Reduction of Barnacle Recruitment on Micro-textured Surfaces: Analysis of Effective Topographic...

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Reduction of barnacle recruitment on micro-textured surfaces: Analysis ofeffective topographic characteristics and evaluation of skin frictionK. M. Berntssona; H. Andreassonb; P. R. Jonssonb; L. Larssonb; K. Ringb; S. Petronisc; P. Gatenholmd

a Tjärnö Marine Biological Laboratory, Göteborg University, Strömstad, Sweden b Department of NavalArchitecture and Ocean Engineering, Chalmers University of Technology, Göteborg, Sweden c

Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden d Departmentof Polymer Technology, Chalmers University of Technology, Göteborg, Sweden

First published on: 01 November 2000

To cite this Article Berntsson, K. M. , Andreasson, H. , Jonsson, P. R. , Larsson, L. , Ring, K. , Petronis, S. and Gatenholm,P.(2000) 'Reduction of barnacle recruitment on micro-textured surfaces: Analysis of effective topographic characteristicsand evaluation of skin friction', Biofouling, 16: 2, 245 — 261, First published on: 01 November 2000 (iFirst)To link to this Article: DOI: 10.1080/08927010009378449URL: http://dx.doi.org/10.1080/08927010009378449

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Reduction of Barnacle Recruitment on Micro-texturedSurfaces: Analysis of Effective TopographicCharacteristics and Evaluation of Skin FrictionK M BERNTSSON1,*, H ANDREASSON2, P R TONSSON1, L LARSSON2,K RING1, S PETRONIS3 and P GATENHOLM4

1 Tjärnö Marine Biological Laboratory, Goteborg University, SE-452 96 Strömstad, Sweden;Department of Naval Architecture and Ocean Engineering, Chalmers University of Technology, SE-412 96 Goteborg, Sweden;

2 Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goteborg, Sweden;3 Department of Polymer Technology, Chalmers University of Technology, SE-412 96 Goteborg, Sweden

(Received 18 November 1999; in final form 23 March 2000)

This study investigates five designed micro-texturedsurfaces and their effects on barnacle fouling andhydrodynamic drag. Three of the micro-textures weredeveloped in the present study and evaluated togetherwith two commercial riblet films. All micro-structureswere arranged as longitudinal grooves with differentprofile depths, widths and angles of inclination. In fieldtests the recruitment of the barnacle Balanus improvisuson micro-textured surfaces and smooth controls wasevaluated. All micro-textured surfaces reduced recruit-ment, and the most efficient texture reduced recruit-ment by 98%. For some micro-textures the reduction ofrecruitment declined as settlement intensity increased.In a correlative analysis, the trigonometric inclinationof the micro-structures explained most of the recruit-ment reduction. The steepest angle of inclination causeda massive reduction in barnacle settlement. Surfacemicro-structures may affect the boundary-layer flowand the hydrodynamic drag (skin friction) of thesurface. The skin friction was empirically measured ina flow channel using a sub-set of the tested micro-textures. The measurements of skin friction showedthat the orientation of the microstructures is important,with a minimum friction when the grooves are parallelto the flow. For one of the micro-textures the skinfriction was ca 10% lower compared to a hydraulicallysmooth surface. It is concluded that, depending on the

flow speed, micro-textures will not significantlyincrease skin friction when arranged parallel to theflow, even at moderate protrusion through the viscoussub-layer.

Keywords: antifouling; barnacle; Balanus improvisus;micro-riblets; micro-texture; recruitment; biofouling;skin friction; boundary layer; drag; drag reduction

INTRODUCTION

Marine invertebrate larvae, in general, settle morereadily on rough as compared to smooth surfaces(Crisp, 1974). Surface topography in relation tolarval settlement has often been characterised astexture (scales of surface irregularities below thatof the size of the larvae) and contour (scales ofirregularities larger than that of the larvae) (Crisp& Barnes, 1954; Crisp, 1974; Le Tourneux &Bourget, 1988). Preferential settlement on surfacecontour structures, such as cracks and pits, by thebarnacle Semibalanus balanoides has been shown

* Corresponding author; fax: +46 526 68607; e-mail: [email protected]

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246 K M BERNTSSON et al.

by Crisp and Barnes (1954), Barnes (1956), Crisp(1961) and Wethey (1984,1986). The preference forrough surfaces has been ascribed to the need offinding a refuge to maximize adhesion and hidefrom high shear stress (Walters & Wethey, 1996) orto prevent desiccation (Raimondi, 1990).

In contrast, few authors have reported anynegative influence of surface texture on barnaclesettlement and recruitment. Barnes and Powell(1950) found that glassfibre filaments, with adiameter of 8|xm and a length of lmm, wereavoided by Balanus crenatus cyprid larvae.Furthermore, Crisp and Barnes (1954) reportedthat cyprids showed a greater tendency to swimoff surfaces at ridges and convexities. In theirexperiments, panels with numerous sharp ridgesand grooves showed low settlement of barnacles.Recently, Lemire and Bourget (1996) and Lapointeand Bourget (1999) investigated settlement pre-ferences of Balanus sp. on surfaces with differentscales of V-shaped grooves (0,1,10 and 100 mm).They found that recruitment tended to be loweron surfaces which included grooves with lateraldimensions of 1 mm compared to surfaces withlarger grooves, although settlement in grooveswas higher than on adjacent smooth surfaces.

Several factors may explain differential settle-ment on rough and smooth surfaces, includingadhesion strength, flux of larvae and larvalbehaviour. Crisp (1974) described the exploratorybehaviour of cyprids as three subsequenrialphases, viz. broad exploration, close exploration,and, if a site on the surface is sufficiently attrac-tive, a third inspection phase which precedesadhesion and metamorphosis. Le Tourneux andBourget (1988) attempted to determine factorsused as cues by cyprids of Semibalanus balanoidesin the field and to classify these factors accordingto their relative importance at each of the spatialscales examined by the larva during the threephases of settlement. They suggested that cypridsused a biological cue during broad exploration,sought clean sites during close exploration and,sought heterogeneous micro-sites to ensure opti-mal adhesion during the inspection phase. Recent

studies of cyprid behaviour seem promising interms of explaining responses to settlement cuesat different spatial scales (Hills et al, 1998; Waltersetal.,1999).

The literature dealing with the responses ofinvertebrate larvae to surface topography is vast,although comparisons between studies aredeceptive since there is no consensus on how todefine surface topography (Crisp, 1984). The mainbody of information on responses to surfaceroughness as a regulating factor for barnaclesettlement comes from studies of the speciesSemibalanus balanoides (Crisp & Barnes, 1954;Barnes, 1956; Crisp, 1961; Wethey, 1984; 1986;Chabot & Bourget, 1988; Le Tourneux &Bourget, 1988; Hills & Thomason, 1996; 1998a;1998b; Hills et al, 1998); information for otherspecies is more scarce (although see, Crisp &Barnes, 1954; Hudon et al, 1983; Wethey, 1986;Raimondi, 1990; Walters & Wethey, 1996;Lemire & Bourget, 1996). Most studies haveshown that surface roughness in general pro-motes barnacle settlement. Furthermore, moststudies on surface roughness effects on barnaclesettlement have focused on roughness elements> l m m (Crisp & Barnes, 1954; Crisp, 1961;Wethey, 1984; 1986; Chabot & Bourget, 1988;Walters & Wethey, 1996; Lemire & Bourget, 1996;Hills & Thomason, 1998a; Lapointe & Bourget,1999). Surface roughness of scales below 1 mm hasreceived less attention (Le Tourneux & Bourget,1988; Holmes et al, 1997; Hills & Thomason,1998a). Le Tourneux and Bourget (1988) foundthat S. balanoides showed a significant preferencefor sites with a micro-heterogeneity of > 35 nmand < 1 mm and suggested that cyprids settlingon micro-topographic surfaces gained strongerantennular adhesion. Holmes et al (1997) foundthat finely grained, 0-10 nm, crystalline rocksincreased S. balanoides settlement more thanany other rock type, although the chemical pro-perties of the rock type may have influencedcyprids at settlement. Furthermore, Hills andThomason (1998a) found higher settlementdensities of S. balanoides on fine (<0.5mm) and


BARNACLE RECRUITMENT ON MICRO-TEXTURED SURFACES 247

medium (<2.0mm) scales of surface rough-ness compared to smoother and rougher sub-strata, suggesting that cyprids preferred to settlein crevices with dimensions similar to theirbody size.

Most studies on surface roughness havefocused on S. balanoides but settlement responsesto surface topography may vary between barnaclespecies. This is particularly true of the micro-texture interval and implies that more studiesconsidering the responses of other barnaclespecies are needed. Furthermore, responses tosurface topography are likely to vary with thetopographic characteristics of the surface, inparticular the geometry and the size of the rough-ness elements. Although the effects of small-scaleheterogeneity have been documented for somespecies, more information on barnacle settlementresponses to micro-topography with differentgeometrical characteristics are needed in themicro-texture interval 0-0.5 mm.

The barnacle Balanus improvisus is one of themajor fouling organisms in Scandinavian watersand the settlement of this species has attractedrecent interest from a fouling control perspective.The present study was conducted after previousobservations showed that settlement and recruit-ment of B. improvisus were negatively influencedby surface texture ranging from 30-45 nm inprofile height (Berntsson et al., 2000). In theprevious study, surface topographies with long-itudinal ridges and grooves reduced recruitmentmore than surfaces with separate topographicstructures (peaks and pits). The present studyfocuses on the recruitment of B. improvisus onmicro-textured surfaces designed as longitudinalgrooves arranged parallel to the ocean surface.Specifically, the aim was to determine the geome-trical characteristics that contribute most to thereduction of barnacle recruitment. If micro-tex-tured surfaces are to be used as a component infouling control, it is essential that this does not initself increase skin friction of the surface. In aseries of empirical flow tests analyses are pre-sented of skin friction on surfaces with different

scales of grooves arranged in different directionsto the flow velocity.

MATERIALS AND METHODS

Manufacturing and Characterisation ofMicro-textured Surfaces

Micro-structured surfaces were prepared onPlexiglas® (PMMA), polyvinylidenefluoride(PVDF), polyvinylchloride (PVC) and silicone(Rhodorsil®RTV 1556). The micro-structures onPMMA were fabricated by sanding PMMA panels(110 x 110 mm2) with a grinding machine (sand-paper grade P80) (Figure la). The micro-textureon PVDF and PVC consisted of a film mouldedwith micro-riblets (Figure lb, lc). The PVDFriblet film was developed and prepared by 3MIncorporated (USA) as a drag-reducing surfacefor aeroplanes (this project is terminated and thePVDF riblet film is not available on the market).The PVC riblet film was developed by Ingenjors-byran Ragnar Winberg AB (Sweden) as an anti-fouling surface for marine environments (the PVCriblet film is still under development and notavailable on the market). The polydimethylsilox-ane (PDMS) based elastomer, Rhodorsil®RTV1556 (Rhone-Poulenc Silicones, France), was usedto prepare smooth and micro-structured siliconefilms for the present study. A method for creatingsurface microstructure with uniform riblets onsilicone samples was based on casting liquid RTV1556 in the inverse surface moulds and peelingoff the cured film. The moulds for casting weremicro-fabricated using photolithography andanisotropic silicone etching (Petronis et ah, 2000)(Figure Id, le). PVDF, PVC and PDMS riblet films(110 x 110 mm2) were glued on PMMA panels andthen soaked in a water bath for 24 h.

Surface topography of all the micro-structuredsurfaces was visualized by scanning electronmicroscopy (SEM) (Figure 1). Micro-texturedPMMA panels, and PVDF and PVC films, werecovered with a thin layer of gold to avoid charging



FIGURE 1 SEM images of the micro-textured surfaces, (a) = sanded PMMA panel; (b) = riblet film moulded in PVDF;(c) = riblet film moulded in PVC; (d) = riblets moulded in PDMS with 46|im high structures (riblet 1); (e) = 70(am highstructures (riblet 2).



when viewing them with a Zeiss DSN 940 A SEMoperating at lOkeV. PDMS riblets were imagedwithout metallic coating using a JEOL JSM 6301FSEM operating at 2 keV.

An assessment of surface texture on sandedPMMA was made using a mechanical stylusprofilometer, Surfascan 3D, from Somicronic.The equipment had a cone-shaped (90°) diamondtip stylus with a radius of 2 urn. The surface,covering an area of 2.5 x 2.5 mm2, was scannedand data were collected every 2 |xm. The verticalresolution was 5.2 nm (Ohlsson, 1995). The rough-ness, or R-parameters, were calculated with adigital Gaussian filter according to a standardizedmethod (EN ISO 4287:1998, 1998), and a cut-offwavelength (Ac) of 0.8 mm was applied. Thechosen roughness parameters were the ampli-tude parameter Rz (the sum of the largest profilepeak height and the largest profile valley depthwithin a sampling length) and the spacing param-eter Rsm (the mean value of the profile elementwidths within a sampling length) (EN ISO4287:1998,1998). As the surface texture on sandedPMMA panels was expected to show somevariation, both within and between panels, rough-ness profiles were assessed on four replicatepanels with three samples of 2.5 x 2.5 mm2 fromeach panel (Table I; Figure la). The surface textureof the PVDF riblets was not measured in this studysince the regular profile was known from themanufacturer (Table I; Figure lb). The sizes of

TABLE I Roughness profiles and trigonometry of the sur-face treatments evaluated

Name of surface

PMMA sanded1

PVDFriblet2

PVC riblet3

PDMS riblet I4

PDMS riblet 24

Rz (nm)

32.9 ±1164

352.5 ±9.34669

Rsm fam)

204 ±93.764

134 ±1.06697

Inclination (°)

« 2 063795555

Rz = maximum height of profile; Rsm = mean width of profileelements and the trignometric inclination of the structures;Jamong panel variability, n = 4; 2regular texture with dimen-sions from the manufacturer (3M Inc.); 3within panel varia-bility, n = 200; 4regular texture measured from cross sectionSEM images

the PVC riblets were determined by measuring200 profile widths and profile heights from twosurface structure samples under a compoundmicroscope (Olympus BH-2,100 x) (Table I; Figurelc). Surface roughness parameters were mea-sured directly from cross section SEM images ofthe ribbed PDMS samples (Table I; Figure ld-e).The trigonometric inclination of micro-structureswas calculated from the profile amplitudes andprofile element widths (Table I).

Field Studies of B. improvisusRecruitment on Micro-textured Surfaces

The recruitment of B. improvisus on micro-tex-tured surfaces was evaluated in field studies atTjarno Marine Biological Laboratory (TMBL) onthe west coast of Sweden (58°53'N, 11°8'E). Thecomposition of the fouling community on artifi-cial substrates is not very diverse in the Tjarnoarchipelago. Some temporal variation in speciescomposition has been recorded during the foulingseason (June to September, Figure 2). Althoughrecordings on PMMA panels (110 x 110 mm2)from 1997 to 1999 showed that the barnacleB. improvisus was by far the most common andpredictable fouler, Mytilus edulis may be animportant species in June and July, especially ifthe substrate is covered by hydroids (June,Figure 2). Hydroids may be common in June,diminish during the warmer summer months,and then increase again in September (Figure 2).B. improvisus was the only fouler recorded in 1999;the abundance of other species, e.g. M. edulis andhydroids, was particularly low.

Recruitment on micro-textured surfaces

Recruitment on five different micro-textured sur-face modifications (PMMA, PVDF, PVC and twosizes of PDMS riblets) was compared with smoothcontrol panels of the same materials. Test panelswere randomly fixed to two aluminum frames,five panels on each of ten transverse beams, andthen submerged from a stationary raft to a depth


250 K M BERNTSSON el ah

100 -i

10-

so

2oPi

1 -

0.1 -

0.01

I

11

June July August

Month

• B. imptvvisusD M. edulisn C. hemisphaericaH B. pyramidata

L. flexuosaS. viduatus

FIGURE 2 Recruitment (mean ± 95% CI, n = 12) of invertebrates on smooth PMMA panels at monthly intervals from Juneto September 1997, averaged over two adjacent locations in Tjarno bay at 0-3 m depth. Recorded foulers were B. improvisus( • ) / M. edulis (O), Clytia hemisphaerica O)< Bougainvillia pyramidata (gS), Laomedea flexuosa Q2i) and Sagartiogeton viduatus 0 .The Y-axis is on a logarithmic scale.

interval of 1-2 m. The raft was located in a 15-mdeep bay with weak water currents close to theTMBL. Panels were arranged vertically withgrooves and micro-riblets directed parallel to theocean surface. The study included ten replicatesof the smooth controls and the micro-texturedsurfaces, respectively. Panels were submerged for4 weeks, then collected for analysis and replacedwith new panels. This procedure was repeated inJune, July and September 1999 for PMMA andPVDF modifications to test for temporal differ-ences in cyprid propensity for settlement ontextured surfaces. Limited supplies of PVC- andPDMS-textured surfaces only allowed for a singletest month. The number and species of attachedcyprids and metamorphosed juveniles were de-termined using a dissecting microscope. To avoidpossible edge effects, individuals which hadattached within 1 cm of the panel edge were

ignored. The recruitment intensity is presented asthe number of individuals cm"2 and the reductionin recruitment was calculated as the proportion ofbarnacles on smooth surfaces as compared tomicro-textured surfaces. Recruitment intensitywas compared among materials (smooth sur-faces) and reduction in recruitment was com-pared among micro-textured modifications.

Recruitment on micro-textured surfaces as afunction of exposure time

To test the effect of topography on settlement aftervarious exposure times, recruitment on smoothand micro-structured PMMA and PVDF wasevaluated after 4 and 8 weeks, respectively.Twenty replicates of each smooth and micro-textured surface were fixed to two aluminumframes (ten replicates from each surface texture


BARNACLE RECRUITMENT ON MICRO-TEXTURED SUREACES 251

were randomly fixed to each frame) and sub-merged from a raft at the beginning of June. Tenreplicates (one frame) of each surface modifica-tion were collected after 4 and 8 weeks, respec-tively, and analysed as described above.

Effects of Micro-structures on theBoundary Layer and Skin Friction

It is essential that a method that prevents bio-fouling by adding a surface structure does notsignificantly increase the hydrodynamic skinfriction by making the surface hydraulicallyrough. To evaluate the skin friction empirical testswere done in a flow channel where the pressuredrop was measured along a duct lined with micro-textured riblet surfaces.

Critical roughness

The turbulent boundary layer over a surface isdivided into regions with different characteristics.Nearest the surface is the viscous sub-layer,reaching approximately five viscous length units,I+, from the surface.

(1)

where v is the kinematic viscosity and uT is thefriction velocity defined as

Mr = 4 / — . (2)

rw is the wall shear stress and p is the fluid density.A roughness within this sub-layer will not affectthe skin friction and this surface is termedhydraulically smooth. To evaluate TW for a vessel,White (1994) gives an empirical expression for flatplates that will work as an approximation for aship hull.

0.0135 • i/1 /7 • p •1

where x is the distance from the leading edge ofthe plate and U is the free-stream velocity. Theabove equation is only one of many suggestions.The approximation of a hull as a flat plate whencalculating the skin friction gives qualitativelygood results since the surface gradients of the hullare very small except in the bow and aft regions.An expression for the roughness limit, h, ofhydraulically smooth surfaces can be obtained ifit is assumed that the sub-layer is five Z+ thick

few 43-

Effect of structure geometry

(4)

Although roughness height is the main character-istic for the skin friction magnitude, structuregeometry is of significant importance. Structurescalled riblets decrease skin friction and these haveto be extremely well defined, with a constantlength scale in viscous units. To calculate the effectof the roughness over a flat plate, Johansson(1984) has given the approximate relation

(5)

where c^ and c^ is the local friction factor forsmooth and rough surfaces, respectively, definedas

cf =2-TW

p-U2 (6)

(3)

U is free-stream velocity, and h+ is the viscousroughness height defined as

Knowing c and the roughness efficiency param-eter C, for a type of structure it is then possible tocalculate Cp. from Eqn 5.

Measurements of skin friction

To estimate the C value for the different structures,measurements were performed in a channel flow



friction measurement facility (Figure 3a). Thetest structures were attached to the longer sidesof a 900 x 20 x 5 mm rectangular duct (Figure 3b),which covered 80% of the inner duct surface.The duct was arranged vertically in the test rig(Figure 3a). A head tank was connected on the topof the duct. This tank had a much greater crosssectional area than the duct to maintain thepressure as constant as possible. At the lowerend there was an outlet with a valve to controlthe flow through the duct. To determine theviscosity, v, the temperature was measured atthe outlet. A water level gauge in the head tankwas used to measure the velocity through the

T

ISS

FIGURE 3 (a) The test facility. 1 = head tank; 2 = test plate;3 = side plate; 4 = outlet control valve; 5 = outlet; 6 =pressure taps; 7 = differential pressure gauge; 8 = surfacelevel meter; 9 = amplifier; 10 = computer, (b) Rectangularduct — the pipe arrangement with the test film pressedbetween the test plates and side plate.

duct. Knowing the water surface velocity in thehead tank, which had a constant water surfacearea, it was possible, by the law of mass conserva-tion, to establish the volume flux through the ductand thus the mean velocity. The pressure gradientwas determined by two pressure taps, one 32 cmfrom the inlet of the test section and the other10 cm from the outlet.

According to Schlichting (1979) and White(1994) the pressure drop, Ap, for flow through agiven length of a rectangular duct obeys therelation

Ap _L 4 • • w • s

(8)

where L is the duct length between pressuretaps, / the friction factor, p the fluid density, wthe duct width, s the spacing between plates andUm the mean flow velocity in the duct. $ is anempirical correction factor introduced by Jones(1976) to increase the reliability for rectangularducts. According to Jones (1976) $ is around0.86 for the duct used in these experiments. Thisgive a friction factor around 5% higher than forcircular ducts. The friction factor can be comparedwith a reference value for a hydraulically smoothsurface at the same Reynolds number, Re, as ameasure of the friction changes caused by thesurface structures.

Francis (1954) showed that an approximaterelation between Cf and / is

(9)

which gives the following modification of Eqn 5,

(10)

in which the difference should be taken for thesame value of uT • dh/v where dh is the hydraulicdiameter. The surface resistance formula as a



function of uT • d^jv is given as

+ 3.05. (11)

Skin friction was measured on the PVDF ribletsurface and the sanded PMMA surface. PVDFriblets were measured with grooves arranged inthe direction of the flow (0°) and sanded PMMAwith grooves arranged at different yaws to theflow (0,45, and 90°). Each measurement requiredtwo panels (900° x 20 mm2) with the tested struc-tures attached to the longer sides of the rectan-gular duct. The measurements were replicatedwith a new set of the tested structures for each run.

All experimental data on the pressure differ-ence and water velocity were sampled with afrequency of 25 Hz for 50 s, which gave 1250samples. The temperature interval in the experi-ments was 5 to 45°C. Pressure and velocity mea-surements were averaged periodically from 100samples. From these values the quantities /, Rea,h+ were calculated and averaged. Eqn 10 was usedto calculate C, which could then be substituted inEqn 5 to predict Cp- for a flat plate with correspond-ing roughness geometry. Each experiment wasrepeated six times on the same surface. Theroughness geometry (h) was fixed for the testedsurfaces (h = 64nm for PVDF riblets, h«33umfor sanded PMMA). To examine the frictionfactor for a range of viscous roughness heights(h+), the strong dependency of viscosity on tem-perature was used together with changes in flowvelocity.

Statistical Methods

If not otherwise indicated, data are reported inthe text, tables or figures as means with 95%confidence intervals. Analysis of variance(ANOVA) was used to test for differences inrecruitment among panel treatments (a = 0.05).A posteriori comparisons were performed usingStudent-Newman-Keuls procedure and tests for

homogeneity of variances were performed usingCochran's test (Winer et ah, 1991).

RESULTS

Roughness Parameters of Micro-texturedSurfaces

Table I shows the roughness parameters Rz andRsm for the tested micro-textures. PVDF, PVC andPDMS had highly regular micro-structures fromwhich the trigonometric inclinations were calcu-lated (Figure lb-e). Structures on PVDF and PVChad average inclinations of 63° and 79° respec-tively (Table I). PDMS modifications with theprofile heights 46 |am and 70 \im both had inclina-tions of 55° (Table I). The trigonometric inclinationof the irregular grooves on sanded PMMA(Figure la) could only be roughly estimated, withan average angle of 20°.

Recruitment of B. improvisus onMicro-textured Surfaces

Analysis of recruitment as a function ofsurface texture

Figure 4a shows total recruitment on smoothtreatments for different months. Recruitmentwas recorded on PMMA and PVDF during allmonths, on PVC in July and on PDMS inSeptember. There was a difference in recruitmenton different materials during the whole period.In June, significantly more barnacles settled onsmooth PMMA (4.5 ± 0.7 individuals cm"2) thanon smooth PVDF (2.7 ±1.0 individuals cm"2)(F1/18 = 11.4, p<0.05). The peak settlementoccurred in July and the mean recruitment onsmooth PVC (27.9 ±1.5 individuals cm"2) dif-fered from smooth PMMA (18.7 ±1.2 individualscm"2) and smooth PVDF (17.7 ±2.7 individualscm"2) (F2,27 = 54.7, p<0.05, SNK, p<0.05). InSeptember, smooth PDMS (0.5 ±0.1 individualscm"2) attracted more recruits than PVDF(0.4 ±0.2 individuals cm"2) and PMMA



a 30 n

I 20 H

10

L

• June• July• September

PMMA PVDF PVC

Surface material

b loo

80

3 40

20

0

PDMS

ESJune• July• September

PMMASanded

PVDFRiblet

PDMSRiblet 1

Surface treatment

PVCRiblet

PDMSRiblet 2

FIGURE 4 (a) Recruitment (mean ±95% CD of B. improvi-sus on smooth PMMA, PVDF, PVC and PDMS panels inJune (E2), July ( • ) and September ( • ) 1999. (b) Percentagereduction (mean ±95% CI) in recruitment on micro-tex-tured PMMA, PVDF, PVC and PDMS panels compared tosmooth panels of the same materials.

(0.3 ± 0.1 individuals cm"2) (F2,27 = 6.58, p < 0.05,SNK, p< 0.05).

The reduction in recruitment on differentmicro-textured modifications compared withsmooth controls is shown in Figure 4b. This effectdiffered among modifications and months. InJune PVDF-riblets reduced recruitment by79 ± 9% compared to smooth controls and sandedPMMA reduced recruitment by 44 ±22%. Thereduction in recruitment was significantly differ-ent between textured PVDF and PMMA (FU 8 =10.5, p < 0.05). During the peak settlement in Julyall modifications differed from each other(F3,36 = 11-0, p < 0.05, SNK, p < 0.05) and the mostefficient were PVC-riblets which reduced recruit-ment by 98 ±0.4%, followed by PVDF-riblets

where the reduction was 46 ±13% and thensanded PMMA at 22 ±5%. In spite of the lowrecruitment in September, micro-textured mod-ifications showed significant effects on recruit-ment. PVDF-riblets differed from all othermodifications (F3/36 = 13.0, p < 0.05, SNK, p < 0.05)and reduced recruitment by 85 ± 6%. PDMS, with46 |im and 70 nm-deep riblet structures, reducedrecruitment by 56 ± 12% and 67 ± 12%, respec-tively. Sanded PMMA reduced recruitment by38 ±17%.

A key issue in this study is whether thereduction in recruitment on micro-textured sur-face can be explained in terms of differences insize and trigonometry of topographic structures.Figure 5a shows the reduction in recruitment onmicro-textured panels plotted against measuredprofile heights (Rz). This plot suggests a weakrelationship between increasing roughnessheight and reduced recruitment. Further studiesare needed, e.g. with profile heights between100 nm to 300 urn. The profile widths (Rsm) ofmicro-structures plotted against the reduction inrecruitment explains little of the variability inrecruitment (Figure 5b). The strongest relation-ship is found between the reduction in recruit-ment and the trigonometric inclination oftopographic structures as shown in Figure 5c.This plot suggests that the reduction in recruit-ment increases as the inclination of the structuresincreases. The PVC modification of regular ribletswith an average inclination of 79° reducedrecruitment by 98%.

Analysis of recruitment as a function ofsurface texture and exposure time

The effect of micro-textured surfaces on recruit-ment appeared to decrease with time. PVDF-riblets reduced recruitment by 79 ±9% after 1month exposure and by 46 ± 13% after 2 monthsexposure. Recruitment on sanded PMMA wasreduced by 44 ± 22% after 1 month and only by9 ±7% after 2 months. The lower efficiency ofmicro-textured surfaces was probably reinforced



by an intensified settlement of B. improvisus inJuly compared to June. In June initial recruitment

while in July the initial recruitment increased

was 4.5 ±0.6 and 2.7 ±1.0 individuals cm onsmooth PMMA and smooth PVDF respectively,

a 100-

£ 80-

•a§ 40 H

50 100 150 200 250 300 350

Profile height (|jm)• June BJuly A September

b 100 -I

g80-| 60-j

g 40-

1 20H0

[ JI .

50 100 150

Profile width (pm)• June HJuly A September

200

100-

r 8o-

1 6 ( H

§ 40 -\

0 20 40 60 80

Trigonometric inclination (°)• June BJuly A September

FIGURE 5 Summary of the percentage reduction in re-cruitment (mean ± 95% CD of B. improvisus on surfaces withdifferent sizes of micro-textures. Recruitment reduction isplotted against the profile height (a), profile width (b) andthe structures trigonometric inclination (c).

to 18.7±1.2 andrespectively.

17.7 ±2.7 individuals cm- 2

Effects on Boundary Layer andSkin Friction by Micro-textured Surfaces

PVDF riblet film

Two replicate surfaces of the PVDF riblet filmwere tested (Figure lb). The data plotted inFigure 6 shows a drag reduction that correspondsto that measured by Bechert et al. (1997). The meanvalue of C is -51.3 (Table II). The inverse C has aSD of 0.0045, which gives a variance of 22.9%(Table II). It was impossible to achieve higher h+ inthe test rig with the given structure. However, thistype of structure is relatively well documentedand it is possible to predict the effect frommeasurements published by others e.g. Walsh(1990), Vukoslavcevic et al. (1991), Bechert et al.(1997) and Coustols (1998). According to thesepapers drag reduction increases with increasingh+ until a certain level where h+ = 15. After thislimit there follows a decrease in performance, i.e.the optimal height of the riblets is in the rangeh+ = 10-15. Within this range, the reduction ofskin friction is close to 8% for the PVDF symmetricV-shaped riblet with height/width ratio of 1.0

Viscous roughness height, h*

- l •

<2 "5H

I -7

- 9 J

Bechert 1997

• PVDF riblets (repl. 1)

A PVDF riblets (repl. 2)

FIGURE 6 Difference in friction factor for two replicatesof PVDF riblet films as a function of dimensionless ribletheight. Data from Bechert et al. (1997) are included forcomparison.



(Table I). Above h+ = 25, the riblets are nolonger efficient, and drag increases significantly(Coustols, 1998). Therefore, it is plausible that theC constant will change to positive values whenh is greater than 25 Z+.

Sanded surfaces

These test series were conducted with panelssanded with P80 sandpaper (Figure la). The testswere performed to establish the efficiency factor,C, for this structure and also to determine whetherthere are any effects due to the yaw of the groovesto the flow. Three angles were tested 0,45 and 90°

to the flow (Table II; Figure 7). The results with thegrooves in the direction of flow, show that for themeasured interval the surface can almost be takenfor hydraulic smooth (Figure 7). There was a smallincrease in friction, of between 0-3%, which isclose to the detection limit. For the surfaces withthe grooves at 45° to the flow there was a slightincrease in average friction, but one of thereplicates showed a low friction increase similarto that for the panels with the grooves in thedirection of flow. The surfaces with the grooves atright angles to the direction of flow showed a greatincrease in the friction factor (Table II; Figure 7).

TABLE II Measurement of skin friction on PVDF and PMMA modifications; the mean efficiency factorC, SD and variance for the different test cases are presented

Surface treatment

Angle of grooves to the flow

Mean value of C1/C Standard deviation1/C Variance

2 0 -

15-

PVDF riblets

0°

-51.30.004522.9%

X

PMMA sanded

0°

107.80.007883.9%

X *

a

PMMA sanded

45°

60.40.012575.6%

X X

•

PMMA sanded

90°

11.10.02021.7%

• 0°• 0°A 45°

X45°X90°•90°

2o

4-*

u

E

5-

-5-1

X

X X

•x

10 11

Viscous roughness height, h*

FIGURE 7 The difference in friction factor for the sanded surfaces with different yaws to the flow (0, 45 and 90") as afunction of dimensionless roughness height.



The estimated mean C value for this surface was11, with a variance of 22% (Table II).

DISCUSSION

Reduction of Fouling on Micro-texturedSurfaces

All the tested micro-textures showed a reducedrecruitment rate compared to smooth controls(Figure 4b). This reduction ranged from 22%(sanded PMMA) to almost 100% (PVC riblets).Recruitment intensity (as measured on smoothcontrols) differed over the season with lowrecruitment in June and increasing approximatelyfive fold in July (Figure 4a) and then declining tolow levels in September. The antifouling effi-ciency of the micro-textured surfaces decreasedwith increasing recruitment intensity (Figure 4a,4b). A likely explanation for this lower efficiencyis that during intense settlement there is a lack ofsuitable settlement substrata and cyprids areforced to settle on any free surface regardless ofthe texture. Bertness et al. (1992), for example,found that S. balanoides showed less selectivesettlement once preferred locations were satu-rated with settlers. Another possible explanationis that cyprids become less selective for settlementcues with age, as shown for conspecific adultextract in S. balanoides (Jarrett, 1997). Furthermore,if some barnacles start to settle on a texturedsurface this will stimulate other cyprids to settlenearby, since the presence of conspecifics isprobably a stronger cue for settlement than thetexture of the surface (Chabot & Bourget, 1988)However, the PVC-riblet micro-texture remainedessentially free of barnacles even during the timeof peak settlement.

The negative response shown by B. improvisusto surface texture in the range 0-0.5 mm in thisstudy is opposite to the results obtained forS. balanoides (Le Tourneux & Bourget, 1988; Hills& Thomason, 1998a). Lemire and Bourget (1996)examined the effect of larger topographic scales

(0,1,10 and 100 mm V-shaped grooves) on Balanussp. and Tubularia crocea recruitment in a short-term experiment (5 days). They found that thedensity of Balanus sp. was higher on panels whichdid not include the 1 mm grooves and decreasedwith increasing panel complexity. In a long-termexperiment (12 months), with the same V-shapedsurfaces, Lapointe and Bourget (1999) found thatafter 5 months recruitment of Balanus sp. waslowest on panels with the 1 mm scale. Later in theseason, however, when overgrowth occurred andspace became a limiting factor the effect of the1 mm scale disappeared. A difference betweenthese studies is that in the studies of Lemire andBourget (1996) and Lapointe and Bourget (1999)settlement in grooves was higher than on adjacentsmooth surfaces while in the present study,settlement was always higher on smooth surfacesthan on surfaces with grooves. Settlement onmicro-textured surfaces (PVDF and PMMA) wasstill significantly lower than on the smoothcontrols after an exposure time of 2 months inthis study. However, overgrowth by microbiotaand gregorious settlement of barnacles may havecombined to effect a reduced efficiency of thestructures during the second month of exposure.

A preliminary laboratory study (Berntsson,unpublished data) showed that the PVC ribletfilm reduced B. amphitrite settlement by 92% ascompared to smooth PVC. This indicates thatmany species of barnacles may respond nega-tively to micro-texture and that more studies areneeded to elucidate settlement preferences todifferent scales and geometries of micro-texture.

Mechanisms Reducing Recruitment onTextured Surfaces

Several mechanisms can explain how surfacetopography may reduce recruitment of B. impro-visus. Post-settlement mortality may be higher oncertain surface textures, e.g. through reducedadhesion due to inadequate contact with thesurface. Biotic interactions could further change



because of texture-specific changes in the pres-ence of predators and competitors. Post-settle-ment mortality on the micro-textured panelsthrough e.g. predation may be inferred from thepresence of attached basal plates. Few recentlydead individuals were detected and cannotexplain the texture-specific differences observed.Post-settlement detachment (including the basalplate) due to low adhesion on some texturedsurfaces cannot be discounted. However, the mostlikely explanation for the low recruitment onmicro-textured surfaces is that cypris larvaeactively reject these surfaces through behaviouralresponses. In a recent laboratory study, Berntssonet al. (2000) investigated the behaviour of B. impro-visus cyprids as a function of surface topography.Comparisons of the time budget in the beha-vioural categories as a function of texturedsurfaces (Rz: 0-100 nm, Rsm: 0-500 im) showedthat cyprids spent more time exploring smoothsurfaces than textured surfaces. These resultssuggested that surface texture mediates a nega-tive settlement response in B. improvisus thattriggers cyprids to swim off. The rejection oftextured surfaces may be related to the behavioursdiscussed in reviews by Crisp (1974) and Hui andMoyse (1987). Crisp (1974) described how surfaceprojections sometimes caused cyprids to swimoff, or to avoid the obstacle, and settle a shortdistance away and suggested that this effect couldhave an important function in preventing over-crowding. Hui and Moyse (1987) concluded thatintertidal balanomorph barnacles, which areadapted to spreading over the rock surface, tendto avoid settlement in contact with the edge of, aswell as on top of, conspecific adults and suggestedthat this response was an avoidance behaviourthat may protect the barnacles from being over-grown by conspecifics and from wave action(although see Miron et al., 1996). In the presentstudy, B. improvisus tended to recruit predomi-nantly on the upper section of the smoothsurfaces. Crisp and Barnes (1954) and Hills andThomason (1998b) reported stratified settlementpatterns of S. balanoides on smooth surfaces where

cyprids accumulated towards the upper edge ofthe panels and suggested that the net verticalmovement was due to positive phototaxis. How-ever, on textured surfaces the rugophilic cypridscrawled for much shorter distances as a suitablesettlement site was more readily found (Hills &Thomason, 1998b). Studies of B. improvisus cypridexploratory behaviour on micro-textured sur-faces suggest that cyprids are triggered to swimoff surfaces when they confront peaks and ridges(Berntsson et al., 2000). In the present study themicro-riblets were arranged parallel to the watersurface. This orientation, perpendicular to thedirection of the light, probably interferes mostwith exploratory behaviour since more ridges perunit area are confronted by cyprids if they tend tomove towards the light.

What Topographic Features are Effective?

Preliminary behavioural observations, as dis-cussed above, indicate that micro-textures mayinterfere with the exploratory behaviour whencypris larvae search for a suitable settling site. Itmay be hypothesised that certain topographicalgeometries prevent cyprids making appropriatecontact with the antennular discs thereby impair-ing their ability to walk on the surface. Of par-ticular importance is an understanding of whichgeometries are most effective in interfering withcyprid behaviour and thus resulting in poorrecruitment rates. The present study used micro-textures that differed in profile depth (Rz), profilewidth (Rsm) and trigonometric inclination. Thesefeatures were not varied independently due tolimitations in the manufacturing processes. How-ever, each of these topographic features can beplotted against the reduction of recruitment(Figure 5). From such plots it can be seen that thetrigonometric inclination is by far the best pre-dictor of the reduction in recruitment (Figure 5c).It can only be speculated that the adhesionbetween the surface and the antennular discs ofthe cyprid is particularly difficult on surface



elements with high inclinations. Although veryfew barnacles recruited to the most effectivesurface (PVC-riblet), it is possible that this couldbe further improved by changing the relationsbetween inclination, Rz, and Rsm.

Effects of Micro-textures on Skin Friction

Topographic structures may affect surface frictioncompared to a smooth surface. If micro-texturedsurfaces are to be useful as a component in anantifouling technology on boats and ships it isimportant that the surface itself does not inducesignificant friction. If the topographic structuresare smaller than the thickness of the viscous sub-layer the surface is considered as hydraulicallysmooth and the structures will not increase sur-face friction. The approximate thickness of thesub-layer as a function of flow speed and distancefrom the leading edge is shown in Figure 8 usingEqn 4. This indicates the permissible height ofsurface topography, e.g. on the hull of a ship. Thecritical height of topographic structures is onlyapproximate since this will also depend on theirgeometry. It is, however, difficult to assess this

effect theoretically. In experiments where thepressure drop was measured along a flow channelwith micro-textured surfaces, the sanded PMMAsurface appeared hydraulically rough, as shownby the added friction when the riblets wereoriented at 90° to the flow direction (Figure 7).Micro-textured surfaces can also reduce frictionunder certain circumstances. It has been shownpreviously that grooves or riblets, in the directionof flow may reduce surface friction for certaincombinations of riblet dimensions and flowspeed. The mechanisms behind these effects arenot fully understood and thus several theories areavailable. Bechert et al. (1997) proposed that thelongitudinal ribs rectify the turbulent flow in themean flow direction by damping the fluctuatingcross flow component. If the cross flow fluctua-tions close to the surface can be reduced, theturbulent momentum transfer close to the surfacewill also be reduced and, consequently, the shearstress will decrease. The measurements of thepressure drop along riblet surfaces showed anexpected decrease in skin friction for the PVDF-riblet which is close to previous results (Bechertet al., 1997). The pressure-drop measurements

250-1

^ 2 0 0 -

E3-s| l 5 0 -0

Siao

vabl

e rou

8

< 50-

0-

\\\\

\\\

I

Vl m10 m50m250 m

6 8 10

Free stream velocity, U (m/s)12 14 16

FIGURE 8 Allowable roughness as a function of velocity for different distances from the bow or more precisely, from theleading edge (1,10, 50 and 250 m) of a plate.


260 K M BERNTSSON el al.

showed that sanded PMMA surfaces, withgrooves oriented parallel to the flow, createconsiderably less skin friction than surfaces withgrooves oriented perpendicularly. The main con-clusion is that if micro-textures are configured aslongitudinal grooves or riblets, the increase inskin friction caused by the added roughnessmay be compensated for by the rectification ofthe turbulent flow, thus allowing higher struc-tures without negative effects on the overall drag.

Implications for Fouling Control

B. improvisus is a serious fouling organism in theKattegatt and the Skagerrak and will be the majorproblem for pleasure-boat owners when the newSwedish restrictions on toxic coatings are imple-mented from the year 2000 (Eriksson et al, 1998).The substantial reduction in the recruitment ofB. improvisus found on some geometries of micro-textured surfaces could be exploited to improvefuture antifouling techniques. Surface texture maybe combined with silicone coatings (Anderssonet al., 1999) and recent research has explored thepossibility of self-texturising coatings (Berglinet al., 2000). It may also be possible to mechanicallymodify the texture of cured coatings. In somecases, e.g. aquaculture equipment and moldedplastic hulls, surface texture may be pre-moldedinto polymer materials. It is clear that a surfacetexture that reduces settlement of B. improvisusmay be preferred by other foulers, e.g. the blue-mussel M. edulis and hydroids such as Laomedeaflexuosa. The success of including one or severalscales of surface texture is a difficult optimisationproblem, which will differ among applicationsand for different geographical areas.

Acknowledgements

Financial support was provided through theMASTEC programme by the Swedish Foundationfor Strategic Research and by the foundation ofColliander to KMB and PRJ. We finally wish toacknowledge Patrick Gladh for his assistance in

the field experiments, Anders Martensson forsupplying us with SEM pictures, and twoanonymous reviewers who provided insightfulcomments that improved the manuscriptsubstantially.

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