This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.Molecular dynamics simulations will be used to study the effects of surfactants on the spreading of drops on partially-wetting solid substrates. It is known experimentally that only very particular surfactant additives, trisiloxanes, have the ability to allow an aqueous drop to spread completely on a hydrophobic surface. The mechanism, and in particular those feature(s) which distinguish this surfactant from others, is not established, although bilayer formation at the advancing contact line has emerged as a candidate. The simulations will be based on realistic molecular models of water and the surfactants, along with a molecular solid surface, in configurations imitating a laboratory experiment where a surfactant-laden drop is allowed to spread freely. Using the insight provided by the trisiloxane simulation as to the spreading mechanism, we will attempt to identify new surfactant systems capable of complete wetting. In the course of the spreading simulations we will measure the hydrodynamic flow fields near the moving contact lines, which will guide modeling of continuum boundary conditions there, in the presence of surfactant. The results of these full-scale simulations will furthermore be used to guide the development of smaller-scale molecular models which focus on the contact-line region alone.
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