Scholarly article on topic 'Novel Fabrication of Un-coated Super-hydrophobic Aluminum via Pulsed Electrochemical Surface Modification'

Novel Fabrication of Un-coated Super-hydrophobic Aluminum via Pulsed Electrochemical Surface Modification Academic research paper on "Nano-technology"

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Abstract of research paper on Nano-technology, author of scientific article — Abishek B. Kamaraj, Vincent Shaw, Murali M. Sundaram

Abstract Super-hydrophobic and super-hydrophilic aluminum (Al) surfaces were fabricated via electrochemical surface modification (ECSM) in neutral NaClO3 electrolyte without the addition of secondary chemical coatings. The effects of processing time and applied potential on the surface roughness and wettability were studied. The aluminum surface was characterized using stylus profilometer and scanning electron microscope (SEM). Wettability was evaluated using Sessile Drop Test and a high resolution camera. Results show that surfaces obtained hierarchical rough features and super-hydrophilic behavior after pulse electrochemical machining. Heat treatment at 200°C transitioned the substrates to exhibit super-hydrophobic behavior due to the removal of all moisture from within the micro- and nano-meter scale features on the aluminum surfaces, allowing for the reformation of a natural passivation (oxide) layer with atmospheric interaction. The method proposed in this study for producing super-hydrophobic aluminum surfaces does not require the use of acid or base etching or chemical coatings, such as flouroalkylsilane (FAS). Experimental results reveal increase in contact angle, with increase in applied potential, and decrease in sliding angle.

Academic research paper on topic "Novel Fabrication of Un-coated Super-hydrophobic Aluminum via Pulsed Electrochemical Surface Modification"

Procedia Manufacturing

Volume 1, 2015, Pages 892-903

43rd Proceedings of the North American Manufacturing Research Institution of SME http://www.sme.org/namrc

Novel Fabrication of Un-Coated Super-Hydrophobic Aluminum via Pulsed Electrochemical Surface

Modification

Abishek B. Kamaraj, Vincent Shaw, and Murali M. Sundaram

Department ofMechanical and Materials Engineering, University of Cincinnati, Cincinnati, USA

murali.sundaram@uc.edu

Abstract

Super-hydrophobic and super-hydrophilic aluminum (Al) surfaces were fabricated via electrochemical surface modification (ECSM) in neutral NaClO3 electrolyte without the addition of secondary chemical coatings. The effects of processing time and applied potential on the surface roughness and wettability were studied. The aluminum surface was characterized using stylus profilometer and scanning electron microscope (SEM). Wettability was evaluated using Sessile Drop Test and a high resolution camera. Results show that surfaces obtained hierarchical rough features and super-hydrophilic behavior after pulse electrochemical machining. Heat treatment at 200°C transitioned the substrates to exhibit super-hydrophobic behavior due to the removal of all moisture from within the micro- and nano- meter scale features on the aluminum surfaces, allowing for the reformation of a natural passivation (oxide) layer with atmospheric interaction. The method proposed in this study for producing super-hydrophobic aluminum surfaces does not require the use of acid or base etching or chemical coatings, such as flouroalkylsilane (FAS). Experimental results reveal increase in contact angle, with increase in applied potential, and decrease in sliding angle.

Keywords: Superhydrophobic Aluminum, Pulse Electrochemical Surface Modification, Wettability Control

1 Introduction

Aluminum and its alloys have vast applications in all forms of industry, including the aerospace, automotive, aviation, machinery manufacture, and general manufacturing fields. This is due to the material's relative abundance, low density, corrosion resistance, and high strength-to-weight ratio (Davis, 1993). While these properties alone make aluminum a premier manufacturing material, the addition of super-hydrophilic and super-hydrophobic behavior to aluminum surfaces extends the benefits of aluminum to new realms.

Many plants, animals, and insects possess the qualities necessary for producing any degree of wetting ranging from the super-hydrophilic to the super-hydrophobic (Shu et al. , 2011). Of all these

2351-9789 © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the NAMRI Scientific Committee

doi:10.1016/j.promfg.2015.09.081

organisms, the most prominent is the lotus leaf's self-cleaning properties that allow it to remain free of dirt and debris at all times. The discovery of the lotus effect (Barthlott and Neinhuis, 1997) has triggered great interest from researchers due to the application of super-hydrophobic surfaces in producing self-cleaning (Fürstner et al. , 2005), anti-wetting (Srinivasan et al. , 2013), anti-icing (Cao et al. , 2009), anti-corrosion (Xu and Wang, 2009), friction reducing (Choi and Kim, 2006), and antimicrobial (Wu et al. , 2012) surfaces.

There have been many attempts to fabricate aluminum that mimic the lotus' super-hydrophobic behavior. Past success in producing super-hydrophobic aluminum surfaces involve the fabrication of rough surfaces via anodic oxidation, chemical etching, anodizing, and electrochemical deposition followed by a chemical treatment (Darmanin et al. , 2013, Liu et al. , 2013, Liu et al. , 2014, Matsumura et al. , 2012, Song et al. , 2012a, Song et al. , 2013, Xie and Li, 2011). All of the processes stated above need a secondary coating which is toxic (Xie and Li, 2011). Chemical etching uses strong chemicals such as acids or bases which are pollutants (Saleema et al. , 2010). The secondary coating used has a tendency to wear off which limits the long term usability of the material (Arieta and Gawne, 1995).

Electrochemical machining has become a prominent mechanism for producing the rough structures required for super-hydrophobic surfaces. However, all previous attempts to produce super-hydrophobic surfaces via electrochemical machining rely on the use of chemical agents, such as flouroalkylsilane (FAS), to form a surface energy reducing film on the rough surfaces and transform the surface from super-hydrophilic to super-hydrophobic. Chemical etching of aluminum by NaOH, followed by the application of FAS was shown to produce super-hydrophobic surfaces (Saleema, Sarkar, 2010). Another method used to produce super-hydrophobic aluminum surface is by electrochemical machining with NaCl electrolyte containing FAS (Song et al. , 2012b). Simultaneous etching by NaOH in a solution containing FAS was also reported by Bermagozzi et al., 2014, to produce low wetting aluminum surfaces (Bernagozzi et al. , 2014).

The electrochemical process outlined in this paper separates itself in that the addition of a secondary chemical treatment/coating is not required to produce super-hydrophobic aluminum surfaces after electrochemical machining. Secondary chemical treatments are costly, toxic to environment and operators, and slow the manufacturing process (Xie and Li, 2011). Additionally it must be reapplied over time or in the event of physical damage that may cause the film to fail. The secondary coatings that are applied are also sensitive to moisture leading to their deteriorating performance over time (Ulman, 1996). Manufacture of super-hydrophobic aluminum surfaces without the need for chemical films will increase production efficiency while reducing cost and risk during production and during the use of the produced surface in various applications.

In this paper, the effects of ECSM processing parameters on the roughness and wettability of aluminum substrates were studied.

2 Literature Review

Wettability is characterized by the apparent contact angle (CA) of a droplet on a surface. The contact angle is defined as the angle between the solid-liquid and liquid-air interface, as illustrated in Figure 1. Young's model is used to describe the contact angle (0) for the physically and chemically homogenous surface:

cos 9 =

CYsg - Y si)

where ysg, Ysi, and ylg are the surface tensions of the solid-gas, solid-liquid, and liquid-gas interfaces, respectively (Ashokkumar et al. , 2012). However, this model fails to account for the roughness of an actual solid surface and its effect on wettability. These issues were addressed in the Wenzel model

cos 9W = r cos0

where r is the roughness ratio between the real and projected solid-liquid contact area, and 9W and 0 are the contact angles for the rough and smooth surfaces, respectively (Wenzel, 1936). For hydrophobic surfaces, contact angles are greater than 90° and become super-hydrophobic when contact angles exceed 150°(Yan et al. , 2011). A surface is hydrophilic for contact angles are less than 90° and become super-hydrophilic when contact angles fall below 10° (Drelich et al. , 2011). In the Wenzel model, a droplet placed on a rough surface will penetrate the rough grooves of the surface and exhibit a high-adhesive force between the liquid and solid surface (Figure 2) (Wenzel, 1949).

A ^^^Droplet •1 r„

Aluminum

Figure 1 Droplet contact angle is determined by measuring the angle between the solid-liquid (ySL) and

liquid-gas (yLG) interfaces.

Wenzel Model

Cassie-Baxter Model

Figure 2 The Wenzel model is used to describe a surface in which droplets penetrate the rough structures leading to high adhesive forces. The Cassie-Baxter Model describes a rough surface that is non penetrable by liquid droplet. Instead, droplets rest on top of the rough structures of the surface, trapping air beneath them. To produce a Cassie-Baxter effect, smaller scale features are needed on top of larger

scale features.

To explain the low-adhesive super-hydrophobic surface, the Wenzel model is replaced by the Cassie-Baxter model (Figure 2). In the Cassie-Baxter model, droplets placed on the surface do not penetrate into the rough grooves. Instead, the droplet will rest on top of these grooves, trapping air in the spaces beneath. When the liquid-gas contact area is larger than the liquid-solid contact area, the droplet contacts a smaller surface area of the surface and will roll off if slightly tilted. The Cassie-Baxter model is described via the following equation:

cos 8C = f1 cos 9 — /2

where 9C and 0 represent the contact angles on the rough and smooth surfaces, respectively, and and/2are the area fractions of the solid and air on the surface, respectively (Zhao et al. , 2013).

Though super-hydrophobic surfaces have been fabricated via anodizing (Liu, Luo, 2013), chemical etching (Xie and Li, 2011), laser processing (Liu, Liu, 2014), electrochemical machining (Darmanin, Taffin de Givenchy, 2013, Song, Xu, 2013), stamping (Matsumura, Iida, 2012), and deposition (Song, Xu, 2012a), in-depth literature review has failed to yield a process that involves the production of a purely aluminum super-hydrophobic surface without any addition of secondary surface coatings.. Some of the secondary surface coatings used on metal surfaces include chemical: examples include flouroalkylsilane (FAS) , polypropylene (PP) (Liu, Luo, 2013), polystyrene (PS) (Xu and Wang, 2009), dodecane-1-thiol (Dong et al. , 2011) and other organic reagents (Kang et al. , 2009), and non-chemical: deposition, in which additional chemical coating are then applied to the deposited structures (Song, Xu, 2012a).

As stated in the introduction, secondary chemical treatments are costly, toxic to the environment and operators, and slow the manufacturing process (Xie and Li, 2011). Additionally, since the chemical treatment must be applied, it must be reapplied over time or in the event of physical damage that may cause the film to fail. Coated super-hydrophobic aluminum surfaces have been shown to deteriorate in instances of anti-icing (Farhadi et al. , 2011) and physical abrasion (Arieta and Gawne, 1995). Development of super-hydrophobic aluminum surfaces that do not require additional coatings will remove all ill health, economical, and degenerative effects associated with modifying aluminum substrates with secondary substances. While pure, uncoated, super-hydrophobic metal surfaces maintain the functional properties of the initial metal surface, coated super-hydrophobic metal surfaces are reduced to operate within the functionality of the coating.

3 Experiments

3.1 Materials and Sample Preparation

Commercially available aluminum (purity > 99%) and brass plates were used as work pieces and cathodes, respectively. Untreated 25x12.5x2 mm3 aluminum plates were polished using 1500# SiC paper and ultrasonically cleaning in deionized water for 1.5 minutes and 6 minutes, respectively. After drying the aluminum plates with compressed air, the anodic aluminum and a cathodic brass plate were separated by a distance of 4 mm and electrochemically machined in 0.2 M aqueous NaClO3 solution with a constant 5 V potential difference for 250 seconds to 1000 seconds at room temperature. After machining, samples were ultrasonically cleaned for 6 minutes and dried with compressed air. Samples were then heated at 200° C to remove any remaining water from the aluminum surfaces and to allow the aluminum to reform a passivation layer, as direct atmospheric interaction is able to occur on the dried surface (Badawy et al. , 1999). An experimental schematic can be found in Figure 3.

Pulse Power Supply

Anode (Aluminum)

Electrolyte

Cathode (Brass)

Electrolytic Tank

Figure 3 ECSM Experimental setup with aluminum anode (work-piece) and brass cathode (tool).

3.2 Measurements

Surface morphology was analyzed at each step of the ECSM process using a scanning electron microscope (SEM, XL 30-ESEM D1398). Surface roughness Ra was measured using a profilometer (Mitutoyo Surftest SJ-410). The water droplet contact angle was measured according to the sessile drop test using a high speed camera (Olympus i-Speed 2) with angle measurement software. Images were taken at 60 frames per second. The average contact angle was determined by taking the average of three 5 ^l drops at different locations on the sample.

4 Results And Discussion

4.1 Processing Time

The effects of electrochemical machining time (processing time) were studied by varying the time in which aluminum samples were electrochemically modified while keeping all other conditions constant. Experimental process parameters can be found in Table 1. It was observed that as ECSM processing time increased, the roughness also increased, as shown in Figure 4 (a). After ECSM and prior to heat treatment droplets on the aluminum surfaces behaved according to the Wenzel model and demonstrated increased hydrophilicity as roughness was increased through longer processing times as shown in Figure 4 (b). However, after heat treatment at 200° C for 2 hours, droplets placed on the electrochemically modified surfaces behaved according the Cassie-Baxter model and transitioned to a super-hydrophobic state in Figure 4 (b).

Table 1 Processing Time Experimental Parameters Parameter Value

Processing Time

NaClO3 Concentration

Gap Size

Potential

Full Duty Cycle

Pulse (on/off) Time

250 - 1000 seconds 0.2 M 4 mm 10 V 50%

0.5 ms/0.5 ms

-After ECSM

-After ECSM and Heat Treat

™ eco kb loco ECSM Processing Time (s)

200 400 S00 800 1000 1200 ECSM Processing Time (s)

Figure 4 Effect of processing time on (a) the roughness ofthe surface (b) the contact angles immediately after ECSM and after the heat treatment process

Figure 5 shows the photographs of the water droplets on untreated aluminum surfaces, polished aluminum surfaces, super-hydrophilic aluminum surfaces treated via ECSM prior to heat treatment, and super-hydrophobic aluminum surfaces treated via ECSM after heat treatment. It can be observed that untreated aluminum, while hydrophilic(0 = 80°,Ra = 0.283 pm), can be made more hydrophilic (0 = 56°,Ra = 0.169 jum) by smoothing the surface via polishing. Roughness profiles for each step of the process can be found in Figure 6.

(Super-Hydrophilic) (Super-Hydrophobic)

Ra = 0.283 Ra = 0.169 Ra = 16.922 Ra = 16.922

Figure 5 Roughness and wettability of an aluminum substrate throughout treatment. a) shows the wettability and roughness ofthe aluminum substrates prior to any processing. The wettability is increased in b) by reducing roughness through polishing with 1500 grit sandpaper. After ECSM c), an aluminum substrate exhibits super-hydrophilic behavior. Upon heating the aluminum substrates d), wettability transitions from super-hydrophilic to super-hydrophobic.

a) Evaluation Profile: Untreated

,!2Ja Ra = 0.2S0 jim

O 0 0 OS

tO 14 2* 2.» 10 »ft *9

Drive Distance [mm]

_ b> Evaluation Profile: Sanded (1500)

E " I------------

•M»«

q> o»

• S-0* Ra =0085 um

00 0» t

1.» 10 II SO SS 4 0

Drive Distance [mm] c) Evaluation Profile: After ECSM

V>M0 5-o

(1 . il [\ 1) , 1

iv M, \ll f\ 1l«A J\ A i v

111 \ 1 un « J J f w V ^ V ** V 11

Ra > 16.130 ym

Drive Distance [mm]

Figure 6 Roughness profiles for a) untreated, b) polished, and c) ECSM aluminum substrates are

shown.

After ECSM and before heating, aluminum surfaces exhibit super-hydrophilicity (0 = 0°,Ra = 16.922 jum). A droplet placed on the roughened sample quickly spreads through a large portion of the machined surface. The behavior observed at this stage of the machining process depicts the transition from the Wenzel model to the surface model in which the aluminum sample can be considered to be absorbent. SEM images of the modified aluminum surfaces at different processing times are presented in Figure 7. At lower processing times, the aluminum substrates have not been sufficiently modified

and maintain portions of the polished aluminum surface (B,C). Uniform roughness is necessary to develop a consistently super-hydrophobic surface.

Figure 7 SEM images of aluminum surfaces at varying processing times are presented. 0 seconds at 1500X and 5000X (A,a) respectively, 250 seconds at 1500X and 5000X (B,b) respectively, 500 seconds at 1500X and 5000X (C,c) respectively, 750 seconds at 1500X and 5000X (D,d) respectively, and 1000 seconds at 1500X and 5000X (E,e) respectively, are shown. (Left Bars = 20 ^m; Right Bars = 5 ^m)

4.2 Electric Potential

The effects of electric potential on the fabrication of super-hydrophobic aluminum surfaces were studied by varying the magnitude at which aluminum surfaces were machined. Experimental processing parameters can be found in Table 2. It was observed that as the machining potential increased, roughness was also increased (Figure 8). As theory suggests, with the rougher surfaces produced at higher potentials, the wettability was observed to decrease (Figure 9). Constant pulsed potentials allowed the current to increase as the aluminum surface was machined. With the increase in current the depth of the rough pits increased leading to a rougher surface overall.

Table 2 Constant Potential Experimental Parameters Parameters Value

Processing Time NaClO3 Concentration Gap Size Potential Full Duty Cycle Pulse (on/off) Time

1000 seconds 0.2 M 4 mm 2 - 10 V 50%

0.5 ms/0.5 ms

2 3 4 5 6

Applied Potential Difference (V) Figure 8 Roughness is shown to increase with higher potentials.

о 110

12 3 4 6 6

Applied Potential Difference (V)

Figure 9 Contact angle is shown to increase with higher potentials

4.3 Droplet Sliding Angle

Droplet sliding angle was measured by placing 5 ^l droplets on the aluminum surfaces and gradually tilting these surfaces. The tilting angle was recorded at the point at which the droplet begins to move from its initial resting position down the inclined surface. The sliding behavior was observed to improve as contact angle increased (Figure 10).

Contact Angle (cteg)

Figure 10 Water sliding angles were observed to decrease as contact angle increased.

4.4 Discussion

The super-hydrophilic behavior of the aluminum substrates after the machining process can be attributed to two main sources. (1) Small pockets of water (moisture) remain entrenched in the micro-and nano- meter scale pits of the aluminum surface and on contact with a droplet, these small pockets of water draw the droplet into the surface resulting in contact angles 9 = 0°(Figure 11). (2) After machining, the natural passivation layer that forms a corrosion resistant film on aluminum surfaces is removed and is unable to re-form until the aluminum surface is able to interact completely with the surrounding oxygen in the air. These issues were overcome by heating samples that have undergone ECSM at 200° C for 2 hours. Heating of aluminum in air has been shown to form a passivation layer (Shih and Liu, 2006). Allowing the reformation of a natural passivation layer not only protects the aluminum surface from further corrosion, but also adds a naturally reforming atomic-scale layer of roughness to the manufacture micro- and nano- scale structures (Lee et al., 2009).

...........I I I I I I I I I I I I I

Aluminum

Figure 11 Small pockets of liquid remaining in the rough structures of the aluminum substrate pull droplets into the surface causing a super-hydrophilic state.

Figure 12 Once dried, the roughened aluminum surface is able to reform a layer of passivation as direct interaction is possible between the surface and atmosphere.

Though measured Rvalues of the aluminum surfaces before and after heat treating the samples experience no change, the droplet contact angle transitions from super-hydrophilic (0 = 0°), to super-hydrophobic (0 > 150°). It should be noted that once the moisture is removed from the surface, and the Cassie-Baxter model comes into effect, the air trapped in the surface that provides low droplet adhesive forces is stable in that it is not easily re-wettable. Even when submerged in water for extended periods of time (1 hour), the surfaces remain dry. Produced surface only experience rewetting as water penetrates less rough areas. However, even then, the advance of the wetted area is slow and eventually halts. Areas subject to allow penetration due to lesser roughness are on the sides of the samples. It is proposed that the use of tools that wrap completely around the workpiece would provide the high roughness necessary for super-hydrophobic behavior on all surfaces of the workpiece. Once all surfaces of the workpiece exhibit the necessary levels of roughness to produce super-hydrophobic behavior, the metal would be free from any areas of lesser roughness that can cause the super-hydrophobic surface to fail.

5 Conclusions

Super-hydrophobic aluminum surface are fabricated without the use of secondary chemical coatings. Constant-potential pulsed conditions were found to be preferable to pulsed constant-current conditions for producing the high roughness required for super-hydrophobic surfaces. Drying the modified samples at 200° C for 2 hours was sufficient for transforming samples from super -hydrophilic to super-hydrophobic with contact angle greater than 150° due to the transition of the substrate to exhibit the Cassie-Baxter state. Contact angle was also found to increases with increase in applied potential and decrease in sliding angle. The formation of passivation layer on the aluminum after oxidation at high temperature is believed to be the mechanism behind the observed behavior. Further study considering the process parameters such as electrolyte concentration, pulse parameters, and inter-electrode gap would give better insight into the process.

Acknowledgements

Financial support provided by the National Science Foundation under Grant No CMMI-1400800 is acknowledged. Vincent and Abishek thank the GSUM/SUMR-UC program at the University of Cincinnati for financial support.

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