Reversible exchange of wetting state of a hydrophobic surface via phase change material coating

Abstract

Reversible exchange of the wetting state of a hydrophobic surface is examined. Solution crystallization of a polycarbonate surface is carried out to form hierarchically distributed micro/nano size spherules and fibrils on the surface. Although the solution crystallized surface has hydrophobic characteristics, the contact angle hysteresis remains high. Functionalized silica particles are deposited on the crystallized polycarbonate surface to improve the droplet contact angle and lower contact angle hysteresis. The liquid film of n-octadecane with 1.5 μm thickness is formed on the functionalized silica particles deposited crystallized surface, which results in hydrophilic surface characteristics. The n-octadecane film solidifies upon reducing the temperature on the surface and solid flakes of n-octadecane are formed. This arrangement changes the surface wetting state to hydrophobic. Liquefaction and solidifying of the n-octadecane film at the functionalized silica deposited surface gives rise to reversible exchange of surface wetting state. This behavior is attributed to exposure of emerging functionalized silica particles to the free surface in the region of the solid n-octadecane flakes. The water droplet is cloaked by the liquid n-octadecane while forming a ridge around the droplet. In this case, the water droplet becomes mobile at the surface because flow develops in the n-octadecane liquid film at the onset of liquefaction.

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Introduction

Self-cleaning of surfaces has crucial importance for efficient operation of solar energy harvesting devices. In general, surface hydrophobicity is associated with surface texture and free energy of surface. Surfaces composing of micro/nano pillars and low surface free energy result in hydrophobic characteristics. The liquid droplet mobility on hydrophobic surface is related to the lateral adhesion of the droplet, which is governed by the droplet contact angle hysteresis and droplet liquid surface tension.¹ Several methods are introduced towards creating surfaces with hydrophobic characteristics.^3–8 Some of these methods are associated with multi-step processing in harsh conditions and require specialized reagents with high cost. Some of these methods are phase separation,² electrochemical deposition,³ plasma treatment,⁴ sol–gel processing,⁵ electrospinning,⁶ laser texturing,⁷ and solution immersion.⁸ In these processes, the free energy of the surfaces is altered through changing chemical composition of the surface. Texturing and generating hierarchical structures of micro/nano pillars and fibrils on the surfaces becomes essential for hydrophobic surfaces. One-step method of achieving such texture structures remains favorable among the other methods. One of the one-step texturing techniques for hydrophobic surface is the solution crystallization, which is favorable for the polycarbonate surfaces; in which case, crystallized surface composes of micro/nano size spherules and fibrils.⁹ Although the crystallized surface has hydrophobic characteristics, because of high contact angle hysteresis, the textured surfaces require further treatment.⁹ However, the deposition of the functionalized silica particles on the crystallized surface generates the Lotus effect and lowers the contact angle hysteresis significantly.¹⁰ The reversible exchange of surface hydrophobicity is interesting and it can reduce the droplet pinning on the surface while contributing to the droplet mobility. On the other hand, some of the phase change materials have low phase change temperatures, such as n-octadecane (T_solidus = 301.15 K and T_liquidus = 303.15 K,¹¹) can be used for reversible exchange of wetting characteristics of a surface.¹² The surface free energy of n-octadecane in liquid phase is high (21.6 mN m⁻¹,¹³) and demonstrates hydrophilic characteristics in the liquid phase.¹² The thin film of solid phase of n-octadecane does not significant effect on the hydrophobic characteristics of the surface; in which case, the surface can remain as hydrophobic after solidification of n-octadecane film.¹² However, the liquid phase of the n-octadecane film covers the hydrophobic surface while changing the wetting state from hydrophobic to hydrophilic. Since the reversible exchange of wetting state has the practical importance in advanced multifunctional systems, such as biomaterials, and microfluidic devices,^14–16 investigation of processes and physical insight into the reversible exchange of the wetting state on the surface becomes essential.

Considerable research studies are carried out to examine phase change material and hydrophobicity. The cold thermal energy storage system incorporating the absorbing dodecane phase change material into the hydrophobic fumed silica was studied by Chen et al.¹⁷ They indicated the hydrophobic fumed silica was beneficial for penetration and infiltration of dodecane, which could be utilized minimizing a leakage problem in the storing system. Super-wetting state of polypropylene aerogel incorporating the phase change material was investigated by Hong et al.¹⁸ They reported the new fabrication technique for polypropylene aerogel with a super-wetting state after incorporating the form-stable phase change material composites. A study for a new fabricating method of the super-hydrophobic polyvinylidene fluoride membrane was carried out by Wu et al.¹⁹ They showed that the spherical micro-structure was formed at the surface due to the addition of the hydroxyl rich silica particles. The silica particles acted as the nuclei sites for crystallization and the hydrogen bond formed between hydroxyl groups and polyvinylidene fluoride chains could alter the surface texture towards achieving the hydrophobic characteristics. Superhydrophobic coatings based on non-solvent induced phase separation during electro-spraying were examined by Gao et al.²⁰ They demonstrated that both smooth and porous surfaces of the electro-sprayed microspheres could be achieved via choosing different non-solvent additives and resulting surface remained superhydrophobic. The surface hydrophobic modification of barium titanate incorporating the acrylonitrile–styrene–acrylate copolymer was investigated by Xiang and Zhang.²¹ The findings revealed that the addition of unmodified barium titanate could significantly improve the solar reflectance of acrylonitrile–styrene–acrylate copolymer, particularly; the near infrared (NIR) reflectance increased significantly when the addition amount increased. However, serious agglomeration occurred when the addition amount increased further; therefore, it led to a relatively smaller increase in solar reflectance. The influence of polymer molecular weight and non-solvent content on the phase separation, surface morphology, and hydrophobicity of polyvinyl chloride films was studied by Khoryani et al.²² They showed that the instantaneous demixing was the dominant mechanism once higher contents of non-solvent were used. In addition, a delayed demixing mechanism was detected when the lower molecular weight polyvinyl chloride was used, which in turn resulted in a pore-less and dense skin layer. The thermal control of hydrophilicity/hydrophobicity changes of hybrid films was examined by Ralbag et al.²³ They demonstrated that the changes in the wettability in all cases were due to phase separation processes, the directionality of which was determined by the treatment of the substrate on which the films were deposited. In addition, by monitoring the change of wettability at various temperatures, an Arrhenius plot was presented from which the activation energy and Arrhenius pre-exponential factor for the phase separation could be derived. The phase behavior in quaternary ammonium ionic liquid–propanol solutions and hydrophobicity was studied by Abe et al.²⁴ The anomalous behavior of pure crystal polymorphism and its mixtures was derived from hydrogen bonding of the hydroxyl group of cation coupled with the hydrophobicity and packing efficiency of propanol. A study on morphology, thermo-mechanical properties, and surface hydrophobicity of nanostructured epoxy thermosets modified with triblock copolymer was carried out by Parameswaranpillai et al.²⁵ They indicated that incorporation of triblock copolymer improved impact strength and tensile properties of the epoxy system. The thermal stability of the epoxy system was retained while hydrophobicity was improved in the presence of triblock copolymer. The influence of calcination temperature on hydrophobicity of microporous layers prepared with two different molecular weights of polydimethylsiloxane polymer was examined by Ozturk and Yurtcan.²⁶ They demonstrated that the calcination temperature had considerable effect on the hydrophobic characteristic of the micro-porous layer. The condensation enhancement in a small channel with hydrophobic characteristic was studied by Chen et al.²⁷ Dropwise condensation was observed in the hydrophobic channel, although the addition of nitrogen suppressed the nucleation rate. In the hydrophobic channel, heat transfer was enhanced significantly as compared to that corresponding to the hydrophilic channel. In addition, they showed that heat transfer coefficients in the hydrophobic channel with presence of nitrogen were identical or higher than those of pure steam in the hydrophilic channel at the same mass flux and quality. The microphase separation and hydrophobicity of urethane/siloxane copolymers with low siloxane content was investigated by Santiago et al.²⁸ They demonstrated that the copolymers had a phase separated structure, which depended on the conversion of the reaction and casting temperature. The water droplet contact angle became the maximum at the intermediate reaction conversion. In addition, the siloxane concentration at the surface increased with the reaction time while the phase separation was higher at intermediate conversions. The effect of processing conditions on the surface morphology and hydrophobicity of polyvinylidene fluoride membrane was examined by Peng et al.²⁹ The findings revealed that different membrane morphologies and hydrophobicity could be obtained via changing the processing conditions; in which case, low air temperature and high polyvinylidene fluoride contents facilitated the crystallization process while increasing the hydrophobicity of the surface.

Reversible exchange of wetting state of surfaces from hydrophilic to hydrophobic or vice versa has a great interest for advanced multifunctional systems,¹⁶ extraction, separation, surface chemistry, life science and as organic solvents.³⁰ Although reversible exchange of the wetting state from hydrophobic to hydrophilic was studied previously,¹² the main focus was to examine the phase change material characteristics on the silicon nano-wires and nano-walls. The reversible exchange of wetting state due to presence of phase change material on the surface texture composing of micro/nano spherules and fibrils is left for the future study. Hence, in the present study, reversible exchange of wetting state of the surface from hydrophobic to hydrophilic or vice versa is examined. Solution crystallization process is adapted to texture polycarbonate surfaces. In order to reduce the water droplet contact angle hysteresis, functionalized silica nano-particles are synthesized and, later, they are deposited on the crystallized polycarbonate surface. The resulting surface is coated by using n-octadecane phase change material, which provided the reversible exchange of the wetting of the surface through melting and solidification. The textured surface is characterized incorporating the analytical tools. The water droplet mobility is also analyzed on the surface when the phase change material is in liquid phase.

Experimental

A polycarbonate wafer (3 × 20 mm × 20 mm, thickness, width, and length) and acetone were used for solution crystallization. The polycarbonate wafers were immersed in 60% concentrated acetone in acetone–water mixture for 4 minutes in line with the previous study.³¹ This process resulted in a hydrophobic surface (θ_w = 130°) with the large contact angle hysteresis (36°). In order to improve the surface contact angle and reduce the contact angle hysteresis, crystallized surface was coated by functionalized silica particles. The silica nano-size particles were produced through synthesizing process. Tetraethyl orthosilicate (TEOS), 3-aminopropyltrimethoxysilane (AMPTS) and isobutyl trimethoxysilane (OTES), ethanol, and ammonium hydroxide were used in the synthesizing process. In this case, 14.4 mL of ethanol, 1 mL of ultrapure water, and 25 mL of ammonium hydroxide were mixed and stirred for 12 minutes. Later, 1 mL TEOS diluted in 4 mL ethanol was added in the mixture. Following 25 minutes after this process, 0.5 mL of TEOS diluted in 4 mL ethanol was added. After 5 minutes, a modifier silane molecule was added in molar ratio of 3 : 4 with respect to the second edition of TEOS. The final mixture was stirred for 18 hours at room temperature, and later centrifuged and washed with ethanol complete removal of reactants. The solvent casting was applied to coating the solution crystallized polycarbonate surfaces. Upon vacuum drying until all solvent was evaporated, characterization of resulting surfaces was carried out. This arrangement gave rise to the water contact angle in order of 160° with contact angle hysteresis of 2°.

The surface morphologies and texture characteristics of the solution crystallized and functionalized silica particles deposited surfaces were characterized with a focused ion beam (FIB) field emission dual beam scanning electron microscope (FESSEM) and atomic force microscope (AFM). The AFM tip was made of silicon nitride probes (r = 20–60 nm) with a manufacturer-specified force constant, k, of 0.12 N m⁻¹.

The coating of n-octadecane, as a phase change material, was introduced on the functionalized silica deposited crystallized polycarbonate surface. The dip coating technique was used for n-octadecane coating of the surface while keeping n-octadecane in the liquid phase at constant temperature (304 K) during the coating process. The deep coating was performed incorporating the coating unit (Chemat Scientific KW 4AH, Chemat Technology Inc., USA). The sample was immersed into the deep coating chamber, which had a constant spinning speed, for four minutes while avoiding any jitters. The sample was withdrawn from the chamber with a constant speed 0.1 mm s⁻¹. This arrangement provided thin film of n-octadecane liquid on the functionalized deposited silica sample surface. n-Octadecane liquid film formed on the functionalized silica particles covered the whole sample surface and it was remained chemically stable. The liquid film thickness of n-octadecane on the surface was measured using the ellipsometer (Model: M-2000 Manufacturer: J.A. Woolam Co., USA). The liquid n-octadecane film thickness measurement relied on the change in polarization state as defined by the quantities: (a) amplitude ratio (Ψ), and (b) phase difference (Δ), i.e. tan Ψe^iΔ = R_p/R_s, here R_p and R_s are the Fresnel reflection coefficients for the p- and s-polarized light, respectively. A spectroscopic ellipsometer provided the measured data for Ψ and Δ value for each wavelength of incident optical radiation and generated the spectrum accordingly. The measured liquid n-octadecane film thickness was in the order of 1.5 μm ± 20 nm.

A goniometer (Kyowa, model DM 501) was used to conduct sessile drop tests for the measurement of the droplet contact angle. Desalinated water was used in the sessile drop experiments, and the droplet volume was controlled with an automatic dispensing system. The images of the droplets were taken one second after deposition of the water droplet on the surface.

Results and discussion

The reversible wetting state of the hydrophobic surface coated with a thin film of n-octadecane phase change material is investigated. The surface characteristics and water droplet mobility on the liquid phase of n-octadecane are analyzed.

Fig. 1 shows SEM micrographs of solution crystallized polycarbonate surface. The texture of solution crystallized surface composes of spherules and fibrils (Fig. 1a and b). Spherules are closely spaced and almost uniformly distributed on the surface while forming a hierarchical texture. Nano-size fibrils emanate from the spherules surface while contributing to the characteristics of the surface texture. In the crystallization process, the size of spherulites increases with processing time and the nanosize fibrils are initiated from the spherulites surfaces. As the immersion time of polycarbonate wafer in acetone progresses further, spherulites aggregate and fibrils cover the surface of spherules, particularly in the top region (Fig. 1b). The spherules initially growth radially from the nucleation sites in the surface region and during the radial growth, few branches are formed and the intermittent branching gives rise to further growth of crystals towards forming large spherules at the surface.^32–34

Fig. 1 SEM micrograph of crystallized polycarbonate surface and functionalized silica particles deposited on crystallized surface: (a) spherules on crystallized surface, (b) fibrils on spherules surface, (c) functionalized silica particles covering crystallized surface, and (d) agglomerated functionalized silica particles.

As the polycarbonate wafer is removed from the acetone immersion bath, residues of acetone evaporate from the surface and spherules remain at the surface, which is similar to the previous findings.^35,36 The crystallization occurs in three consecutive phases including crystallization initiation, primary formation of crystals, and secondary crystal growth.³⁷ During the initiation of crystallization, a nucleus site emerges and the polymer chains align in a parallel, which are added to the emerging nucleus. The crystal growth remains spontaneous once the size of nucleus reaches to the critical size.³⁷ The nucleation gives rise to a bundle-like or a lamellar crystallization. The difference between the types of crystallization is related to the size of primary nucleus and free energy of the surface, which is normal to the chain direction per unit area.³⁸

The crystallization mainly initiates from the molten state of the swollen film. In this case, the mixture of bundle-like and lamellar nucleus are developed because of series of addition of the repeating units during the crystallization. In addition, the presence of some small size cavities and pores result in micro/nano waviness at the surface while modifying the texture of the crystallized surface. Fig. 2 shows X-ray diffraction of crystallized and as received polycarbonate wafers. As received polycarbonate wafer demonstrate amorphous behavior, since it does not yield a visible peak in the diffractogram. In the case of crystallized wafer, two peaks are evident from the diffractogram. These peaks appear at diffraction angle 17.1° (020) and 25.7° (222). The peak heights and full width at half maximum (FWHM) of the peaks are different in the diffractogram.

Fig. 2 X-ray diffractogram of crystallized and as received polycarbonate.

The crystallinity of the polycarbonate surface can be obtained from the ratio of the sum of integrated intensities of the reflections from the crystalline phases (peaks) to the total scattered intensity after background subtraction.³⁹ This arrangement results in the crystallinity (f_c) values in the order of 34%, which is slightly higher than that reported in the previous study.³⁹ It should be noted that Crystal Impact Match 3 software⁴⁰ is used to determine the crystallinity. The water droplet contact angle and contact angle hysteresis are measured for crystallized surface in line with the previous study.¹⁴

The findings revealed that the contact angle on the crystallized is in the order of 130° and the contact angle hysteresis is in the order of 36°, which is considerably high. Consequently, the water droplet pins on the crystallized surface because of high contact angle hysteresis despite the fact that the crystallized surface demonstrates the hydrophobic characteristic.

In order to lower the contact angle hysteresis, functionalized silica particles are deposited onto the crystallized polycarbonate surface. Fig. 1c and d show SEM micrographs of functionalized silica particles deposited crystallized polycarbonate surface. Functionalized silica particles demonstrates sponge like structures, which are formed by the agglomerated silica particles and some voids. During the synthesizing of silica particles, orthosilicate (TEOS) is used and the surface roughness of the particles can be altered slightly by the functionalized shell and the cell size increases slightly in the region of spongy structures.^41,42 This is, mainly, caused by the condensing units of monomer, which can grow at a faster rate than the nucleation rate.⁴³ This, in turn, enhances the agglomeration of functionalized silica particles. The average size of the silica particles is in the order of 30 nm. In order to assess the surface morphology of the functionalized silica particles deposited crystallized surface, atomic force microscopic examination of the surface is carried out. Fig. 3 shows AFM images of 3-dimensional surface (Fig. 3a) and line scan of the functionalized silica particles deposited surface (Fig. 3b). The appearance of spherules is evident from AFM image (Fig. 3a). Some small oscillation-like variations are observed along the surface peak heights (Fig. 3b), which are associated with the presence of functionalized silica particles on the spherules surfaces (Fig. 3b). These particles can generate Lotus effect while lowering the contact angle hysteresis. The roughness of the surface is in the order of 0.8 μm. The texture parameters are defined in terms of fraction of the projected area of the textured surface that is not occupied by the solid spherules (φ), and the ratio of total surface area of the textured surface to its projected area (r), which are in the order of φ = 0.32 and r = 0.88. The modified crystallized surface by deposition of the functionalized silica particles improves the water droplet contact angle significantly and lowers the contact angle hysteresis. In this case, the droplet contact angle remains almost uniform across the surface at 160° on the surface and the contact angle hysteresis reduces to 2°. The functionalized silica particles deposited surface is coated by the phase change material through implementing the dip coating technique in the liquid phase of n-octadecane. The liquidus and solidus temperatures of n-octadecane are 303.15 K and 301.15 K, respectively. n-Octadecane remains in the solid phase at temperatures lower than the solidus temperature while it becomes in liquid phase for temperatures higher than the liquidus temperature.¹¹ This allows n-octadecane remaining in the solid phase at room temperature once the coating is accomplished. Fig. 4 shows the optical image of the liquid and solid phases of n-octadecane. The liquid phase of n-octadecane forms a continuous film on the functionalized silica surface. The liquid film thickness of n-octadecane is measured using the ellipsometer and the liquid film thickness is in the order of 1.5 μm, which demonstrates that the liquid n-octadecane impregnates the functionalized silica particles deposited onto the crystallized polycarbonate surface. However, the state of impregnation of liquid phase of the n-octadecane on functionalized silica particles deposited onto the crystallized surface is related to the interfacial energy and the texture parameters.⁴⁴ The state of impregnation is associated with the ratio of surface energies of liquid phase and the solid surface, i.e. (γ_sa − γ_ls)/γ_la = f((1 − φ)/(r − φ)). Here, γ_sa is the surface energy of solid phase in air, γ_ls is the solid–liquid interfacial energy, and γ_la is the surface energy of liquid phase in air. In addition, φ is the fraction of the projected area of the surface that is not occupied by the solid (spherules) and r is the ratio of the total surface area to the projected area of the solid. Incorporating the Young's equation, cos θ_w = r(γ_sa − γ_sl)/γ_la into the equation that relates the interfacial energy and surface texture parameters, the Hemi-Wicking criterion for the liquid n-octadecane impregnation can be obtained.^45,46 This yields cos θ_os(a) = (γ_sa − γ_ls)/γ_sa − γ_ls, here γ_sa is the interfacial energy between solid surface and air, γ_ls is the interfacial energy between liquid n-octadecane and solid surface, and γ_la is the interfacial energy between liquid phase of n-octadecane and air. The condition of liquid phase of n-octadecane impregnation and encapsulation of the textured surface need to satisfy the condition of θ_os(a) < cos⁻¹(1/r) and spreading rate S_ls(a) ≡ −γ_ls(r − 1/r) or S_ls(a) ≥ 0,^45,46 where S_ls(a) is the spreading rate of liquid n-octadecane on the textured solid surface in air. The spreading rate of the liquid phase of n-octadecane on the textured surface is in the order of S_ls(a) = 2.95 mN m⁻¹, which is greater than zero. It should be noted that the surface energy of the liquid phase of n-octadene is 21.6 mN m⁻¹,.¹³ Therefore, the liquid phase of n-octadecane forms a film and encapsulates the surface of the functionalized silica particles deposited onto the crystallized polycarbonate surface. On the other hand, upon the solidification of n-octadecene film, it forms granules like structures on the functionalized silica particles deposited crystallized polycarbonate surface.

Fig. 3 AFM images of functionalized silica particles deposited crystallized polycarbonate surface: (a) 3-dimensional image of surface, and (b) line scan on the surface showing the texture height.

Fig. 4 Optical image of solid and liquid phases of n-octadecane. Image is taken during solidification of liquid phase of n-octadecane.

This situation can be seen from Fig. 5a and b, in which SEM micrographs of solid phase of n-octadecane are shown on the functionalized silica particles deposited textured surface. The granules like structures cover the entire surface while forming connected islands of granules on the surface. The functionalized silica particles emerge and expose to the free surface among the granules-like solid phase of n-octadecane. In this case, the region of functionalized silica particles exposing to the free surface covers the area of almost 20% of the total area of the surface. Consequently, density and free surface energy change of the n-octadecane during the solidification results in formation of granules like structures on the functionalized silica particles deposited textured surface.

Fig. 5 SEM micrograph of solidified n-octadecane on functionalized silica particles deposited crystallized polycarbonate surface: (a) solid flakes covering the surface, and (b) functionalized silica particles emerging around solid flakes and exposing to free surface (marked in dotted circle).

The water droplet contact angle on the surface of the n-octadecane is measured when the n-octadecane is in liquid and solid phases on the functionalized silica particles deposited textured surface. Fig. 6 shows the image of water droplet on the liquid and solid phases of the n-octadecane coated functionalized silica particles deposited surface. The functionalized silica particles deposited surface with presence of liquid phase of n-octadecane film in the surface region gives rise to hydrophilic characteristic with a water droplet contact angle 86° and contact angle hysteresis 1°. However, the surface remains hydrophobic once the n-octadecane is in the solid phase and the water droplet contact angle becomes in the order 140° and contact angle hysteresis 8°. The hydrophobic behavior of the surface with presence of solid phase of n-octadecane is related to the surface area where the functionalized silica particles emerge among the solidified granules-like n-octadecane flakes and expose to the free surface. However, upon liquefying of n-octadecane on the functionalized silica particles deposited surface, the surface characteristic returns to the hydrophilic state. The hydrophilic characteristic of the surface changes and becomes hydrophobic again once the liquid phase of n-octadecane changes to solid phase on the functionalized silica particles deposited surface. Liquefying and solidifying of n-octadecane film on the functionalized silica particles deposited surface is repeated fifteen times, the reversible change of hydrophobic to hydrophilic states of surface is observed for each repeat. This situation is schematically shown in Fig. 7. The contact angle hysteresis of the water droplet on the surfaces of the solid and liquid phases of n-octadecane is in the order of 5° to 1°, respectively. Consequently, the water droplet mobility remains high on the liquid phase of n-octadecane film on the functionalized silica particles deposited surface.

Fig. 6 Water droplet images on crystallized polycarbonate, silica particles deposited crystallized polycarbonate, n-octadecane coated (solid phase) and functionalized silica particles deposited crystallized polycarbonate surface, and n-octadecane coated (liquid phase) and functionalized silica particles deposited crystallized polycarbonate surface.

Fig. 7 Schematic view of surface wetting characteristics and reversible exchange of hydrophobic to hydrophilic states.

On the other hand, the water droplet moves on the surface once the solid phase changes to the liquid phase on the functionalized silica particles deposited surface. This behavior is attributed to the flow field developed within n-octadecane layer during the phase change process. In this case, an experimental setup is designed to monitor the velocity of the particles in the liquid phase of n-octadecane under controlled melting. The clusters of carbon nanotubes are used as the particles to trace the flow field. Fig. 8a shows optical micro-images of clustered carbon nanotubes in different locations inside the melted zone of n-octadecane. The corresponding velocity is estimated as 186 μm s⁻¹. In addition, further experiments are carried out to monitor the velocity of the water droplet located on the liquid phase of n-octadecane onset of phase change. Fig. 8b shows optical images of moving droplet at two locations on the liquid phase of n-octadecane. The corresponding velocity is determined as 238 μm s⁻¹. The experiments are repeated 12 times to estimate the experimental errors and in both experiments the error related to the measurement is in the order of 7%.

Fig. 8 Optical images: (a) carbon nanotube cluster motion in liquid n-octadecane due to phase change, and (b) droplet location on n-octadecane deposited functionalized silica particles surface during droplet motion onset of liquefaction.

The water droplet velocity on the liquid phase of n-octadecane onset of melting almost agrees with the flow velocity in the liquid phase of the n-octadecane. However, the small difference is associated with the flow boundary layer in the liquid phase of n-octadecane. The flow velocity measurement is carried out at some depth below the surface inside the liquid n-octadecane, which becomes less than that of the flow velocity at the surface, i.e. the velocity profile in the liquid layer of n-octadecane may be similar to the Couette flow characteristic due to small layer thickness (∼1.5 μm); in which case, the maximum flow velocity occurs at the liquid surface of n-octadecane. Moreover, the cloaking of the water droplet takes place in the liquid phase of the n-octadecane. The cloaking velocity of the liquid n-octadecane is monitored using the high-speed camera. Fig. 9a shows the images of liquid phase cloaking of the water droplet and the cloaking velocity. The cloaking is associated with the spreading coefficient of the liquid phase of n-octadecane while satisfying the condition of S_lw > 0.

Fig. 9 Cloaking velocity and optical image of droplet stain: (a) cloaking velocity and image of droplet during cloaking, and (b) droplet stain left on solidifying n-octadecane film while showing ridge residue on solidified n-octadecane surface (marked in dotted circle).

Here, S_lw is the spreading coefficient of the liquid phase of n-octadecane on the droplet water, which is S_lw(a) = γ_w − γ_lw − γ_l, where γ_w is the surface energy of water at air interface (52 mN m⁻¹), γ_lw is the interfacial energy at water–liquid n-octadecane interface, and γ_l is the surface energy of liquid n-octadecane at air interface (21.6 mN m⁻¹,¹³).

Since the liquid phase of n-octadecane forms a ridge around the water droplet (Fig. 9) because of positive spreading rate (S_lw > 0), the interfacial energy between the droplet water and the liquid phase of n-octadecane (γ_lw) should remain less than 30.4 mN m⁻¹. The cloaking velocity increases to reach the peak at early time period (∼0.62 s, Fig. 9a). Hence, the partial encapsulating of the droplet water takes place by the liquid phase of n-octadecane at a fast rate. This, in turn, enables the water droplet to move with the velocity of the liquid phase on the surface almost at onset of the liquid phase initiation of n-octadecane.

Consequently, droplet becomes mobile almost unset of melting of n-octadecane layer on the functionalized silica particles deposited polycarbonate surface. Moreover, in order to assess the cloaking and the ridge height formed around the water droplet, further experiments are carried out. The water droplet is dispensed onto the liquid phase of n-octadecane film and n-octadecane film is left for solidification at room temperature with presence of water droplet.

Later water droplet is left for evaporation over some time period. Fig. 9b shows optical image of the solidified n-octadecane film on the functionalized silica particles deposited and crystallized polycarbonate surface. The residue of ridges is evident and liquid n-octadecane does not completely encapsulate the water droplet. This is because temperature of the water droplet, which is lower than that of liquid n-octadecane film. Consequently, heat transfer from liquid n-octadecane to water droplet lowers liquid film temperature during cloaking. In this case, liquid n-octadecane solidifies before encapsulation of water droplet. In this case, cloaking terminates when solidification of liquid n-octadecane initiates around the ridge.

Conclusion

The reversible exchange of wetting state of hydrophobic surface is examined. Solution crystallization of a polycarbonate surface is carried out using the acetone immersion technique. Although the crystallized polycarbonate surface has hydrophobic characteristic, the contact angle hysteresis remains high while causing the pinning of the water droplet on the surface. In order to generate Lotus effect on the crystallized polycarbonate surface, nano-size functionalized silica particles, which are synthesized chemically, are deposited on the crystallized surface. The wetting state of functionalized silica particles deposited surface is modified through coating by n-octadecane phase change material. The selection of n-octadecane is due to its low solidus (T_solidus = 301.15 K) and liquidus (T_liquidus = 303.15 K) temperatures. The liquid phase of n-octadecane cloaks the water droplet on the surface while influencing the droplet mobility; therefore, the cloaking velocity is measured. Since the water droplet moves on the surface onset of melting of n-octadecane, the velocity of water droplet is measured and the finding is related to the flow velocity developed inside the liquid n-octadecane layer during the phase change. It is found that solution crystallization of polycarbonate surface results in formation of spherules and fibrils, which are hierarchically distributed on the surface. The solution crystallized polycarbonate surface has hydrophobic characteristics with water droplet contact angle 130° and contact angle hysteresis 36°. The deposition of functionalized silica particles on crystallized polycarbonate surface improves the water droplet contact angle to 160° and lowers the contact angle hysteresis to 2°. The coating of functionalized silica particles deposited surface by the continuous film of liquid n-octadecane with the thickness of 1.5 μm changes the wetting state of the surface to hydrophilic state with the water droplet contact angle 86° and contact angle hysteresis 1°. Once the continuous film of liquid n-octadecane solidifies via reducing temperature, flakes of n-octadecane are formed on the functionalized silica particles deposited surface. This, in turn, gives rise to exposure of functionalized silica particles to the free surface via emerging beside the flakes of solid n-octadecane. This alters the wetting state from hydrophilic to hydrophobic with the droplet contact angle 140° and contact angle hysteresis 8°. Once the flakes of solid n-octadecane is melted on the functionalized silica particles deposited surface with increasing temperature to liquidus temperature, a continuous film of liquid n-octadecane film is formed on the surface. In this case, the wetting state of the surface changes to hydrophilic. Consequently, the reversibly exchange of wetting state occurs once the n-octadecane coating undergoes phase change on the functionalized silica particles deposited crystallized polycarbonate surface. The liquid n-octadecane forms ridge around the water droplet due to cloaking. The water droplet moves on the surface onset of the liquefaction of n-octadecane film. This is mainly because of the flow current developed in the liquid phase of n-octadecane onset of melting. In this case, n-octadecane ridge formed around the water droplet anchors and moves the droplet on the surface of the liquid n-octadecane film. The present study provides.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support of the Deanship of Research at King Fahd University of Petroleum and Minerals (KFUPM) and King Abdulaziz City for Science and Technology (KACST) through project #11-ADV2134-04.

Notes and references

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