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Contact Angle Hysteresis on Polymer Substrates Established with Various Experimental Techniques, Its Interpretation, and Quantitative Characterization Edward Bormashenko,*,† Yelena Bormashenko,† Gene Whyman,† Roman Pogreb,† Albina Musin,† Rachel Jager,† and Zahava Barkay‡ Ariel UniVersity Center of Samaria, The Research Institute, 40700 Ariel, Israel, and Wolfson Applied Materials Research Center, Tel AViV UniVersity, Ramat-AViV 69978, Israel ReceiVed December 12, 2007. In Final Form: January 16, 2008 The effect of contact angle hysteresis (CAH) was studied on various polymer substrates with traditional and new experimental techniques. The new experimental technique presented in the article is based on the slow deformation of the droplet, thus CAH is studied under the constant volume of the drop in contrast to existing techniques when the volume of the drop is changed under the measurement. The energy of hysteresis was calculated in the framework of the improved Extrand approach. The advancing contact angle established with a new technique is in a good agreement with that measured with the needle-syringe method. The receding angles measured with three experimental techniques demonstrated a very significant discrepancy. The force pinning the triple line responsible for hysteresis was calculated.
1. Introduction The phenomenon of contact angle hysteresis (CAH) has been one of the most studied topics in surface science in two past decades.1-15 Interest in CAH is stipulated by the fact that CAH governs the wetting properties of the solid surface to a large extent. It is already well accepted not only that the equilibrium (Young) contact angle is essential for understanding the wetting but also that CAH dictates a diversity of phenomena occurring at the liquid/solid interface, including the spreading and sliding of drops. Thus, CAH becomes essential in a variety of technological processes, such as filtration, printing, impregnation of textiles, and so forth. At the same time, experimental and theoretical data concerning CAH are still contradictory, and the low reproducibility of experimental results is noteworthy, especially for widespread polymer substrates. Various experimental techniques were applied to the study of CAH. These are the needle-syringe method, the incline plane * Corresponding author. E-mail:
[email protected]. † Ariel University Center of Samaria. ‡ Tel Aviv University.
(1) de Gennes, P. G.; Brochard-Wyart, F.; Que˘ re˘ , D. Capillarity and Wetting Phenomena; Springer: Berlin, 2003. (2) Erbil, H. Y. Surface Chemistry of Solid and Liquid Interfaces; Blackwell Publishing: Oxford, England, 2006. (3) Yaminsky, V. V. Molecular Mechanisms of Hydrophobic Transitions, in Apparent and Microscopic Contact Angles, Drelich, J., Laskowski, J. S., Mittal, K. L., Eds.; VSP BV: Utrecht, The Netherlands, 2000; pp 47-95. (4) Lam, C. N. C.; Wu, R.; Li, D.; Hair, M. L; Neumann, A. W. AdV. Colloid Interface Sci. 2002, 96, 169-191. (5) Tavana, H.; Neumann, A. W. Colloids Surf., A 2006, 282-283, 256-262. (6) Chibowski, E.; Perea-Carpio, R. AdV. Colloid Interface Sci. 2002, 98, 245-264. (7) Chibowski, E. AdV. Colloid Interface Sci. 2003, 103, 149-172. (8) Shanahan, M. E. R. Langmuir 1995, 11, 1041-1043. (9) Nosonovsky, M. J. Chem. Phys. 2007, 126, 224701. (10) Li, W.; Amirfazli, A. AdV. Colloid Interface Sci. 2007, 132, 51. (11) Marmur, A. Soft Matter 2006, 2, 12-17. (12) Extrand, C. W.; Kumagai, Y. J. Colloid Interface Sci. 1997, 191, 378383. (13) Extrand, C. W. J. Colloid Interface Sci. 1998, 207, 11-19. (14) Extrand, C. W. J. Colloid Interface Sci. 2002, 248, 136-142. (15) Extrand, C. W. Langmuir 2003, 19, 3793-3796.
method, and the captive bubble method.4,5,14,16,17 The needlesyringe method yields significant discrepancies due to the variation of the rate of liquid introduction and withdrawal with the syringe.17 The results of the inclined plane method showed even worse discrepancies because of the drop size effect. Shanahan, Erbil, and other investigators used the evaporation of the drop for the estimation of a receding contact angle.2,17-19 Several groups applied vibration for the estimation of advancing and receding angles.20-25 When the experimental results obtained by various investigators are compared, it could be concluded that different techniques supply very different CAH values for the same solids. In spite of the fact that a microscopic explanation of the phenomenon has already been proposed (see the very detailed microscopic analysis of CAH in ref 3 relating the CAH phenomenon to the static friction at the solid/liquid interface), the quantitative characterization of CAH remains problematic. There exist very different approaches to the CAH energy evaluation, beginning from the classical Dupre´ formula and ending with new approaches presented by Extrand, Chibowski, Nosonovsky, and other investigators.6,7,9,13-15 The large discrepancy in the evaluation of CAH energy has to be emphasized. Thus, we conclude that the CAH phenomenon is not profoundly understood from both experimental and theoretical points of view. (16) Drelich, J. Instability of the Three-Phase Contact Region and Its Effect on Contact Angle Relaxation, Drelich, J., Laskowski, J. S., Mittal, K. L., Eds.; VSP BV: Utrecht, The Netherlands, 2000; p 27. (17) Erbil, H. Y.; McHale, G.; Rowan, S. M.; Newton, M. I. Langmuir 1999, 15, 7378-7385. (18) Bourge`s-Monnier, C.; Shanahan, M. E. R. Langmuir 1995, 11, 28202829. (19) Tavana, H.; Yang, G.; Yip, C.; Appelhans, D.; Zschoche, St.; Grundke, K.; Hair, M. L.; Neumann, A. W. Langmuir 2006, 22, 628-636. (20) Andrieu, Cl.; Sykes, S.; Brochard, Fr. Langmuir 1994, 10, 2077-2080. (21) Decker, E. L.; Garoff, S. Langmuir 1996, 12, 2100-2110. (22) Meiron, T. S.; Marmur, A.; Saguy, I. S. J. Colloid Interface Sci. 2004, 274, 637-644. (23) Della Volpe, C.; Maniglio, D.; Morra, M.; Siboni, S. Colloids Surf., A 2002, 206, 47-67. (24) Bormashenko, E.; Pogreb, R.; Whyman, G.; Bormashenko, Ye.; Erlich, M. Appl. Phys. Lett. 2007, 90, 201917-1-201917-2. (25) Bormashenko, E.; Pogreb, R.; Whyman, G.; Bormashenko, Ye.; Erlich, M. Langmuir 2007, 23, 6501-6503.
10.1021/la703875b CCC: $40.75 © 2008 American Chemical Society Published on Web 02/27/2008
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Figure 1. Device for measuring the receding and advancing contact angles by pressing the droplet.
2. Experimental Section 2.1. Materials. The phenomenon of CAH was studied on the extruded polymer films of low-density polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polysulfone (PSU, trademark Thermalux), and polyvinylidene fluoride (PVDF). We performed our experiments with non-poled PVDF films (trademark Kynar) and PSU supplied by Westlake Plastics Inc, and poled PVDF supplied by Precision Acoustics Ltd. PVDF is a polymer of vinylidene fluoride with the predominant repeating unit established as (-CH2CF2-). After poling in a high electric field (50 MV/m), PVDF is pyroelectric as well as piezolelectric.26 Substrates were cleaned thoroughly with acetone and ethyl alcohol and rinsed with a large amount of distilled water. 2.2. Methods. Contact angles were measured with three experimental techniques: namely, the needle-syringe method, evaporation of the drop, and a new technique described below. 2.2.1. Technique 1: Needle-Syringe Method. Drops with an initial volume of 5 µL were deposited carefully on the substrates with a microdosing syringe. The drop was inflated and deflated with a needle-syringe without the contact line moving. Threshold values within which the triple line did move were referred to as the advancing and receding angles. The contact angles were measured with a homemade goniometer and an image-processing technique. A horizontal laser beam illuminated the drop profile and produced its enlarged image on the screen using a system of lenses. Measurements were made on both sides of the drop and were averaged. A series of 10 experiments were carried out for every polymer substrate. The results related to this technique are denoted by the subscript 1. Thus, rec advancing and receding angles are denoted θadv 1 and θ1 , respectively. 2.2.2. Technique 2: New Technique for CAH Measurements under Constant Drop Volume. The novel device intended for CAH measurements is presented in Figure 1. The idea of this technique follows the approach developed by Lafuma and Que´re´, who pressed a drop deposited on a rough substrate for the study of the Cassie(26) Bormashenko, Ye.; Pogreb, R.; Stanevsky, O.; Bormashenko, E. Polym. Test. 2004, 23, 791-796.
Wenzel transition.27 Pressing of a drop was also used recently for a qualitative study of wetting phenomena by Jiang and Crosby et al.28-29 We implemented this technique for a CAH study on flat surfaces. The drop deposited on the investigated substrate was compressed using a micrometric screw, as depicted in Figure 1. The maximal static angle measured with a homemade goniometer and image-processing technique was referred to as an advancing angle. Then the experiment was repeated with a drop of the same volume under the reverse motion of a micrometric screw. The drop was drawn by the Teflon plate (Figure 1), thus the receding (minimal) angle was established. A horizontal laser beam illuminated the drop profile and gave its enlarged image on the screen using the system of lenses as presented in Figure 1. Measurements were made on both sides of the drop and were averaged. A series of 10 experiments were carried out for every polymer substrate. The results related to this technique are denoted with the subscript 2. Thus, advanced and rec receding angles are denoted as θadv 2 and θ2 , respectively. It has to be emphasized that the technique uses drops of the same volume (5 µL), in contrast to the existing experimental techniques.4,5,12-17 2.2.3. Technique 3: Establishing Receding Angles under Drop EVaporation. Receding angles were also established using an evaporation technique, as recently reported by Shanahan, Erbil, and other investigators.8,17-19 The stage when the height and radius of the evaporated drop decreased while the contact angle remained stable was observed. The contact angle at this stage corresponds to a receding angle on smooth surfaces.8,17-19 It was argued that the drop evaporation method allows a minimum rate of liquid withdrawal that minimizes the linear rate of the retreat effect on a receding contact angle measurement. Thus, the contact angle established with the evaporation technique was defined as a true receding contact angle. We will demonstrate that the situation is actually more complicated and the physical meaning of the receding contact angle needs much more clarification. The receding angles related to the evaporation technique are denoted by subscript 3, namely, θrec 3 . (27) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457-460. (28) Zhang, J.; Gao, X.; Jiang, L. Langmuir 2007, 23, 3230-3235. (29) De Souza, E. J.; Gao, L.; McCarthy, T. J.; Arzt, E.; Crosby, A. J. Langmuir published online Oct 20, 2007, http://dx.doi.org/10.1021/la702188t, .
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Table 1. Advancing and Receding Contact Angles, Dielectric Constants, and Specific Energy of Hysteresis for Different Polymer Substrates polymer PE PP PET PVDF non-poled PVDF poled PSU
θadv 1 , deg
θrec 1 , deg
θadv 2 , deg
θrec 2 , deg
θrec 3 , deg
103.5 ( 1 102.5 ( 1.5 84 ( 2 86.5 ( 1.5
70.5 ( 2 74 ( 3 40.5 ( 2.5 48 ( 4
105 ( 2 94 ( 2 83.5 ( 2 92 ( 2.5
54 ( 3 79 ( 2 53.5 ( 2.5 52 ( 2
78 ( 1 91 ( 1 56 ( 1 74 ( 1
51 15 30 40
87 ( 2
38 ( 3
83.5 ( 3
51 ( 2.5
48 ( 1
32.5
90.5 ( 2
37.5 ( 2
86.5 ( 3
45 ( 2
47 ( 1
41.5
Figure 2. Contact angle of a droplet under evaporation. The plateaus in the middle parts of the curves correspond to the receding angle. 2.2.4. ESEM Study. The fine structure of the triple line has been investigated with a Quanta 200 FEG (field emission gun) ESEM (environmental scanning electron microscope). The substrate was fixed on the Peltier stage held at temperatures of 2 and 5 °C. A water droplet of 2 µL was deposited on top of the substrate prior to the pump-down process and stabilized to 2 and 5 °C. After the pump down, the pressure in the sample chamber was stabilized to just about the dew point. In addition, prior to the pump-down process, a few droplets of water were added to regions around the stage held at room temperature. Thus, this minimized the possibility of fluctuations and evaporation of the 2 µL water droplet during the pump-down process. The triple line was imaged with a GSED (gaseous secondary electron detector) in the ESEM wet mode. Imaging at 2 °C under a pressure of 5.4 Torr and at 5 °C under a pressure of 5.6 Torr guaranteed the observation of the triple line in the water vapor equilibrium state with minimum water condensation.
3. Results and Discussion 3.1. Results of CAH Measurements. Advancing and receding angles established experimentally as described above with various experimental techniques are summarized in Table 1. The change in the contact angle under drop evaporation is presented in Figure 2 for three polymer substrates: poled and non-poled PVDF and PP. The stage at which the contact angle remains constant while the radius of the drop diminishes could be seen distinctly. The analysis of experimental data leads to several conclusions: (1) excellent agreement of advancing angles measured with the needle-syringe method and pressing the drop could be recognized for PE, PET, and PSU substrates and (2) for poled and non-poled PVDF and PP this agreement could be recognized as satisfactory; the maximal discrepancy between advancing contact angles was observed for PP substrates, which was as
∆θ2
2.3 2.2 2.9-3.2
3.14
∆g, mJ/m2
(dF/dl) (θadv), mJ/m2
17.7 3.3 5.1 15.1
150 150 100 110
13.6
110
4.1
110
high as 8° (the high discrepancy of advancing angles, established with various experimental techniques on PP substrates, has already been mentioned by Chibowski30), whereas receding angles established for all polymer substrates demonstrate a significant discrepancy as high as 24° for PE substrates. Thus, our results support the opinion already expressed by Neumann and Chibowski: “The models ascribing contact angle hysteresis to features of the solid surface such as roughness and heterogeneity may well be applicable in certain situations, but not on carefully prepared films of polymeric materials”, and “Receding contact angles on a dry surface are experimentally conceptually inaccessible”.4,30-31 Several physical reasons justifying these conclusions were brought forth, including the interaction between liquid and substrate.31 We want to focus on the additional reason to be taken into account, namely, the fine structure of the triple line. 3.2. Fine Structure of the Triple Line and the Receding Contact Angle. Shanahan has stated that “if a sessile drop is simply allowed to remain on a substrate, it may be observed that the contact angle will descend below the accepted value of receding angle, tending to zero as the drop disappears by evaporation.”8 Our experimental findings illustrated with Figure 2 support this idea. Actually the continuous spectrum of receding angles is observed, as can be seen in Figure 2. Hence, our decision to accept that the true receding contact angle corresponds to the stage when the height and radius of the evaporated drop decreased while the contact angle remained stable was fixed is arbitrary. To understand the origin of the continuous spectrum of receding angles, we studied the fine structure of the triple line with the use of environmental scanning electron microscopy (ESEM). An ESEM study of the drops deposited on the polymer substrate revealed that the drop was surrounded by a thin precursor film, as depicted in Figure 3A,B, of width ∼10 µm. Precursor films were observed on all substrates at both experimental temperatures (i.e., 2 and 5 °C). Thin precursor water films on the polymer substrates were also evidenced recently with neutron reflectivity experiments and synchrotron X-ray reflectivity measurements.32,33 When the precursor film is taken into account, the total surface energy of the drop could be written as
E ) Edrop + Eprecursor
(1)
where Edrop is the energy of the drop without a precursor and Eprecursor is the energy of the precursor film. The energy of the drop is given by (30) Chibowski, E. AdV. Colloid Interface Sci. 2007, 133, 51-59. (31) Lam, C. N. C.; Kim, N.; Hui, D.; Kwok, D. Y.; Hair, M. L.; Neumann, A. W. Colloids Surf. A 2001, 189, 265-278. (32) Steitz, R.; Gutberlet, T.; Hauss, T.; Klosgen, B.; Krastev, R.; Schemmel, S.; Simonsen, A. C.; Findenegg, G. H. Langmuir 2003, 19, 2409-2418. (33) Poynor, A.; Hong, L.; Robinson, I. K.; Granick, S.; Zhang, A.; Fenter, P. A. Phys. ReV. Lett. 2006, 97, 266101-266104.
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Figure 4. Scheme of a droplet with a precursor film.
Figure 5. Dependence of the force per unit length of a triple line on the contact angle.
R ) 2H
Figure 3. (A) ESEM image of a droplet edge with a precursor film. The substrate is a PE film. (B) ESEM image of a droplet edge with a precursor film. The substrate is a PSU film.
Edrop ) πR2(γf(θ) + γSL sin2 θ)
(2)
where γ and γSL are the surface tensions at the liquid/air and solid/liquid interfaces, respectively, R is the radius of the drop expressed in terms of the contact area radius r and contact angle as R ) r/sin θ (Figure 4), and f(θ) is the dimensionless function depending on the drop spatial form, which includes the deflection of the drop from the spherical form due to gravity. The energy of the precursor film is given by
∆Eprecursor ) (γ + γSL + P(e))2πrH
γ + γSL + P(e) γf(θ) + γSL sin2 θ
∆g ) -
RgT sin θadv ln A sin θrec
(5)
where Rg is the gas constant, T is the absolute temperature, and A is the molar surface area given by
A)
(34) Derjaguin, B. V.; Churaev, N. V. J. Colloid Interface Sci. 1978, 66, 389-398. (35) Derjaguin, B. V.; Churaev, N. V. Prog. Surf. Sci. 1992, 40, 272-285.
(4)
When R ≈ H ≈ 10 µm, the contact angle value is dictated to a large extent by the precursor film. Thus, a continuous spectrum of receding contact angles turns out to be possible when the drop evaporates (Figure 2). For large drops (R ≈ 1 mm), the impact of the precursor film on the apparent contact angle is negligible, and the phenomenon of CAH is due to other reasons, such as the chemical and physical heterogeneity of a substrate, surface swelling, sorption, and the phenomenon of viscous dissipation in a polymer substrate.1-3,18,30,31,36 The diversity of these factors also yields the continuous spectrum of contact angles. 3.3. Quantitative Characterization of Hysteresis: Calculation of CAH Energy. It has to be noted that there exist at least two approaches to the calculation of the hysteresis energy.9,12-15 We follow the approach developed by Extrand who related the phenomenon of CAH to the change in the chemical potential µ of particles at the solid/liquid interface due to the change in the Laplace pressure.13-15 He has obtained for the free energy of hysteresis ∆g the following equation
(3)
where e and H are the thickness and width of the precursor film, respectively (Figure 4), and P(e) is a term due to the disjoining pressure Π(h) introduced by Derjaguin, Π(h) ) -(dP/de) (refs 1, 34, and 35). It can be easily seen from eqs 2 and 3 that the energies of the drop and precursor film turn out to be of the same order of magnitude when condition 4 takes place:
sin θ
( ) M0 F
2/3
NA1/3
(6)
with F being the density, M0 being the monomer weight, and NA being Avogadro’s number. Extrand started from the assumption that the triple line is pinned (the contact solid-liquid area is constant) and the drop volumes corresponding to the advancing and receding angles are different. It is noteworthy that the approach proposed by Extrand is faced with difficulties when apparent contact angles are obtuse. (36) Shanahan, M. E. R.; Carre´, A. Langmuir 1995, 11, 1396-1402.
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(Note, for example, a negative value of ∆g.) In particular, if θrec + θadv ) π then ∆g in eq 5 vanishes. Although the pressures in the drop are the same in this case, it is not obvious why the energies are also the same. We proposed for the sake of quantitative characterization of CAH the comparison of contact angles of the same drop (i.e., under the condition of constant volume). Thus, the calculated energy of the hysteresis will correspond to the experimental situation realized under technique 2, when the drop is compressed using a micrometric screw. Following ref 13, we start with the Gibbs adsorption equation
dg ) -
1 dµ A s
(7)
where g is the surface free energy of the solid and µs is its chemical potential. Note that dg relates to the solid-surface unit. The chemical potential of the solid surface µs is equal to that of the liquid drop µl, and the latter is connected with the pressure p; therefore, from eq 7 it follows that
dg ) -
Rg T d ln p A
(8)
Let us introduce the following definition of the hysteresis energy
∆g ) gr - ga
(9)
Then from eqs 8 and 9 and the equation for the Laplace pressure the final result follows:
∆g )
RgT ∆padv RgT Rrec ) ) ln ln A ∆prec A Radv adv 2 adv 1 RgT (1 - cos θ ) (2 + cos θ ) (10) ln 3 A (1 - cos θrec)2(2 + cos θrec)
The expression for the spherical cap volume V ) (πR3/3)(1 cos θ)2(2 + cos θ)is also used for derivation of eq 10. The energies of hysteresis obtained with technique 2 are summarized in the Table 1. In spite of the fact that eq 10 improves the approach to the calculation of CAH energy proposed by Extrand,13-15 it still suffers from certain shortcomings. The calculation according to eq 10 implies latently that a spherical drop deposited on a polymer substrate will keep θadv and θrec established with technique 2 when V ) const. 3.4. Quantitative Characterization of the Pinning Force. We want to propose an alternative approach to the quantitative characterization of CAH phenomena (i.e., the force-based approach). It was noted by Yaminsky that contact angle hysteresis resembles static friction, thus the droplet could be inflated or deformed until some critical pinning force acting on the triple line is not exceeded.3 After the critical pinning force is exceeded, the triple line moves.25 The critical force is attained when θ ) θadv. This approach could be especially suitable for polymer substrates when a receding angle is not well defined. The calculation of the force acting on the unit of the triple line has been already carried out in ref 25
Figure 6. Dielectric constants of polymers and the force per unit length of the triple line: (1) polytetrafluoroethylene (PTFE), (2) PP, (3) PE, (4) PET, (5) PSU, and (6) polyamide 66 (PA66). The experimental data for advancing contact angles of PTFE and PA66 are taken from refs 12 and 14. Dielectric constants of polymers are taken from refs 38 and 39.
The behavior of the function (dF/dl) (θ) is illustrated with Figure 5. The values of (dF/dl) (θadv) for various polymers are summarized in Table 1. These values, when multiplied by the elementary displacement of the triple line δr, will give the potential barrier δU to be surmounted by the triple line to jump to its next equilibrium position. If δr is of the order of magnitude of atomic radius δr ≈ 0.1 - 1 nm, then we obtain δU ) (dF/dl)(θadv)δr ≈ 10-10-10-11 J/m. It is noteworthy that this value coincides with the linear surface tension of water, obtained experimentally by Pompe and Herminghaus.37 It also could be demonstrated that (dF/dl)(θadv) correlates with the dielectric constants of polymer substrates. (Table 1 and Figure 6; the dielectric constant of PVDF is not a well-measured value. Even non-poled PVDF comprises inclusions of the so-called β phase responsible for the piezoelectric properties of PVDF.26) This correlation is easily understood if we will take into account that the dielectric constant, representing on the macroscopic level the polarity of polymer molecules, correlates with the surface energy of the polymer substrate.1 And the contact angle in turn correlates with the surface energy of the substrate.1,40 In spite of the critics of the receding angle notion, it may be still useful for certain semiqualitative reasoning. It is well known that the deviation of the receding angle from the Young angle is always larger than that of the advancing one. This could be also concluded from eq 11 and Figure 5. Indeed, suppose that the heights of energy barriers separating the equilibrium (Young) angle θY from θadv and θrec are the same. Then
dF adv dF dF dF rec (θ ) (θ ) ) (θ ) (θ ) dl dl Y dl Y dl
(12)
and θadv- θY > θY - θrec since (dF/dl)(θ) is an increasing function of θ. This assertion is confirmed by our observations, as summarized in Table 1 and as reported by other investigators.30
Conclusions
(11)
We present a new experimental technique for the contact angle hysteresis measurement based on the slow deformation of the droplet. The measurement is performed under a constant drop volume. The fixed volume of the drop allowed the calculation of the hysteresis energy in the framework of the improved Extrand approach. The comparison of the new technique with existing
where p is an excess over the atmospheric pressure inside the droplet given by the sum of the Laplace and hydrostatic pressures p ) (2γ/R) + Fgh.
(37) Pompe, T.; Herminghaus, S. Phys. ReV. Lett. 2000, 85, 1930-1933. (38) van Krevelen, D. W. Properties of Polymers; Elsevier: Amsterdam, 1997. (39) Engineering Plastics; Engineered Materials Handbook; ASM International: Metals Park, OH, 1988; Vol. 2. (40) Good R. J.; Girifalco, L. A. J. Phys. Chem. 1960, 64, 561-565.
Radvp dF ) (2θadv - sin 2θadv) dl 4 sin θadv
Contact Angle Hysteresis on Polymer Substrates
ones has been carried out. The advancing contact angle turned out to be in good agreement with advancing angles established with the needle-syringe method for various polymer substrates. However, the receding angle demonstrated significant discrepancies when compared with needle-syringe and evaporation techniques. We conclude that the value of a receding angle is sensitive to an experimental technique for polymer substrates, thus the calculated hysteresis energy could be related to the hysteresis observed for deformed drops only, when the volume of the drop is conserved. The new approach to the qualitative characterization of contact angle hysteresis based on the calculation of the force pinning the triple line is proposed. The pinning force is evaluated for different
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polymer substrates. It is demonstrated that the pinning force correlates with the dielectric constant of the polymer substrate. The fine structure of the triple line was studied with ESEM. It was revealed that a water drop deposited on a flat polymer substrate is surrounded with a thin precursor film with a width of 10 µm. The precursor film provides the continuous spectrum of contact angles under evaporation of the drop. Acknowledgment. This work was supported by the Israel Ministry of Absorption. We are grateful to Professor M. Zinigrad for his generous support of our experimental activity. LA703875B
