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Engineering nanoscale roughness on hydrophobic surfacepreliminary assessment of fouling behaviour

Engineering nanoscale roughness on hydrophobic surfacepreliminary assessment of fouling behaviour A preliminary investigation of the fouling behaviour of smooth and roughened superhydrophobic coatings is reported. The effect of nanoscale interfacial roughness on the adhesion of single (SW8) and mixed cultures of micro-foulant for periods of up to 6 months was assessed using visual and wettability measurements. Detailed analysis indicated virtually no micro-organism attached to the superhydrophobic surfaces in the first weeks of immersion. As a result by comparison with smooth substrates, which exhibited fouling within a day, very rough (roughness ratio O2.7) surfaces exhibited high resistance to fouling over a 6-month period. However, after periods exceeding 2 months under ocean conditions, both films showed limited anti-fouling properties. There appears to be a correlation between the nature of the nanoscale roughness in the creation of superhydrophobic coatings and their potential anti-fouling properties. The future architecture of such a correlation is investigated. q 2005 Elsevier Ltd. All rights reserved. Keywords: Anti-fouling; Superhydrophobic; Contact angle; Surface roughness 1. Introduction non-stick surfaces. Recent advances in paint and surface technology has produced a range of superhydrophobic The control of marine biofouling is of considerable coatings that have water contact angle of greater than 1508 [7,8]. The aim of this work is to investigate their potential interest both technically and economically [1–3]. Tra- to be applied as anti-foulants. ditionally anti-fouling paints and coatings are designed Anti-fouling coatings act through a combination of with the biological nature of ‘pest’ in mind and typically biological (poison) deterrents and chemical (non-stick) are formulated to continuously release poisons, mostly incompatibility. In the present work, the latter aspect is based on heavy metals such as tin or copper [2,4]. explored. The ‘chemical design’ of such interfaces can be Although successful in providing effective long-term anti- modified through the adjustment of micro- and nano- fouling performance, the anti-fouling biocides released structure. This allows naturally hydrophobic surfaces from the paints are now recognised to cause adverse [contact angle (CA) !90] to become superhydrophobic impacts in the marine environment and are increasingly (CAO150). Evaluation of this change to the fouling posing risks to human health. As a result, biocides properties of these materials examined. containing organotin have recently been reported [5], and copper-based anti-fouling paints are now under scrutiny [6]. 2. Experiment These concerns have lead to a search for alternatives. One approach is to remove poisons altogether and focus on Superhydrophobic material was made from fumed silica (primary particle size around 50 nm), alkyltrialkoxysilane * Corresponding author. Tel.: C61 2 93854678; fax: C61 2 96621697. and polysiloxane. A detailed description of the preparation E-mail address: [email protected] (H. Zhang). method was described elsewhere [7,8]. Three superhydro- phobic films having variations on chemistry and surface 1468-6996/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.stam.2005.03.003 geometry were prepared. The chemical, surface structural H. Zhang et al. / Science and Technology of Advanced Materials 6 (2005) 236–239 237 Table 1 Surfaces for fouling attachment tests polysiloxane smooth Chemical composition Roughness Contact angle surface polysiloxane rough ratio (degrees) surface PTFE moderate rough Polysiloxane 1.1 75 surface" PTFE particles 1.3 150 Polysiloxane and fine particles 2.7 169 1 day 6 days 21 days 6 months features, and water resistance of the film together with the standard polytetrafluoroethylene (PTFE) particle treated Fig. 1. Wetting behaviour after immersion. surface are summarised in Table 1. Thin film samples were prepared by dip coating glass microscope slides. the control of fouling as described in the following The roughness of these films was characterised using sections. atomic force microscope (Dimension-3000 AFM). Tapping Previous studies [11] have shown that when super- mode was selected during analysis at an analytical range of hydrophobic surfaces are immersed, air bubbles are 5 mm. The resulting roughness ratio r is calculated by entrapped into micro- and nano-sized pores at the solid dividing surface area by projected surface area [9]. surface. A mix of solid–liquid and solid–gas interfaces is Macroscopic fouling behaviour assessed through short- created. The extent of solid–gas interface is proportional to and long-term fouling attachment tests described are below. the degree of hydrophobicity of the material. The higher the An Immersion test investigated the correlation between hydrophobicity, the larger the solid–gas interface will be. fouling and the change of wettability on surfaces after The air bubble layer creates a barrier that may prevent submerging in tap water for 1 day, 6 days, 21 days and 6 adsorption of micro-organisms in the short-term. In the months. Surfaces were exposed to mixed cultures of early stage of the immersion, air bubbles occupy most of the naturally occurring micro-foulant in mains water. Contact surface. With increasing immersion time, organisms dis- angle was used as the quantitative indicator of changing place the air. The net effect is the creation of a new solid– surface hydrophobicity. liquid interface to replace the original solid–gas interface. A Static test further examined surfaces remaining The key is the design of surfaces that effectively ‘hold’ hydrophobic after 1 day immersion. These were exposed the air in place. to a sole culture SW8 in artificial seawater for 10 min. The amount of air bubbles attracted on hydrophobic The number of attached bacteria was determined using surface depends on surface roughness. The rougher the an epifluoroscene microscope. This provides a rapid surface, large percentage of solid–gas interface is created. assessment of the likelihood of long-term fouling However, the resistance of that layer to micro-organisms properties. will depend on the design of the air adsorption sites. In Field-trial experiments long-term through immersion Loss of superhydrophobicity occurs in a variety of ways. in seawater was tested for 2 months [10]. Observation of One way is that air may be dissolved in water for a fouling attachment was carried out after 1 and 2 months. long-term period submersion. More importantly, it occurs when the chemistry or structure of surface is changed due to 3. Results and discussion 3.1. Anti-fouling and the roughness of a hydrophobic surface Non-wet area after immersion for a certain period of time has been estimated and the results summarised in Fig. 1. Contact angles were also measured before and after the immersion test and the results are shown in Fig. 2. 01 0.5 1.5 2 2.5 3 It was found that contact angles of surfaces with r!2 decreased drastically with increasing immersion time. The Roughness Ratio decrease of contact angle is an indicator of loss of both Before immersion test After 21 days immersion hydrophobicity and roughness. Hydrophobic surfaces with After 1 day immersion rw2.7 suggests micro- and nano-scale roughness (Fig. 4). Fig. 2. Change on contact angles before and after the immersion test. This multi-level roughness has the direct contribution to Contact Angle (degrees) Wet area percentage (%) 238 H. Zhang et al. / Science and Technology of Advanced Materials 6 (2005) 236–239 delivery of the micro-organism is largely reduced due to the attraction of air bubbles. Such effects lead to a persistence in anti-fouling performance over short periods of time. This result is consistent with that obtained from the immersion test. The conditioning film which precedes full scale coloni- sation usually contains macromolecules and proteins, which change the overall surface chemistry. Fig. 3. Static test: micro-organism attachment on hydrophobic and 3.3. Long-term anti-fouling properties of superhydrophobic superhydrophobic surfaces. surfaces the attachment of a conditioning layer of macromolecules. After 1 month of immersion, coatings with nanoscale A simple indicator of cleanliness in this instance is the non- rough polysiloxane surfaces accumulate only a small wetting property of a surface. coverage of green algae (!5% surface cover) and few barnacles (!2% cover) (Fig. 5). Closer inspection shows 3.2. Short-term anti-fouling properties of superhydrophobic signs of fish grazing, particularly near the edges of the test surfaces surface. The observation that fish appeared attracted to the test The static test provides a direct method to determine the surfaces is of interest. It is assumed that the attraction is amount of bacteria attached to the surface under standard initiated by the attached nutrients, and the subsequent lack of conditions. The photos shown in Fig. 3 were taken under adhesion of the micro-organism layer to the coating makes it epifluoroscene microscope. Quantification of the number of easer to remove the ‘food’. It suggests that the contamination the micro-organism attached on surface leads to the has weak interaction with the coating surface. This secondary comparison between a hydrophobic and a superhydrophobic mechanism may also explain the reduced barnacle numbers surface. on the test surfaces with natural cleaning through fish grazing It has been found that, in a 10 min period there is only rather than first stage adsorption. one visible bacterium in view on rough polysiloxane It was also noted that coatings with smooth polysiloxane surfaces while on smooth polysiloxane surface considerably surface had many more barnacles, covering an estimated more have been attracted. 10% of the surface, but almost no algae (Fig. 5). Untreated Rough polysiloxane surfaces therefore have a better control panels immersed during this same period were anti-fouling performance due to the combination of its colonized by macroalgae (z50%), tubeworms (O50%), unique geometrical feature on surface and chemical origin bryozoans (z5%), barnacles (z50%) and ascidians of low surface energy material. It was shown under AFM (z20%). (Fig. 4) that such a surface presents extreme roughness After 2 months, surfaces of both samples were more rw2.7 which is much larger than that of a smooth surface. heavily fouled by macroalgae (10–20%), barnacles (5–10%), The actual surface in contact with water, and the area for Fig. 4. AFM of a rough polysiloxane surface. Fig. 5. Field-trial (after 1 months) results. H. Zhang et al. / Science and Technology of Advanced Materials 6 (2005) 236–239 239 and bryozoans (50–60%). The net barnacle population on Acknowledgements test surfaces remained less than that on the inert control plates. The results indicate that the superhydrophobic The present research is funded by Faculty Research coatings did influence macrofouling colonization but their Grants Program (FRGP 2004) of University of New South application for long-term anti-fouling needs further Wales, Sydney, Australia. investigation. References [1] D.P. Edwards, T.G. Nevell, B.A. Plunkett, B.C. Ochiltree, Int. 4. Conclusions Biodeterior. Biodegrad. 34 (1994) 349. [2] J.A. Lewis, Mater. Forum 22 (1998) 41. Superhydrophobic surfaces delay fouling occurrence. [3] M.J. Cowling, T. Hodgkiess, A.C.S. Parr, M.J. Smith, S.J. Marrs, Sci. For a short period exposure to fouling environments, Total Environ. 258 (2000) 129. superhydrophobic surface showed excellent fouling repel- [4] A. Milne, G. Hails, Paint composition for ships’ hull, DE 2514574, Ger. Offen., 1975. lency while the hydrophobic film was stained by a large [5] IMO, Anti-Fouling Systems: International Convention on the Control amount of micro-organisms. However, after a long period of Harmful Anti-Fouling Systems on Ships, 2001. exposure to seawater, both films lose anti-fouling properties. [6] K. Schiff, D. Diehl, A. Valkirs, Mar. Pollut. Bull. 48 (2004) 371. This is clearly associated with the strength of air bubble [7] R. Lamb, H. Zhang, C. Raston, Hydrophobic coatings, PCT Int. Appl. (1998) WO 9842452. adsorption and further studies on the geometry of bubble [8] H. Zhang, R. Lamb, A. Jones, Durable superhydrophobic coating, capture are required. PCT Int. Appl. (2004) WO 2004090065. Nevertheless superhydrophobic coatings show promise [9] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988. for short-term anti-fouling application with no detrimental [10] In Port Phillip Bay, Vic. Australia side effects that are common in poison based formulations. [11] P. Attard, Langmuir 16 (2000) 4455. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Science and Technology of Advanced Materials IOP Publishing

Engineering nanoscale roughness on hydrophobic surfacepreliminary assessment of fouling behaviour

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Copyright
Copyright 2005 Elsevier Ltd
ISSN
1468-6996
eISSN
1878-5514
DOI
10.1016/j.stam.2005.03.003
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Abstract

A preliminary investigation of the fouling behaviour of smooth and roughened superhydrophobic coatings is reported. The effect of nanoscale interfacial roughness on the adhesion of single (SW8) and mixed cultures of micro-foulant for periods of up to 6 months was assessed using visual and wettability measurements. Detailed analysis indicated virtually no micro-organism attached to the superhydrophobic surfaces in the first weeks of immersion. As a result by comparison with smooth substrates, which exhibited fouling within a day, very rough (roughness ratio O2.7) surfaces exhibited high resistance to fouling over a 6-month period. However, after periods exceeding 2 months under ocean conditions, both films showed limited anti-fouling properties. There appears to be a correlation between the nature of the nanoscale roughness in the creation of superhydrophobic coatings and their potential anti-fouling properties. The future architecture of such a correlation is investigated. q 2005 Elsevier Ltd. All rights reserved. Keywords: Anti-fouling; Superhydrophobic; Contact angle; Surface roughness 1. Introduction non-stick surfaces. Recent advances in paint and surface technology has produced a range of superhydrophobic The control of marine biofouling is of considerable coatings that have water contact angle of greater than 1508 [7,8]. The aim of this work is to investigate their potential interest both technically and economically [1–3]. Tra- to be applied as anti-foulants. ditionally anti-fouling paints and coatings are designed Anti-fouling coatings act through a combination of with the biological nature of ‘pest’ in mind and typically biological (poison) deterrents and chemical (non-stick) are formulated to continuously release poisons, mostly incompatibility. In the present work, the latter aspect is based on heavy metals such as tin or copper [2,4]. explored. The ‘chemical design’ of such interfaces can be Although successful in providing effective long-term anti- modified through the adjustment of micro- and nano- fouling performance, the anti-fouling biocides released structure. This allows naturally hydrophobic surfaces from the paints are now recognised to cause adverse [contact angle (CA) !90] to become superhydrophobic impacts in the marine environment and are increasingly (CAO150). Evaluation of this change to the fouling posing risks to human health. As a result, biocides properties of these materials examined. containing organotin have recently been reported [5], and copper-based anti-fouling paints are now under scrutiny [6]. 2. Experiment These concerns have lead to a search for alternatives. One approach is to remove poisons altogether and focus on Superhydrophobic material was made from fumed silica (primary particle size around 50 nm), alkyltrialkoxysilane * Corresponding author. Tel.: C61 2 93854678; fax: C61 2 96621697. and polysiloxane. A detailed description of the preparation E-mail address: [email protected] (H. Zhang). method was described elsewhere [7,8]. Three superhydro- phobic films having variations on chemistry and surface 1468-6996/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.stam.2005.03.003 geometry were prepared. The chemical, surface structural H. Zhang et al. / Science and Technology of Advanced Materials 6 (2005) 236–239 237 Table 1 Surfaces for fouling attachment tests polysiloxane smooth Chemical composition Roughness Contact angle surface polysiloxane rough ratio (degrees) surface PTFE moderate rough Polysiloxane 1.1 75 surface" PTFE particles 1.3 150 Polysiloxane and fine particles 2.7 169 1 day 6 days 21 days 6 months features, and water resistance of the film together with the standard polytetrafluoroethylene (PTFE) particle treated Fig. 1. Wetting behaviour after immersion. surface are summarised in Table 1. Thin film samples were prepared by dip coating glass microscope slides. the control of fouling as described in the following The roughness of these films was characterised using sections. atomic force microscope (Dimension-3000 AFM). Tapping Previous studies [11] have shown that when super- mode was selected during analysis at an analytical range of hydrophobic surfaces are immersed, air bubbles are 5 mm. The resulting roughness ratio r is calculated by entrapped into micro- and nano-sized pores at the solid dividing surface area by projected surface area [9]. surface. A mix of solid–liquid and solid–gas interfaces is Macroscopic fouling behaviour assessed through short- created. The extent of solid–gas interface is proportional to and long-term fouling attachment tests described are below. the degree of hydrophobicity of the material. The higher the An Immersion test investigated the correlation between hydrophobicity, the larger the solid–gas interface will be. fouling and the change of wettability on surfaces after The air bubble layer creates a barrier that may prevent submerging in tap water for 1 day, 6 days, 21 days and 6 adsorption of micro-organisms in the short-term. In the months. Surfaces were exposed to mixed cultures of early stage of the immersion, air bubbles occupy most of the naturally occurring micro-foulant in mains water. Contact surface. With increasing immersion time, organisms dis- angle was used as the quantitative indicator of changing place the air. The net effect is the creation of a new solid– surface hydrophobicity. liquid interface to replace the original solid–gas interface. A Static test further examined surfaces remaining The key is the design of surfaces that effectively ‘hold’ hydrophobic after 1 day immersion. These were exposed the air in place. to a sole culture SW8 in artificial seawater for 10 min. The amount of air bubbles attracted on hydrophobic The number of attached bacteria was determined using surface depends on surface roughness. The rougher the an epifluoroscene microscope. This provides a rapid surface, large percentage of solid–gas interface is created. assessment of the likelihood of long-term fouling However, the resistance of that layer to micro-organisms properties. will depend on the design of the air adsorption sites. In Field-trial experiments long-term through immersion Loss of superhydrophobicity occurs in a variety of ways. in seawater was tested for 2 months [10]. Observation of One way is that air may be dissolved in water for a fouling attachment was carried out after 1 and 2 months. long-term period submersion. More importantly, it occurs when the chemistry or structure of surface is changed due to 3. Results and discussion 3.1. Anti-fouling and the roughness of a hydrophobic surface Non-wet area after immersion for a certain period of time has been estimated and the results summarised in Fig. 1. Contact angles were also measured before and after the immersion test and the results are shown in Fig. 2. 01 0.5 1.5 2 2.5 3 It was found that contact angles of surfaces with r!2 decreased drastically with increasing immersion time. The Roughness Ratio decrease of contact angle is an indicator of loss of both Before immersion test After 21 days immersion hydrophobicity and roughness. Hydrophobic surfaces with After 1 day immersion rw2.7 suggests micro- and nano-scale roughness (Fig. 4). Fig. 2. Change on contact angles before and after the immersion test. This multi-level roughness has the direct contribution to Contact Angle (degrees) Wet area percentage (%) 238 H. Zhang et al. / Science and Technology of Advanced Materials 6 (2005) 236–239 delivery of the micro-organism is largely reduced due to the attraction of air bubbles. Such effects lead to a persistence in anti-fouling performance over short periods of time. This result is consistent with that obtained from the immersion test. The conditioning film which precedes full scale coloni- sation usually contains macromolecules and proteins, which change the overall surface chemistry. Fig. 3. Static test: micro-organism attachment on hydrophobic and 3.3. Long-term anti-fouling properties of superhydrophobic superhydrophobic surfaces. surfaces the attachment of a conditioning layer of macromolecules. After 1 month of immersion, coatings with nanoscale A simple indicator of cleanliness in this instance is the non- rough polysiloxane surfaces accumulate only a small wetting property of a surface. coverage of green algae (!5% surface cover) and few barnacles (!2% cover) (Fig. 5). Closer inspection shows 3.2. Short-term anti-fouling properties of superhydrophobic signs of fish grazing, particularly near the edges of the test surfaces surface. The observation that fish appeared attracted to the test The static test provides a direct method to determine the surfaces is of interest. It is assumed that the attraction is amount of bacteria attached to the surface under standard initiated by the attached nutrients, and the subsequent lack of conditions. The photos shown in Fig. 3 were taken under adhesion of the micro-organism layer to the coating makes it epifluoroscene microscope. Quantification of the number of easer to remove the ‘food’. It suggests that the contamination the micro-organism attached on surface leads to the has weak interaction with the coating surface. This secondary comparison between a hydrophobic and a superhydrophobic mechanism may also explain the reduced barnacle numbers surface. on the test surfaces with natural cleaning through fish grazing It has been found that, in a 10 min period there is only rather than first stage adsorption. one visible bacterium in view on rough polysiloxane It was also noted that coatings with smooth polysiloxane surfaces while on smooth polysiloxane surface considerably surface had many more barnacles, covering an estimated more have been attracted. 10% of the surface, but almost no algae (Fig. 5). Untreated Rough polysiloxane surfaces therefore have a better control panels immersed during this same period were anti-fouling performance due to the combination of its colonized by macroalgae (z50%), tubeworms (O50%), unique geometrical feature on surface and chemical origin bryozoans (z5%), barnacles (z50%) and ascidians of low surface energy material. It was shown under AFM (z20%). (Fig. 4) that such a surface presents extreme roughness After 2 months, surfaces of both samples were more rw2.7 which is much larger than that of a smooth surface. heavily fouled by macroalgae (10–20%), barnacles (5–10%), The actual surface in contact with water, and the area for Fig. 4. AFM of a rough polysiloxane surface. Fig. 5. Field-trial (after 1 months) results. H. Zhang et al. / Science and Technology of Advanced Materials 6 (2005) 236–239 239 and bryozoans (50–60%). The net barnacle population on Acknowledgements test surfaces remained less than that on the inert control plates. The results indicate that the superhydrophobic The present research is funded by Faculty Research coatings did influence macrofouling colonization but their Grants Program (FRGP 2004) of University of New South application for long-term anti-fouling needs further Wales, Sydney, Australia. investigation. References [1] D.P. Edwards, T.G. Nevell, B.A. Plunkett, B.C. Ochiltree, Int. 4. Conclusions Biodeterior. Biodegrad. 34 (1994) 349. [2] J.A. Lewis, Mater. Forum 22 (1998) 41. Superhydrophobic surfaces delay fouling occurrence. [3] M.J. Cowling, T. Hodgkiess, A.C.S. Parr, M.J. Smith, S.J. Marrs, Sci. For a short period exposure to fouling environments, Total Environ. 258 (2000) 129. superhydrophobic surface showed excellent fouling repel- [4] A. Milne, G. Hails, Paint composition for ships’ hull, DE 2514574, Ger. Offen., 1975. lency while the hydrophobic film was stained by a large [5] IMO, Anti-Fouling Systems: International Convention on the Control amount of micro-organisms. However, after a long period of Harmful Anti-Fouling Systems on Ships, 2001. exposure to seawater, both films lose anti-fouling properties. [6] K. Schiff, D. Diehl, A. Valkirs, Mar. Pollut. Bull. 48 (2004) 371. This is clearly associated with the strength of air bubble [7] R. Lamb, H. Zhang, C. Raston, Hydrophobic coatings, PCT Int. Appl. (1998) WO 9842452. adsorption and further studies on the geometry of bubble [8] H. Zhang, R. Lamb, A. Jones, Durable superhydrophobic coating, capture are required. PCT Int. Appl. (2004) WO 2004090065. Nevertheless superhydrophobic coatings show promise [9] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988. for short-term anti-fouling application with no detrimental [10] In Port Phillip Bay, Vic. Australia side effects that are common in poison based formulations. [11] P. Attard, Langmuir 16 (2000) 4455.

Journal

Science and Technology of Advanced Materials – IOP Publishing

Published: Apr 30, 2005

There are no references for this article.