How did the swan-neck flask experiment prevent contamination of boiled broth?

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Painting by Albert Edelfelt showing Louis Pasteur in his laboratory, holding a glass flask and examining it intently, surrounded by scientific equipment on a workbench. — Louis Pasteur In the 19th century Louis Pasteur's work established the germ theory of disease, proving that microorganisms cause fermentation and sickness, which led to the development of pasteurization, aseptic techniques in medicine, and effective vaccines for diseases such as rabies. (Painting by Albert Edelfelt, 1885.)

Louis Pasteur utilized the swan-neck flask to disprove the theory of spontaneous generation by demonstrating that microbial life does not arise from dead matter.

The design of the flask featured a long, bending neck that acted as a physical trap. When the beef broth was boiled to achieve sterilization, the steam pushed air out of the flask. As the broth cooled, air was drawn back in, but the unique shape of the neck caused dust particles and airborne contaminants to settle in the curve of the glass rather than reaching the liquid.1

Because these contaminants were trapped in the neck, the broth remained sterile indefinitely. Pasteur proved that contamination only occurred if the neck was broken or tilted to allow the trapped particles to contact the broth, which subsequently led to microbial growth. This experiment was foundational to the development of germ theory and the modern science of bacteriology.1