Pulsating bubble technique for evaluating pulmonary surfactant.

Surface tension is determined with an apparatus which records pressure across the surface of a bubble, expanded in the sample liquid and communicating with ambient air. The disposable sample chamber, with a volume of 20 microliter, communicates with a pulsator and a pressure transducer. The volume displacement of the pulsator's moving piston is hydraulically geared down 1,000 times, which gives the pulsator a stroke volume of 0.43 microliter. When this volume is moving into the sample chamber, it causes the bubble radius to change from a maximum of 0.55 mm, accurately measured through a microscope, to a minimum of 0.4 mm. The pulsator speed is usually 20 rpm, but it can be changed from 0.02 to 80 rpm. From the known pressure gradient across bubble surface, and bubble radius, surface tension is calculated with the law of Laplace.