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Tensiometric characterization of superhydrophobic surfaces, as compared to the sessile and bouncing drop methods Valentin Hisler, Hiba Jendoubi, Camille Hairaye, Laurent Vonna, Vincent Le Houérou, Frédéric Mermet, Michel Nardin, and Hamidou Haidara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01886 • Publication Date (Web): 13 Jul 2016 Downloaded from http://pubs.acs.org on July 21, 2016
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Tensiometric characterization of superhydrophobic surfaces, as compared to the sessile and bouncing drop methods
Valentin Hisler,a Hiba Jendoubi,a Camille Hairaye,c Laurent Vonna,a* Vincent Le Houérou,b Frédéric Mermet,c Michel Nardin,a and Hamidou Haidaraa
a
Institut de Science des Matériaux de Mulhouse (IS2M) CNRS - UMR 7361, Université de Haute Alsace 15 rue Jean Starcky BP2488, 68057 Mulhouse, France b
Institut Charles Sadon (ICS) (UPR22-CNRS), Université de Strasbourg 23 rue du Loess BP 84047, 67034 Strasbourg, France c
IREPA-LASER Boulevard Gonthier d'Andernach, Parc d'Innovation, 67400 Illkirch-Graffenstaden, France
*Corresponding author:
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ABSTRACT We have considered in this work the Wilhelmy plate tensiometer to characterize the wetting properties of two model surface textures: (i) a series of three superhydrophobic micropillared surfaces, and (ii) a series of two highly water repellent surfaces microtextured with femtosecond laser. The wetting forces obtained on these surfaces with the Wilhelmy plate technique were compared to the contact angles of water droplets measured with the sessile drop technique, and to the bouncing behavior of water droplets recorded at a high frame rate. We showed that it is possible with this technique to directly measure triple line anchoring forces that are not accessible with the commonly used sessile drop technique. In addition, we have demonstrated on the basis of the bouncing drop experiments, wetting transitions induced by the specific test conditions associated to the Wilhelmy plate tensiometer for the two series of textured surfaces. Finally the tensiometer technique is proposed as an alternative test for characterizing the wetting properties of highly liquid repellent surface, especially in immersion conditions.
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INTRODUCTION Liquid repellency of textured surfaces has been extensively studied these two last decades. The repellency properties of natural or recently developed artificial surface textures may be such that liquid droplets do not adhere at all to these substrates, even in the case of liquids of low surface tension.1–4 For these highly liquid repellent surfaces, the sessile drop technique usually applied to characterize the water repellency is no longer applicable. Indeed, because of the low adhesion of the liquid on such surfaces, the droplet often hardly detaches from the syringe tip, and if does so, the droplets often roll off the surface or only slightly adhere, making the measurement of the extremely high advancing and receding contact angles (i.e. very low hysteresis) impossible or inaccurate. In addition, it is often essential to understand and characterize the robustness of the repellency properties of a textured surface when immersed in a liquid. This is the case for immersed natural textures like the leaves of the water fern Salvinia5 or functional surface textures applied for drag reduction,6–8 anti-corrosion9–11 or anti-fouling12 for example. This robustness, related to the ability of the surface to resist the wetting of the texture, is of fundamental importance since it defines the stability of the air pockets at the origin of the Cassie-Baxter wetting regime, which indeed was shown to be a metastable regime. The total or partial replacement of these air pockets trapped in between the liquid and the surface, leads to a transition from the Cassie-Baxter wetting regime to a Wenzel or a mixed wetting regime respectively, and consequently, to the loss of the interfacial properties resulting from the air trapping. In the case of immersed liquid repellent surfaces, such transitions were shown to occur with time,13 but also with varying the surface tension of the liquid,14 the pressure15 or the flow16 over the superhydrophobic coating for example. Again, the sessile drop technique is here no longer adapted for this specific characterization which requires new approaches to simulating the pressure and confinement conditions associated to the immersed state. The droplet bouncing experiment is a quite simple way to get insight in the liquid repellency properties of highly non-wettable surfaces which are not accessible with the sessile drop technique. In this approach it is the impact velocity of the droplet hitting the surface which defines the pressure of
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the liquid on the texture. Different parameters can be considered in order to characterize the interaction of the liquid and the surface such as the number of bounces,17 the contact time,18,19 the sticking20,21 or the shape of the droplet22 for example. Although more rarely followed, other approaches exist to study the wetting resistance of a textured surface with a sessile drop (as a function of the surface topography), such as those in which the droplet is squeezed in between two surfaces,23,24 or those based on electrochemical spectroscopy25 or on acoustic wave propagation for example.26 For the study of the liquid repellency properties of textured surfaces under immersion, the stability and evolution of the air pockets trapped between the liquid and the substrate is most often characterized optically.13,14,27–31 The results obtained with all these techniques are qualitative and it has to be mentioned here that only a few of them give a direct measure of the pinning force of the triple line on the surface, which assess for the interaction (i.e. adhesion) of the liquid with the textured surface. Among all the techniques applied to evaluate the wetting properties of highly non-wettable surfaces, the Wilhelmy plate tensiometer was so far rarely used. This technique based on a microbalance was initially developed to determine the surface tension of liquids. It gives a direct measure of the triple line anchoring force, the contact angles being calculated from the corresponding capillary force (cosθ = Fw / (γ p), with Fw, the wetting force, γ, the surface tension of the liquid, and p, the section of the sample of the contact line). Another difference with the sessile drop technique which gives the retention force only indirectly (after measuring the advancing and receding contact angles), is the specific experimental configuration of the Wilhelmy plate tensiometer which allows modeling the immersion of the sample and varying all related experimental conditions such as immersion speed, immersion depth, residence time and temperature of the liquid for example. Although the exact relation between the dynamic contact angles measured with the sessile drop technique and those measured with the Wilhelmy plate tensiometer is still under debate,32–34 this latter technique was already used for example to determine the dynamic contact angles of low-wettable thiol SAMs35 or the stability of SAMs to acidic and basic solutions.36 Despite the relevance of the Wilhelmy plate tensiometer to the sessile drop, only few studies are based on this technique to characterize the wetting properties of immersed non-wettable surfaces. Among these, Kleingartner et al.37 for example measured with the Wilhelmy plate tensiometer the wetting force on low-wetting surfaces and non4 ACS Paragon Plus Environment
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wetting woven meshes. More recently, Kim et al.38 studied the influence of the plate velocity and capillary number on the dynamic contact angles and contact angle hysteresis of hydrophobic and superhydrophobic surfaces. We discuss in the following the wetting forces and dynamic contact angles measured on model superhydrophobic surfaces with the Wilhelmy plate tensiometer. A first set of samples consists in superhydrophobic surfaces decorated with micropillars and showing a scale invariant wetting behavior as already discussed in a previous study based on the sessile drop technique.39 A second set of samples consists of two highly water repellent silicone surfaces textured with a femtosecond laser technique, for which the sessile drop technique is not applicable since water droplets systematically roll off the textured surfaces. The dynamic contact angles retrieved from the wetting forces measured with the Wilhelmy plate tensiometer are compared to those obtained with the sessile drop technique (only applicable to the micropillared surfaces). In addition, the bouncing of water droplets on the different textured surfaces was considered to discuss wetting transitions observed during the Wilhelmy plate measurements. Finally, we highlight in this work the differences between test conditions associated to the sessile drop and to the tensiometer respectively, which lead to different liquid/solid interfacial wetting states (air trapping), and consequently, to different wetting forces. The Wilhelmy plate tensiometer is proposed in this frame as an alternate method for evaluating the liquid/surface interaction of highly water repellent surface texture in immersed conditions.
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EXPERIMENTAL DETAILS Surface characterization. The surface textures were characterized by scanning electron microscopy (FEI Quanta 400 ESEM operating at energy between 15 and 20 kV) after metallization of the samples with gold. The root mean squared roughness of the smooth PDMS was measured with an atomic force microscope in the tapping mode (Nanoscope IIIa from Digital Instruments - Veeco, Santa Barbara CA, USA). Fabrication of the textured surfaces. All the samples were made of Sylgard 184 (Dow Corning, Midland, Michigan, United States). This polydimethylsiloxane (PDMS) elastomer was used to fabricate the micropatterned surfaces as well as the highly water repellent PDMS surfaces textured with the femtosecond laser technique. The Sylgard 184 prepolymer and cross-linker were mixed at 10:1 ratio, cured at 80°C during 4 hours and peeled off for further use. In the case of the surfaces textured with micropillars, the mixed prepolymer and cross-linker were poured then cured, on silanized silicon moulds fabricated by photolithography and dry etching. The silicon moulds were micropatterned to obtain PDMS substrates decorated with hexagonal micropillars which width d and interpillar distance l are varied homothetically over the samples (with d = l) as shown in Figure 1. The micropillars are distributed on a hexagonal lattice and thus display a constant surface fraction φ (with φ = 25% in this case). We have considered the three pillar widths and interpillar distances, d = l = 8 µm, 16 µm and 32 µm respectively. The heigth of the pillars are H = 16 µm in the three cases. Figure 1 shows the electron micrographs of these three different micropatterned surfaces named in the following 8/8, 16/16 and 32/32 respectively.
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Figure 1 : (A) Schematic of the surface pattern with l the interpillar distance and d the width of the pillars. (B - D) Scanning electron micrographs of the different micropillars arrays (grazing view) with l = d = 8 µm (B), l = d = 16 µm (C), l = d = 32 µm (D), named 8/8, 16/16 and 32/32 respectively (height of the pillars H = 16 µm in the three cases).
In the case of the laser texturing, the PDMS surfaces were irradiated with a linear amplified Yb fiber laser with a center wavelength of 1030 nm, a repetition rate up to 2MHz and pulses of 300 fs (Tangerine from Amplitude Systèmes, Bordeaux, France). The beam was focused to a spot size of 20 µm. It was already shown that the femtosecond laser texturing of PDMS can lead to superhydrophobic textures for given processing parameters.40–42 For this work we have identified two different sets of processing parameters leading to two different topographies. The first set (sample named R1 in the following) consists of an irradiation of 1.5 J·cm-1 at 80% overlap, a scan speed of 5.1 m·min-1 and a spacing between line of 4 µm. The second set (sample named R2 in the following) consists of an irradiation of 3.5 J·cm-1 at 50% overlap, a scan speed of 12.8 m·min-1 and a spacing between line of 11 µm. The scanning electron micrographs of the two topographies R1 and R2 obtained with these two sets of processing parameters are shown in Figure 2. Although of different topographies, these two surfaces both show similar superhydrophobic features such that sessile water drops roll off these surfaces, making impossible any reasonable contact angle measurements. It is to be pointed here that
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the surfaces just after processing are hydrophilic and become hydrophobic then superhydrophobic, only after many hours when exposed to air.
Figure 2 : Scanning electron micrographs of the surfaces R1 (A to C: top view, and D: side view) and R2 (E to G: top view, and H: side view) textured with femtosecond laser.
Wetting characterization with the sessile drop technique. The contact angles of sessile droplets were measured with a manual goniometer. In the case of the advancing and receding contact angles measurements, a droplet of water (∼ 8 µL) was first formed at the tip of a syringe. The drop was then slowly moved down until touching the substrate. This procedure ensures an exclusive contact between the liquid and the top of the pillars, especially in the case of large interpillar distances for which the Cassie-Baxter regime (corresponding to a suspended droplet) is particularly instable. The advancing contact angle was then measured by growing the size of the droplet, and the receding contact angle by decreasing the size of the droplet. A droplet volume of ∼ 8 µL leads to a droplet contact base diameter of ∼ 4 mm which is large compared to the maximal interpillar distance of 32 µm considered in this work. For these experiments we used fresh deionized water (Elix from Merck, Millipore, Germany), with a surface tension of γw = 71.8 mN·m-1 ± 0.5 mN·m-1 (as measured with the Wilhelmy plate tensiometer).
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Bouncing experiments. The bouncing events were recorded with a high-speed camera (4M180-CL from IO Industries Inc., London, ON, Canada) at a frame rate of 500 fps. The impact velocities were calculated from the five last images before the contact between the droplet and the surface. The impact velocities considered in this work were all lower than 1.1 m·s-1 ± 0.1 m·s-1. Moreover, we have used a microsyringe which tip geometry allows delivering water droplets with a diameter of 2.5 mm ± 0.2 mm. In these conditions (impact velocities and size of the water droplets), the hydrodynamic of the impact is characterized by Weber numbers, We = ρV²R/γ, lower than 60 (with ρ the density of the liquid, V the impact velocity, R the radius of the droplet, and γ the surface tension of the liquid). Working with such low Weber numbers allows observing low adhering droplets that eventually rebound, without fragmenting during impact.40,41 As for the sessile drop experiments, we used fresh deionized water (Elix from Merck, Millipore, Germany) as described above. Tensiometric measurements. The tensiometric measurements were performed using a Wilhelmy plate tensiometer (K12 from Krüss GmbH, Hamburg, Germany). In this technique, the sample is connected to a microbalance and the liquid reservoir is rising up and down in order to immerse and emerge the sample from the liquid. During the experiment, the net force FT measured with the microbalance and acting on the sample immersed in the liquid, is the sum of the wetting force Fw = p γ cosθ and the buoyancy force FB = - V ∆ρ g :
FT = p γ cosθ - V ∆ρ g
eq. 1
with p the section of the sample, γ the surface tension of the liquid, θ the contact angle, V the volume of the sample immersed (or volume of liquid displaced), ∆ρ the difference in density between the liquid and the air (if air is the surrounding media), and g the acceleration of gravity. All experiments were performed at an immersion and emersion speed of 20 mm·min-1 so that the viscous forces also participating to the net force can here be neglected. In the case of water, this speed corresponds indeed to a capillary number Ca ≈ 5.10-6 for which the dynamic contact angles are similar to the advancing and receding contact angles measured with the sessile drop technique (i.e. in a quasi-static mode).38 The sample was attached to the balance with clamp of the K12 tensiometer. 9 ACS Paragon Plus Environment
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Figure 3A shows the characteristic variation of the net force FT as a function of the sample position, obtained with the Wilhelmy plate tensiometer during an immersion/emersion cycle. The associated schemes on this figure (Figure 3B) show the position of the sample and the related wetting meniscus shapes in the case of a non-wetting surface, as those investigated in this work (both advancing contact angle θa and receding contact angle θr , < 90°). Snapshots of the sample at three different positions of this cycle are given in Figure 3C to Figure 3E.
Figure 3 : (A) Variation of the net force measured during an immersion/emersion cycle using the Wilhelmy plate tensiometer and a non-wetting sample (advancing and receding contact angles > 90°). (B) Sketches of the sample positions (relative to the liquid surface) and the associated meniscus profiles. (C-E) Snapshots of the meniscus (lengthwise view of the sample) at position c, between d and e, and between g and h, respectively (the bright arrow on these snapshots shows the displacement of the interface).
At the beginning of the experiment, the sample is out of the liquid. From this point, the force is zeroed and the liquid reservoir is raised (Figure 3A, from a to b) until the surface of the liquid touches the sample (Figure 3A, from b to c, and Figure 3C). The segment c to d on the graph corresponds to the variation of the force associated to the reconformation of the meniscus (for a detailed description of this process see Kleingartner et al.37). The force then shows a linear variation corresponding to a 10 ACS Paragon Plus Environment
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constant meniscus shape and a constant advancing contact angle (Figure 3A, from d to e, and Figure 3D). Since the wetting force is now constant, the variation of the net force is due to the sole variation of the buoyancy force (for Ca
