Studying of fast interfacial loading of surfactants by PANDA pages:115
IMA7 - 7th Conference of the International Marangoni Association edition:7 location:Vienna date:23-26 June 2014
I. Oscillating pendant drops: indirect imaging of surface tension
Surfactant molecules are important in household, industrial procedures (such as cleaning in semiconductor production) and, nowadays, as a study object for biological1 and micro-organism related processes
2. Many use pendant drop experiments to measure dynamic surface tension. However, the method is severly limited by inertia and tends to be slow. Hence, it is hard to probe for the fast molecular dynamics of surfactant ad- and desorption to the interface.
II. Fast surfactant dynamics: Interfacial surfactant loading dominates flow and its time scale
An oscillating pendant droplet was used, yet instead of a pendant droplet, a small volume of air was trapped into a aqueous surfactant solution (see figure 1, left). 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 (see figure 2, left). 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 behaviour (see figure 2, right). 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.