Super‐liquid‐repellent thin film materials for low temperature latent heat thermal energy storage: A comprehensive review of materials for dip‐coating

When discharging latent heat thermal energy storage (LHTES) systems, performance is influenced by the formation and adherence of a solid layer of phase change material (PCM) on heat eXchange (HX) surfaces. Super‐liquid‐repellent thin films (STFs) may be able to reduce solidifying PCM adhesion on HX surfaces during discharging, delay PCM solidification to lower temperatures, and by modifying nucleation sites potentially enable long‐term seasonal thermal storage. Techniques employed previously to fabricate sintered polymeric STF coatings include chemical vapour deposition, dip‐coating, spray‐coating, spin‐coating, layer‐by‐layer (LbL) assembly, sol‐gel, anodizing, electrodeposition, electrospinning, so on. Dip‐coating is considered attractive for fabricating thin films on simple and complex surface geometries due to process maturity, scalability, flexibility and cost‐effectiveness. To identify suitable materials for preparing STFs on metal HX surfaces using the dip‐coating process, more than 200 journal articles published in English during the period 2010 to 2022 were reviewed and the potential role of STFs in LHTES applications was assessed. The review identified key areas and applications stimulating STF material developments and formulations. The dip‐coating of potential STF materials was classified under three major themes driving current research and development (R&D) activities, that is, high performance thin films, eco‐friendly thin films and fundamental research formulations. This review provides a platform from which to develop coatings and HX systems to enable the cost‐effective implementation of STFs for improved heat transfer in future mobile/stationery LHTES systems.

Latent Heat Thermal Energy Storage (LHTES) systems are attractive in realising compact thermal storage due to their potential to provide heat at a stable temperature with a high heat storage to volume ratio. 1 They can help address heat demand-supply variations, exploit waste heat recovery and extend renewable electricity supply into power-to-heat applications. 2An important weakness of LHTES is the formation of a solid layer of Phase Change Material (PCM) on the heat eXchange (HX) surfaces during heat discharge. 3,4ctive methods to remove the solidifying PCM layers from HX surfaces, for example, mechanical scrappers 5,6 and electrical resistance heaters 7 have been considered, but these require additional system components and associated parasitic power demand to operate them, which compromises the overall efficiency of the thermal store.Less complex, and cost-efficient passive techniques are urgently needed to modify the solidification of PCM on HX surfaces during discharge and improve the heat output.
Super-liquid-repellent Thin Films (STFs) are a promising route for modifying the formation and adhesion of PCM-solid-layers on metal surfaces in LHTES.[10][11] Since the interaction of PCMs and HX surfaces depends on other factors, for example, the molecular polarity of liquid PCMs (ie, surface tension factors), the desired thin films and their characteristics need to be adequately considered.Thin films can be differentiated 12 according to their interaction with different test liquids, including, high surface tension liquids, for example, water (γ ≈ 72:7 mNm À1 ) and low surface tension liquids, for example, oils/waxes (γ ≈ 14 À 34 mNm À1 ). 13,14Waterrepellent thin films are termed superhydrophobic because the surface tension within the polar water molecules is significantly greater than the surface energy of the thin film surface, resulting in unstable spherical water droplets.Thin films repellent to non-polar molecules (lower surface tension liquids, eg, oils/waxes) are termed superoleophobic. 15Surfaces that repel non-polar liquids are also typically repellent to polar liquids, 16 so most superoleophobic surfaces are also superhydrophobic.Such surfaces are termed as superamphiphobic or superomniphobic, 17 but can be difficult and expensive to fabricate with currently available techniques.Fabrication methods for STFs differ widely in cost-effectiveness, availability, flexibility and complexity.The safety and durability of thin films achieved by any fabrication process and their intended application on metallic and non-metallic surfaces are other important factors to consider.Due to a wide scope of these topics, this paper focuses on the fabrication of STFs on rigid metal substrates by dip-coating.The dip-coating of rigid metal surfaces is considered one of the simplest, most flexible and readily available techniques, particularly in applications where the physical characteristics of the resulting thin films are less restricted.
STFs find applications in many technical disciplines, but reports are lacking related to their application in low temperature process heat LHTES technologies.Research on dip-coating is also widely dispersed with thin films employed on metallic and non-metallic surfaces to target different operating requirements.This review identifies materials that can be used to fabricate STFs using the dip-coating process and their characteristics for potential application in LHTES systems.The STFs are targeted for application in compact thermal stores used for industrial process waste heat recovery, transport and storage in the low temperature range (up to $250 C).Dip-coating to produce thin films is often cost-effectively conducted based on sol-gel processing of inorganic, organic or composite mixtures that are also adaptable at scale for other fabrication processes including spray-coating 18 and spin-coating. 19Other fabrication approaches for example, plasma etching, laser and/or optical lithography, electrodeposition, anodic oxidation, chemical vapour deposition (CVD), so on, that can achieve STFs are widely reported [20][21][22][23] but their cost-effectiveness and potential application to low temperature compact LHTES systems is uncertain and beyond the present scope.

| REQUIREMENT OF STF SURFACES FOR LHTES APPLICATIONS AND KEY CHARACTERISTICS 2.1 | The shedding/detachment of PCMsolid-layers
STFs could promote the shedding of PCM-solid-layers from HX surfaces in LHTES and modify the phase change process during heat discharge, thereby enhancing heat delivery efficiency.Although they find extensive application in many fields (ie, drag reduction, 24 industrial boiling and condensation, 25 antifouling, 26,27 de-icing [28][29][30][31][32][33] and defrosting, [34][35][36] etc.), their contextual application in LHTES systems for recovery, storage and transportation of low temperature process heat (up to $250 C) is novel.STFs may be a key technical component required to develop efficient LHTES systems that can exploit the significant potential 37,38 of low temperature waste heat streams from industrial thermal processes.
Commercial non-stick coatings were first explored by Laing-Nepustil et al. 2 on LHTES system HX surfaces for temperatures above $300 C. Active electrical resistance heaters were integrated with the coated HX surfaces for power-to-heat applications and to presumably aid the shedding of solidifying PCM from heat transfer surfaces during discharge.It was concluded that non-stick coatings were unsatisfactory for shedding the PCM-solidlayer.The coatings were damaged or came away from the fabricated experimental HX surfaces.The potential application of the coatings for long-term seasonal thermal energy storage at low process heat temperatures (up to $250 C) was also not explored.
Table 1 summarises the research studies where different methods were utilised to detach the PCM-solid-layer from HX surfaces during heat discharge.Shedding of PCM-solid-layers to enhance heat transfer for the discharge process was mostly achieved using active mechanical scrappers. 39,40Studies to date do not exist on employing non-stick STFs for the temperature range up to $250 C. In mechanical scrapping, detaching the PCM-solid-layers from HX surfaces is typically accomplished using an electrical motor that provides shaft power for the scrapper.The total energy expended in shedding PCM-solid-layers includes the energy to overcome friction, and mechanical energy to drive the scrapper.The required scrapping power also depends on the adhesion strength between HX surfaces and PCMs, the configuration of the mechanical scrapping system, the PCM-solid removal rate, so on. 41STFs can eliminate such active power demands by promoting gravity-driven shedding of the PCM-solid-layers if a brief heat pulse is provided to release the PCM solid adhering to the HX surface.Using STFs, the adhesion of solidifying PCM layers on heat exchanger walls can be reduced, minimising the heat input required to detach the solid.Different methods can detach the PCM solid, for example, electrical heating 7 or a reverse heat pulse provided by the Heat Transfer Fluid (HTF) at the system's charging temperature.
The availability of many non-toxic PCMs which have a phase change temperature below $250 C opens significant research and market opportunities for low temperature LHTES. 43,44Key focus areas are industrial energy efficiency and energy system decarbonisation through power-to-heat applications.Assessment of the potential use of STF in the design and development of efficient compact LHTES for waste heat recovery and other stationary/mobile thermal storage applications is required.From current literature on dip-coating to fabricate STFs for a diverse range of interdisciplinary applications, the main research areas can be summarised into five broad categories (Table 2): • Clean water, sanitation, environment and climate, • Enhancing functionality in transparent and luminescent materials, • Efficient industrial processes, transport and renewable energy infrastructure, • Safety/material risks in aviation, marine, infrastructure and energy, • Fundamental R&D of novel thin film materials and formulations.
To date, the effect of applying STFs on HX surfaces in low temperature (up to $250 C) LHTES systems has not been evaluated.The passive shedding/detachment of PCM-solid-layers in LHTES HX surfaces compares to the control of ice adhesion in anti-icing applications, [45][46][47][48][49] which occurs under freezing operating conditions.It also relates to the promotion of dropwise condensation in heat transfer equipment 50 where hydrophobic thin films are commonly employed. 51,52Similar to ice phobic surfaces which delay freezing and promote ice shedding, [53][54][55][56] STF surfaces should suppress/delay T A B L E 1 Investigations into heat discharge enhancement of LHTES systems by the removal of PCM-solid-layers from HX surfaces.

Temperature range ( C) Reference
Rotating drum scrapper Active scrapping of PCM-solid-layers ≤31. 5  5   Electrical resistance heating Gravity driven detachment of PCMsolid-layers by briefly heating the HX surface using electrical resistance heaters >200 7 Fixed blade scrapper with rotating heat transfer tube Active scrapping of PCM-solid-layers ≤70 6 Rotating screw heat exchanger Active scrapping of PCM-solid-layers ≤550 42 the nucleation of the PCM solid and limit PCMsolid-layer adhesion in LHTES systems.Compared to most ice phobic surfaces, STF surfaces in LHTES will be exposed to less fluctuating and less damaging physical conditions (eg, pressure, dust and friction).Nevertheless, their durability and long-term thermal stability at the required operating temperature is an important criterion for the dip-coating fabrication process.Other important criteria including the surface roughness and contact angle of the achieved STF surfaces are considered in Section 2.2.1.

| Surface texture considerations for STFs and contact angle
Surface texture is fundamental in fabricating STFs and can be adjusted to achieve extreme liquid-repellent behaviour.This section provides a brief overview of the theorical models commonly employed when clarifying the role of surface roughness in achieving STFs.In liquid-solid adhesion science, the 'Young's contact angle (CA), θ Y ' characterises the interaction between the liquid and an ideal surface (ie, flat, smooth and chemically homogeneous) according to Equation (1).1][112][113][114][115] The equation is derived by considering the interfacial surface tension forces of three phases (solid, liquid and vapour) in equilibrium, that is, the solid-vapour (γ SV ), solid-liquid (γ SL ) and liquid-vapour (γ LV ) as shown in Figure 1A.Therefore, θ Y is the apparent CA measured if the solidliquid-vapour system were to be in an equilibrium thermodynamic state. 112s For real surfaces, the Young's CA, θ Y is commonly corrected for imperfections due to inherent texture and anisotropic surface chemical composition.The Wenzel (homogeneous/complete wetting) and the Cassie-Baxter wetting (heterogeneous wetting) models describe two limiting cases of wetting attributed to the roughness on real surfaces.When the liquid permeates the texture of a rough surface as shown in Figure 1B, the Young's CA is corrected using the Wenzel equation by introducing a roughness ratio, r to estimate the apparent CA, θ W using Equation (2).The roughness ratio is the quotient of the total wetted solid area and the projected area geometry, implying that always r > 1 for rough surfaces.By increasing the roughness ratio, a hydrophobic surface (θ Y ¼ 120 ), becomes more hydrophobic whilst a hydrophilic surface (θ Y ¼ 60 ) becomes more hydrophilic as shown in Figure 1C.The lowest surface energy thin film coating has been found to produce a hydrophobic surface with θ Y ≈ 119 À 130 on a smooth solid surface. 16,116cordingly using Equation (2), the predicted Wenzel's roughness ratio to transform a smooth surface to superhydrophobic would be r ≈ 1:35 À 1:79.
For the heterogeneous wetting model, a liquid droplet is considered suspended on the roughness tips with air trapped beneath and within roughness troughs as shown in Figure 2A.][119][120][121][122][123][124][125] The Cassie-Baxter model in Equation ( 3) is used for theoretical modelling of liquid droplets on practical surfaces, that is, surfaces with large roughness of the order up to 30 μm 117 or very hydrophobic surfaces, 119 cos where θ CB is the apparent contact angle on the rough surface 112 and f S is the fraction of the liquid droplet base in contact with the surface.The Cassie-Baxter, θ CB and the Wenzel, θ W are the approximate apparent contact angles, θ rough used in the mathematical modelling of heterogeneous and homogeneous wetting states, respectively, for rough surfaces.STF surfaces can be analysed by considering the initial Young's CA, θ Y formed by a liquid droplet on a flat/smooth surface.When the Wenzel model fails, for example, for surfaces with a high roughness (ie, when the right-hand side of Equation 2 exceeds 1), the Cassie-Baxter model is employed.).The Cassie-Baxter model predicts that STF surfaces with a contact angle (θ CB > 150 ) are theoretically achievable for small f S < 1 irrespective of the initial wetting state of the substrate.The requirement for smaller f S to achieve STF surfaces is less restrictive when the initial CA is high.For example, Figure 2B shows that substrates with initial CA of θ Y ¼ 40 ,90 and 130 require theoretical fractions of the wetted solid area f s = 0.08, 0.13, 0.38, respectively, to produce superhydrophobicity with CA of 150 .The foregoing theoretical insights have also been expressed and derived using wetting diagrams. 9,119,126n important consideration is the magnitude of practical surface roughness values required to achieve stable STFs.8][129] Hierarchical textured surfaces can provide a low solid-liquid contact area compared with single scale textured surfaces, for example, coarser (Figure 3A) or finer (Figure 3B) texture. 16Cao et al. 126 created hierarchical surface textures with defined geometries of nanometre and micrometre scales to achieve superhydrophobic behaviour on hydrophilic F I G U R E 1 Illustration of (A) the three interfacial surface forces acting on a liquid droplet on a solid surface, that is, the solid-vapour (γ SV ), solid-liquid (γ SL ) and liquid-vapour (γ LV ), (B) the Wenzel's wetting state of a liquid droplet on a rough surface and (C) the theoretical variation of Wenzel's apparent contact angle, θ W with the surface roughness ratio, r for different Young's contact angles, θ Y ¼ 60 , 120 according to Equation (2). 125urfaces.The aspect ratio and periodicity of fabricated surface morphologies are considered key parameters. 130The re-entrant surface texture shown in Figure 3E is one of the most recent fundamental design recipes 131 for superomniphobic surfaces that are repellent to low surface tension liquids, for example, oils and waxes. 132Lotus leaves feature natural roughness with typical heights and radii of 10 to 20 μm with water contact angles (WCAs) >150 . 127Surface topography with microscopic ($50 nm-10 μm) and nanoscopic (up to $100 nm) roughness features are a general guideline for metallic substrates. 54A similar length scale of 0.1 to 10 μm for surface roughness on polymeric surfaces is required for superhydrophobicity. 133 The dip-coating process described in Section 2.2.2 is well adapted for achieving hierarchical surface texture.Technologies (eg, photolithography 134 and laser-based texturing 135 ) that can produce well-defined microarray-like surface texture 136 such as those in Figure 3D,E are presently expensive to scaleup for STF applications demanding larger surfaces.The fabrication of STF surfaces typically involves four processes, namely: (a) cleaning and surface pretreatment, (b) creating surface texture, (c) applying thin film materials or coatings and (d) post-treatment of the thin film.To remove surface oxides, the surfaces are washed in a dilute acid solution, such as hydrochloric acid (HCl). 137Sequential cleaning using chemical detergents and solvents (eg, acetone, isopropyl alcohol [IPA] and deionised [DI] water) ensures removal of organic contaminants from surfaces.Ultrasonic cleaning can be used to achieve high-quality surface cleaning. 138The cleaned surface is then dried using several methods, for example, clean nitrogen 138 or oven drying.Surface texture can be created using appropriate surface abrasion methods, for example, polishing or sandblasting.Other F I G U R E 3 Surface texture can stabilise liquid droplets in the Cassie-Baxter wetting state 16 by employing (A) coarser texture, (B) finer texture and (C) hierarchical texture to reduce the contact area between the liquid and the surface.The hierarchical texture (Figure 3C), reduces surface wetting significantly with lower resistance to liquid droplet motion and the thin film achieves a lower contact angle hysteresis (CAH). 134An additional requirement limits liquid break-through (by hydrostatic pressure, ie, the downward capillary force) and avoids surface wetting using (D) concave texture and (E) convex (re-entrant texture) depending on the surface tension of the targeted liquid.
approaches, for example, direct surface texturing of low energy surfaces 139 as well as stamping 140 could be used to impart super-liquid-repellence on surfaces.][143][144][145][146] To characterise surface roughness, several parameters are determined on a line profile or a defined area using surface analysis techniques, for example, Scanning Electron Microscopy (SEM). 126,134Surface roughness parameters can be derived from the surface image captured by the microscope using specialist software.Among the surface texture parameters evaluated on a sampled length of the line profile are the arithmetic mean roughness, R a , the maximum surface roughness (peak-to-valley height), R z , the root-mean-square deviation, R q ; skewness, R sk and kurtosis, R ku .The skewness of the surface height probability density function, R sk , is used to determine the symmetry of the roughness profile about the mean.The height probability density function of Gaussian surfaces is symmetrical and has a skewness of R sk ¼ 0. To achieve superhydrophobicity, a negatively skewed roughness profile (R sk < 0) is required because it features a surface composed of fewer peaks than valleys above the mean line, which results in lower values of CAH. 147Kurtosis (ie, the measure of the sharpness of the surface height distribution) must be such that, R ku < 3, which provides a surface with more rounded peaks and valleys and is less susceptible to wetting. 148Regarding R a and R z , microscopic and nanoscopic length scales are required as earlier indicated.Corresponding surface roughness parameters (S a , S z , S q , S sk and S ku , etc.) have been defined based on areal non-contact techniques. 149,150Finally, additional performance requirements of STFs, for example, wear/adhesion strength, resistance to harsh conditions, and thermal stability/ resistance are typically evaluated depending on the targeted application.Section 2.2.2 describes the fabrication of STFs using the dip-coating technique.A comparison of coating methods, their pros and cons, schematic illustrations; and the theory on surface wetting phenomena is given by Rasouli et al. 23

| Dip-coating of low surface energy thin films on surfaces to achieve STFs
Liquid-repellent properties are typically achieved by adding low surface energy thin film materials on textured surfaces.They are also achievable without using thin films, for example, due to a change in carbon content on the surface after laser treatment and/or the possible adsorption of airborne hydrocarbon contaminants on laser treated surfaces. 151,152Dip-coating can produce hierarchical and randomly distributed thin films 136 by controlling the dipping speed and dipping time, and has high flexibility/compatibility with many materials and geometries.
The surface-textured substrate is dipped in a coating mixture and retained in the mixture for a selected duration before withdrawing to create a thin film.The dipping speed, retention time, withdrawal speed and process conditions can affect the thin films and thus, are typically controlled and well-defined.Dip-coating can be employed to fabricate STFs using materials of inorganic, organic and hybrid mixtures.The mixtures are typically synthesised as colloidal sols (suspensions of solid particles in a liquid) or colloidal emulsions (suspensions of liquid droplets in another liquid) which are adaptable for dip-coating.Hybrid coatings are typically colloidal sols of suspended inorganic micro/nano particles in a composite matrix of organic materials. 153Therefore, dip-coating depends primarily on sol-gel processing, a fact relatively less obvious in recent literature, 154,155 despite its early emphasis by past authors. 19,79,153,156he dip-coating process to fabricate thin films on surfaces is depicted in Figure 4.During substrate withdrawal, the coating mixture becomes a thin film via aggregation, gelation and drying.Sol-gel processed mixtures are desirable since they are continuously reusable, 99 recoverable and renewable with limited loss.Controlling the withdrawal speed and the fluid properties of the sol-gel mixture (eg, viscosity and surface tension) is essential in governing the thickness, mechanical properties, and the CA of the STF. 156Perhaps the most important advantage of sol-gel processing over conventional coating methods is the ability to control precisely the microstructure of the deposited thin film, that is, the pore volume, pore size and surface area. 153Finally, thermal curing or postheat-treatment is often needed to sinter the thin films for a defined duration without causing degradation.This enhances the adhesion strength (or bonding) and operational stability of the thin film coatings. 23For example, increasing the temperature ramping rate can densify thin films deposited by dip-coating to enhance adhesion and durability. 157For film thicknesses in the range 0.05 μm to 0.2 μm, the adhesion of sol-gel derived thin films is generally considered less problematic compared to greater film thicknesses. 153Thin film coatings can be sacrificial or permanent, dictating their lifetime performance and their potential impact on the environment. 158Sacrificial coatings are potentially ecologically unfriendly since they deteriorate with time, demanding periodic application.

| Methodology and rationale
A detailed literature review was conducted to identify the materials for fabricating STF surfaces using the dipcoating method.This considered research articles and review papers published in English during the period: 1 January 2010 to 31 March 2022.Scopus (ie, the largest database for scientific peer-reviewed literature) was employed for the literature search.Due to the broad scope of STF materials, the review focused on waterrepellent (superhydrophobic) thin films.Table 3 shows the selection of keywords for searching the relevant publications and their groupings.The string of search terms consisted of functional terminology, form, process, application and exclusions using the listed keywords.
Many studies considered the fabrication of superhydrophobic thin films on non-metallic surfaces.For example, fabrics, 160,161 sponges, 57 porous and non-porous membranes, 23 wood and paper, 162,163 polymeric substrates, 164,165 and meshed materials, 166 so on.Although such surfaces have the needed surface texture for fabricating STFs, their properties and intended uses are incompatible with LHTES.Durable metallic surfaces are required for HX surfaces in LHTES systems, thus studies using non-metallic surfaces were excluded.Also excluded were studies focused on fabricating micro/ nanostructure texture using specialised techniques. 167pecialised techniques, for example, laser lithography, electrodeposition, CVD, electrodeposition, so on, are currently expensive to scale-up for HX surfaces required for LHTES applications.The search string excluded studies with 'photovoltaic', and 'pv' in their title, abstract and keywords.Nevertheless, the dip-coating of transparent STF materials was addressed in many publications included in this review.
To identify general themes and classify the knowledge structure of the identified publications, bibliometric The fabrication steps for dip-coating of thin films on surfaces to achieve super-liquid-repellent thin films.During the withdrawal stage the coating mixture becomes a thin film on the surfaces of the substrate, via aggregation, gelation and drying. 159nalysis was conducted.This utilised the 'Biblioshiny' functionality found in the 'Bibliometrix' package in the R programming language. 168The literature was organised into five categories.Four categories covered the chemical nature of thin film materials, that is, (1) inorganic (INO), ( 2) organic (ORG), (3) inorganic-organic hybrids (HYB) and (4) using micro-nano texture without thin film coatings (TEX).The fifth category (5) covered all the identified review papers.Literature was further classified into three broad themes which are presently inspiring the R&D of superhydrophobic thin film materials.The identified themes are summarised in Table 4. Section 3.2 presents the synthesis of literature and the results of the bibliometric analysis.

| Bibliometric analysis
Table 5 summarises the top 20 sources with the most publications on superhydrophobic thin film materials during the review period.The sources are diverse across the engineering and natural sciences disciplines with an annual growth rate of 33.13%.A total of 355 peerreviewed documents were captured using the search string in Table 3.These included 284 research articles and 71 reviews and represented 141 sources (journals).Surprisingly, core sources in the 'energy' subdiscipline of engineering are absent from the top 20 tier.Worth noting are solar energy materials and solar cells (two papers 80,169 ), energy and buildings (one paper 88 ), journal of energy storage (one paper 170 ), international journal of heat and mass transfer (one paper 171 ) and energies (one paper 172 ).Only six studies 123,151,152,[173][174][175] were found under the TEX category.The TEX category was eliminated because it lacked materials for dip-coating of STF surfaces.Thus, four categories, that is, HYB, INO, ORG and Reviews were finally retained, which altogether contributed 349 documents in the bibliometric analysis.
Figure 5 shows the identified publications covering the review period of 13 years.There has been a clear trend indicating increasing research interest over this period in relation to the dip-coating of superhydrophobic thin film materials.Total publications per year grew significantly from 2014 and reached a high of 71 in 2021.Reviews and research articles have increased annually, except research on the dip-coating of purely inorganic materials.Inorganic-organic hybrid materials, have the greatest number of research investigations reported annually since 2014.
Figure 6 shows the percentage distribution of publications (excluding reviews) within the three general  T A B L E 4 Themes currently inspiring the R&D of superhydrophobic thin films.

High-performance superhydrophobic thin films
Research targeting the development of water-repellent thin films that are resistant and resilient to harsh corrosive environments, and for mechanical, chemical and thermal stability

Eco-friendly superhydrophobicity
The R&D of superhydrophobic thin films using environmentally friendly thin film materials

Fundamental research formulations
Fundamental R&D work on the formulation and synthesis of inorganic, organic, and colloidal hybrid mixtures to achieve alternative superhydrophobic thin films using a diversity of surface immersion techniques and dipcoating to target diverse application challenges coating of STFs are summarised in Sections 3.3, 3.4 and 3.5 according to the general themes.
Many materials are reported for STFs, their applications, theory, challenges and fabrication methods including dip-coating.Zhang et al. 9 reviewed and presented design strategies for durable STF surfaces and grouped them under passive and active strategies.Passive strategies can modify the composition by including hard and/or elastic material components to the thin film coating mixture.The hard materials help in strengthening the adhesion of the coating on metal surfaces whilst elastic components absorb shocks.Excessive physical or chemical abrasion and thermal degradation can irreversibly damage superhydrophobicity.Therefore, active strategies were proposed that enable thin film regeneration to achieve self-healing when they fail in a self-similar manner.For example, bulk/volumetric superhydrophobic coatings remain liquid-repellent even when the surface gets damaged. 178Consequently, the development of self-healing 183 and/or easily repairable 184 liquidrepellent surfaces is gaining significant research interest.The formulation of bulk materials by additive manufacturing 146 and selection of self-regenerating substances, for example, in corrosion 185 was also considered.The cost-effective application of additive manufacturing and high-resolution micro-/nano machining techniques to fabricate liquid repellent HX surfaces is unclear.Mechanical robustness aspects, for example, surface texture aspect ratio limitations, and the need for STFs to resist critical, shear and normal forces were discussed.Lack of durability testing standards for superhydrophobic thin films was identified as a challenge.To address this, testing standards frequently used in the coatings industry were recommended, 9 for example, Taber abrasion ASTM D060.
Hybrid thin film mixtures composed of organic materials and inorganic nanoparticles were widely considered for achieving robust STFs.Typical inorganic nanoparticles included carbon nanotubes (CNTs) and graphene oxide (GO).Also, metal-organic frameworks (MOFs) 102 and transition metal carbides (MXenes) 9 were considered as potential nanomaterials for fabricating superhydrophobicity.Xue et al. 184 considered the fixation of the fragile nanostructure using nanocomposite coatings for long-lasting mechanical and corrosion strength.The durability of STF surfaces was also extensively covered by Ellinas et al. 103 They mentioned STF materials used to fabricate damage-tolerant superhydrophobicity on non-metallic surfaces.These included composites achieved by combining Polypropylene (PP), TiO 2 nanorods, SiO 2 nanoparticles and polydimethylsiloxane (PDMS) or other silanes, fluorinated compounds, and metal oxides.Such composite materials are also typically employed on metallic surfaces.Zhu et al. 45 conducted a review on the adhesion behaviour of STFs.It was shown that the adhesion efficiency of STFs can be controlled using surface compositions, surface texture, and external stimulation. 45hasemlou et al. 181 reviewed eco-friendly fluorine-free STF materials and their design, synthesis and challenges in scaling-up.Organic and hybrid material mixtures were discussed in several exemplar studies on environmentally friendly thin film surfaces.The thin films were produced by different techniques including, sol-gel, spray-coating and dip-coating.The thin film materials included long alkyl chain thiols, natural waxes, oxide nanoparticles (eg, TiO 2 , SiO 2 , ZnO, etc.), polysiloxanes and silicones.Additionally, 19 tradenames of usable commercial products for fabricating STF surfaces were listed.These included, Nasio, NeverWet, Ultra-Ever Dry, FluroroPel, FluorAcryl, Fluorothane, HydroFoe, Aculon, Drywired Nanocoating, Hirec Paint, NANOMYTE, The annual variation of research articles published over the considered review period within the three themes, (i) Eco-friendly superhydrophobicity, (ii) Fundamental research formulations and (iii) Highperformance superhydrophobic thin films.
studied the potential of superwetting (superhydrophilic) surfaces in enhancing mass transport dynamics of liquids, gases and chemical ions in reactors, but the dip-coating of such surfaces was not discussed.The fundamentals are typically based on natural surfaces 8,136,177 for innovations aimed at achieving multifunctional or stimulus-response 65,136,176,179 ; and miniaturisation of components. 10,101The fundamental R&D focus areas of STFs include operational longevity, human and environmental safety, test standards, theoretical modelling, and characterisation challenges.The corpus had two review papers 14,170 reporting the emerging Slippery Liquid-Infused Porous Surfaces (SLIPS) and their application potential for anti-icing.Infusing suitable phases into the micro-/nanopore texture at design stage is considered useful in addressing the failure of STFs. 14,171,186,187This has been found to improve the dynamic stability of the Cassie-Baxter wetting regime for long-term stability of superamphiphobic surfaces. 188The dip-coating of SLIPS is limited by weak interaction between the lubricant and the surface, leading to poor durability. 14The contextual application of SLIPS for low temperature LHTES could find a niche in the ultra-low temperature range (<120 C). 37 Azimi Yancheshme 170 considered infusing the surface microstructure with PCMs confined within micro-capsules (diameters 1-1000 μm) and nano-capsules (diameters <1 μm).The emphasis was on PCM encapsulation methods and functionality as opposed to their dip-coating.

| High performance superhydrophobic thin films
The materials used to fabricate STFs in this category were summarised from 24 sampled papers 132,157,171, synthesised in Table 7. A post-at treatment process was often used after dip-coating.This can be considered an essential step for achieving thin films capable of handling the temperature requirements of LHTES.Therefore, the post-heat treatment temperature of the identified thin films was included in Table 7.The methodological aspects regarding pre-processing, dipping/withdraw speed, resident time and post-processing are accessible from the provided references.A searchable corpus is provided in the accompanying supplementary file as per the methodology described in Section 3.1.
Hybrid organic-inorganic materials (HYB) containing nanoparticles, inorganic or organic powders, and nano/micro fibres are frequently investigated under this theme.Topical themes, that is, weather/chemical/corrosionresistance, 96,132,192,196,199,201,203,205,206,[211][212][213][214][215][216] wear/abrasion-resistant/adhesion strength 132,190,191,193,194,[197][198][199][200]204,207,208,[217][218][219] and long-term thermal stability 157,171,189,196,207,209,211,213,214,217,[219][220][221][222] were found. Recent research is ptimistic about realising robust, reliable, easily reparable 216,223 and long-lasting STFs for a wide range of engineering applications.Gradual damage of surface texture and degradation of thin film performance under humid conditions was reported by Kulinich et al. 210 Raimondo et al. 132 fabricated functional thin films on aluminium alloy surfaces using sol-gel processing. The mean surface roughness, R a , of the sandblted reference samples (aluminium alloy, Al1050 99% H24) was measured as 4 À 5 μm using 3-D profilometry.The samples were dip-coated in separate steps. First, n an aqueous colloidal sol of alumina nanoparticles (Al 2 O 3 ) (dipping/ withdrawing speed of 120 mm/min and holding time of 5 s), followed by thermal transformation of the alumina to γ-alumina (characterised as a flower-like structure).Second, dip-coating in fluoroalkylsilane (FAS) solution (dipping/withdrawing speed: 2 mm/s, residence time: 120 s) to achieve a hybrid inorganic-organic thin film (with an air-filled nanostructure for superhydrophobicity). Dip-coating enhanced the water CA and CAH from 95:8 and 40:0 , respectively, to 179:7 and < 1 for the reference sample.Lastly, the resulting hybrid thin film was carefully immersed in a FC-43 lubricant solution (of high-density and low surface tension).The lubricant replaced the air in the nanopores of the surface texture, to achieve a SLIP surface.The SLIPS had high CAs for other test liquids including diiodomethane (167:1 ), hexadecane (144:1 ) and tetradecane (138:9 ) and was superomniphobic.High-performance durability testing of the thin films was demonstrated through their resistance to chemical attack and mechanical wear.Thermal stability of dip-coated thin films 79,157,171,196,[207][208][209]211,213,214,217,222 was considered. This is an importan performance requirement for employing STFs in LHTES applications.Thermal stability depends on the thermal degradation of the chemical and physical bonds of the molecular species in the coating.Ahmad and Eshaghi 157 evaluated the thermal stability of sol-gel derived thin films.They found that destruction of superhydrophobicity above 450 C resulted from thermal degradation of ÀCH 3 molecules bonded on the surfaces.Gulfam et al. 171 suggested that by infusing appropriate molecular phases into the micro-/nano structure, such destruction can be addressed.Zhang et al. 213 studied the effect of nano-additives on thermal stability of composite thin film material using thermogravimetric Their review was focused on the implications for boiling heat transfer at temperatures up to 300 C. They found one study 225 that had considered superhydrophobic surfaces and observed that the WCA remained stable for temperatures below 100 C. Nanda et al. 207 dip-coated a hybrid thin film of silica (SiO 2 ) microparticles in a matrix of octadecyltrichlorosilane (OTS) on glass.The thin film was dried at 40 C for 5 h to achieve significant thermal stability, and superhydrophobic properties were retained for annealing temperatures of up to 160 C. For high-performance thin films derived using organic materials (ORG), the corpus contained 30 papers.Thirteen selected papers are included in Table 7. Surface texture on substrates is typically fabricated separately before applying a low surface energy organic material using dip-coating.Khaskhoussi et al. 192 used Beck's HF/HCl solution for etching of cleaned aluminium alloy substrate (grade 6082-T6) to generate surface texture.The etched sample was dipped in octadecyltrimethoxysilane/ toluene solution for 10 min at room temperature to fabricate a self-assembled silane monolayer. Usig a Mitutoyo profilometer, the measured arithmetic mean roughness, R a was increased from 0:254 μm as received to 1:427 μm after dip-coating and a low kurtosis parameter, S ku of 0.243 was achieved.The organic thin film had zero sliding angle and high static WCA of 179 AE 3 , measured using the Attension Theta Tensiometer (Biolin Scientific, Sweden).The sample's corrosion resistance was enhanced due to water repellence capability of the thin films.This was demonstrated by a low corrosion voltage (À0:472 V) in seawater compared to the as received surface À0:931 V ð Þ .Similar research used anodising to fabricate surface roughness followed by dip-coating in a solution of 1H,1H,2H,2H perfluorodecyl-trichoxysilane/ ethanol. 191The fabricated STFs achieved a static WCA ranging from 155.6 to 167.1 for marine applications.Hydrophobic agents, for example, surfactants differ in chemical structures and in their boding mechanism on micro-/nano-textured surfaces as examined by Garnweitner et al. 226 The influence of this important issue on the durability of superhydrophobic surfaces was investigated by Boinovich and Emelyanenko. 96They used infrared nanosecond laser treatment to fabricate reproducible multimodal roughness on different chemically identical substrates.Four different surfactants were selected to lower the surface energy of the samples.Two surfactants: (i) stearic acid, and (ii) methoxy-{3-[(2,-2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-oxy]-pro-pyl}-silane (MAF) adsorb chemically on the aluminium surface to form self-assembled monolayers, whilst the other two: (iii) perfluoropentadecane (PFP), and (iv) docosane adsorb physically due to van der Waals forces.Extensive degradation studies of the derived superhydrophobic surfaces were conducted.They found that chemically adsorbed surfactants (eg, MAF) performed better at protecting surfaces in chemically aggressive media.
The corpus had no studies considering the singular dip-coating of inorganic materials (INO) to generate high performing superhydrophobic metal surfaces.Indeed, most inorganic materials cannot independently impart superhydrophobicity due to their high surface energy characteristics.Most substrate materials with low surface energy are typically non-metallic, for example, polycarbonate.The surface energies of solvent wiped mild steel and aluminium alloy surfaces are 46 and 50mN=m, 227 respectively, compared to 34mN=m for polycarbonate.Thus, these metals are more hydrophilic than polycarbonate.The typical surface energy of nonmulti-component inorganic compounds, for example, particles, powders, fibres, so on, is even higher relative to metals.Inorganic compounds may enhance the surface hydrophobicity on bulk substrate materials with inherently low surface energy.Qin et al. 228 dip-coated polycarbonate in an ethanol solution of silica nanoparticles followed by hot compression with 500 mesh screen.The process altered the surface microstructure of polycarbonate, achieving CAs as high as 145 , with an R a of 11:8 μm.
Importantly, the dip-coating of inorganic STFs can necessitate hot-dipping, 229 which can reduce process safety and add complexity.Hot dip-coating of superhydrophobic surfaces was mentioned for organic materials such as Alkyl ketene dimer (AKD), 230,231 where the coating temperature ranged from 70 C to 90 C. Stable superhydrophobic surfaces were also achieved by dip-coating a substrate in polyvinyl chloride and spraying several layers of modified silica nanoparticles at surface temperature of 50 C. 204

| Eco-friendly superhydrophobicity
An increasing number of studies discussed the environmental threat posed by STFs derived using fluoropolymer coatings (see Figure 7).They degrade into toxic and ecologically persistent molecules, such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS).Most literature on environmentally friendly STFs focused on inorganic-organic hybrid 185,219,[232][233][234][235][236][237][238] and organic 239 materials.The employed dip-coating process involved combining different surface texture fabrication strategies 174 and less environmentally damaging chemical materials. 151,152Table 8, syntheses the selected literature for materials involved in the dip-coating of eco-friendly STFs.The methodological aspects regarding preprocessing, dipping/withdraw speed, resident time, and post-processing are accessible from the provided citations.
Micro-nanoparticles/nano-fillers typically used to formulate inorganic-organic composites for sol-gel dipcoating included oxides of titanium (TiO 2 ), zinc (ZnO), and silicone (SiO 2 ), so on.Liang et al. 185  modified with a corrosion inhibitor (1-hexadecyl-3-methylimidazolium bromide, HMID), and self-healing acid/alkali dual-stimuli-release properties.The material was proposed as an alternative superhydrophobic coating to replace non-eco-friendly chromate conversion coatings used for metal passivation.Dip-coating was employed to fabricate a STF on aluminium alloy, AA2024 substrate with dipping/withdrawal speeds of 10 mm/min and resident time of 5 min.The roughness of the thin film was characterised using Atominc Force Microscopy (AFM).It was observed that the randomly deposited hydrophobic SiO 2 nanoparticles (with average diameter 30 nm) increased the final surface roughness to an R a value of 51 nm, but CAs were not measured.The self-healing property of the fabricated STF was evaluated using scanning vibrating electrode technique (SVET).The corrosive current measured after 6 h with surface defects was eventually inhibited and eliminated after 24 h.This was attributed to surface protection via surface physisorption and chemisorption of the inhibitor HMID in saline water.Xi et al. 234 formulated an eco-friendly composite which achieved superhydrophobicity with CAs of 159 to enhance the corrosion resistance of AZ91D magnesium alloys.The post-heat treatment process after dip-coating was done at 50 C for 30 min.Zulfiqar et al. 235 used micro-nanoparticles derived from readily available and renewable organic materials, for example, sawdust to achieve superhydrophobic composite thin films.Polymers and fatty acids, for example, stearic acid are the typical organic materials for the dip-coating of ecofriendly STFs.Razavi et al. 239 etched an aluminium substrate with HCl to generate hierarchical roughness.A water-based solution of perfluoro acrylic copolymer (PMC) was utilised as an environmentally safe alternative for dip-coating.The dip-coating was performed at a dipping/withdrawal speed of 50 mm/min for 10 s, resulting in a WCA of >160 and a sliding angle less than 5 .Zou et al. 238 generated a so-called 'trampoline' surface texture on aluminium substrate.The substrate was dip-coated in an ethanol solution of stearic acid to achieve superhydrophobicity with WCA greater than 160 .Most studies retrieved from literature and selected under the 'ecofriendly superhydrophobicity' theme examined mainly STFs for corrosion protection.

| Fundamental research formulations
This category covers fundamental research activities aimed at gaining theoretical and practical understanding of technical challenges in achieving STFs.A grey line exists between this and the above two themes.
Literature was assigned to this theme if the ecofriendly and/or mechanical, chemical, thermal durability performance aspects of the thin films were considered inadequate.The selected papers reported the dipcoating of inorganic-organic hybrid, 79,241,242 and organic 231,[243][244][245][246][247][248] materials for different substrates and pre-/post-treatment processes as summarised in Table 9. Papers employed a variety of creative methods to generate surface texture prior to dip-coating.Zhang et al. 246 utilised the ultrasonic-cavitation effect to enhance metal surface etching, thus achieving 'mountain-like' micro-texture together with 'coral-reef-like' nano-texture.Tilebon and Norouzbeigi 247 optimised the surface structure and the dip-coating process through a statistical Taguchi experimental design.
Inorganic and organic materials were used separately to generate surface texture and low-surface energy, respectively.The resulting thin film coating was regarded as hybrid, in contrast to using inorganic-organic composite sol-gel mixtures.Fleming and Zou 241 achieved surface texture by dip-coating a titanium alloy plate in a solution of silica nanoparticles.A passivation process was employed, that is, a deep reactive ion etcher (DRIE) to produce an inductively coupled plasma of perfluorocyclobutane (C 4 F 8 ).The process deposited low surface energy a thin film of thickness less than 10 nm.Surface roughness was evaluated using contact profilometry (over a scan length of 3 mm) for the different titanium (ground finish grade 5, Ti6Al4V alloy) samples.The samples were (i) as received (AR), (ii) sandblasted (SB), (iii) as received and dip-coated with SiO 2 nanoparticles (DC) and (iv) sandblasted and dip-coated (SB + DC).Table 10 shows the roughness parameters estimated for different samples before and after applying the low surface free energy (SFE) thin film using DRIE.The micrometre scale roughness produced by sandblasting was larger and more randomly distributed, as shown by the SD.After dipcoating, a nanometre scale film of silica nanoparticles was introduced on the existing micrometre scale surface texture.This facilitated the Cassie-Baxter wetting regime for the SB + DC + DRIE sample.Jian et al. 248 found similar results on the impact of surface roughness on WCA after coating.
Zhou et al. 244 considered the effect of dip-coating parameters and pre-/post-processing steps on the properties of fabricated STFs.The investigated dip-coating parameters included dipping time, number of dipping cycles, surface cleaning and drying time between immersions.They used dip-coating to produce hydrophobic polyelectrolyte multilayers (PEMs) on glass slides.The coating obtained by the LBL assembly of branched polyethylenimine (BPEI) and Nafion was characterised by a porous surface microstructure.
The porous microstructure was infused with a lubricant (Krytox 100) to form a SLIPS and the above dip-coating process parameters were evaluated.Superhydrophobicity was influenced by the number of bilayers (corresponding to the number of dipping cycles).This is shown by the improved WCA after three dipping cycles in Figure 8A and a corresponding reduction in sliding angles in Figure 8B. Figure 8C shows the variation in the water droplet shapes achieved for the fabricated film surfaces.It was found that increasing the number of deposition cycles sustained superhydrophobicity with the emergence of new roughness features.At 15 deposition cycles, the contact angle was compromised.This was attributed to the formation of a sheetlike structure that covered the essential roughness features due to limited material diffusion within the nanotextured structure.The number of dipping cycles must be optimised to obtain hierarchical roughness without forming the sheetlike structure. 244he dipping time (Figure 8D) and the drying time between washing and rinsing during the fabrication of each bilayer (Figure 8E) were also examined.The dipping time determines the quantity of materials adsorbed onto  F I G U R E 8 Wetting characteristics of polyelectrolyte multilayer (PEM) surfaces showing the effect on number of bilayers, that is, number of dipping cycles on (A) WCA, and (B) sliding angle; and the (C) achieved water droplets shapes. 244The WCA also depends 244 on (D) dipping time, and (E) drying time.
of superhydrophobic surfaces.This involved three deposition cycles (total fabrication time $ 9 min) for a dipping time of 1 min and washing time of 30 s.

| CHALLENGES AND OPPORTUNITIES
Extensive R&D of STFs as self-cleaning/non-stick surfaces has been recognised to be important for many key technical areas identified in Table 2.One study conducted experiments with non-stick coatings for passive heat transfer enhancement in power-to-heat LHTES applications at temperatures above 300 C. 2 The experiments were unsuccessful or inadequate and no further research has been performed to date.The potential use of STFs for LHTES temperatures below 300 C is unexplored, leading to a significant knowledge gap.An extensive body of work has established the design fundamentals of STFs targeting the molecular characteristics of different liquids.Molecular characteristics of PCMs in molten state are expected to dictate the design of STFs required in any LHTES application.The suitability of employing PCMs for LHTES is typically based on their measured thermal physical parameters.These include specific heat capacity, thermal conductivity, latent heat of fusion, volume change, degree of subcooling, so on.To quantify the liquid-repellent behaviour of a surface, the CA exhibited by a liquid as discussed in Section 2.2 is required.The surface tension force of liquid molecules mainly comprises polar and dispersive components which determine their wetting property or CA on solid surfaces. 125Liquids where the polar component contributes greatly to the surface tension force compared to the dispersive component are described as polar liquids.Measured data of the surface tension force (including its polar and dispersive components) are not available at present for typical PCMs employed for LHTES.The thermal physical characteristics of PCM should be supplemented with the measured polarity of PCMs to enable targeted selection of STF surfaces for different PCMs.
The design of controlled hierarchical texture to achieve durable STF surfaces that resist failure was illustrated in Section 2.2.1 (Figure 3).The surface roughness can be optimised depending on the surface tension of selected PCM liquids.This should include the expected heat transfer enhancement achievable by the liquid repellent HX surfaces under realistic LHTES operating conditions.On the one hand, micro-and nanofabrication techniques, for example, laser lithography can achieve surface texture to bespoke geometrical and nano-/ microscale requirements.These techniques are currently expensive for fabrication of STFs on large surfaces.Cost-effective methods are required for achieving scalable hierarchical texture on metal surfaces in combination with dip-coating.This will strengthen the appeal for the dip-coating of durable STFs that can be used for shedding PCM-solid layers from HX surfaces and enhance heat transfer in LHTES.On the other hand, dip-coating was used to fabricate other interesting STF surfaces at a conceptual level, for example, SLIPS, aiming to achieve liquid-repellent behaviour for multiple liquids.Investigations should consider the in-service performance of STF HX surfaces in LHTES, for example, the requirement for resistance to thermal and chemical degradation.

| CONCLUSION AND FUTURE RESEARCH
An extensive literature review revealed that to date no papers have assessed dip-coating to produce STFs on HX surfaces for use in low temperature (up to 250 C) Latent Heat Thermal Energy Storage (LHTES) applications.STFs may delay PCM solid phase nucleation to a lower temperature than when using an uncoated HX surface and improve discharge efficiency and achieve a longer duration of thermal energy storage.STF coatings on the HX surfaces can also promote shedding of solid PCM-layers formed during discharge and improve LHTES discharge performance.Typical materials used for dip-coating of STFs were identified and their technical characteristics including water contact angle (WCA), contact angle hysteresis (CAH), sliding angle (SA); and film type (ORG, INO, HYB) as an initial step to address this knowledge gap.Sol-gel dip-coating of inorganicorganic (hybrid) mixtures and organic materials dominated research studies reported in the literature over the last 8 years.Inorganic thin films were not generally used on their own but were increasingly crucial in fabricating surface texture as micro-/nanoparticles for sol-gel composite thin films and to enhance surface bonding characteristics and thin film performance.Conventional fluorinated materials dominate the dip-coating of STF surfaces, but significant work on environmentally friendly alternatives is emerging.Despite durability challenges, dip-coating has been used extensively to achieve the Cassie-Baxter wetting state by fabricating hierarchical thin film structures on surfaces, but it cannot achieve defined concave and/or re-entrant structures that target super-liquid-repellence of specific polar and non-polar liquids.To employ STF surfaces in LHTES aimed at low temperature process heat applications, the thermal durability of the thin films at the maximum working temperature of the thermal store is a key criterion which can be linked to the post-heat treatment sintering process of the thin films after dip-coating.Whilst the thermal stability of fabricated STFs was rarely evaluated, hot dipcoating and/or post-heat treatment sintering could achieve the needed thin film adhesion strength.Fundamental research continues to explore different sol-gel thin film formulations for long-term stability of STFs, self-healing properties, easily repairable surfaces and superamphiphobic thin film surfaces.Superamphiphobic thin film surfaces can be repellent to different kinds of PCMs when in the liquid state, leading to LHTES heat transfer surfaces that are PCM-agnostic.Research is required to understand the adhesion phenomena between PCM-liquid/solid phases and STF HX surfaces in LHTES, mathematical modelling of the mechanisms involved and optimisation of system performance.

F I G U R E 2
Illustration of the (A) heterogeneous Cassie-Baxter wetting state of a liquid droplet on a rough solid surface and (B) theoretical variation of apparent contact angle, θ CB for Cassie-Baxter model in Equation (3) with the fraction of wetted area, f s for substrates with different Young's contact angles, θ Y ¼ 40

4 F
I G U R E 5 Publishing trends over the years of the 349 selected peerreviewed papers comprised of 278 research articles and 71 review papers.

F
I G U R E 6 Percentage distribution of research articles and material groups in fabricating superhydrophobic thin films by dipcoating in three themes, (i) Eco-friendly superhydrophobicity, (ii) Fundamental research formulations and (iii) High-performance superhydrophobic thin films.
The key areas of Research and Development (R&D) for STF surfaces.
T A B L E 2 Keyword groupings used to construct the database search for literature using Scopus.The number of research articles identified in each topic is indicated on the horizontal axis inside the brackets.'Highperformancesuperhydrophobic thin films' featured the highest publications where the number of articles considering hybrid, organic, and inorganic materials are 97, 31 and 10, respectively.This was followed by 'fundamental research formulations' where the number of articles considering hybrid, organic, and inorganic materials are 77, 25 and 8, respectively.The 'ecofriendly superhydrophobicity' category featured only hybrid and organic thin film materials with 23 and 7 articles, respectively.Figure7shows the annual variation of the research articles within the general themes during the review period.Dip-coating of thin films for the 'ecofriendly superhydrophobicity' category seems to have first appeared in 2013.The dip-coating of 'high performance superhydrophobic thin films' gained increased attention over the years.Annual publishing in the 'high performance superhydrophobic thin films' category exceeded that for the 'fundamental research formulations' category in 2021.The identified materials for dip-Top 20 sources based on citations and the number of superhydrophobic thin film publications.
T A B L E 3 ABS-KEY (cotton) AND NOT TITLE-ABS-KEY (membrane) AND NOT TITLE-ABS-KEY (fabric) AND NOT TITLE-ABS-KEY (rubber) AND NOT TITLE-ABS-KEY (pulp) AND NOT TITLE-ABS-KEY (paper))T A B L E 5 64perCN and SuperCN Plus, Pearl Nano, TUORMAT, so on.Parvate et al.64considered TiO 2 , and ZnO nanoparticles as low cost and eco-friendly materials for producing hybrid mixtures for STFs.Efforts to extend STFs into new applications are evolving under the theme 'fundamental research formulations'.Wu et al.
T A B L E 7 Dip-coating of high performance superhydrophobic thin films for mechanical, chemical and thermal stability.
224lysis (TGA).Composites with nano-additives displayed no weight loss for TGA test temperatures up to 250 C. To achieve thermally durable STFs on HX surfaces in LHTES, the in-service thermal stability studies are considered more appropriate.Song and Fan224pioneered a detailed review on the practical evaluation of thin films and the temperature dependence of WCA.
created a sol-gel mixture containing hydrophobic SiO 2 nanoparticles T A B L E 8 Materials for the dip-coating of eco-friendly superhydrophobic surfaces for environmental sustainability.
241erials in the fundamental synthesis and formulation of materials for dip-coating of superhydrophobic thin films.Roughness parameters measured for different samples before coating and their WCAs 10 days after applying a low surface free energy (SFE) film.241 T A B L E 9 the surface, whilst the washing and rinsing time removes excess material in the bilayer dip-coating process.The washing and rinsing times were likened to the withdrawal speed and substrate withdrawal angle for a single layer dip-coating process.The study concluded with an optimised procedure for the bilayer dip-coatingT A B L E 1 0