Hopping the Contact Line

Monday, March 26, 2018 - 4:00pm - 4:30pm
Lind 305
Stephen Garoff (Carnegie Mellon University)
We will discuss a number of phenomena where molecules cross a contact line and have major impact on the movement of that contact line. Autophobing is a well-known phenomenon where a fluid, often a surfactant solution, begins to spread over a high energy solid surface and then spontaneously retracts, reaching a static contact angle higher than expected given the surface tensions in the problem. We will review the evidence that surfactant crosses the contact line, alters the surface energy over a small distance ahead of the contact, and reverses the movement of the contact line. We will then explore the same behavior when a drop of pure surfactant is placed on liquid surface where it is insoluble. Finally, we will examine the case of a drop of surfactant solution containing a non-surface active solute spreading on the surface of a liquid in which the solvent, solute and surfactant are all soluble. Marangoni driven spreading occurs in this case with the surfactant moving ahead of the contact line but the solvent remaining as a drop and confining the solute for times much longer than the spreading process. This latter process can be harnessed for drug transport in the lung.