Fiber coating with surfactant solutions
Amy Q. Shen (1), Blake Gleason (1), Gareth H. McKinley (2) and Howard A. Stone (1)
(1)Division of Engineering & Applied Sciences, Harvard University, Cambridge, MA 02138, USA
(2)Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 01239, USA
When a fiber is withdrawn at low speeds from a pure fluid, the variation in the thickness of the entrained film with imposed fiber velocity is well-predicted by the Landau - Levich -Derjaguin (LLD) equation. However, surfactant additives are known to alter this response. We study the film thickening properties of the protein BSA (bovine serum albumin),the nonionic surfactant Triton X-100,and the anionic surfactant SDS (sodium dodecyl sulfate).For each of these additives, the film thickening factor α (the ratio of the measured thickness to the LLD prediction)for a fixed fiber radius varies as a function of the ratio of the surfactant concentration c to the critical micelle concentration (CMC).In the case of BSA, which does not form micelles, the reference value is the concentration at which multilayers form. As a result of Marangoni effects, α reaches a maximum as c approaches the CMC from below. However, when the surfactant concentration c exceeds the CMC, the behavior of α varies as a consequence of the dynamic surface properties, of wing for example to different sorption kinetics of these additives, or possibly surface or bulk rheological effects. For SDS, α begins to decrease when c exceeds the CMC and causes the surface to become partially or completely remobilized, which is consistent with the experimental and theoretical results published for studies of slug flows of bubbles and surfactant solutions in a capillary tube and the rise of bubbles in surfactant solutions. However, when the SDS or Triton X-100 surfactant concentration is well above the CMC, we observe that the film thickening parameter α increases once again. In the case of SDS we observe a second maximum in the film thickening factor. For all the experiments, transport of monomers to the interface is limited by diffusion and the second maximum in the film thickening factor may be explained as a result of a non-monotonic change in the stability characteristics of suspended SDS micelles and corresponding changes in the rheology of the solution.