Technical NoteDirect measurement of macro contact angles through atomic force microscopy
Introduction
When a liquid partially wets a solid substrate, a comprehensive understanding of the liquid film near the contact line is critical for various applications including heat transfer [1], [2], flotation separation [3], oil recovery [4], deposition of pesticides on plant leaves [5], nano printing [6], etc. The profile of the thin film is believed to be the most important factor in determining thin-film heat transfer [7] and many modeling studies have been devoted [8], [9], [10]. The liquid film profile near the contact line is complicated for its anisotropy and multi-scale nature [11]. Within a nano to micro region, surface tension [12], line tension [13], [14], complex intermolecular interactions [15], multiphase flow [16] and interfacial effects such as Marangoni [17] are all working together.
Macro/apparent contact angle [18] is an important concept as it is an observable emersion from the complicated solid–gas–liquid interactions. It plays a critical role as a boundary condition in many wetting investigations. Numerous theoretical and experimental efforts [18], [19], [20], [21] have been made on the measurement and prediction of contact angles. Theoretically, the most famous Young’s contact angle is well defined as it is been described by Young’s equation which indicates a single, unique contact angle. However in practice, contact angle phenomena are far more complicated: the smoothness of the substrate, chemical heterogeneity and other factors can easily affect the force balance near the contact line and lead to hysteresis of contact angles [20], [22]. Moreover, Young’s angle is an apparent angle. When approaching the contact line which is at micro even nano-scale, the thin film profile and the micro contact angles are much less understood [14], [20], [23].
Contact angle measurement is a key and fundamental path to study all the issues mentioned above. Several methods have been well developed which can be sorted into direct and indirect groups. The most widely used method is the direct optical measurement by viewing the drop profile and then measuring the angle at the edge [24]. Its accuracy is influenced by the resolution of CCD, the algorithm for the image processing, and the formula to fit the droplet profile etc. [25]. The indirect methods include Wilhelmy method [26], axisymmetric drop shape analysis-profile (ADSA-P) [27], interference fringe method [28], and polynomial fitting method [25]. Among them ADSA-P method is the most famous one, which is based on a series of theoretical assumptions to obtain an ideal macro contact angle. For the contact angles less than 10°, the accuracy of the method decreases dramatically [27].
AFM technique [29] is developed based on scanning tunneling microscopy and has been applied in material science, surface chemistry, biology and other disciplines. Tapping-mode AFM has gentle interactions between its tip and the sample surface, which allows measurements on soft matter like bio samples [30], [31]. However to our best knowledge AFM is seldom used to have a systematical study on contact angles. The most related work is Nguyen et al.’s [32] work in which the line tension along contact line was studied by AFM using a non-ideal artificial surface substrate.
In the present work, attempts are made to use a state-of-the-art AFM (MFP-3D, Asylum Research) to scan the liquid film near the contact line. Tapping mode is employed and the tip is set to work at the attractive region [33] ensuring that the interaction between the tip and the liquid surface is strong enough to generate stable signals, but not too much to distort the liquid surface. Macro contact angles are extracted from the scan results and compared to the ones measured from the direct optical method. Very good agreement is seen but the AFM results show much higher precision. Moreover, benefiting from the sub-micron resolution of the AFM, the scanning reveals important features of the film profile near the contact line. Within the scale of one to tens of microns in film thickness, a highly linear profile with a constant slope is seen, indicating the existence of the contact angle. While if the film thickness is below a sub-micron value, the film profile would be no longer linear and a transition region with varying micro contact angle emerges.
Section snippets
Experimental setup and procedure
Two liquid–solid pairs, glycerol-mica and glycerol-PS (Polystyrene), were employed as samples. Glycerol is widely used in contact angle studies for its stability and non-volatility [19], [20]. Mica and PS have good qualities of smoothness, homogeneity and stability in air. Mica has high surface free energy while PS has low surface free energy thus a wide range of contact angles could be generated for test. Mica was newly cut before the test. The roughness of both substrates is within tens of
Results and discussion
The optical results of glycerol on mica and PS are shown in Fig. 2(a) and (b) respectively. The diameters of the contact areas are 1.71 and 0.38 mm as illustrated, both large enough to neglect the effect of line tensions which is important for tiny droplets [13], [14], [35]. The method proposed by [25] is employed to obtain the liquid film profiles. The profile of the droplet was extracted from the photograph (Fig. 2). Then a fitted curve was obtained:in which a, b, c, d, e are
Conclusions
The accurate geometry of the region near the three-phase contact line was obtained by directly scanning with an atomic force microscopy under tapping mode. Two liquid–solid pairs, glycerol-mica and glycerol-PS (Polystyrene), were tested. At a scale of one to tens of microns, highly-linear film profiles were observed near the contact lines, indicating the existence of the constant angles. The angles turn out to be almost the same with the apparent/macro contact angles measured by the direct
Acknowledgments
This work was supported by the Natural Science Foundation of China (Grant No. 51276003) and Common Development Fund of Beijing.
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