-situ growing approach was utilized in this article to construct the magnesium–aluminum layered double hydroxide (MgAl-LDH) film on the surface of a 1060 aluminum anodized film. To improve the corrosion resistance and friction qualities of aluminum alloy, the MgAl-LDH coating was treated using stearic acid (SA) and thiourea (TU). The aluminum substrate and anodized aluminum film layer corroded to varying degrees after 24 h of immersion in 3.5% (mass) NaCl solution, while the modified hydrotalcite film layer continued to exhibit the same microscopic morphology even after being immersed for 7 d. The results show that the synergistic action of thiourea and stearic acid can effectively improve the corrosion resistance of the MgAl-LDH substrate. The tribological testing reveals that the hydrotalcite film layer and the modified film layer lowered the friction coefficient of the anodized aluminum surface substantially. The results of the simulations and experiments demonstrate that SA forms the dense LDH-TU interlayer film layer by exchanging NO3– ions between TU layers on the one hand and the LDH-SA film layer by adsorption on the surface of LDH on the other. Together, these two processes create LDH-TU-SA, which can significantly increase the substrate's corrosion resistance. This synergistically modified superhydrophobic and retardant hydrotalcite film layer offers a novel approach to the investigation of wear reduction and corrosion protection on the surface of aluminum and its alloys.

Femtosecond Laser Preparation of Terracotta Warrior-like Pit Superhydrophobic Structure on Magnesium Alloy with Mechanical Durability and Corrosion Resistance.

Magnesium alloys are among the most promising materials for medical implants, and by preparing a superhydrophobic surface, the rate of corrosion can be effectively slowed down and durability be improved. However, the anticorrosion surfaces are inevitable to be damaged for the conventional micro-nanostructured superhydrophobic magnesium alloys, which highly limits their application prospects. This work proposes the development of a Terracotta Warrior pit superhydrophobic structure (TWPSS), consisting of a wall structure with a Terracotta Warrior-like pit and a lotus-like surface papillae structure within the wall. For the first time, top-down laser ablation of the pits to prepare the lotus-like surface papilla structure is used in conjunction with a bottom-up laser-guided melt stacking of the raised wall structure to achieve rapid fabrication of a TWPSS on a magnesium alloy surface. The Cassie-Baxter-based design of the wall structure spacing effectively protects the internal lotus-like surface papillae from damage and the disappearance of low surface energy material, and the results show that the superhydrophobic surfaces of magnesium alloys have excellent mechanical durability and repairability. In addition, it was found that the recast layer and laser melting stacked layers produced on the surface of the alloy during femtosecond laser processing refined the grain size of the magnesium alloy and effectively suppressed the corrosion rate. The combination of the superhydrophobic gas layer and the resulting grain refinement slowed down the corrosion of the magnesium alloy. Thus, the rapid preparation of TWPSS structures with mechanical durability and corrosion resistance by femtosecond lasers expands the clinical applications of superhydrophobic surface magnesium alloys in medical devices.

Intermediate-layer strengthened superhydrophobic coating with stable corrosion resistance, delayed icing and long-term weatherability

Improving the mechanical stability and anti-corrosion durability of superhydrophobic material remains a great challenge that has not been solved. In this paper, we designed and prepared two types of superhydrophobic coatings using PTFE nanoparticles and waterborne epoxy resin, viz. superhydrophobic PTFE@epoxy single layer coating and superhydrophobic Epoxy + PTFE@epoxy double layer coating. It was found that the mechanical stability and interfacial bonding of the superhydrophobic composite coatings were further improved by introducing an epoxy resin intermediate layer. The superhydrophobic Epoxy + PTFE@epoxy double layer coating maintained excellent non-wetting superhydrophobicity even after 150 sandpaper abrasion and 160 tape-peeling cycles. The electrochemical testing results showed that the Rct and low-frequency modulus values of the superhydrophobic double layer coating improved by 7–8 orders of magnitude compared with the bare Q235 carbon steel substrate, demonstrating excellent corrosion resistance. In addition, the coating maintains good corrosion protection after 60 days of outdoor atmospheric exposure and 30 days of neutral salt spray acceleration tests, indicating superior weathering resistance and long-term anti-corrosion properties. The coating also features delayed icing, self-cleaning, anti-fouling, chemical stability, and high substrate applicability, which provides long-lasting multifunctional superhydrophobic materials with comprehensive functions.

Facile fabrication of robust superhydrophobic polyurethane sponge modified with polydopamine- silica nanoparticle for effective oil/water separation

Porous superhydrophobic materials with high mechanical stability have a significant amount of potential for oil-water separation in practice. In this study, a superhydrophobic material with strong hydrophobic coating adhesion was developed using a polyurethane sponge. The interior and exterior surfaces of the porous polyurethane material were coated using a two-step technique. The sponge was treated with poly (dopamine) to modify its skeleton, allowing the second layer to adhere readily. The second layer of silica applied to the surface of the sponge functions as a superhydrophobic layer. The modified sponge's water contact angle was approximately 156.2° ± 5°, compared to 78° ±2° for the original sponge, demonstrating its superhydrophobic nature. The fabricated sponge is capable of separating and absorbing 43.3–51 times of its initial mass from an oil-water mixture, depending on the type of oil used in the test. Fifteen cycles of oil-water separation were followed by chemical cleaning of the hydrophobic sponge. After undergoing these cycles, the hydrophobic nature and absorption capacity of the produced sponge were preserved. Over a broad pH range, the hydrophobicity of the superhydrophobic sponge was studied, and the results indicated that hydrophobicity was substantially maintained even under harsh pH operating conditions. The water contact angle of the sponge was measured to be 145° ± 1° and 140° ± 1°, respectively, after 12 h of immersion in strong acids and bases.

Cell reprogramming in a predictable manner on the superhydrophobic microwell array chip

Reprogramming of somatic cells into the pluripotent state is stochastic and inefficient using the conventional culture plates. Novel micro-culture systems employing precisely controlled biophysical cues can improve the reprogramming efficiencies dramatically. Here we perform iPSC induction on our previously developed superhydrophobic microwell array chip (SMAR-chip) where cells undergo distinctive morphology change, switching from 2D monolayers to 3D clumps, and develop into bona fide colonies in more than 90% of the microwells. The PDMS substrate, together with the microwell structure and the superhydrophobic layer constitute a well-controlled microenvironment favorable for the morphogenesis and pluripotency induction. Investigation of the molecular roadmap demonstrates that the SMAR-chip promotes the transition from the initiation phase to the maturation phase and overcomes the roadblocks for reprogramming. In addition, the SMAR-chip also promotes the reprogramming of human cells, opening our method for translational applications. In summary, our study provides a novel platform for efficient cell reprogramming and emphasizes the advantages of employing the insoluble microenvironmental cues for the precise control of cell fate conversion.

Design and performance optimization of self-cleaning coating on decorative UHPC surface

After long-term use, decorative UHPC may adhere to pollutants due to its hydrophilicity, which affects its aesthetics. Due to the smooth and dense exposed surface of UHPC, ordinary self-cleaning coatings have poor adhesion on its surface and are prone to peeling off due to rain wash in the environment, which cannot maintain the self-cleaning performance of UHPC for a long time. In this paper, a modified self-cleaning coating with a double layer structure was designed. The bottom layer is an epoxy resin (ER) adhesive layer to improve the adhesion performance of the coating to UHPC. The surface layer is a polymethylhydrosiloxane (PMHS) modified nano-SiO2 superhydrophobic layer, which satisfies the self-cleaning characteristics while constructing a regular rough superhydrophobic surface on the UHPC surface. The prepared superhydrophobic UHPC showed good self-cleaning performance, with a static water contact angle of 156.5and a sliding angle of only about 2.2. The performance test results show that the double-layer structure design makes the superhydrophobic coating have good wear resistance, water erosion resistance and excellent chemical stability. The surface functional groups, micro-area morphology and roughness were characterized by Fourier transform infrared spectrometer, scanning electron microscopy, optical profiler and atomic force microscopy, respectively. The reasons for the excellent self-cleaning performance and long-term stability of the coating were explained.

Thermodynamic and kinetic analyses of Janus membrane scaling in membrane distillation for zero liquid discharge engineering

Scaling phenomena are the principal obstacle to the factual application of membrane distillation (MD). In this study, we fabricated a Janus membrane composed of a commercial polytetrafluoroethylene substrate, a superhydrophobic surface layer, and a hydrophilic reverse side coated by MXene nanosheets via a simple spraying method. The virgin and modified membranes were carried out and compared in zero liquid discharge (ZLD) MD experiments. MD flux analysis showed that all the membranes experienced severe flux decline during the ZLD process. Saturation index, Gibbs free energy and nucleation energy barrier were calculated to explain the observed difference in scaling behavior. Screenage assessment of the homogeneous nucleation and heterogeneous nucleation showed that the severe scaling developed from some small clusters of the new phase, wherein the thermodynamic energy barrier played a critical role in determining the anti-scaling behavior for these unsmooth surfaces. Model analysis and data fitting were used to conclude the kinetic behavior of the virgin and modified membranes in the ZLD engineering. Finally, we evaluated the reversible and irreversible scaling after twice washing. Therefore, this study may provide instructive insights to design novel anti-fouling membranes and promote practical applications such as industrial brine treatment and ZLD engineering.

Preparation of Cu/CuO@Stearic acid layer with superhydrophobicity to provide multiple barriers for corrosion protection of carbon steel

Carbon steel is a widely used metallic material. It is prone to corrosion. In this study, we designed and fabricated a superhydrophobic layer with corrosion resistance for protection of carbon steel substrate. Multiple layers were formed that acted as barriers to protect the substrate from corrosion. This superhydrophobic layer reduced the contact area between the substrate and the corrosive medium owing to the superhydrophobicity. The experimental results showed that a copper layer uniformly covered the carbon steel substrate, and the outer surface of the copper layer was converted into copper oxide, which had a multi-scale structure. The Cu/CuO double layer was modified with stearic acid (SA) to form a Cu/CuO@SA superhydrophobic layer. The superhydrophobic layer exhibited a high water contact angle (up to 160°) and a low sliding angle (<3°). The superhydrophobic layer also exhibited repulsive effect to salt, acidic and alkaline solutions. Atmospheric stability test and UV irradiation test were conducted and the results indicated that the superhydrophobic layer was stable throughout the test period. The corrosion potential of the superhydrophobic sample was −0.18 V and the corrosion current density was 7.72 × 10−7 A cm−2, which was one order of magnitude lower than that of the carbon steel substrate. These results indicate that the superhydrophobic sample had a low corrosion tendency and corrosion rate. The polarization resistance and impedance modulus of the superhydrophobic sample were two orders of magnitude higher than the bare steel sample, indicating corrosion resistance and effective protection to the carbon steel substrate.

Stable method for electrodepositing network-like magnesium hydroxide layer with superhydrophobicity and enhanced surface performance under structure direction of xanthan gum/glycerol

Investigation of magnesium hydroxide superhydrophobic layer would significantly improve the application of green and low-cost superhydrophobic materials, for the abundant source, low economic cost, nontoxicity and environment friendliness of magnesium hydroxide. In this work, an obviously developed method which is stable and easy for fabricating network-like magnesium hydroxide superhydrophobic layer on iron substrate has been rendered, simply by cathodic electrodeposition with magnesium nitrate aqueous solution under structure direction of glycerin cross-linked xanthan gum, and subsequent surface modification with 1H,1H,2H,2H-perfluorodecyltriethoxysilane. The preparation process could be carried out at tremendously wide range of bath temperature (from 15 °C to 75 °C), xanthan gum/glycerol dissolving temperature (from room temperature to 100 °C), and storage times (from 0 day to 30 days). The superhydrophobic layer obtained at 35 °C bath temperature with addition of xanthan gum/glycerol dissolved at room temperature and stored for 7 days, shows strong water repellency (160.1° water contact angle, 3.2° sliding angle), and excellent self-cleaning performance. The layer also demonstrates good mechanical property according to cross hatch adhesion and tape peeling test, and owns an inhibition current efficiency as high as 99.98%, taking advantage of its extremely high anti-corrosion resistance (Rct = 2.87 × 105 Ω cm−2, Rf = 1.49 × 109 Ω cm−2).

Efficient preparation of multifunctional superhydrophobic surfaces on Zr-based bulk metallic glass by electrochemical machining

It is well known that metallic glasses (MGs) are ideal superhydrophobic substrate materials. However, the preparation methods and application fields of superhydrophobic MGs are presently limited. For the first time, the superhydrophobic layers on both the cathode and anode surfaces of Zr-based bulk metallic glasses (BMGs) were efficiently fabricated through combining the electrochemical etching with electrochemical deposition. The superhydrophobic Cu/Cu2O coating with cauliflower structure on the cathode surface shows excellent photocatalytic and antibacterial performance. In the meantime, the corrosion inhibition rate of the superhydrophobic anode surface reached ∼99.62% and the surface shows good long-term corrosion resistance. This study provides a new horizon for designing superhydrophobic surface on MGs showing mutifunctional performance.

Preparation and performance study of superhydrophobic layer based on fluffy ZnO rods/PDMS

The preparation of superhydrophobic layers and their research in various fields has always been a hot topic. In this work, we designed a fluffy micro-nano structure model of Zinc oxide (ZnO) to investigate their superhydrophobic properties furtherly. A simple and easily achievable preparation method of superhydrophobic coatings was proposed: The ZnO/polydimethylsiloxane (PDMS) layer was prepared by strictly controlling the ratio of ZnO to PDMS. Based on this, the staggered-growth ZnO rods were prepared on the substrate surface by hydrothermal method. These ZnO rods are structured with micrometer lengths and nanometer diameters, and they form a fluffy ZnO layer on the original coating surface. After being modified by 1H,1H,2H,2H-perfluorooctyltriethoxysilane, the coating possessed excellent superhydrophobic properties with a contact angle of 161° After growing ZnO rods on the surface, the surface roughness was reduced from 326 nm to 181 nm. The superhydrophobic coating has excellent self-cleaning properties, high resistance to acid and alkali corrosion, and superhydrophobic properties against various common liquids. Even after exposure to air for up to 6 months, the contact angle of the superhydrophobic coating can remain above 153° This study provides an excellent idea for future research on superhydrophobic coatings.

Robust Janus Superwetting Textile with Large Pore Sizes for Oil-in-Water Emulsion Separation.

Developing advanced oil-water separation technology is significant for environmental conservation. According to the synergetic effects of the size-sieving mechanism, superwetting materials with small pore sizes have been designed to realize high-efficiency separation for oil-water emulsions. However, the separation flux limited by the pore size and the weakness of the superwetting material impede its practical application severely. Herein, we construct a robust Janus superwetting textile with large pore sizes for oil-in-water emulsion separation. The pristine textile is coated by the as-prepared CuO nanoparticles as the bottom layer with superhydrophilicity and then grafted by 1-octadecanethiol as the top layer with superhydrophobicity to construct the Janus textile. When used as a filter, the superhydrophobic layer acts as the nucleation site to coalesce the small oil droplets facilely. Then, the coalesced oil fills the pores of the superhydrophobic layer and selectively permeates it but is blocked by the superhydrophilic layer with large pore sizes. Utilizing the unique separation mechanism, the Janus textile realizes efficient and rapid separation. Even after multicycle separation, hot liquid immersion for 24 h, tribological test for 60 min, and sandpaper abrasion for 500 cycles, the Janus textile still retains the superwettability and excellent separation performance, manifesting outstanding stability to resist severe damage. This separation strategy provides a novel guideline for high-efficiency and high-flux emulsion separation and practical application.

Construction of superhydrophobic surfaces via dual-scale modified particles and digital light processing 3D printing techniques

This article reports a method of Construction of superhydrophobic surfaces via digital light processing (DLP) and a low-cost, clean, and non-fluoride calcium sulfate particle (CSP) modification method with stearyl chloride. Modified micron-scale CSP and nano-scale hydrophobic fumed silica (HFS) were added into photocurable resin as fillers to prepare 3D printing ink. Relying on the manufacturing features of DLP 3D printing layer-by-layer curing and layer-by-layer stacking. The nano-scale HFS and micro-scale CSP are uniformly assembled to the surface and form a superhydrophobic layer with a dual-scale structure, so 3D objects with superhydrophobic surfaces were successfully prepared. The contact angle (CA) of superhydrophobic surfaces reached 157°, and the sliding angle (SA) reached 2.64°. Relying on the advantages of high precision of DLP 3D printing technology, different 3D samples were printed, whose surfaces also exhibited excellent superhydrophobic, which makes 3D objects expected to be applied in the fields of liquid transportation, micro-reactor, etc.

Waterproof and ultrasensitive paper-based wearable strain/pressure sensor from carbon black/multilayer graphene/carboxymethyl cellulose composite

Huge electronic wastes motivated the flourishing of biodegradable electrically conductive cellulosic paper-based functional materials as flexible wearable devices. However, the relatively low sensitivity and unstable output in combination with poor wet strength under high moisture circumstances impeded the practical application. Herein, a superhydrophobic cellulosic paper with ultrahigh sensitivity was proposed by innovatively employing ionic sodium carboxymethyl cellulose (CMC) as bridge to reinforce the interfacial interaction between carbon black (CB) and multilayer graphene (MG) and SiO2 nanoparticles as superhydrophobic layer. The resultant paper-based (PB) sensor displayed excellent strain sensing behaviors, wide working range (−1.0 %–1.0 %), ultrahigh sensitivity (gauge factor, GF = 70.2), and satisfied durability (>10,000 cycles). Moreover, the superhydrophobic surface offered well waterproof and self-cleaning properties, even stable running data without encapsulation under extremely high moisture conditions. Impressively, when the fabricated PB sensor was applied for electronic-skin (E-skin), the signal capture of spatial strain of E-skin upon bodily motion was breezily achieved. Thus, our work not only provides a new pathway for reinforcing the interfacial interaction of electrically conductive carbonaceous materials, but also promises a category of unprecedentedly superhydrophobic cellulosic paper-based strain sensors with ultra-sensitivity in human-machine interfaces field.