IMA7 - 7th Conference of the International Marangoni Association edition:7 location:Vienna date:23-26 June 2014
An oscillating pendant droplet was used, yet instead of a pendant droplet, a small volume of air was trapped into a aqueous surfactant solution. This scheme with improved temporal resolution (i.e. 20 Hz) is named PANDA or Pressure Assisted Nucleation of a Droplet of Air. To prove its strength, a systematic PANDA study of varying concentrations and surfactant types was done. Furthermore, a complementary fluorescence microscopy study of the internal flow in geometrical identical drops, of those in the PANDA study, was done. Astonishingly (even above the CMC) time and concentration depending properties influence the internal flow and the deposition pattern in the sessile droplets (such as an altered coffee ring effect of the (bio)colloidal fronts). Further experiments in bacterial colonies of surfactant producing micro-organisms (Pseudomonas aeruginosa) show comparable flows. The flow properties were heavily influenced by the soluble surfactants; and their ad- and desorption dominate the observed time scale in the oscillating flow behavior. This could be clarified by the PANDA technique as the needed time of loading and unloading of the interface by the surfactant, steered by the capillary force (induced by evaporation) in combination with a surfactant induced Marangoni force.