Contact angle hysteresis on nano-structured surfaces

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<ul><li><p>nha</p><p>ructur</p><p>18, 69</p><p>4040</p><p>pted</p><p>Abstract</p><p>1. Introduction</p><p>two dierent values depending on the contact line</p><p>motion; it advances with a contact angle hA and</p><p>recedes with hR (hA &gt; hR). In the last 20 years</p><p>has been under investigations for many years, the</p><p>experimental studies performed up to now dealtessentially with surfaces structured at the micro-</p><p>scopic scale, with patterns of a few 10s of mi-</p><p>crometers in size. To date, in spite of the many</p><p>fundamental and potential applications, the ex-*</p><p>Surface Science 540 (2003)Corresponding author. Tel.: +33-0-472-431-218; fax: +33-0-</p><p>472-431-592.It is now well established that the wettingproperties of the real solid surfaces are strongly</p><p>aected by their roughness and chemical hetero-</p><p>geneities. In many practical situations these het-</p><p>erogeneities pin the contact line inducing a contact</p><p>angle hysteresis. In this case, the static angle be-</p><p>tween the solid surface and the uid interface takes</p><p>much theoretical and experimental work has been</p><p>devoted to investigation of the motion of a uidinterface on an heterogeneous surface and of the</p><p>contact angle hysteresis [110]. From an experi-</p><p>mental point of view, it has been shown that</p><p>chemical heterogeneities and geometrical defects</p><p>(roughness) do not have a similar eects on the</p><p>hysteresis. Although the contact angle hysteresisWe present results from an experimental study on the phenomenon of contact angle hysteresis on solid surfaces</p><p>decorated by a random array of nanometric hollows. For weak values of the areal density of defects /d, the hysteresis Hincreases linearly with /d. This evolution is described by a pinningdepinning process of the contact line by individualdefects. At higher values of /d, a collective pinning eect appears and H decreases with increasing /d. In the linearregime, our experimental results are compared to theoretical predictions for contact angle hysteresis induced by a single</p><p>isolated defect on the solid surface. We suggest that the crossover from the individual to the collective pinning eects</p><p>could be interpreted in terms of an overlapping of wetting cross sections. Finally, we analyse the inuence of both the</p><p>size and the morphology (hollows/hillocks) of defects on the anchorage of the contact line.</p><p> 2003 Elsevier B.V. All rights reserved.</p><p>Keywords: Contact; Wetting; Surface defects; Ion bombardmentContact angle hysteresis o</p><p>S.M.M. Ramos a,*, E. Ca Laboratoire de Physique de la Matieere Condenseee et Nanost</p><p>43 Boulevard du 11 Novembre 19b CIRIL, B.P. 5133, F-1</p><p>Received 7 March 2003; acceE-mail address: ramos@dpm.univ-lyon1.fr (S.M.M. Ra-</p><p>mos).</p><p>0039-6028/$ - see front matter 2003 Elsevier B.V. All rights reservdoi:10.1016/S0039-6028(03)00852-5nano-structured surfaces</p><p>rlaix a, A. Benyagoub b</p><p>es (UMR CNRS 5586) Universitee Claude Bernard LYON I,622 Villeurbanne Cedex, France</p><p>Caen Cedex 5, France</p><p>for publication 3 June 2003</p><p>355362</p><p>www.elsevier.com/locate/suscperimental investigations taking into account the</p><p>roughness eects at a nanometer range still remain</p><p>ed.</p></li><li><p>limited, probably due to the diculties usually</p><p>encountered for the preparation of well charac-</p><p>terized surfaces with controlled defect distribu-</p><p>tions.</p><p>Recently, we have attacked the problem of</p><p>wetting on nano-rough surfaces by investigatingthe phenomenon of contact angle hysteresis on</p><p>surfaces structured by a random distribution of</p><p>nanometric hillocks [11].</p><p>The aim of the present work is to study the</p><p>eects of nanometric hollows on the contact angle</p><p>hysteresis. In these experiments, we analyse the</p><p>inuence of both the size and the morphology</p><p>(hollows or hillocks) of defects on the contact linemotion. For this purpose, we have processed our</p><p>surfaces by using swift heavy ion (SHI) irradia-</p><p>tions. As we reported in the recent past, the pro-</p><p>jectile impacts at the glass slices surface induce the</p><p>2. Experimental section</p><p>2.1. Surface processing</p><p>The starting material is constituted of glass sli-ces with a low roughness value purchased from</p><p>Prolabo. The surfaces of these slices were pro-</p><p>cessed twice: rst, hollows of nanometric size were</p><p>created by using swift heavy ions irradiations at</p><p>Grand Acceeleerateur National dIons Lourds(GANIL). In order to modify the hollow dimen-</p><p>sions, two dierent kinds of ions were used: 208Pb</p><p>ions of 250 MeV energy and 78Kr of 240 MeVenergy. All the irradiations were performed at</p><p>room temperature with uences extending from</p><p>5 108 to 8 1010 cm2. For the dierent irradia-tion conditions we have listed in Table 1, the</p><p>corresponding average distance (k) between two</p><p>betw</p><p>the s</p><p>P</p><p>h</p><p>1</p><p>1</p><p>1</p><p>1</p><p>1</p><p>1</p><p>1</p><p>1</p><p>356 S.M.M. Ramos et al. / Surface Science 540 (2003) 355362formation of nanometric hollows [12]. The hollow</p><p>dimensions depend on both the ion mass and the</p><p>irradiation energy, whereas the concentration of</p><p>such structures and consequently the surfaceroughness is governed by the irradiation uence.</p><p>The role played by topographical defects in the</p><p>contact angle hysteresis phenomenon is investi-</p><p>gated here via two complementary techniques:</p><p>atomic force microscopy AFM observations and</p><p>contact angle measurements.</p><p>Table 1</p><p>Irradiation uence /d (hollow concentration), average distance(advancing and receding) contact angles hA and hR collected for</p><p>/d (109ions cm2)</p><p>k (nm) aPb (%) aKr (%)</p><p>Reference</p><p>surface</p><p> 0 0</p><p>0.5 447 1.8 </p><p>1.0 316 3.5 </p><p>3.0 182 10.0 </p><p>5.0 141 1.6</p><p>6.0 129 19.0 </p><p>7.0 119 2.3</p><p>10.0 100 29.7 3.3</p><p>15.0 82 41.0 </p><p>20.0 71 50.5 6.4</p><p>40.0 50 75.5 </p><p>60.0 41 87.9 17.8</p><p>80.0 35 23.2The contact angle measurements were done optically with an accuracneighbouring defects and the fraction of the sur-</p><p>face covered by the defects (a). The a values weredetermined by using the following relation, whichis based on a direct impact model [13]</p><p>a 1 exp/dA 1where A is the defect cross-section and /d is theirradiation uence.</p><p>Second, in order to obtain contact angles large</p><p>enough to be accurately measured and to have a</p><p>een hollows k, coverage rate of the surface by defects a, andamples irradiated with Kr or Pb ions</p><p>b Kr</p><p>A () hR () hA () hR ()</p><p>00.0 89.0 100.0 89.0</p><p>00.0 86.0 </p><p>01.0 84.0 </p><p>02.0 83.0 </p><p> 101.0 89.0</p><p>05.0 83.0 </p><p> 104.0 91.0</p><p>03.0 83.0 103.0 86.0</p><p>02.0 83.0 </p><p>01.0 84.0 106.0 87.0</p><p>99.0 86.0 </p><p>96.0 86.0 101.0 88.0</p><p> 99.0 90.0y of 3.</p></li><li><p>temperature in a saturated atmosphere in order to</p><p>minimize the evaporation problems. At least ve</p><p>dierent measurements were performed on dier-</p><p>ent areas of each sample. This procedure allows</p><p>one to take into account a possible non-uniformity</p><p>of the surface probed by the contact angle.</p><p>3. Results and discussion</p><p>3.1. Surface topography</p><p>Fig. 1 shows two 3D views of AFM micro-</p><p>graphs recorded, respectively, on a pristine and anirradiated surface. The circular hollows visible in</p><p>Fig. 1b result from the individual interaction of</p><p>each energetic ion at the solid surface. As we re-</p><p>ported in the recent past [12], this specic eect</p><p>(i.e., crater formation in the amorphous silica)</p><p>supports the previous observation of the isotropic</p><p>compaction measured by a decrease of the width</p><p>of the irradiated sample [14]. For all samples</p><p>ace Science 540 (2003) 355362 357wide range of variation of the contact angle hys-</p><p>teresis, octadecyltrichlorosilane (OTS) molecules</p><p>were grafted on the tailored surfaces. The prepa-</p><p>ration method of these surfaces is explained below</p><p>briey. Prior to the OTS deposition, the samples</p><p>were cleaned according to the following procedure:rinsing thoroughly in hot water with Micro90 de-</p><p>tergent in an ultrasonic bath for 30 min, andrinsing again in deionised water. The cleaned</p><p>substrates were immersed in a reaction bath con-</p><p>sisting of a freshly prepared millimolar solution of</p><p>OTS in toluene (Prolabo, 99%) for 1 h. To remove</p><p>all excess reactants, the samples were rinsed in two</p><p>successive chloroform ultrasound baths. Finallythe samples were rinsed in deionised water, and</p><p>excess water droplets were blown away with dry</p><p>N2 gas. In order to provide a reference surface the</p><p>OTS molecules were also grafted on a virgin</p><p>sample.</p><p>2.2. Surface characterization</p><p>An Explorer Nanoscope atomic force micro-</p><p>scope (AFM) operating in a tapping mode was</p><p>used. The surfaces were probed under ambient</p><p>conditions with silicon tips having a nominal ra-</p><p>dius curvature of 15 nm. Each sample was pro-bed at least three dierent locations, the images</p><p>were attened (to eliminate the experimentally</p><p>obtained bowing eects) and no other ltering wasdone. Prior to characterize the topographic mod-</p><p>ications induced at the irradiated surfaces, both</p><p>the pristine sample and the as-grafted monolayer</p><p>onto glass slices were examined by AFM. The rms</p><p>roughness values, measured on a scanned zone of 1</p><p>lm2, are of 0.07 and 0.13 nm, respectively. Theselow roughness values evidence the high quality of</p><p>both the starting sample and the monolayergrafting, which constitute our reference surfaces.</p><p>The sessile drop method was used to charac-</p><p>terize the wettability properties of processed sur-</p><p>faces. The samples were introduced in a glass</p><p>chamber and the water drop was put onto the</p><p>substrate through a microsyringe. Steady-state</p><p>advancing contact angles, hA, and receding contactangles hR, were measured using 1 ll drops ofdeionized water (with a surface tension c 72:0</p><p>S.M.M. Ramos et al. / SurfmNm1). All measurements were made at room</p><p>Fig. 1. 3D view of a virgin (a) and an irradiated (/d 1010</p><p>Pb cm2) sample (b).</p></li><li><p>ace S358 S.M.M. Ramos et al. / Surfinvestigated a good agreement between the hol-</p><p>lows density and the irradiation uence was found.</p><p>Prole line scans performed over exposed areasare given in Fig. 2, where the marks indicate the</p><p>border between the damaged zone and the sur-</p><p>rounding region. For the study of the hollow di-</p><p>mensions more than 40 impacts were analyzed and</p><p>both a mean basal diameter D and a mean depth hwere extracted from these measurements. The</p><p>following values were obtained: DPb 72 6 nm;</p><p>Fig. 2. Top view and surface proles recorded on glass slices: (a) irradi</p><p>1010 Pb cm2 of 250 MeV energy and (c) irradiated sample (/d 6cience 540 (2003) 355362hPb 3 1 nm; DKr 22 3 nm; hKr 0:6 0:2nm for Pb ions and Kr ions, respectively. It is</p><p>worth noticing that the condence in the depthmeasurements decreases at low hollow diameters.</p><p>For small diameters, close to the tip probe di-</p><p>mensions, the measured values are probably un-</p><p>derestimated.</p><p>Finally, the surface characterization was com-</p><p>pleted by AFM observations after OTS grafting on</p><p>the irradiated samples. A typical result is displayed</p><p>ated with 5 109 Kr cm2 of 450 MeV energy; (b) irradiated with109 Pb cm2) after OTS grafting.</p></li><li><p> At low defect concentration, /d &lt; /c (where /c 6 109 defects cm2 for Pb and /c 20 109defects cm2 for Kr), H grows linearly with /d.This variation suggests that each defect pins</p><p>the contact line individually. The slope of the</p><p>straight line tting the experimental points cor-responds to the total energy W dissipated byone defect on the hysteresis cycle. From our ex-</p><p>periments we found the following energy values:</p><p>WPb 2:2 1016 Nm and WKr 5:7 1017Nm.</p><p> At higher defect concentration (/d &gt; /c), H de-creases with increasing /d. This behaviour indi-cates that, despite the rather large mean distancewhich remains between two individual impacts</p><p>(b)0</p><p>5</p><p>10</p><p>15</p><p>20</p><p>25</p><p>30</p><p>0 5 10 15 20 25 30</p><p>d (defects.cm-2)</p><p>H(m</p><p>N.m</p><p>-1 )</p><p>Fig. 3. (a) Contact angle hysteresis H and wettability contrastDS versus the areal defect concentration /d and (b) zoom of theH curve in the low /d regime.</p><p>ace SFig. 2c. No signicant patterns formation at the</p><p>surface is observed and the lm seems to follow</p><p>very well the surface topography. Meanwhile, the</p><p>OTS grafting induces a small (7%) decrease inthe hollow dimensions. The following values for</p><p>the average diameter d and for the mean depth h</p><p>were found: dPb 68 7 nm; hPb 2:7 0:8 nm;dKr 20:5 2:5 nm; hKr 0:4 0:2 nm, for Pbions and Kr ions, respectively. From the ensemble</p><p>of our observations we have no evidence about the</p><p>OTS deposition inside the hollows.</p><p>3.2. Wetting properties</p><p>3.2.1. Data processing</p><p>From the experimental measurements of both</p><p>advancing (hA) and receding (hR) contact angles(given in Table 1) we have determined, for each</p><p>processed surface, two fundamental parameters</p><p>for the wetting investigations. The rst one is the</p><p>wettability contrast, i.e., the dierence of spread-</p><p>ing coecient on a solid surface with chemicaldefects Sd and a pristine one, S0. This parameter isdened as DS Sd S0; ccos hd cos h0 wherecos hi (i d or 0) is the average contact angle de-ned as cos hi cos hA cos hR=2. The secondparameter is the contact angle hysteresis, dened</p><p>as H ccos hR cos hA. The values obtained forthese two parameters in our reference (unirradi-</p><p>ated) surface are DS 0 mNm1, H 13:2mNm1, respectively.</p><p>The evolution of both the wettability contrast</p><p>DS and the contact angle hysteresis H versus thedefect concentration is displayed in Fig. 3. The</p><p>gure shows that DS varies very slightly with thedefect density. Although not shown here, a similar</p><p>evolution is observed on surfaces irradiated with</p><p>Kr ions. In both cases, one can estimate from dataof Table 1 and Fig. 3 that the chemical heteroge-</p><p>neity does not exceed DcSV cSl 4 mNm1,indicating that a chemical heterogeneity of the</p><p>defects, due to an eventual small variation in the</p><p>coverage of the surface by OTS molecules, can be</p><p>reasonably neglected.</p><p>From Fig. 3 it is clearly evidenced that the</p><p>contact angle hysteresis is not a monotonic func-tion of the defect concentration. Two dierent re-</p><p>S.M.M. Ramos et al. / Surfgimes are identied:(a)</p><p>0</p><p>5</p><p>10</p><p>15</p><p>20</p><p>25</p><p>30</p><p>0</p><p>5</p><p>10</p><p>15</p><p>20</p><p>25</p><p>30</p><p>0 20 40 60 80 100</p><p>d (defects.cm-2)</p><p>H(m</p><p>N.m</p><p>-1 )</p><p>S(mN</p><p>.m-1)</p><p>PbKr</p><p>cience 540 (2003) 355362 359(e.g., for /d 2 1010 Kr cm2 the average</p></li><li><p>tion. The latter force is proportional to the</p><p>deformation of the contact line. In this approach,the maximum amplitude of the contact line de-</p><p>formation gmax can be determined by the followingrelation: gmax f =K.</p><p>From our experimental results we determine the</p><p>value of three parameters: the spring constant K,the interacting force between the surface and the</p><p>triple line f and the maximum amplitude of thecontact line distortion gmax. In our calculations weassume that the range of deformation of the con-</p><p>tact line L corresponds to the capillary length j1</p><p>of the liquid and we use the following experimental</p><p>data: j1 2:7 mm, c 72 mNm1 and h0 95.The values obtained for f and gmax are presentedin Table 2. These results lead to the following re-</p><p>marks: (i) the defect force f is directly correlated tothe defect size, in agreement with what is expectedfrom the wetting of a heterogeneous surface with</p><p>purely geometrical defects; (ii) the maximum de-</p><p>Fig. 4. Schematic representation of both the distortion of the</p><p>contact line induced by one isolated defect (a) and the inter-</p><p>action of two wetting cross-sections. This interaction occurs</p><p>when k &lt; 2gmax (b).</p><p>ace Samplitude of the line deformation. The deforma-</p><p>tion of the contact line induced by the presence of</p><p>a defect is schematised in Fig. 4a. This deforma-</p><p>tion is assumed to result from a topographical</p><p>defect on a planar surface. Considering...</p></li></ul>

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