Tracker

The Tracker™ is an automated drop tensiometer which can measure variations in surface tension or interfacial tension over time. The instrument can also measure contact angle of a liquid against a solid.

The control unit of the instrument can oscillate the drop or bubble being observed to determine complex elastic modulus, including the elastic and viscous components. By choosing optional lenses, lower limit of interfacial tension measurement can be less than 0.1 mN/m.

The Tracker™ is available in 3 different versions:
Tracker H
Tracker S
Tracker M

Tracker-H

This is the most versatile tensiometer from Teclis.The Tracker H is reinforced to allow use of various pressure chambers. The electronics cabinet contains a power supply to operate heaters in the pressure chamber and a readout for set and actual temperature. Requires a computer (with minimum 2 Gb RAM and one free PCI slot, full height) to run software.

Features 
. Surface and interfacial tension measurement of liquid/liquid or liquid/gas;
. Forming Bubbles or drop automatically;
. Dynamic contact angle measurement;
. Calculated surface elasticity or rigidity by data analysis of measurement;
. Executing the measurements with heavier sample chambers up to 200°C at pressures over the range from 0.1bar to 200 bar (Optional);
. Controlled by WINDROP software;
. Comparing the results of the different measurements easily;
. Exporting measurement results and switching to spread sheet type software as Excel.

Tracker-S

The Tracker S is not reinforced to allow use of the pressure chambers. The electronics cabinet does not contain a power supply to operate heaters in the pressure chamber or a readout for set and actual temperature. Otherwise, the Tracker S has the same features, capabilities, and accessories as the Tracker H. It is possible at any time to upgrade a Tracker-H.

Features
. Surface and interfacial tension measurement of liquid/liquid or liquid/gas;
. Forming Bubbles or drop automatically;
. Dynamic contact angle measurement on solid;
. Calculated surface elasticity or rigidity by data analysis of measurement;
. Controlled measurement temperature facility by a circulating bath (optional) over the range from 15-80°C;
. Controlled by WINDROP software;
. Comparing the results of the different measurements easily;
. Exporting measurement results and switching to spread sheet type software as Excel.

Tracker-M

The TRACKER-M is a less automated instrument than the S or H versions. Drops or bubbles must be formed manually with the TRACKER-M. The TRACKER-M is not reinforced to allow use of the pressure chambers. However, the TRACKER-M still allows measurement of surface or interfacial tension for drops or bubbles manually, as well as measurement of contact angle on solids manually. It is possible at any time to upgrade a TRACKER-S.

Features
. Surface and interfacial tension measurement of liquid/liquid or liquid/gas;
. Forming Bubbles or drop manually;
. Contact angle measurement on solid;
. Controlled measurement temperature facility by a circulating bath (optional) over the range from 15-80°C;
. Controlled by WINDROP software;
. Comparing the results of the different measurements easily;
. Exporting measurement results and switching to spread sheet type software as Excel.

Tracker options

All TRACKER instruments are supplied with a sample cell for use at ambient temperature and pressure.

Optional sample chambers for measurement under different conditions are available for TRACKER versions S and H.
Pressure cell for 1 bar/10 to +80 0C (Tracker-H)
Pressure cell for 6 bar/10 to +80 0C (Tracker-H)
Pressure cell for 15 bar/200 0C (Tracker-H)
Pressure cell for 100 bar/200 0C (Tracker-H)
Pressure cell for 200 bar/200 0C (Tracker-H)
Pressure cell CA200 (Tracker S)
Dense phase exchange cell (Tracker S and H)
Drop phase exchange (Tracker S and H)
CMC: Critical Micelle Concentration Concentration
Surfactant powder interaction (Tracker S and H).

Principles
Using drop shape analysis to obtain surface tension

Wait a minute – who said we had to use drop shape analysis to measure surface tension?
There are many different methods to measure surface tension. Why use something as complex as drop shape analysis?

It turns out that drop shape analysis (DSA) has the widest measurement range of all techniques. DSA allows surface or interfacial tension to be measured on all sorts of systems, such as molten metals and glasses, liquefied gases under pressure, or tiny drops of emulsions where surface tension is 0.0001 mN/m. Versatility of the technique is based on the ability to look at a drop and measure it, rather than having to come in direct contact with it.
When a drop of liquid forms at the end of a needle, it takes a shape which was first described by French mathematician Pierre-Simon Laplace and English scientist Thomas Young around 1802. This shape is the balance between two forces acting on the drop-gravity and surface tension.
Gravity elongates the drop (based on its density), while surface tension opposes deformation, keeping the drop closer to spherical on the needle. A form of the Young-Laplace equation can be used to describe the balance of forces at every point around the drop boundary.
Thus, the Young-Laplace equation can be used to plot the theoretical shape a drop should have if surface tension and density of the drop are known along with the acceleration due to gravity when measured. Gravity and density values are easily obtained experimentally and careful measurement from a drop photograph or other image will provide a good set of x,y coordinates for a drop boundary.
It may therefore be possible to use shape information on a drop with unknown surface tension along with the Young-Laplace equation to solve surface tension. Unfortunately, there is no analytical solution to the Young-Laplace equation. The equation can only be solved for surface tension by successive approximation.

Mechanics of the TRACKER are used to produce a liquid drop [C] at the end of a needle [A] or to deposit a drop on a substrate [B] for contact angle measurement. A drop can be pendant (hanging down) as shown in [A], or it can rise beneath the surface of a more dense liquid (rising drop). A bubble of gas takes the shape of a drop if it is in the rising configuration. Thus it is possible to measure “drops” on gas/liquid or liquid/liquid systems.

Optical system of the TRACKER is shown schematically on the left. A light source [D] illuminates the needle and drop [A and C]. A drop image is captured by a video camera [E] fitted with a telecentric lens [F]. The boundary of the drop edge is then digitized to produce a series of x,y values for analysis.

Before measurement, the needle holding the drop is adjusted to be as vertical as possible. This makes the drop image symmetrical, so only one side needs to be analyzed. A boundary [A] is set in the software so only useable data are acquired. This limit corresponds to the needle tip. During measurement, software generates an initial value for surface tension and drop radius (at top or bottom of the drop) to produce a set of theoretical coordinates for the drop based on the Young-Laplace equation [B]. Values for each experimental and theoretical coordinate are compared to produce an overall error margin. The initial surface tension and drop radius values are then varied slightly and the process is repeated. After a number of iterations, a minimum error between theoretical and experimental values is obtained, and the surface tension and drop radius values yielding this lowest error are reported.

Faster data analysis by TRACKER software

Development of software for the TRACKER has allowed it to measure faster and more precisely than had been thought possible just a few years ago. For example, depending on how many data points (coordinates) are chosen and how low the error limit is set, fitting calculations for the Young-Laplace equation can require millions of iterations. Many powerful mathematical tools have been developed and tested to accelerate these calculations without sacrificing accuracy.TRACKER software can now measure and report up to 25 measurements per second.

This speed allows real time analysis of the variation in interfacial properties and enables many other dynamic measurements:
. Kinetics of surface tension change
. Measurement of Gibbs elasticity
. Drop oscillation for measurement of interfacial rheology

What can the TRACKER measure ?

. Surface or interfacial tension
. Interfacial elasticity
. Surface rheology
. Components of cohesion of pure solvents
. Adhesion in liquid-liquid or liquid-solid systems
. Absorptivity of a solid surface (contact angle)
. Characteristics of a surfactant
. Adsorption of surfactant onto a powder

 

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 influences directly 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 large, 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 at the time of a stress 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 a 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 being 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 technique of surface rheology 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).

Components of cohesion of pure solvents

Interactions between molecules in a pure substance can be characterized by a quantity known as the energy of cohesion. This energy of cohesion can be expressed per unit of volume (Hildebrand theory of regular solutions or Hansen parameters for solubility) or per unit of area (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 energy of cohesion 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 too 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 energy of adhesion results from interactions shared between the two materials. These interactions control liquid spreading, absorptivity and adhesion, whose importance is critical for many applications, especially for coatings and adhesives.

 

Absorptivity of a solid surface

Modifications to a solid surface can be easily followed by contact angle measurement of Tracker. 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. Values of critical surface energy of a surface (Zisman) or van der Waals components for surface energy (regular interfaces of Good) can be readily 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 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 module “CMC” allows automatic measurement of 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. A quality control (QC) software option is available which allows management of control charts on three of the parameters just described. The TRACKER can then be used in the QC lab of surfactant producers 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 which has occurred. The TRACKER equipped with the “CMC” module and the powder adsorption cell allows this measurement to be made easily.