(147e) Autophobing On Liquid Subphases Via Interfacial Transport of Amphiphilic Molecules

Autophobing is a phenomenon in which a low surface tension drop of pure amphiphilic molecules or a solution of amphiphiles fails to completely wet a high energy substrate and instead forms a lens. Considerable research has examined autophobing on solid substrates, where questions still remain about the balance of surface tensions in the final state and what molecular structures produce those surface tensions. Autophobing has also been observed on liquid subphases. Autophobing on liquids has been attributed to molecular transport through one of the bulk phases rather than to interfacial transport. Motivated by Marangoni driven spreading on liquid subphases, we investigate the mechanism of autophobing on liquid subphases via interfacial transport of amphiphiles. We deposit drops of a pure amphiphilic compound (oleic acid) or of a surfactant solution (dodecanol in hexadecane) on water or glycerol-water mixture subphases. The low solubility and vapor pressure of the deposited amphiphiles make it highly unlikely that bulk phase transport of the molecules can cause the drop to autophobe on the short timescales observed here. We measure surface tensions relevant to the spreading both before and after drop deposition, determine if the amphiphiles have moved from the drop onto the surrounding subphase surface outside the lens, and estimate the packing of those molecules on the surface. In all cases, we observe the rapid escape of amphiphiles across the drop contact line promptly after drop deposition and formation of a monolayer that is close to collapse on the subphase outside the drop. The monolayer formation decreases the subphase-air surface tension external to the drop and reverses the sign of the spreading coefficient from positive (before the monolayer forms) to negative, forcing the drop to retract into a lens with an increased contact angle. Measurement of all three surface tensions that exist at the end of drop spreading reveals that it is the reduction in the surface tension of the subphase outside the drop that is solely responsible for inhibiting the spreading of the drop and initiating its retraction. Further, we capture the dynamic retraction of an oleic acid drop on a glycerol-water mixture subphase by simultaneously  tracking the  movement of the drop contact line and the capillary ridge that advances ahead of the drop contact line. We clearly see that the drop does not spread completely to a thin wetting film with zero degree contact angle despite the initially positive spreading coefficient. Rather it first spreads only to a finite extent and then starts retracting once the amphiphilic molecules have oriented themselves to form a dense monolayer outside the drop. In addition, we also develop a mathematical model to predict the diameter of the lens given all the surface tensions, densities and the deposited drop volume. We compare the experimentally measured lens diameters with the mathematical model and find the values to be in good agreement.