Facile Adhesion-Tuning of Superhydrophobic Surfaces between

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Facile Adhesion-Tuning of Superhydrophobic Surfaces between “Lotus” and “Petal” Effect and Their Influence on Icing and Deicing Properties Md J. Nine, Tran Thanh Tung, Faisal Alotaibi, Diana N. H. Tran, and Dusan Losic* School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia S Supporting Information *

ABSTRACT: Adhesion behavior of superhydrophobic (SH) surfaces is an active research field related to various engineering applications in controlled microdroplet transportation, self-cleaning, deicing, biochemical separation, tissue engineering, and water harvesting. Herein, we report a facile approach to control droplet adhesion, bouncing and rolling on properties of SH surfaces by tuning their air-gap and roughness-height by altering the concentrations of poly dimethyl-siloxane (PDMS). The optimal use of PDMS (4−16 wt %) in a dual-scale (nano- and microparticles) composite enables control of the specific surface area (SSA), pore volume, and roughness of matrices that result in a wellcontrolled adhesion between water droplets and SH surfaces. The sliding angles of these surfaces were tuned to be varied between 2 ± 1 and 87 ± 2°, which are attributed to the transformation of the contact type between droplet and surface from “point contact” to “area contact”. We further explored the effectiveness of these low and high adhesive SH surfaces in icing and deicing actions, which provides a new insight into design highly efficient and low-cost ice-release surface for cold temperature applications. Low adhesion (lotus effect) surface with higher pore-volume exhibited relatively excellent ice-release properties with significant icing delay ability principally attributed to the large air gap in the coating matrix than SH matrix with high adhesion (petal effect). KEYWORDS: lotus effect, petal effect, roughness, porosity, superhydrophobicity, deicing

1. INTRODUCTION

force that sticks a water droplet at the interface, where the surface is turned upside-down.4 These different states of wettability can be explained by classical theory of wettability on rough surfaces based on Cassie−Baxter (suspended state) and Wenzel (penetrated state) models.5,6 Water can either penetrate the asperities or suspend above the asperities to create highly adhesive SH surfaces or very low adhesive SH surfaces, respectively.2 Here, “lotus effect” actually follows Cassie−Baxter model addressing suspended droplet on the air-pockets trapped into rough surface, while “petal effect” is defined as an Cassie impregnating wetting state.4 Many of the SH surfaces with various adhesion forces are actually tuned between Cassie and Wenzel states,7 where this Cassie impregnating wetting state is an intermediate adhesive state.8 The SH surfaces with the tunable adhesion forces have emerging applications in the field of controlled microdroplet transportation,9 biochemical separation,10 self-cleaning,11 deicing,12 cell adhesion/tissue engineering,13,14 and vapor condensation and collection.15 A number of methods, such as

Superhydrophobic (SH) surfaces exhibit an extremely high water repellent behavior with a static contact angle (CA) greater than 150° and possess low CA-hysteresis.1 Geometrical structures in nano/micro scale with different chemical compositions affect surface free energy, roughness, specific surface area (SSA), and porosity of matrices, which are the key parameters to define different types of SH surfaces.2 These active parameters by time have been revealed to play a significant role in tuning surface air pockets and contact area between water droplets and the SH surfaces, hence influences adhesion behaviors. In the early 1990s, the investigation of the microstructure of extremely water-repellent plant-leaves eventually brings this concept of superhydrophobicity.1 Subsequently, different representative terms such as “lotus effect” and “petal effect” were coined to define antiadhesive and highly adhesive state of SH surfaces particularly observed in N. nucifera (indian lotus leaves) and rosea Rehd (red rose petal), respectively. A SH state with antiadhesive abilities, bouncing droplet, and small angle roll-off properties was called “lotus effect” coined by Barthlott and Neinhuis.3 Recently, compared with the popular “lotus effect”, a new term “petal effect” was coined by Feng et al.4 to define a SH state with a high adhesive © 2017 American Chemical Society

Received: December 21, 2016 Accepted: February 13, 2017 Published: February 13, 2017 8393

DOI: 10.1021/acsami.6b16444 ACS Appl. Mater. Interfaces 2017, 9, 8393−8402

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ACS Applied Materials & Interfaces

Figure 1. Schematic presentation for the fabrication of low and high adhesive SH surfaces.

electrodeposition,16 electromachining jet technique,17 e-beam deposition,18 linear templated texturing,19 femtosecond laser weaving,20 and heterogeneous chemical composition (mixture of hydrophobic and hydrophilic compounds), have been explored to control adhesion of SH surfaces.9,21 Besides, the use of PDMS in fabricating SH surfaces with controlled adhesion includes PDMS microcell array,22 femtosecond parallel array,20 and other 3D pattern dependent structures.19 However, most of these methods require the use of expensive facilities and fabrications which are not scalable for industrial level. More importantly, many of these approaches generate irreversible transitions induced between Cassie and Wenzel states, with a loss of the antiadhesive properties if the textures are filled with water, the material loses its water-repellent properties over the time.7 This is a common problem of using heterogeneous chemical composition (synergy of hydrophilic and hydrophobic compounds),9 and hydrophilic dimples introducing on hydrophobic surface,17 in order to create controllable adhesion on SH surfaces. The terms “superhydrophobic” and “ice-phobic” were simultaneously used in literature for many years,23 which is recently found questionable considering facts that superhydrophobicity is not the only criteria to create highperformance ice-phobic surfaces.24 The synergistic effect of van der Waals forces, chemical bonding, and direct electrostatic interactions plays a significant role in ice adhesion process. In general, ice-phobicity implies the ability to prevent ice formation on the surface which depends on whether a droplet of supercooled water is repelled showing bouncing-off properties at the temperatures below freezing point.12 Another criterion of “‘ice-phobic”’ surfaces include shear strength between 150 and 500 kPa and even as low as 15.6 kPa.25 Further definition of ice-phobicity refers to the ability to prevent the ice formation on the surface, which is characterized by observing time delay of heterogeneous ice nucleation.12 However, there is a critical role of the micro/nanostructures which are significantly important in the design and fabrication of an ice repellent surface. For example, nanohairs,26 micro/ nanostructure of butterfly,27 and microratchets combined with nanohairs12 exhibit better ice-phobic properties with significant icing delay effect than the surface without any structure (smooth). Therefore, an efficient design of surface structure is believed to save a million dollar of yearly loss attributable to the icing and ice-releasing of many devices such as aerofoils, power towers, ships, radars, and even pipes of air conditioners or refrigerators.

In this paper, we describe a facile method, addressing these problems of complex and expensive fabrication with irreversible wettability, to demonstrate adhesion-tuning approach of SH surfaces between “lotus” and “petal” effect and their influence on icing and deicing. A simple suspension is developed which can be easily applied on any surface to control the surface wettability in a way by optimizing pore-volume and roughness height as schematically presented in Figure 1. It is well-known that the magnitude of the adhesive force of a water droplet on a SH surface ascends in the following order “‘point contact”’ < “line contact” < “area contact”. We described here a simple, low-cost, and scalable method to alter these contact types for obtaining controllable adhesion of SH surfaces combining dualscale roughness of “TiO2 (P25)/Al2O3” modified with PDMS at various concentrations. The adjustment in localized microstructures with tunable pore volume, SSA, roughness height is proposed that enables the surface to exhibit different bouncing droplet effects. The prepared SH surfaces with various porosity, SSA, and roughness were explored to demonstrate their icing and ice-release characteristics for the fabrication of an efficient ice-phobic surface.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. The experiments in this work were carried out using aluminum oxide (Al2O3) (150 neutral, Type-T, Merck) microparticles and titanium dioxide (TiO2) (P25, Degussa, USA) nanoparticles which were used as received. The suitable hydrophobic modifier for this work was chosen to be polydimethylsiloxane (PDMS) prepolymer (Sylgard 184) with curing agent collected from Dow Corning Corporation (USA). Other chemicals include 99.95% toluene (Chem-Supply) as solvent. Milli-Q water (PURELAB Option-Q, 18.2 MΩ cm) was used in CA measurement and in icing/ ice-release experiments. 2.2. Preparation of Superhydrophobic Coatings. The TiO2 (P25) nanoparticles (30 to 50 nm) and Al2O3 microparticles (∼1 μm) were used as received to make a dry mixture in a weight ratio of 1:3. The PDMS prepolymer with curing agent at a weight ratio of 10:1 were dissolved in 12 mL (∼10 g) of toluene with different concentrations between 1 and 20 wt % to prepare different coating formulations. Finally, each solution was used to disperse 2 g of premixed TiO2 (P25) and Al2O3 particles using a mechanical stirring for 20 min at a temperature of 50 °C to obtain a homogeneous and slightly thick/semiliquid solution. The as prepared solution was instantly used for dip coating to fabricate homogeneous films on glass slide (2 cm × 2 cm) followed by a dry at 80 °C for 3 h. This solution can also be used to spray on a glass or metal substrate to create a thin coating layer (used for icing and ice-releasing application). 2.3. Characterization. The as prepared coatings were gold sputtered for imaging by a scanning electron microscope (SEM-FEI 8394


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Figure 2. Effect of PDMS concentration on surface morphology to tune air-pockets of composites, (a) 4 wt % of PDMS, (b) 7 wt % of PDMS, (c) 10 wt % of PDMS, (d) 13 wt % of PDMS, (e) 16 wt % of PDMS, and (f) schematic representation of PDMS concentration effect on roughness height and air-gap. Scale bars in a−e are 10 μm. QUANTA 450, Japan) to analyze the morphological properties at a voltage between 20 and 30 kV. A benchtop 3D optical profiler (Bruker ContourGT-K1) was used for imaging surface profile and to quantify surface roughness. The measurement was conducted in vertical scanning interferometry (VSI) mode using 20X objective. The powder XRD patterns were recorded by X-ray diffraction (XRD) (RigakuMiniflex 600, Japan) for TiO2 (P25), Al2O3 and their mixture before and after hydrophobic modification with PDMS at 40 kV and 15 mA in the range of 2θ = 10−80° with a scanning speed of 10°/min. The vibrational stretching modes of different molecular bonds in PDMSmodified samples were studied by Fourier transform infrared spectroscopy (FTIR) (Nicolet 6700 Thermo Fisher, USA). To measure SSA and pore size distribution in the coated matrix, we used BELSORP-max, MicrotracBEL Corp. Japan by N2 gas absorption and desorption isotherm. The samples for the measurement of SSA and pore size distribution were prepared as powder form treated with PDMS with a concentration between 4 and 16 wt %. The measurement of a specific dry sample required approximately ∼0.3 g (between 2.5 and 3.5 g) degassed at 150 °C for 24 h to calculate exact mass before the final mass input in the system for each adsorption and desorption run. 2.4. Measurement of Contact Angle, Contact Angle Hysteresis, and Bouncing Effect. Sessile drop water contact angle (WCA) and supplementary videos were recorded using an Attension theta optical tensiometer (KSV instruments, Finland). For WCA measurements, at least five different locations on the samples were probed by auto dropping a set volume (5 μL) of Milli-Q water. The static contact angles were recorded after 60 s of droplet stability. Sliding angles were measured with the assistance of an autotilting stage after a gentle release of a droplet of 5 μL from capillary tip. Moreover, the measurements of CA-hysteresis were achieved automatically using the “tilted plate” method introduced by McDougall and Ockrent.28 The procedure was a modified sessile drop method by which both maximum advancing-CA and minimum receding-CA were obtained with gradual inclination of the sample stage at a speed of 0.5°/s and recorded until the drop just begins to move. The “Attension theta optical tensiometer” automatically displayed real time dynamic angles and recorded every single frame. Further analysis of individual recorded frame facilitates to investigate CA-hysteresis and sliding angle. The tip-to-surface droplet-interaction was automatically achieved by a built-in capillary tip with a droplet of 3.5 μL. The transportation of micro droplet from low adhesion surface to high adhesion surface and bouncing affect phenomena were executed using a water droplet of 5 μL. The auto video records of bouncing

phenomena were recorded by setting the time sequence of 0.02 s for each frame to the end of droplet stability. 2.5. Icing and Deicing Experiment. A chiller with digital PID control (Model. DH.WCR 001/01, Daihan Sci. Co. Ltd., Korea) was used for icing purpose of water droplet (50 μL) placed on the surfaces of various wettability (Al-control, PDMS thin coating, PDMS 4 wt % composite, and PDMS 16 wt % composite). The Al surface modified with different formulations was tightly attached to the stage of the icing device to minimize thermal resistance. The temperature was precisely maintained at −14 ± 0.1 °C. The cooling unit involves a cooling circuit through the experimental rig covered with asbestos insulation and acrylic lid on the targeted surface area. The droplet of 50 μL was placed on the surface after the system reaching to the thermal equilibrium state that takes about 20 min. The ice-release experiment involves a special setup of preiced droplet (500 μL) on targeted surfaces (Bare-Al, PDMS on Al, SH surface with 4 and 16 wt % PDMS) at an inclination angle of 5.5 ± 0.2° to allow the droplet to glide on. The measured ambient temperature and humidity were 22 ± 2 °C and of 34−39% RH, respectively.

3. RESULTS AND DISCUSSION The dual scale mixture consists of nanosized (20−30 nm) TiO2 (P25) and microsized (∼1 μm) Al2O3 particles with spherical in shape as shown in Figure S1. Crystallinity of these metal oxide components (TiO2 (P25) and Al2O3) and their chemical structures were studied by X-ray diffraction pattern and FTIR spectra before and after modification with PDMS, respectively as displayed in Figure S2. The crystallinity of TiO2 (P25) shows the major characteristic diffraction peaks as indicated in the Figure S2a with crystalline plane that can be assigned to the anatase (XRD JCPDS Card No. 78−2486) and rutile (JCPDS 21−1276) phases of TiO2 (P25).11 The characteristic peaks for Al2O3 at 2θ = 32.8, 37.0, 42.5, 45.6, and 67.2° could be assigned to cubic phases (JCPDS 00−004−0880).29 The PDMS modification of these oxides shows another little bump at 2θ position of 12.7°. The chemical bonding and corresponding functional groups of the oxide particles and PDMS have been shown by FTIR spectra in Figure S2b. The modification of the particles with different concentration of PDMS shows enhanced intensity of the peaks at 1259 and 790 cm−1 arise from in-plane bending and out-of-plane oscillations of the Si− 8395


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Figure 3. Surface roughness analysis of different coating formulations. (a) 3D surface topography images of coatings on glass-slide, (a) ControlPDMS, (b) modified Al2O3/TiO2 (P25) with 4 wt % PDMS, (c) modified Al2O3/TiO2 (P25) with 10 wt % PDMS, (d) modified Al2O3/TiO2 (P25) with 16 wt % PDMS, (e) roughness profile of coatings at different PDMS concentration, (f) comparison between mean roughness height (Rtm) and mean roughness spacing (Sm) with respect to PDMS concentration, and (g) relation between area roughness (Sa) and static CA with respect to PDMS concentration.

Figure 4. Adhesion tuning of superhydrophobic surfaces, (a) WCA and tip-to-surface droplet (3.5 μL) interaction for various SH surfaces, (b) sliding angles captured for different superhydrophobic surfaces, (c) influence of PDMS concentration on advancing-CA, receding-CA, and CAhysteresis.

CH3, 1015 and 1100 cm−1 from the high coefficient of absorption of siloxane bonds (Figure S3a).11,30 Other peaks at around 1400 cm−1 and at 3400−3700 cm−1 are corresponding to Ti−O−Ti bonding and −OH spectral region of alumina.31,32 The elemental analysis of the sample with 4 wt % of PDMS revealed in Figure S3b, where the corresponding peaks for Al and Ti with Si and O confirm the existence of PDMS-modified TiO2/Al2O3 composite in the matrix.

Morphological changes in microstructure of the prepared SH surface modified with different concentrations of PDMS are presented in Figure 2a−e. The SEM images of the as-prepared PDMS modified samples were observed to be different in surface pore-volume, SSA and roughness parameters (discussed in following dedicated section). The enhanced PDMS concentration in the matrix decreases the nanoparticle hierarchy over microparticles, which is expected to play a 8396


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Figure 5. Dynamic properties SH surfaces, (a) free flow bouncing of water droplet (5 μL) on superhydrophobic surfaces modified with different concentrations (4−16 wt %) of PDMS, (b) bouncing time length and number of bouncing cycles with respect to different concentrations of PDMS, (c) droplet transformation from low adhesion superhydrophobic surface to high adhesion superhydrophobic surface. The droplet (5 μL) is still suspended when the high adhesion surface is turned upside down resembling “petal effect”.

significant role to shift the pattern of contact type from “point contact” to “area contact” between droplets and surfaces. Furthermore, the pores and roughness valley depth in a sampling length of the matrix look significantly clogged by the PDMS in an ascending order with the increase of PDMS concentrations. This is proposed to be another factor to enable droplets either suspended with “point contact” facilitating larger air-gap at low concentration or pinned at higher concentration of PDMS by “area contact” as schematically shown in Figure 2f. To justify our hypothesis and provide clear evidence for change in roughness and air gaps, we characterized the surface roughness parameters of the prepared samples as shown in Figure 3a−d. These measurements include arithmetic mean of profile roughness (Ra), root-mean square (RMS) of profile roughness (Rq), roughness height (Rtm), area roughness (Sa), RMS of area roughness (Sq), and mean spacing between peaks (Sm) to quantify surface morphology with different coating formulations (Table S1). The roughness parameters, with the change in PDMS concentrations, were found to be significantly different in line profile as shown in Figure 3e. The height of line profile decreases with the increase of PDMS concentration which is an important evidence to support the surface morphology illustrated in Figure 2. The mean spacing between roughness peaks also increases with the decrease in roughness height (Figure 3f). Considering that the relation between area roughness and static contact angle (CA) of SH surfaces are proportional to each other (Figure 3g), it can be predicted that a surface with low Sa, shorter Rtm and greater Sm will exhibit a low static CA between droplet and surface and vice versa. The preliminary wettability and adhesion behavior of these surfaces were demonstrated by measuring static-CA and tip-tosurface droplet release phenomena, respectively as shown in Figure 4a. With the morphological change, as displayed in Figure 2, for using different PDMS concentrations between 4 and 16 wt % enables surface to interact with water droplet differently. A trend in decrease of the CA (from 164° ± 2 to 152° ± 1) was observed as PDMS concentration increases. The static-CA further decreases below 150° if the PDMS concentration exceeds 16 wt % for the formulation used in

this investigation, hence it is considered as critical PDMS concentration. This slight decrease in static-CA is an indication of contact transformation from “point contact” to “area contact” between droplet and surface. Similarly, if the PDMS concentration becomes less the 4 wt %, the composition behaves differently as shown in Figure S4. The PDMS with 1 wt % is not enough to modify all particles thus cannot reach high CA, while 2 wt % showed adhesive superhydrophobicity which is possibly attributed to the presence of uncoated hydrophilic oxide particles exposed on the surface (heterogeneous mixture of hydrophilic and hydrophobic particles). However, the drawback of using very low concentration of PDMS is the delicacy of surface that are not bonded as film with PDMS. A tip-to-surface droplet (3.5 μL) interaction was observed to analyze adhesion behaviors of the surfaces as shown in Figure 4a and Movie S1. The droplet (3.5 μL) was not released and came up with the tip from the adjacent layer of SH surface with low concentration of PDMS (4−10 wt %), whereas the droplet was easily released from the needle, and pinned on the surface modified with higher concentration of PDMS (13 and 16 wt %). The phenomena indicate that the wetting contact between droplet and the surface varies with the change of PDMS concentration attributed to the change in air-gap and roughness. We performed sliding angle test to reveal the adhesion behavior of the surfaces, where the maximum sliding angle (87° ± 2) was achieved by the surface modified with 16 wt % of PDMS as displayed in Figure 4b. As the percentage of PDMS increases, the pore-volume and SSA decreases simultaneously with the Rtm that results in a shift from “point contact” to “area contact” that enables controlling sliding angle between 2 ± 1° to 87 ± 2° (discussed in following dedicated section). All the CA-hysteresis were measured using the “tilted plate” method introduced by McDougall and Ockrent.28 The increased PDMS concentration found to give rise to more pronounced hysteresis effects as shown in Figure 4c. Such a difference in CA-hysteresis would lead to different dynamic performances of the droplets. To further investigate the dynamic properties of surfaces with various adhesion effects, we performed bouncing droplet 8397


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Figure 6. Pore volume distribution, and SSA on unmodified and modified samples, (a) the adsorption and desorption isotherms of N2 for control Al2O3/TiO2 (0 wt % PDMS) and inset shows PDMS modified samples with 4 and 16 wt %, (b) pore volume distribution of the control Al2O3/TiO2 (0 wt % PDMS), (c) pore volume distribution of modified samples with 4 and 16 wt % PDMS, (d) comparison between total pore volume and SSA of the superhydrophobic composite coatings, and (e) schematic of low and high adhesive superhydrophobic surface with different pore volume and contact type based on PDMS concentration.

behaviors and microdroplet transportation as shown in Figure 5. The release of water droplet (5 μL) from capillary tip was carried out maintaining a tip distance of 6 mm from the substrate on a leveled platform (within 1°) prior to the recording to investigate bouncing off properties. The ability to bounce-off incoming droplets constitutes the third aspect of the superhydrophobicity,1 which is even recently proposed to define the art of SH surfaces.33 Effort has been made in previous literature to establish a relationship between water-CA and number of bounces, which is dependent on the surfaces microstructure.33 Here, an interesting phenomena relating bouncing length/cycles and surface adhesion was observed and explored where the bouncing cycle decreases with respect to the degree of adhesion of the SH surfaces. The relation might be useful to define type of adhesion behavior of different SH surfaces. Figure 5a, b (Movie S2) revealed that the increase in adhesion (pinning droplet behavior) is inversely proportional to droplet bouncing cycles and time length. The use of 4 wt % PDMS provides maximum static-CA and bouncing effect with lowest CA-hysteresis. Therefore, the magnitude of superhydrophobicity of a surface can be realized by translating the number of water droplet bouncing, in other way, a surface with an average of 2 bouncing cycle (13 wt % PDMS) is less

superhydrophobic than that of an average bouncing cycle of 7 (4 wt % PDMS).33 The transportation of microdroplet (5 μL) from low adhesion surface to high adhesive surface have been performed against gravity from a surface with 10 wt % of PDMS (lower counterpart) to the surface with 16 wt % PDMS (upper counterpart) as shown in Figure 5c (Movie S3). This tunable microdroplet transportation against gravity is resembling the phenomena observed in rose petal.4 The application of such controllable droplet transportation can have significant application in the field of microfluidics for spatial and temporal manipulation of droplets within microfluidic architectures.34,35 To quantify the matrix pore structures, we performed the gas adsorption and desorption isotherm of unmodified (control) and PDMS modified samples (4 and 16 wt %) in powder form in order to correlate the effects on adhesion behavior of these SH surface and their porosity and pore volume. Results presented in Figure 6a showed that the unmodified TiO2 (P25)/Al2O3 (1:3 ratio) surface has the N2 adsorption and desorption isotherms II/IV type with macropore/mesopore characteristics. The type II isotherm represents unrestricted monolayer-multilayer adsorption. The initial part of the type IV isotherm is attributed to monolayer−-multilayer adsorption since it follows the same path as the corresponding part of a 8398


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Figure 7. Effect of SH surfaces with different pore-volume and SSA on icing and deicing, (a, b) icing delay investigation of different surfaces at −14 ± 0.5 °C for the droplet of 50 μL, (c) free-flow deicing at an inclination angle of 5.5 ± 0.2° at 22 ± 2 °C and 34−39% RH (droplet 500 μL). (d) Ice moves upward during meting, allowing water to accumulate at the interface because of density difference.

type II isotherm.36 The increase in adsorption amount for control- TiO2 (P25)/Al2O3 at p/p0 (0.6−0.9) indicates the existence of mesopores with the calculated mesoporous (2−50 nm) volume of 0.1870 cm3 g−1. However, the PDMS modified samples shows significantly decreased in adsorption with a shift of pore volume distribution to macropores as shown in Figure 6b, c. The reduced pore volume was identified for the highest PDMS concentration (16 wt %) which is ∼8 times less in the sample modified with 4 wt % of PDMS. The SEM cross-section of the coating modified with 4, 10, and 16 wt % has been shown in Figure S5, which indicates a little change in morphology of the matrix with an average thickness of ∼60 μm. Similarly, the overall SSA of the sample with 16 wt % (petal effect) was reduced by ∼3 times compared to the sample with 4 wt % PDMS (Lotus effect) as shown in Figure 6d. The SSA and pore size distribution of the powder samples significantly depend on the agglomeration size and particle attachments affected by different PDMS concentration that binds and builds surface. The visual observation of 100 g of the control and PDMS modified samples (4 and 16 wt %) have been shown in Figure S6 to realize the volume contraction after PDMS modification. The particles simply spreading on the surfaces exhibits its own

characteristics showing hydrophilicity (control TiO2/Al2O3) and superhydrophobicity over the agglomerated particles (PDMS modified samples). These transformation and alteration of surface roughness and pore volume affects the adhesion of SH surfaces which are explained and schematically presented in Figure 6e. The nonadhesive state (lotus effect) is considered as classical Cassie−Baxter wetting state, where air is trapped in the pores between sharp peaks of a rough hydrophobic surface. The trapped air results in a greater contact angle θ(Cassie) of the rough surface than the smooth surface with a young contact angle, θy.5 The relation can be explained as follows cos θ(Cassie) = f − 1 + rf cos θy

(1)

Where, “r” is the roughness ratio between the true area of surface to its projected area and the area fraction of the projected wet area is symbolized by “f ”. The contact type will shift from “point contact” to “area contact” when the pore volume in the matrix reduces that will result different adhesion properties with the rise in CAhysteresis at 16 wt % of PDMS. This can be considered as intermediate state between Cassie−Baxter state and Wenzel 8399


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ACS Applied Materials & Interfaces state,37 which was defined as Cassie impregnating state as the water droplet on the surface results in higher surface wettability due to the increase in contact area (shown in Figure 6e).4 This intermediate state from Cassie−Baxter state to the Wenzel state is dominated by the Laplace pressure in the present case, which is actually the competition between the energy barrier and external forces from droplet.37 However, this state is not a fully wetting state where a droplet attaches to the solid surface to create a contact angle θWenzel for a surface with young contact angle of “θy”. The relation can be expressed as follows6 cos θ(Wenzel) = r cos θy

either by naturally under gravity or by means of external force applying hot water, deicing fluid, and electrical heating. Movie S5 is shown at a speed of 8× and 4× the original footage for the surfaces with “petal effect” and lotus effect, respectively. The surface with frost droplet of 500 μL was set to observe the icereleasing rate while gaining temperature from the surrounding ambient. The SH surface with low adhesion released frost droplet by gliding as early just after 30 s of the placement while rest of the surfaces including SH surface with high adhesion (petal effect) did not allow the frost droplet to glide even after a complete melting (Figure 7c). Again, it happens because of the adhesion behavior of these surfaces where the melting of frost droplet at the interface enables its easy gliding to roll on the low adhesion surface (lotus) in contrast to the high adhesion surface (petal effect). We observed the upward movement of solid ice-crystals during heat gain allowing melting ice (water) to accumulate at the interfaces (Figure 7d). This interfacial melt down is independent to the heat source as water density is higher than ice that results to carry ice over the low adhesion surface shown in Movie S4. Micro/macroscale roughness and roughness height are significantly responsible for altering the heat transfer in the matrix.38 The heat transfer in both the icing and deicing mode also depends on whether a droplet of water freezes or defrost by losing or gaining heat at the interfaces. A three-phase interface (solid−liquid−air) has been considered for the droplet on the surface to describe thermodynamics following the literature.12 The droplet gains heat from air and loses heat to the cold surface that happens in forms of contact heat conduction and thermal radiation among these solid−liquid− air interfaces. The relationship in terms of heat gain and loss in this three phases of interface can be expressed as12,39

(2)

Therefore, we speculate that the precondition to tune adhesion properties is to modulate the surface topography and surface energy to decrease the ratio of solid−liquid interfaces on a lowsurface-energy surface. The materials we used to prepare SH surfaces also show good hydrophobicity at a temperature of below freezing point with a reduction in CA of ∼15% measured for both low adhesion and high adhesion surfaces. This indicates that both the SH surfaces with different porosity, roughness and SSA are capable to repel water droplet at subzero temperature. This is the reason why many of SH surfaces are anticipated to be icephobic.25 The reversible superhydrophobicity (Figure S7) shows that both the low and high adhesive surface are also capable to regain their superhydrophobic behavior once the frost solid droplet (10 μL) archives the ambient temperature greater than 22 °C despite having relatively low CA at frost state. The reason behind the quick regain of WCA can be explained by the phenomena observed during melting of frost droplet. Being less dense, the ice crystals are identified to move upward leaving melted ice (water) at the bottom as shown in Figure S7, Figure 7d, and Movie S4. By the time, surface gains room temperature and allows heat transfer to the water molecule at the adjacent to the surface. However, to distinguish the effect of matrix pore-volume of these SH surfaces, we performed a more precise experiment to execute icing delay effect during freezing of the droplet by dropping down the temperature from ∼23 °C to −14 °C. To examine this icing delay ability,12 we set the surfaces (Al-control, Al-PDMS thin coating, PDMS 4 wt % composite, and PDMS 16 wt % composite)to be attached well to the cooling stages subjected to the temperature of −14 °C. The reference droplet (50 μL) was placed on each surface, where the bare Al plate shows negligible resistance in icing displaying quick freezing time of maximum 70 s as shown in Figure 7a, due to strong electrostatic force between water and metal.24 The droplet on the PDMS coated Al still transparent after freezing of about 460 s. The drop finally becomes nontransparent on PDMS coated Al at ∼540 s (frame 4), indicating it is frozen. In the similar way, the SH surface with petal effect (16 wt % PDMS) exhibits icing resistance until 840 s while the SH surface with lotus effect (4 wt % PDMS) shows maximum icing delay effect resisting water droplet to be crystallized for 1660 s Figure 7a (Lotus effect-frame 5) and b. We believe, as discussed above in Figure 6, the pore volume in the matrix with large amount of trapped air underneath droplet plays significant role in icing delay that makes the SH surface with lotus effect a better icephobic surface than the SH surface with petal effect. Finally, we set an experiment to observe the ice release properties of the surfaces at a temperature of 22 ± 2 °C and 34−39% RH with an inclination angle of 5.5 ± 0.2° as shown in Figure S8a, b (Movie S5). Deicing is an action of ice-removal

Δh = hg + hg − hl − hl (3) ′ ′ Here, Δh represents net heat gain in unit time (Δh is greater than zero for an ice-phobic surface at higher temperature); hg and hg′ represent heat gains from air through contact heat transfer and thermal radiation in unit time; hl and hl′ are presented for heat loss from droplet to solid surface through contact heat transfer and thermal radiation in unit time. Furthermore, the decrease in droplet temperature (tf) can be calculated using the net heat gain (obtained from eq 1) and incorporating other parameters such as water density (ρw); heat capacity (Cp); and temperature difference between initial droplet temperature (t0) and surface temperature (ts). The final expression is as below where a large Δh can cause a small tf.12

tf =

ρw Cp(to − ts) Δh

(4)

The relation explains the heat gain/loss from/to solid surface to/from liquid droplet is significantly affects the icing delay, hence less heat loss/gain through contact heat conduction will occur if the surface possesses more pore volume/trapped air. Therefore, the surface with a relatively large pore volume showed significantly improved icing-resistance with ease of deicing effects.

4. CONCLUSION In summary, a simple and scalable approach to alter adhesion properties of superhydrophobic surfaces has been reported using dual-scale roughness of “TiO2 (P25)/Al2O3” modified 8400


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with PDMS at different concentrations. Our results revealed that the various adhesion effects arise from the corresponding change in surface roughness, SSA, and pore volume in the matrix. These physical changes in localized microstructure enabled the surface to transform the contact type between water droplet and the surface from “point contact” to “area contact”, which plays a significant role for altering adhesion behaviors. In addition, it enables the surface to exhibit a controllable bouncing effect for different time length and bouncing cycles. Finally, the application of such high and low adhesive SH surfaces in icing and deicing action have been found to be affected with their physical properties. The icing delay was dominated by the surface with low adhesive state of the SH surface (lotus effect) as possessing higher pore volume and roughness height that results in low contact area between liquid and solid surface. Similarly, excellent deicing (ice-release) performance was achieved by the SH surface with low adhesion as the ice removal starts with melting down solid-ice to liquidwater accumulated at the interface. The present work suggests that the hydrophobicity with pore volume for sufficient air gap is necessary to design efficient anti-icing surface with ease of deicing for the applications in cold environments.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16444. Additional figures and information (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI) Movie S4 (AVI) Movie S5 (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dusan Losic: 0000-0002-1930-072X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the support of the Australian Research Council (Grant IH 150100003 ARC Research Hub for Graphene Enabled Industry Transformation) and The University of Adelaide for this work.

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REFERENCES

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