Calibrating Schlieren imaging for understanding local film deformation for a range of wetting and intrusions in soap films

Abstract

Bubbles and films play a pivotal role in the dispersion of pathogens. Intrusions in thin films directly affect the final spray size and concentration of droplets. In this study, we seek to develop a method that can inform the direct observation of contaminants interacting with the film in which they are trapped. We develop an apparatus to directly observe microscale interactions within films from a macroscopic perspective. We penetrate soap films with rods (d rod >> h film) to investigate which properties affect the size of the optical spot created by the intrusion into the soap film. The experimental design is careful to account for parameters that could confound results by ensuring consistent environmental parameters, precise positioning of the object, and accounting for thickness of the film over time. We use high levels of pure surfactant to generate films that can sustain themselves despite being punctured by large objects. Using this approach, we are able to validate and extend the results found by Su and Bourouiba [30] by showing that the Schlieren spot size produced by large objects is not affected by size or wetting properties of the object. The spot size in soap films is primarily affected by the film thickness, consistent with prior results on water films (without surfactant).