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Experimental Study on the Evolution of Contact Angles with Temperature Near the Freezing Point Rachid Karmouch* and Guy G. Ross INRS-Centre E´nergie Mate´riaux Te´le´communications, 1650 BouleVard Lionel-Boulet, Varennes, Que´bec, J3X 1S2 Canada ReceiVed: NoVember 25, 2009; ReVised Manuscript ReceiVed: January 27, 2010
Measurements of the water contact angle as a function of temperature down to freezing gives valuable information for the development of anti-icing coatings. Advancing (ACA) and receding (RCA) contact angles were measured by depositing drops of water on different material surfaces for temperatures ranging from room temperature to 0 °C. No changes in the contact angles as a function of temperature have been observed for polished silicon, polished aluminum, roughened silicon, gold, high density polyethylene, PTFE (polytetrafluoroethylene), and PMMA (poly(methyl methacrylate)) for the entire temperature range. However both the ACA and RCA decrease and the hysteresis increases at temperatures below 5 °C for all nanostructured materials used in this study, such as nanopatterned PMMA, PTFE nanoparticles film, and HIREC-100 (a super water-repellent coating blended with TiO2; developed by NTT Advanced Technology Corporation (http:// www.ntt-at.com)). This behavior was attributed mainly to the condensation from the vapor phase of the water drop for temperatures below 5 °C. The resulting thin water film decreases the contact angles, especially for the receding contact, enhancing the hysteresis and water drop adherence. These experimental results could explain the adherence of ice on superhydrophobic nanostructured surfaces. 1. Introduction Control of the surface wettability is crucial in many practical applications.1-6 Microfluidics for biotechnology, thin film technology, purification, textiles, self-cleaning coatings (used on home appliance surfaces, skyscraper windows, car windshields, aircrafts), and anti snow sticking surfaces are a just few examples of applications where the wetting property plays a key role. Many structures such as antennas, electrical networks, steel masts, bridges, radar, and satellites could strongly benefit from a hydrophobic treatment. Once applied on surfaces, hydrophobic treatment providing water contact angles greater than 145° can cause water to simply roll off the surface instead of sticking. It has a self-cleaning mechanism, which is simulated by deliberate surface chalking. A direct indicator of the wettability is the contact angle of a liquid drop deposited onto a surface, which can easily be measured. Contact angles are the result of the molecular interactions of solid-liquid, solid-gas, and gas-liquid at the three phases contact point. A great deal of information of solid-liquid interaction can be obtained by contact angle study. It has become one of the most effective and sensitive methods for characterizing the materials surface chemistry.7,8 The sessile drop method has been widely used to measure the contact angle,9 although the determination of the advancing (ACA) and the receding (RCA) contact angles is far more rigorous and provides more complete information. For both techniques, it is well-known that the contact angle measurement is affected by experimental conditions, such as drop volume, contact time of liquid-solid, and surface temperature. Until now, most studies were focused on drop volume and time dependence of the contact angle.10-12 On the other hand, temperature dependence of contact angle has usually been neglected in most published reports. Ruiter et al.13 suggested * Corresponding author. E-mail address:
[email protected]. Phone: (450) 929-8178. Fax: (450) 929-8102.
that the dynamic contact angle is weakly dependent on temperature and showed a decrease of squalane contact angle deposited on polyethylene terephtalate from 10 to 55 °C by using the hydrodynamic and molecular kinetic models.14 To our knowledge, no previous report has been published, which presents the temperature dependence of contact angle from RT to 0 °C. Nevertheless, the study of the water contact angle at temperatures near freezing is of prime importance for the development of anti-icing superhydrophobic coatings. Speculation about a possible anti-icing property of superhydrophobic surfaces, especially when supercooled water droplets strike such surfaces, has been debated for many years. One aim of this paper is to explain the lack of correlation between superhydrophobic and icephobic surface properties. It should be mentioned that most of the superhydrophobic surfaces have been produced by the formation of nanostructures on the materials surface. In addition to the extraordinary water-repellent and self-cleaning properties of superhydrophobic surfaces, another attractive application is their potential capability of reducing accumulation of snow and ice, possibly completely preventing the formation of ice on solid surfaces. In this paper, we have measured the ACA and RCA on well polished, rough, or nanostructured surfaces for temperatures ranging from room temperature (RT) to the freezing point (0 °C). We have used a wide range of materials: Au, Si (SiO2), Al (Al2O3), HDPE (high-density polyethylene), PTFE (polytetrafluoroethylene), PMMA (poly(methyl methacrylate)), and HIREC-100. This last material is a super water-repellent coating blended with TiO2 developed by NTT Advanced Technology Corporation.15 PTFE and PMMA were either rough or nanostructured. These materials were selected in order to compare the temperature dependence of the contact angles for surfaces with different structural properties and to show that the superhydrophobic property of rough surfaces does not necessarily entail good anti-icing properties. The morphology of these different surfaces was
10.1021/jp911211m 2010 American Chemical Society Published on Web 02/12/2010
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Figure 1. Sketch of the apparatus used for contact angle measurement at different temperatures.
measured by SEM and AFM; images of nanostructured materials are presented in this paper. 2. Experimental Procedure The contact angle measurement was carried out by depositing the droplets on a flat, horizontal test specimen of 25 mm diameter. A schematic of the experimental apparatus is shown in Figure 1. The test specimen is fixed onto a Peltier cooling stage by means of a sample holder. A thermocouple is gently taped onto the sample with sufficient pressure to establish a firm connection with the sample. The substrate, the Peltier module and the droplet delivery system were located inside an evacuated stainless steel chamber. A nitrogen flux was used for both lowering the relative humidity (RH) and removing the water vapor condensation from the surface. The contact angle measurements were carried out in a nitrogen environment. The droplet of deionized (DI) water deposited on the sample surface was increased (to a volume of ∼10 µL) and decreased by means of a microsyringe coupled to a programmable pump. The injection and withdrawal speeds are slow enough (typically 5 µL/min) to keep the 3-phase contact line in equilibrium. A highresolution CCD camera is used to record the time evolution during the whole process. The drop images are digitized and analyzed with software that determines the contour of the drop and evaluates the angles made by the tangent at the point of contact between the drop and the sample surface at both edges of the contact line, yielding θleft and θright. This characterization is performed at different temperatures from RT to 0 °C. The topographical microstructures of the different surfaces were observed by scanning electron microscopy (SEM; JSM6300f), and the surface roughness was measured by atomic force microscopy (AFM). Examples of SEM pictures of the nanostructured surfaces are shown in Figure 2. Typically, the structures range from a few tens of nm to 1 µm. The rms roughness calculated from AFM pictures for all materials are shown in Table 1. 3. Results and Discussion 3.1. Silicon: Roughness Effect. The evolution of the advancing (ACA) and receding (RCA) contact angles and the hysteresis as a function of the sample temperature are shown in Figure 3a) and b) for polished and roughened Si surfaces, respectively. The results indicate that both ACA and RCA (and the hysteresis) are not affected by a variation of the temperature between RT to 0 °C. The same behavior is observed for a roughened silicon surface with an rms roughness of 476.6 nm (compared to a value of 0.9 nm for polished silicon); thus there is no effect of the surface morphology of Si on the evolution of the contact angle with temperature at least for temperatures
Figure 2. SEM images of sample surface coated with (a) nanostructured PTFE and (b) HIREC-100. (c) Nanopatterned (embossed) PMMA.
TABLE 1: Root-Mean-Square (RMS) Roughness of Different Materials As Measured by AFM materials
rms roughness (nm)
polished silicon silicon backside gold aluminum HDPE PTFE nanostructured PTFE HIREC-100 nanostructured PMMA
0.9 476.6 1.6 5.3 19.3 23.5 40.0 732.2 365.0
ranging from RT to 0 °C. However, the hysteresis is larger on the rougher Si surface with ACA and RCA larger and smaller than that of polished surface, respectively. Such an effect, reported in many papers, was expected. 3.2. Gold and Aluminum: Oxidation Effect. These two materials were chosen because a relatively thick oxide (Al2O3) layer is always present on aluminum surfaces with rms roughness of 5.3 nm while no oxide is formed on the gold sample surface with an rms roughness of 1.6 nm. The temperature dependence of ACA and RCA (and the hysteresis) for Au and Al2O3 are presented in Figure 4, panels a and b, respectively. Once again, no variation of contact angles is observed for temperatures ranging from RT to 0 °C. Thus, it seems that the water contact angles remain unchanged on flat, rough, oxidized
Evolution of Contact Angles
Figure 3. Contact angle for (a) polished and (b) roughened face of silicon sample; 9, advancing contact angle (ACA), 2, receding contact angle (RCA), and b, hysteresis.
Figure 4. Contact angles for (a) Au and (b) Al2O3, as a function of temperature; 9, ACA, 2, RCA, and b, hysteresis.
Figure 5. Contact angles of HDPE (a) and PTFE (b) as a function of temperature; 9, ACA, 2, RCA, and b, hysteresis.
or nonoxidized surfaces for the temperature range and the materials considered in this study. 3.3. Polyethylene (HDPE) and Polytetrafluoroethylene (PTFE). The temperature dependence of the contact angles has been tested on another category of materials, polymers. Figure 5a shows the results obtained with high density polyethylene (HDPE) surfaces and Figure 5b those of polytetrafluoroethylene (PTFE) surfaces. The two materials have similar rms roughness, 19.3 nm for HDPE and 23.5 nm for PTFE. For sample temperature ranging from RT to 0 °C, no variation of both ACA and RCA were observed. Thus, it appears that contact angles of water drops deposited on flat and uniform surfaces are not affected by a variation of temperature in the temperature range considered in this study. 3.4. Nanostructured PTFE and HIREC-100. Figure 6a shows the evolution of the contact angle of water on a HIREC-
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Figure 6. Contact angles of HIREC-100 (a) and nanostructured PTFE (b) as a function of temperature; 9, ACA, 2, RCA, and b, hysteresis.
100 surface as a function of temperature. The HIREC-100 is a nanostructured surface (Figure 2b; few hundred nm diameter particles and rms roughness of 732 nm) with superhydrophobic behavior. The results indicate that both ACA and RCA remain constant for temperature ranging from RT to 5 °C. However, below 5 °C, the ACA decreases from 147° to 141 °C and the RCA is still more affected, decreasing from 144° to 107°. In consequence, the contact angle hysteresis is greatly modified, increasing from 3° to 40° at -1 °C. All these observations are a signature of a so-called Cassie-Wenzel transition.16-20 Indeed, a similar effect has been reported for condensing water vapor onto a nanostructured surface17 or by cooling the substrate below the dew point (without contact angle measurement).20 Thus, this effect seems to be more related to water vapor condensation than to a simple temperature decrease. We have also observed that a water drop, which slipped easily (a tilt angle
