Langmuir 2002, 18, 8929-8932
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Experimental Procedure To Measure the Blocking Energy of Liquid on Smooth Hydrophobic Surfaces Olivier Albenge,*,†,§ Colette Lacabanne,† Jean-Denis Beguin,‡ Alain Koe¨nen,§ and Catherine Evo§ Laboratoire de Physique des Polyme` res, CIRIMAT, Universite´ Paul Sabatier, 118 route de Narbonne, 31065 Toulouse Cedex, France; LGT, ENIT, 47 avenue d’Azereix, 65016 Tarbes Cedex, France; and Valeo Syste` mes d’Essuyage, La Verrie` re-Issoire, France Received June 19, 2002. In Final Form: September 2, 2002 Smooth hydrophobic polymer treatments on inorganic surfaces were prepared. Their macroscopic properties (static contact angle and advancing and receding contact angles) were evaluated, and each surface structure was finely characterized using AFM measurements. Our work dwells on the difficulties of characterizing hydrophobic surfaces: it must be done not only in terms of work of adhesion but also in terms of blocking energy. A new mechanical protocol is used to directly measure this blocking energy of water droplets between smooth hydrophobic surfaces, and the results obtained using this simple model are in close agreement with theoretical energies. This measure allows us to distinguish two surfaces that otherwise should be considered as having the same properties considering their work of adhesion.
I. Introduction The studies of Young,1 with his famous equation connecting surface tension to contact angle between solid and liquid at the triple point, are at the origin of the modern perception of wettability. Among the various possible situations of damping, the partial wetting of solids by liquids with a static contact angle θ superior to 90° is of increasing interest, as well at the fundamental level as at the practical level. Indeed, various fields of research, such as petrochemistry, biology, and adhesion, to quote only some of them, are highly interested in the comprehension and the control of wettability. Since Shafrin and Zisman’s work,2 several determination and/or calculation methods of surface energy were developed to finely characterize hydrophobic surfaces.3 But despite the apparent simplicity of those techniques (measurement of contact angles of liquids on solids), many controversies still exist today on the concept of hydrophobicity. This is mainly due to the fact that one of the most difficult subjects to comprehend, as explained recently by Della Volpe,4 relates to the classification of hydrophobic surfaces. First of all, the concept of hydrophobic behavior can be defined in terms of work of adhesion of a liquid drop on a hydrophobic surface. The work of adhesion is related to the formation (or to the elimination) of interfacial area between, on the one hand, the liquid and the solid as two separated bodies and, on the other hand, a system composed of the drop lying on the solid. This work, taking into account the base area of the drop, is defined as follows:5 * To whom correspondence should be addressed. E-mail:
[email protected]. † Laboratoire de Physique des Polyme ` res, CIRIMAT, Universite´ Paul Sabatier. § Valeo Syste ` mes d’Essuyage. ‡ LGT, ENIT. (1) Young, T. Trans. R. Soc. (London) 1805, 95, 65-87. (2) Shafrin, E.; Zisman, W. J. Phys. Chem. 1957, 61, 1046. (3) Good, R. J. Adhesion Sci. Technol. 1992, 6, 1269-1302. (4) Della Volpe, C.; Siboni, S.; Morra, M. Langmuir 2002, 18, 14411444.
Wadh ) γLV(1 + cos θ) ×
{ [(3Vπ ) π
]}
sin θ (2 + cos θ)1/3(1 - cos θ)2/3
1/3
2
(1)
where γLV is the liquid surface tension, θ is the contact angle, and V is the volume of the drop. This equation is commonly expressed without the part between brackets. This part corresponds to the contact area of the drop on the solid, which is calculated using a geometric consideration. This equation is based on the hypothesis of an “ideal solid surface” having only one single static contact angle (according to the primary definition of Young). Thus, for a given drop volume, the more hydrophobic the solid material is, the higher the contact angle will be. The consequence is a weaker solid/liquid contact surface and a weaker work of adhesion. But “real” solid surfaces cannot be considered to have one single static contact angle. From surface roughness, on the one hand, and physical and chemical heterogeneities, on the other hand,6-8 numerous local energy minima are generated for the system, so that it is not possible to determine one single static contact angle θ . This fact is characterized by the hysteresis of the contact angle H, defined by
H ) θ A - θR
(2)
where θA and θR are respectively the advancing and the receding contact angle. The existence of such a hysteresis explains, for example, the sticking of droplets on a tilted hydrophobic surface:9 there is a competition between the gravitational energy (5) Dupre´, A. The´ orie Me´ canique de la Chaleur; Gauthier-Villars: Paris, 1869; p 369. (6) Wenzel, R. Ind. Eng. Chem. 1936, 28, 988-994. (7) Schuttleworth, R.; Bailey, G. Discuss. Faraday Soc. 1948, 3, 1622. (8) Elkin, B.; Mayer, J.; Schindler, B.; Vohrer, U. Surf. Coat. Technol. 1999, 116, 836-840. (9) Que´re´, D.; Azzopardi, M.-J.; Delattre, L. Langmuir 1998, 14, 2213-2216.
10.1021/la026092i CCC: $22.00 © 2002 American Chemical Society Published on Web 10/15/2002
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Scheme 1. Scheme of the Device Used To Determine Static Contact Angles (Left Part) and Advancing and Receding Contact Angles (Right Part) of Droplets on Samples
Figure 1. Image of the film of a 15 µL drop on sample A tilted just before the drop sliding, for measuring advancing and receding contact angles.
and the blocking energy of the drops (Eb) on the surface. This energy (by surface area) is expressed by the Furnidge’s equation:10
Eb ) γLV(cos θR - cos θA) )
mg sin R w
(3)
where m is the mass of the drop, g is the gravity, R is the tilting of the plane with respect to horizontal, and w is the transverse diameter of the drop. From the preceding equation, one can deduce that the blocking energy, which only exists on “real” surfaces, is related to local defects of the solid surface. This point is clearly shown on the left-hand side of this equality by the presence of the advancing and receding contact angles and on the right-hand side of the equality by the presence of w. The local minima of energy created by those defects are blocking the sliding of liquid on a tilted solid.11 Eb is then representative of the local hindrance of the solid/ liquid interface: the lower Eb is, the lower the sliding angle is. Until now, the determination of the blocking energy can only be deduced starting from the measurement of advancing and receding angles and/or by the use of the critical sliding angle. In this paper, we will first characterize macroscopically and microscopically solid surfaces; then we will describe a new method allowing us to determine experimentally the blocking energy of a surface from mechanical measurements.
Figure 2. AFM image of sample A in tapping mode (arbitrary unit on z axis). Scheme 2. Schematic Representation of Droplets between Parallel Sample Plates in the Dynamical Mechanical Spectrometer
II. Materials and Methods II.1. Materials. We used for this study three types of hydrophobic surface treatments deposited onto glass sheets of 3 cm × 3 cm. Two coatings contained fluorinated polymers (samples A and B), and the third one was a deposit of poly(dimethylsiloxane) (PDMS) (noted C). All these samples were prepared according to the same protocol. After the glass plates were degreased with acetone and cleaned with 2-propanol, deposition was carried out in a liquid way by spreading out using soft tissue (two horizontal runs and two vertical ones). After 24 h, we eliminated the excess deposit by cleaning with 2-propanol. II.2. Methods. Advancing and Receding Contact Angles. To determine experimentally the principal parameters of hydrophobic surfaces, an apparatus was constructed as schematically illustrated in Scheme 1. It is composed of a plane sample-holder on which the hydrophobic glass was set. This sample-holder can be tilted from 0 to 90°. A numerical camera with an optical zoom ×10 was fixed at the sample-holder. After a 15 µL drop was deposited on the treated glass, an image was taken to determine the static contact angle θ (left part of Scheme 1). Then the plate was tilted and the drop was filmed (right part of Scheme 1). The film was analyzed in order to determine the exact image where the drop starts to slip onto the sample surface. The preceding image of the film (as shown on Figure 1) was extracted for determining the advancing contact angle θA and the receding (10) Furnidge, G. J. Colloid Interface Sci. 1962, 17, 309-324. (11) Jonhson, R.; Dettre, R. Adv. Chem. Ser. 1964, 43, 112-136.
contact angle θR of the liquid on the sample. The hysteresis of the contact angle H could then be calculated. For each sample, measurements were repeated nine times. Surface Morphology. We have finely characterized the surface morphology of the various samples by atomic force microscopy (Nanoscope III, Digital Instruments). Each measurement was performed on a 100 µm × 100 µm area. Average roughness (Ra) and an average value of the specific increase in surface area (%S dr) were calculated on the basis of five different images per sample (Figure 2). Dynamic Mechanical Analysis. To determine the blocking energy between hydrophobic glass and water droplets, we set up a protocol of measurement with a dynamic mechanical spectrometer (ARES, Rheometric Scientific) in “Dynamic Strain Sweep” mode and in parallel plates configuration. After four 2 µL droplets were deposited onto the lower plate and the gap between plates was set at 100 µm (Scheme 2), the mechanical stress is gradually increased at a frequency of 10 rad‚s-1 at
Blocking Energy of Liquid on HydrophobicSurfaces
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Table 1. Morphological Data Obtained by Analysis of AFM Images sample
Ra (nm)
%S dr (%)
ra
A B C
25.68 ( 0.03 30.30 ( 0.02 58.41 ( 0.09
0.57 ( 0.08 0.60 ( 0.04 1.60 ( 0.05
1.006 1.006 1.016
a r represents the Wenzel’s coefficient calculated from the specific surface area increase of the samples (%S dr): r ) 1 + %S dr.
constant temperature (20 °C). The torque transmitted to the higher plate is recorded thanks to the integrated torque sensor.
III. Results and Discussion AFM measurements allow us to characterize the surface of the three samples. The values of average roughness (Ra) and the average value of the specific increase in surface area (%S dr) are displayed in Table 1. The low values of Ra allow us, in a rough estimate, to consider the plates as smooth samples. This is confirmed by the low values of %S dr, which imply a Wenzel’s coefficient (r) very close to 1: the surface roughness created by the deposits on the glass plates can be considered as negligible, and thus the static contact angle of water θ will not be artificially increased. The contact angle can thus be given on these surfaces while being reasonably regarded as the static contact angle within the meaning of Young. We have experimentally determined the values of static contact angles thanks to our apparatus; droplets of 15 µL have been used to overcome the sticking of droplets when the sample-holder is tilted. As the radius of the solid/ liquid contact area of 15 µL droplets (about 1.8 mm in our cases) is lower than the capillary length of water κ-1 (which is 2.7 mm), we can neglect gravitational effects. The static contact angles of samples A, B, and C are respectively 102.7°, 102.8°, and 95.4°. Considering those values, one can deduce that, for a drop of a given volume, the surface area of contact between the drop and the solid will be the lowest for sample A and the highest for sample C. Those contact angle values also allow us to classify these surfaces in term of work of adhesion. Results are shown in Table 2. According to those results, we can deduce that samples A and B have the same hydrophobicity (in terms of work of adhesion) and that C is less hydrophobic. Thanks to our apparatus, we also have measured advancing and receding contact angles for the samples. According to eq 3, it is then possible to determine the theoretical blocking energy Eb/mth of those surfaces using those angles (see data in Table 2). Figure 3 shows the variation of the torque transmitted to the higher plate by one, two, three, or four droplets of 2 µL between two surfaces of type A during a strain increase. We can easily notice the linearity of the response according to the number of drops in contact between the two plates. The curves can be broken up into two parts. The first one (noted a) corresponds to the progressive elastic deformation of the drops forced between the two parallel plates: when the deformation imposed by the lower plate increases, the liquid transmits an increasing strain to the higher plate (i.e. the liquid structure less
Figure 3. Transmitted torque plotted against deformation for one (times sign), two (diamond), three (up-triangle), and four (square) 2 µL droplets between two glass sheets of sample A.
Figure 4. Transmitted torque plotted against deformation for four 2 µL droplets between two glass sheets of sample A (square), sample B (up-triangle), and sample C (dot).
and less compensates the strain). In part b, the transmitted torque is rapidly decreasing until it reaches a plateau. After elastic and viscoelastic deformation of the drops, the slipping of the drops onto the hydrophobic surface starts. Thus, the maximum of this second zone corresponds to the beginning of friction at the liquid/solid interfaces. If the experiment is continued by further increasing the strain (not represented in Figure 3), the torque suddenly decreases and then gradually increases to reach its preceding maximum level. This evolution is due to a jerked friction of the drop onto one surface. Figure 4 represents the variation of the torque transmitted to the higher plate as a function of deformation by four 2 µL droplets between two identical plates, for samples A, B, and C. The maximum torque values (Γmax) are displayed in Table 3. First of all, one can see that the behaviors of samples A and B are very different, even if their work of adhesion is quite analogous! It proves that the work of adhesion is not sufficient to finely characterize a hydrophobic solid. As there are four drops in contact with two surfaces, there are eight areas of contact. At its maximum, the transmitted torque corresponds to the total energy of
Table 2. Work of Adhesion and Blocking Energy Calculated from Eqs 1 and 3 and Experimental Contact Angles Obtained from 15 µL Droplets
a
sample
θ (deg)
θA (deg)
θR (deg)
Wadha (J)
Eb/m2th b (mJ/m2)
A B C
102.7 ( 1.0 102.8 ( 0.4 95.4 ( 1.1
112.6 ( 2.0 112.4 ( 2.8 96.8 ( 1.1
92.8 ( 1.4 95.7 ( 2.0 84.5 ( 1.4
5.2 × 10-7 ( 0.2 × 10-7 5.2 × 10-7 ( 0.1 × 10-7 7.0 × 10-7 ( 0.3 × 10-7
24.45 ( 1.18 20.53 ( 1.26 15.61 ( 0.72
Work of adhesion calculated using eq 1. b Theoretical blocking energy, expressed per unit of area, calculated with eq 3.
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Table 3. Comparison between Theoretical and Experimental Blocking Energy Values Γmax (×10-6 N‚m)
sample
3.97 ( 0.01 3.28 ( 0.01 2.48 ( 0.03
A B C
Ebexp (×10-7 J) 4.96 ( 0.01 4.09 ( 0.01 3.09 ( 0.03
Eb/m2th (mJ/m2) 24.45 ( 1.18 20.53 ( 1.26 15.61 ( 0.72
Sa (m2) 10-5
2.00 × 2.00 × 10-5 2.00 × 10-5
Ebth (×10-7 J)
∆[Ebexp/Ebth]b (%)
4.90 ( 0.24 4.11 ( 0.25 3.12 ( 0.14
1.21 0.49 0.97
a The contact area between droplets and plates. b ∆[E exp/E th] is the percentage of discrepancy between the theoretical and experimental b b blocking energies.
Table 4. Comparison between Experimental and Calculated Advancing and Receding Contact Angles
a
sample
θAa (deg)
θRa (deg)
Hc (deg)
θAb (deg)
θRb (deg)
Hc (deg)
A B C
112.6 ( 2.0 112.4 ( 2.8 96.8 ( 1.1
92.8 ( 1.4 95.7 ( 2.0 84.5 ( 1.4
19.8 16.7 12.3
113.0 ( 1.1 111.2 ( 0.4 101.5 ( 1.2
92.8 ( 1.1 94.7 ( 0.4 89.3 ( 1.2
20.2 16.5 12.2
Angles experimentally obtained. b Angles calculated using eq 5. c Hysteresis deduced using eq 2.
friction. The experimental blocking energy Ebexp for a solid/ liquid interface is thus equal to 1/8 of the maximum torque measured (Γmax). As the distance between the plates is relatively small (fixed to 100 µm), we can approximate the 2 µL drops’ geometry to a cylinder. In those conditions, the solid/liquid contact area for each drop is expected to be 2 × 10-5 m2. The knowledge of this value allows us to deduce the theoretical blocking energy for our system Ebth, starting from the theoretical blocking energy per unit of surface previously calculated Eb/m2th. Results displayed in Table 3 exhibit the discrepancy between theoretical and experimental values. The obtained results are showing a very good correlation, which validates, on the one hand, the experimental system and, on the other hand, our starting assumption, that is, hydrophobic surfaces can be considered as “ideally smooth”. So, we prove that it is possible to experimentally measure the blocking energy of such a system. Using this measurement, it is also possible to determine other parameters of the surface. From the formula stated by Della Volpe in ref 12, it is possible to express the static contact angle of “real” surfaces according to their advancing and receding angles:
cos θ )
{
1 1 cos θA + cos θR 2 2
(4)
By combining eqs 3 and 4, one obtains
cos θA ) cos θ -
( ) ( ) Ebexp 2SγLV
Ebexp cos θR ) cos θ + 2SγLV
(5)
Contact angles obtained by the experimental method, and those deduced from the measurement of the energy of blocking using eq 5, are reported in Table 4. Note that (12) Della Volpe, C.; Maniglio, D.; Siboni, S.; Morra, M. Oil Gas Sci. Technol. 2001, 56, 9-22.
there is a really good correlation between these values. For all samples, hysteresis data obtained using measured contact angles and calculated using eq 5 angles are very closed. For sample C, the calculated advancing and receding contact angles are slightly higher than the measured ones. This lag might be explained by a Wenzel’s coefficient higher than that for the other samples: sample C is slightly rougher than the others (see Table 1). Then eq 4 is not strictly applicable, and results obtained using eq 5 are slightly higher. Nevertheless, θA and θR increase in equal proportions, so that the calculated value of H is in good agreement with its measured value. IV. Conclusion The present study is devoted to the analysis of smooth hydrophobic polymeric treatments on inorganic surfaces that we prepared for this purpose. Their surface properties were characterized by static contact angle and advancing and receding contact angle, and their surface structure was finely analyzed using AFM measurements. Our work first put a stress on the difficulties of characterizing hydrophobic surfaces: it must be done, not only in terms of work of adhesion but also in terms of blocking energy. A new mechanical protocol has been proposed to directly measure this blocking energy between those surfaces and water droplets, and we have shown that the corresponding results are in close agreement with calculated energies. This measure allows us to distinguish two surfaces that should be considered as having the same properties, considering their work of adhesion. This direct measurement is nevertheless restricted to surfaces with low roughness (i.e. having a Wenzel’s coefficient very close to 1). Then, by simply using a measurement of static contact angle and an experimental determination of the blocking energy, it is possible to calculate the characteristics of smooth hydrophobic materials, such as advancing and receding contact angle (and thus hysteresis). Such an approach might improve the knowledge of wettability and of hydrophobic properties of solid surfaces. LA026092I
