"Surfactant Structures Controlling Spontaneous Dewetting" and

“Hydrodynamics of Dip-coated Thin Films in the Presence of Evaporation”

Dan Qu (Advisor: Stephen Garoff)  <personal>

Surfactant Structures Controlling Spontaneous Dewetting     PDF slides (285kB)

When an aqueous solution of ionic surfactant spreads on an oppositely charged substrate, the contact line will first advance and then spontaneously retreat across the surface.  We have discovered that a rearrangement of adsorbed surfactant structures at the three interfaces near the contact line governs the spontaneous retreat; and in turn, the contact line dynamics determines the instantaneous surface rearrangement of surfactant molecules.  To investigate the surfactant structures, We studied a negatively charged SiO₂ substrate forced into a low-concentration, cationic solution. Using AFM, optical scanning ellipsometry, X-ray reflectometry, scanning Auger electron microscopy and water condensation figures, We observed three distinctive regions near the contact line, corresponding to different wettabilities, topographies and surfactant densities. The molecular structure in these regions is a transition from disordered surfactant molecules lying on the substrate, to loosely packed molecules standing out from the surface, to densely packed molecules forming a monolayer.  Using video microscopy, we have also analyzed the dynamics of the spontaneous retreat and correlated the contact line motion with the spatially varying surfactant structures.   The contact line motion rearranges the surfactant structures as it sweeps across the region.  Different retreating speeds cause different molecular structures.  In turn, the instantaneous structures of the self-assembled surfactant molecules in the microscopic vicinity of the contact line determine the time varying Young's force which drives the spontaneous retreat.  The essence of spontaneous dewetting behavior of ionic surfactant solutions lies in the interplay of these two processes.

Hydrodynamics of Dip-coated Thin Films in the Presence of Evaporation     PDF slides (176kB)

Dip coating with evaporating solutions is a widely used industrial process.  We have measured and analyzed the profiles of dip-coated evaporative thin films and used them to characterize the hydrodynamic properties of such films.  Furthermore, we have developed a systematic method to probe the contribution of gravity, capillary force, viscous force, vapor recoil, as well as evaporation and Marangoni flow on the velocity field, pressure field and flux field, which consequently reveals the mass transport mechanism of such systems. In contrast to films produced by withdrawing a substrate from a non-evaporative fluid, fully developed evaporative films have fixed lengths that do not increase with time.  With laser interferometry, we investigated the film profile of several low molecular weight, volatile silicone oils with different evaporation rates (similar to those of water and ethanol) as the substrate is withdrawn from the bulk liquid at different speeds.  The film length, cross-sectional area and characteristic thickness all have power-law dependences on the withdrawing speed.  Moreover, the apparent contact angle and the curvature at the film tip both have power-law dependences on the withdrawing speed of the substrate.  They decrease with speed, but increase with evaporation rate, showing the effect of  both viscous force and evaporation.  A full picture of the hydrodynamics of these fluids also requires consideration of the thermocapillary flow caused by the temperature gradient due to evaporation.  Gravity, viscous drag, capillary force, evaporation and Marangoni flow compete in mass transportation and in forming the pressure field inside the film.  For typical evaporative systems like ours, the mass flux in the film is to the first order governed by the balance of viscous drag, evaporation and Marangoni flow.