What is Surface Tension?
Fluid surface tension is the tangential force that keeps a fluid together at the air/fluid interface. It is the intermolecular force of attraction between adjacent molecules, expressed in force per unit width, as dynes/centimeter (Dynes/Cm.) or milliNewtons/meter (mN/m). Water, at ambient temperature, has a high surface tension in the range of 72 Dynes/Cm. while alcohols are in a much lower range of 20 to 22 Dynes/Cm. Solvents, typically, are in the 20 to 30 Dynes/Cm. range.
Why is Surface Tension important?
Quite simply, if any formulation changes at the molecular level then the surface tension changes. If the formulation changes due to the addition of another chemical, the addition of a surfactant, or if anything contaminates the fluid in question, then the surface tension changes. Measuring surface tension is a direct indicator of the quality of any chemical and any formulation.
What is Dynamic Surface Tension?
Many fluids and formulations contain surfactants or other impurities that are surface active and time dependent. A surfactant is a chemical whose molecules have a hydrophobic segment and a hydrophilic segment. This partial solubility allows this molecule to migrate to, and occupy the space at, the gas/fluid interface, but to do so takes time and this is dependent on the surfactant's mobility.
Surfactants greatly reduce the surface tension of solvents, water and water-based solutions, inks and fountain solutions, adhesives, and other coating formulations. To reduce surface tension, however, the surfactant molecules have to migrate to the interface, and this takes some finite amount of time. Given enough time, the formulation will eventually reach equilibrium (static) surface tension. This takes several seconds or even minutes depending on the type of surfactant and its concentration. If you do not allow enough time for the solution to reach equilibrium you are then operating in the dynamic zone, and the critical measurement parameter is Dynamic Surface Tension.
How do Surfactants work?
Classified by the ionic charge of the surface acting part of the molecule, anionic surfactants have a negative molecular charge, cationics positive, and nonionics no charge. In amthoterics, there are both positive and negative charges. Anionics and nonionic surfactants provide most of industrial surfactant requirements and are the most common. Selection of surfactants is based on specific needs and often mixed surfactants are used.
In general, surfactants with a smaller (lighter) molecule mass (short hydrophobic tail) diffuse more rapidly to the interface, and are vertically adsorbed at the interface, causing a compressive force to act on the surface, thereby reducing surface tension. Most surfactants at higher concentrations exert strong molecular attractions between adjacent molecules causing strong surface films, the strength of which determines the surface properties of the surfactant solutions. Nonionic surfactants with ethylene oxide groups usually diffuse very rapidly to the surface while fluorinated surfactants are slower and more effective at equilibrium
Can you give me an example of how Dynamic Surface Tension changes?
On a printing press, ink is picked up from the ink tray by a roller that rotates at a certain speed, through a number of degrees, until the ink is deposited on the substrate to be coated (paper, plastic, etc.) The time it takes for the ink to get from the ink tray to the substrate is the process time for these press conditions, and during this time surfactant molecules in the ink migrate to the newly created air/ink interface, where their function is to lower surface tension.
A positive spreading coefficient results when the dynamic surface tension of the ink is lower than the surface energy of the substrate. If the speed of the press is increased, so that migration time (and the process time) of the surfactant is reduced, the press will operate higher on the dynamic curve and the ink's dynamic surface tension may be equal to, or higher than, the surface energy of the substrate, resulting in poor printing quality.
Is anyone using the instrument for my application?
Please refer to the Application Notes and Technical Papers for references. If you cannot find an application of interest, please contact us for assistance. We know of many usual and unusual applications that are not listed. We can also run one or two feasibility tests, at no charge, to determine if our instruments are applicable to your specific needs.
Why should I buy a SensaDyne tensiometer?
Surface tension relates directly to a wide range of practical relationships and applications, so that even when surface tension itself is not of direct interest, it relates directly to other important properties of the chemical or formulation. Examples of this are: surface tension versus additive concentration in plating baths; surface tension versus contamination level; and how knowing the Critical Micelle Concentration (CMC) for your surfactant will lower cost and optimize use.
How can I use this tensiometer?
Most users of SensaDyne Tensiometers begin by generating dynamic surface tension curves that characterize their fluids and formulations. Generating curves on various (good and bad) samples allows formulation to be compared. This gives users definitive information on why certain formulations work better than others. Users then can develop criteria that defines quality levels and allowable quality variances for their formulations. Once they know what the quality criteria is, they can use the tensiometer for routine QC testing.
Smart users extend the use of the tensiometer to check incoming chemicals for quality, so that what comes in the door, is what is expected in terms of quality. In cases where the manufacturing is a batch or continuous process, instruments are often adapted to batch or on-line monitoring and control.
The net results from using a SensaDyne Tensiometer are:
Enhanced Research and Development capability,
Tighter control of incoming chemical quality,
Tighter formulation control,
Defect reduction and yield enhancement, and