What can the TRACKER measure?

Surface or interfacial tension

The TRACKER is able to measure surface tension (gas/liquid interface) and interfacial tension (liquid/liquid interface) in a simple way. This measurement can be realized on liquid or solid, in condition of equilibrium (tension static) or out of equilibrium (dynamic tension). This measurement is often the most useful information for basic and applied research in many fields.

Interfacial elasticity

Surface or interfacial tension is used extensively to measure liquids that contain surface active molecules. These could be systems used to make foams or emulsions or simply mixtures used to clean surfaces. When a gas/liquid or liquid/liquid boundary is established, surface active molecules will accumulate at the interface between the two phases. When the properties of an interface are due to adsorption of amphiphilic molecules, surface density of these molecules directly influences the interfacial tension. A local reduction in the number of surface active molecules per unit area increases the local interfacial tension.
This local variation in turn induces an elastic response of the interface. This phenomenon occurs in foam film (aqueous film separating two bubbles), and determines longevity of the film. If the elastic response is strong, the film will last longer. Identical physics exist when two droplets of an emulsion are in contact, and must resist coalescence. TRACKER makes it possible to measure intensity of the elastic response when stress is imposed on film (when the interfacial area is increased or decreased during drop oscillation). Adsorption kinetics for this phenomenon can also be measured with TRACKER software.

Surface rheology

A computer controlled syringe pump on the TRACKER can be used to vary volume of a bubble of air or a drop of liquid while it is being measured. By periodic variation of the drop volume (sinusoidal with time), it is possible to impose an extension or compression of any interfacial film present. This variation of the interfacial surface is accompanied by variation of the interfacial tension.
Speed of the TRACKER software makes it possible to measure this variation. Two sinusoids result from the measurement, variation of interfacial area and interfacial tension with time. By comparing these sinusoids, it is possible to determine phase angle between them. If the sinusoids are perfectly aligned (with no phase angle between them) the layer of molecules adsorbed at the interface is described as being purely elastic (or as having a purely elastic modulus, denoted by the symbol G'). A phase angle of 90° indicates a purely viscous interface (modulus denoted by the symbol G"). An intermediate phase angle (between 0° and 90°) indicates a viscoelastic interfacial system which has both elastic, G', and viscous, G", character. TRACKER software calculates the elastic and viscous module (G' and G") for the interface measured. A common example of a three dimensional material with viscoelastic properties is a rubber band, for which an elastic modulus (G')and a viscous modulus (G") can be given.
The surface rheology technique is essential for the study of interfaces occupied by compounds strongly adsorbed and able to modify mechanical properties of the interfacial zone (such as proteins and amphiphilic polymers).

Cohesion components of pure solvents

Interactions between molecules in a pure substance can be characterized by a quantity known as the energy of cohesion. This cohesion energy can be expressed per volume unit (Hildebrand theory of regular solutions or Hansen parameters for solubility) or per area unit (gas/liquid interface). Surface tension gives direct access to this last value. With the help of some simple assumptions, it is possible to subdivide the cohesion energy into Lifshitz-van der Waals and Lewis acid/base components. It then becomes possible to classify solvents by their capacity to share van der Waals interactions and possibly hydrogen bonds. Classifying solvents in this manner avoids use of the excessively general term "polarity", which is widely used but so indefinite as to be nearly useless.

Adhesion in liquid-liquid or liquid-solid systems

In addition to energy of cohesion for a pure liquid, it is possible to have quantitative information concerning interactions shared by two materials at their zone of contact (interface). Measurement of interfacial tension between two liquids or contact angle for a drop placed on a solid substrate makes it possible to evaluate the energy of adhesion.
This adhesion energy results from interactions shared between the two materials. These interactions control liquid spreading, absorptivity and adhesion, the importance of which is critical for many applications, especially for coatings and adhesives.

Absorptivity of a solid surface

Modifications to a solid surface can easily be followed by contact angle measurement by the Tracker. The contact angle of a drop placed on a surface will vary directly in proportion to the surface treatment applied. With the use of empirical techniques (method of Zisman) or physically grounded equations (decomposition of Fowkes, approximation of Good-Girifalco), it is possible to characterize a surface. Critical surface energy values of a surface (Zisman) or van der Waals components for surface energy (regular interfaces of Good) can be easily calculated from contact angle data and surface tension of liquids used for the measurements.

Characteristics of a surfactant

The term "surfactant" or "surface active compound" is applied to any material able to lower the surface tension of water (usually) even at low concentrations (0.1% by weight or 0.001 Mole/L). Surfactants are essential ingredients in countless formulated products. They stabilize emulsions, facilitate foam formation, enable dispersion of powders in liquids, improve spreading of liquids, increase absorptivity of solid substrates, and provide the key properties of detergents. Although synthesized using the same overall concept, from the chemical point of view, surfactants can have very different structures. This structure influences their performance enormously.

The TRACKER equipped with the "CMC" module automatically measures seven characteristic surfactant parameters. These include parameters of Rosen (pC20 and maximum lowering of surface tension), critical micelle concentration (CMC), surface elasticity, kinetics of adsorption, and interfacial coverage rate. If chemical structure of the surfactant is known, evaluation of parameters according to Israelachvili, Niham, and Mitchell can also be made. Quality control (QC) software option is available which allows management of control charts on three of the aforementioned parameters. The TRACKER can then be used in surfactant producers’ QC lab and for incoming QC by surfactant users.

Adsorption of surfactant onto a powder

Tensiometry makes it possible to evaluate adsorption of surface active compounds on powders from aqueous solution if they have a critical micelle concentration (CMC). CMC of the surface active material is measured in the solvent alone and again in the presence of the powder. A comparison of the two values indicates the degree of adsorption that has occurred. The TRACKER equipped with the "CMC" module and the powder adsorption cell easily allows this measurement.