The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Hydrodynamics of Dip-Coated Thin Films in the Presence of Evaporation and Surfactant Structures Controlling Spontaneous Dewetting

by Dan Qu



In this dissertation, we discuss the investigation of two problems in dynamic wetting: the hydrodynamics of dip-coated, finite-length films of evaporative fluids and the surfactant structures controlling the spontaneous dewetting of a surfactant solution.
While films pulled from non-volatile fluids on a vertical substrate are essentially infinite in length, films pulled from volatile fluids have a finite length. We examine such finite films using three well-controlled oligomer liquids as well as surfactant solutions. We find that the finite length of the film is controlled by a global balance between mass lost by evaporation and mass input by viscous forces. While the attendant thermally driven Marangoni flows have small impact on the mass balance, they do alter the velocity field in the film in the direction parallel to the substrate. Using measured film profiles, we have developed a novel method to determine the combined effects of evaporation and Marangoni flow on velocity and pressure fields in the film. This method is independent of any specific model of the evaporation process. In preliminary studies with surfactant solutions, we observed strong effects of solutal Marangoni flows on dip-coated films.
For the second problem, we examine the structures of self-assemblies left on a solid as a contact line spontaneously retreats across a surface during an autophobing event. We find that surfactants of a continuous structural gradient are deposited: from molecules lying down on the surface with low packing densities in a region never touched by the solution, to molecules standing up with higher packing densities in a region where the contact line has moved slowly. Despite significant free volumes within the self-assemblies, we see no evidence of clustering of molecules. We see a clear correlation between contact line speed and the surfactant structures. We show that the dynamics during at least a later period of the autophobing event is dominated by the time evolution of Young's force dictated by the self-assembly near the contact line.