The general goal of dynamic surface tension measurements is an isotherm showing expansion/ compression:

Surface area may be measured as a fraction of original area, in square angstroms per molecule, or in standard units of area. The gap between lines implies hysteresis, or energy loss. (c.f. Schürch et al., 1994, binder, Goerke and Clements, 1985, Introduction binder, and Notter et al., 1980, binder)

Researchers sometimes report the surface pressure of surfactant instead of surface tension. These are related by the formula:

(Enhorning, 2001, online)

The "Pressure-Area" isotherm is a reflection of the surface tension graph:

Other parameters measured include compressibility and the minimum surface tension achieved. (c.f. Schürch et al., 1994, binder, and Wang et al., 1995, Introduction binder)

Dynamic surface tension measurements are made at different cycling rates, concentrations, compressions, and temperatures, and with different combinations of substances. All of these vary widely; for example, surfactant concentrations at orders between .001 and 10 mg/ ml have been studied. (c.f. Otis et al., 1994, Theory binder and Krueger and Gaver, 2000, Theory binder) Physiological concentrations are probably at the high end of this range. (c.f. Putz et al., 1994, binder)

The first dynamic surface tension measurements (of stearic acid) were made in 1897 by Agnes Pockels, using a bowl and a button. (Kaganer et al., 1999, Film Structure online)

The Langmuir-Wilhelmy Balance

This is the oldest method of measuring pulmonary surfactant surface tension. It was first used for this purpose by John Clements in the 1950s. (Possmayer et al., 2001, online)

The trough and barrier are usually made of Teflon. (Goerke and Clements, 1985, Introduction binder) Surfactant may be spread at the water's surface by a solvent which later evaporates, or it may adsorb from a solution. (Enhorning, 1977, binder)

The fluid forms a meniscus at the plate. One can determine surface tension from the increased weight of the plate or the height of the meniscus.

Variants of the Langmuir-Wilhelmy balance include the de Noüy ring. (Adamson, 1990, binder)

Advantages of the Wilhelmy balance:

• Simple
• Strong control over the amount of surfactant at the surface
• Easy to remove and examine compressed film
• The effects of adsorption and expansion/ compression can be isolated.

Disadvantages of the Wilhelmy balance:

• Leakage
• Slow cycle rate
• High volume of fluid and surfactant
• Flat geometry is not realistic for the lungs.

(Enhorning, 1977, binder; Notter, 1989, binder; see also Film Structure)

Pulsating Bubble Surfactometer

The pulsating bubble surfactometer (PBS) has been in use since the 1970s. It was first developed by Goran Enhorning.

The entire sample chamber is placed beneath a microscope. The full radius and the radius at 1/2 the original surface area are measured. A sensor monitors the pressure in the solution directly. If we assume a spherical bubble, we may calculate surface tension from the Law of Laplace:

(Enhorning, 1977, binder)

Commercial pulsating bubble surfactometers are calibrated so that the surface area varies sinusoidally:

(Chang and Franses, 1994, binder)

Filming the bubble's oscillations can provide a more accurate estimate of variations in surface area. (Putz et al., 1994, binder)

Advantages of the PBS:

• Quick cycle rate
• Low sample volume
• Easy to change samples
• Commercially available

Disadvantages of the PBS:

• No control over surface concentration
• Leakage
• Bubble deformation from spherical shape
• Overcompression
• Can require repeated cycling to reach low surface tension

(Enhorning, 1997, binder; Enhorning, 2001, online; Notter, 1989, binder; Putz et al., 1994, binder)

Captive Bubble Surfactometer

The captive bubble surfactometer was invented in the late 1980s by Samuel Schürch to address some of the problems with the pulsating bubble surfactometer. An air bubble, created with a syringe, floats in a surfactant-containing solution. Above the bubble is a slightly curved layer of hydrophilic agar gel.

The bubble is filmed, and its shape is analyzed (using the ratio of height to diameter) to determine its surface tension. Researchers have observed a phenomenon known as bubble "clicking" where the bubble's shape suddenly changes to greater surface tension and lower surface area.

(Schürch et al., 2001)

Advantages of CBS

• Little or no leakage
• Rapid expansion and compression
• Fairly small sample chamber.

Disadvantages of CBS

• Complex data analysis
• May not be commercially available
• Hard to switch samples
• Control over surface amount is difficult
• Maximum surfactant concentration of about 3 mg/ ml (due to visibility issues)

(Putz et al., 1994, binder; Schürch et al., 2001, online)

Test Droplet Method

Though not a standard method of measuring pulmonary surface tension, this represents one of the few attempts to investigate surfactant function in the lungs.

The test droplet method uses the fact that the shape of a droplet on top of an air-water interface depends on the surface tension of the interface.

The surface tensions labeled have a simple relationship:

(Schürch et al., 2001, online)

This relationship shows that the angle of droplet contact with the surface depends on surface tension. If the droplet volume is constant, this means that its diameter will also depend on the surface tension. Researchers calibrate the relationship between droplet diameter and surface tension on a Langmuir-Wilhelmy balance (modified to make the subphase very thin). They then place these drops on the surface of excised lungs and take pictures as the lungs are "quasi-statically" expanded. (Schürch et al., 1978, binder)