Force tensiometry is a powerful and accurate technique to measure static surface tension and interfacial tension of liquids. These direct measurements allow determination of material and surface properties, such as dynamic contact angle, surface free energy.
Force tensiometry provides information necessary for the control, development and modification of liquid and solid surfaces. It enables precise characterization of a number of material properties. Analysis of surface/interfacial tension and contact angles provides valuable information on the interactions between gas, liquid and solid phases. These interactions play a key role in the study of:
- Surfactant development
Force tensiometry is the method of choice for many industrial standards related to characterization of liquids. It is used in the testing and quality control of insulator and transformer oils in compliance with the standard, ASTM-D971. Force tensiometry is also the most used technique for measuring critical micelle concentration (CMC) for the optimization of surfactant concentration. In addition, it is the only method available for determining the absorption and contact angle of packed powder, pigments or fiber beds with the Washburn method. It is also commonly used for single fiber measurements.
The basic principle of all force tensiometry experiments is to record and analyze the forces exerted onto a probe or solid sample using a sensitive microbalance.
Measure Surface Tension
When a solid touches the surface of a liquid, the liquid tends to be drawn up in a meniscus. The meniscus creates forces on the solid that are correlated to surface tension. Using probes that completely wet, such as a platinum Du Noüy ring or a Wilhelmy plate, simplifies calculations and enables Sigma Force Tensiometers to precisely measure surface and interfacial tension. Correction calculations for rings are made using models from Huh and Mason (the model from Zuidema & Waters can also be used).
Critical Micelle Concentration
CMC is determined by measuring surface tension of a solution at different concentrations. CMC is the concentration at which the surface tension becomes independent of surfactant concentration.
Dynamic contact angles are measured by dipping a solid into a liquid (advancing contact angle) and then withdrawing (receding contact angle). The forces exerted by the liquid on the sample are recorded and used to calculate the advancing and receding contact angles. The solid samples must have uniform size and surface properties (e.g. single fibers, sensor plate, metal rod). By measuring contact angles with different liquids, the surface free energy of the solid can be defined.
A container filled with powder (or a fiber bundle) is lowered to the liquid level. The instrument monitors the mass change while the liquid wets the powder.
A sedimentation probe is hung from the Sigma microbalance. The instrument records the mass of the sediment collected in the probe over time. The downward movement of particles due to gravity can be studied.
The density probe is pushed through the liquid surface. The forces exerted on the probe are used to calculate the liquid density.
Standard Test Methods
ASTM D1331‑11 Surface and interfacial tension of solutions of surface active agents.
ASTM D971‑12 Interfacial tension of oil against water by the ring method.
ISO 1409:2006 Plastics/rubber — Polymer dispersions and rubber lattices. Determination of surface tension by the ring method.
OECD 115 OECD Guideline for the testing of chemicals. Surface tension of aqueous solutions.
EN 14210 Surface-active agents. Determination of interfacial tension of solutions of surface active agents by the stirrup or ring method.
EN14370 Surface-active agents. Determination of surface tension.
For information about the standard test methods above, see:
Standards for Tensiometers