Interfacial Tension Measurements

Liquid surface/interfacial tension measurements are done through the so-called Pendant Drop method with a contact angle meter. The shape of the hanging drop is measured using the established Young-Laplace theory. Both static and dynamic interfacial tension measurements are possible with the same set of hardware. Interfacial tension measurements through the Pendant Drop method require less sample liquid than other measurement techniques, making it ideal for analysis of small volumes of liquids such as inks, alcohol mixtures and creams.

Typical Experimental Results

Figure 1. Image capture of a water droplet suspended in toluene used for interfacial tension analysis.

Figure 2. Image capture of a water droplet suspended in hexane used for interfacial tension analysis.

Figure 3. Captured image of an oil droplet for interfacial tension analysis in distilled water; the Young-Laplace method analysis routine selected; the blue line is the curve fitted line for analysis.

Droplet used for interfacial tension analysis in air

Applications

For more information please read our application notes:
Surface and Interfacial Tension of Liquids, PDF
Density and Surface Tension of Ink, PDF

Instrument: The DM-701 Fully Automatic Contact Angle Meter

Instrument Key Specifications

Surface and Interfacial Tension of Liquids

The surface tension and interfacial tension are the result of imbalanced inter-molecular forces at the surface of a liquid or at the interface between liquids. Inside a liquid, any molecule has an equal number of neighboring molecules. Therefore, the net force acts on any inside molecule is zeroed out. On the other hand, the surface molecule has reduced number of neighboring molecules and this arrangement results in an inward cohesive force acting on the surface molecules. Liquid surface tension and solid surface free energy are from the same origin and could be used interchangeably in some situations. Existence of surface and interfacial tension may be evidenced by phenomena like a small part made of a denser material floating on a liquid surface or morning dew beading up on leaves. In general, low surface tension liquids have better surface wetting properties. High surface tension liquids have a higher tendency to form droplets. Surface and interfacial tension have infinite numbers of industrial applications where liquids, liquid to liquid interface, or liquid to solid interface are of interest. One of the most known applications could be in surfactant. Surfactant is a surface active agent to reduce surface tension of a liquid in order to increase the solution’s wettability to surfaces or increase cleaning efficiency.

Kyowa’s Contact Angle Meters all have the ability to measure the surface and interfacial tension of a liquid. For conducting the measurement, the largest possible droplet is created on the end of the needle of liquid dispenser. Using either the ds/de or the Young-Laplace analysis routine, the surface and interfacial tension of the liquid is found. Both analysis routines are part of the FAMAS software available on the contact angle meters. This method is also referred to as the Pendant Drop method.

Any material that can be expelled through the dispenser tip can be measured by the Pendant Drop method. These materials include aqueous solutions, beverages, chemicals, cosmetic creams, food pastes, inks, oils, paints, surfactant solutions, tooth pastes, etc. To accurately measure the surface tension, the droplet has to reach equilibrium. Most liquids will reach an equilibrium point quickly. Thicker fluids may take a bit longer time to reach equilibrium. As demonstrated in Figure 1, the surface tension of a pure epoxy was found to be 44.2mN/m using the Young-Laplace routine.

The Pendant Drop method is not limited to surface tension measurements in air. A liquid can be used as the surrounding medium provided sufficient light is allowed to pass through. As shown in Figure 1, the interfacial tension of water in toluene could be easily determined. To ensure accurate measurement, the droplet was left suspended in toluene to reach equilibrium. Once equilibrium was reached, the interfacial tension of water in toluene was found to be 18.0mN/m using the Young-Laplace routine.

Density and Surface Tension of Ink

Inkjet printers can produce high quality pictures in a short amount of time. One important aspect to the print quality is the surface tension of the inks. Controlling the surface tension of the inks can help to improve their surface wetting properties to the printing media. One method to determine the surface tension of a liquid is the so-called Pendant Drop method. For surface tension measurements using the Pendant Drop method, a single droplet is suspended in air from a needle tip. The drop shape is then captured by a high speed camera for analysis. A fitting routine is used to analyze the captured image and determine the surface tension of the liquid.

The Pendant Drop method requires the density of the liquid to be known or measured. Other surface tension measurement techniques, such as the Wilhelmy Plate and du Noüy Ring, do not require the liquid density to be known. Nonetheless, the Pendant Drop method requires significantly less of a liquid sample for analysis. Just a few milliliters are sufficient for multiple surface tension measurements with the Pendant Drop method. In addition, the needle tip does not need to be cleaned using burning heat between measurements and is much more resilient to deformation than the Wilhelmy plate or the du Noüy Ring.

Two common printer ink colors are cyan and magenta. To determine the density, the cyan and magenta printer inks were measured with a DDM 2911 Density Meter manufactured by Rudolph Research Analytical (USA). Each ink was carefully injected into the Density Meter at room temperature. The results of the density tests for the cyan and magenta printer inks are shown in Table 1.

With the densities of both the cyan and magenta inks measured, the surface tension of each ink can be determined through the Pendant Drop method. The surface tension measurements were performed with a DM-701 Contact Angle Meter made by Kyowa Interface Science Co. Ltd. (Japan). The DM-701 allows for automatic liquid dispensing and drop size control. Figure 1 shows typical droplets formed by the cyan and magenta printer inks. The drop shapes were analyzed using the Young-Laplace theory.

The surface tension for each ink was approximately 30 mN/m with the cyan ink being slightly greater in value than the magenta ink. Both measured surface tension values fall within typical surface tension values for printer inks. Even though the Pendant Drop method requires more information about the liquid properties to be known than other methods, it still has advantages over other surface tension measurement techniques for certain applications where liquid amount is rather limited.