Contact angle Labs (sessile drops, captive bubbles, Axisymmetric Drop Shape Analysis-ADSA, Wilhelmy balance technique)
Our labs are equipped with commercially available contact angle devices and with in-house developments. All measurements are carried out under constant climate (temperature: 23 °C, relative humidity: 50 %). The choice of a particular contact angle technique depends on the geometry of the system and the size and the shape of the solid sample. The accuracy of contact angle measurements can be affected by the quality of the solid surface (heterogeneity, roughness), the purity of the measuring liquids, the skill of the experimenter, but also by the methodology and procedure.
Sessile drop and captive (adhering) bubble techniques
Wilhelmy balance technique
DSA10 (Krüss GmbH)
Tensiometer K12 (Kruess GmbH)
DataPhysics OCA40 micro (DataPhysics)
Tensiometer K14 (Kruess GmbH)
Fibro Dat 1100 (Fibro System AB, Sw)
Tensiometer OBS2 (in-house development)
Fibro Dat 1129 Automatic tilt table (Fibro System AB, Sw)
Axisymmetric Drop Shape Analysis-Profile (ADSA-P) (in-house development)
Many processes in polymer production, processing and application include wetting of solids by liquids. The contact angle θ is an important parameter to quantify the wettability of solid surfaces. It is defined as the angle θ formed between the liquid-fluid (vapor) and the solid-liquid interfaces, at the solid-liquid-fluid (vapor) three phase contact line.
Sessile drop techniques
The sessile drop or, alternatively, the captive (adhering) bubble method are the most commonly used techniques for flat surfaces. In these cases, the contact angles are measured from the drop profile, either by the conventional goniometer-telescope or the more sophisticated and advanced Axisymmetric Drop Shape Analysis-Profile (ADSA-P). Using the goniometer technique, which is the most widely used procedure, the contact angle is determined simply by aligning a tangent with the sessile drop profile at the point of contact with the solid surface. The results of this conventional goniometer technique are somewhat subjective and dependent on the experience of the operator. Although certain training procedures can be used to improve the reproducibility, the accuracy of this method is usually ± 2° at the best.
The measurement of liquid contact angles based on Axisymmetric Drop Shape Analysis-Profile (ADSA-P) is a powerful method with several advantages over conventional contact angle techniques. ADSA-P determines simultaneously contact angles and liquid-fluid interfacial tensions from the profile of sessile liquid drops or captive bubbles. Besides these two parameters, the drop volume and the drop surface area, as well as the drop radius are also output. Input parameters are the density difference across the interface, the magnitude of the local gravity acceleration, and several arbitrary coordinate points selected along the experimental drop profile. ADSA-P can achieve an accuracy of ± 0.05 mJ/m2 or better for surface tensions and ± 0.2° or better for contact angles.
Details of ADSA-P, including background mathematical analysis, fundamental equations and optimization can be obtained from S. Lahooti, O. I. del Rio, P. Cheng, A. W. Neumann, Axisymmetric drop shape analysis (ADSA), in: Applied Surface Thermodynamics, A. W. Neumann and J. K. Spelt (Eds.), Marcel Dekker, New York, 1996, pp. 441-507 and Laboratory of Applied Surface Thermodynamics, University of Toronto
Fig. 1 shows the results of an ADSA-P contact angle measurement with a water sessile drop on a polystyrene surface prepared by spin coating on a silicon wafer. Even on this very smooth polymer surface a contact angle hysteresis of about 10° is observed which is the difference between the maximum (“advancing”) and the minimum (“receding”) contact angles.
Wilhelmy balance technique
The Wilhelmy balance method is an excellent technique to measure contact angles indirectly on a flat plate of known perimeter of the plate cross-section or on thin fibers of known perimeter. In our labs, we use commercially available instruments and in-house developments consisting of a microbalance and a motor driven movable table, on which the liquid container is placed. The contact angle experiments can be performed under inert gas atmosphere and at elevated temperatures.
In the classical Wilhelmy balance experiment the force F measured by an electrobalance is the sum of gravitational, interfacial, buoyancy, and hydrodynamic forces. In the case of low-molecular liquids, shear forces can be neglected. For thin fibers (diameters less than 100 µm), the buoyancy force can also be ignored and the following simple equation results:
The perimeter p of the sample can be determined by using a liquid of known surface tension γlv for which the contact angle θ is zero. The contact angle θ of another liquid with known surface tension γlv can then be determined by measuring the force per unit length of the perimeter p. In the special case, when the contact angle is zero and the perimeter is known, the measured force is related directly to the liquid surface tension.
Several drawbacks of the method should be mentioned. The high sensitivity of the electrobalance employed in the Wilhelmy experiment can exploited only if the perimeter is constant. In addition, the plate must have the same composition and morphology at all surfaces: front, back, and both edges. This condition may be difficult to meet, particularly if one wants to investigate films or anisotropic systems. Swelling of the solid may also become a problem because it may change the perimeter in an uncontrollable manner. Finally, adsorption of the vapor of the liquid at various parts of the gravimetric system may change the balance output.
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