Measuring Dynamic Surface Tension in the Lungs

Langmuir-Wilhelmy Balance
Pulsating Bubble Surfactometer
Captive Bubble Surfactometer
Test Droplet
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The general goal of dynamic surface tension measurements is an isotherm showing expansion/ compression:

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

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:

Disadvantages of the Wilhelmy balance:

(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:

Disadvantages of the PBS:

(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

Disadvantages of CBS

(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)

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