title: Surface shear inviscidity of soluble surfactants.
abstract: Foam and emulsion stability has long been believed to correlate with the surface shear viscosity of the surfactant used to stabilize them. Many subtleties arise in interpreting surface shear viscosity measurements, however, and correlations do not necessarily indicate causation. Using a sensitive technique designed to excite purely surface shear deformations, we make the most sensitive and precise measurements to date of the surface shear viscosity of a variety of soluble surfactants, focusing on SDS in particular. Our measurements reveal the surface shear viscosity of SDS to be below the sensitivity limit of our technique, giving an upper bound of order 0.01 μN·s/m. This conflicts directly with almost all previous studies, which reported values up to 10(3)-10(4) times higher. Multiple control and complementary measurements confirm this result, including direct visualization of monolayer deformation, for SDS and a wide variety of soluble polymeric, ionic, and nonionic surfactants of high- and low-foaming character. No soluble, small-molecule surfactant was found to have a measurable surface shear viscosity, which seriously undermines most support for any correlation between foam stability and surface shear rheology of soluble surfactants.
authors.affiliation: Department of Chemical Engineering, University of California, Santa Barbara, CA 93106-5080.
authors.forename: Zachary A
authors.initials: ZA
authors.lastname: Zell
authors.forename: Arash
authors.initials: A
authors.lastname: Nowbahar
authors.forename: Vincent
authors.initials: V
authors.lastname: Mansard
authors.forename: L Gary
authors.initials: LG
authors.lastname: Leal
authors.forename: Suraj S
authors.initials: SS
authors.lastname: Deshmukh
authors.forename: Jodi M
authors.initials: JM
authors.lastname: Mecca
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Tucker
authors.forename: Todd M
authors.initials: TM
authors.lastname: Squires
mesh_terms: D059346:Magnets; D008956:Models, Chemical; D012212:Rheology; D033081:Shear Strength; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry*; D013501:Surface-Active Agents / Q000737:chemistry*; D014783:Viscosity
chemical_list: D013501:Surface-Active Agents; D012967:Sodium Dodecyl Sulfate