Abstract
We investigate the spontaneous motion of a soap film in a conical geometry connected to a long tube and show how it can be used to measure the dynamic viscosity of air. In contrast to other techniques that are complicated to implement and require expensive and sophisticated equipment, this measurement method relies only on soapy water and three everyday life objects: a smartphone, a funnel, and a hose. More precisely, to determine the viscosity of air, we use a smartphone to record the spontaneous motion of a soap film placed in a funnel when the motion of the film is quasistatic and the flow of air escaping the geometry is viscously dominated. This simple experiment should be of value to undergraduate physics students in learning about effects of both fluid viscosity and surface tension (another fluid property which they could also measure with a smartphone; Goy et al., Phys. Teach. 55, 498–499 (2017)), and the usefulness of reasonable approximations in physics.
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