Levich Institute Seminar Announcement, 12/12/2006

Tuesday, 12/12/2006
2:00 PM
Steinman Hall, Room #312
(Chemical Engineering Conference Room)

Greg Carroll
Columbia University
Chemistry Department
(Advisors: Jeffrey T. Koberstein and Nicholas J. Turro)

"Light-directed Control of Macromolecule Organization on a Surface"


[This is a CCNY/Columbia NSF-IGERT Soft Materials seminar]
Tuesday, 12/12/2006
2:30 PM
Steinman Hall, Room #312
(Chemical Engineering Conference Room)

Jon Halverson/Makonnen Payne
City College of CUNY
Chemical Engineering Department
(Advisors: Alex Couzis, Joel Koplik, Charles Maldarelli)

"An Investigation of the Mechanism of Superspreading and the Search for Alternative Superspreading Systems"


[This is a CCNY/Columbia NSF-IGERT Soft Materials seminar]

ABSTRACT

This report describes a versatile method to photo-generate and control self-organized polymer patterns on a surface within a larger pattern. Thin polymer films are cross-linked by irradiation with UV light. Crosslinked thin polymer films resist dewetting when heated above the glass transition temperature. Combining pattern formation via instability with pattern formation via photolithography allows the dewetting patterns to be localized to specific areas of a surface, resulting in a self-organized pattern within a light-directed pattern. By confining the uncrosslinked polymer to an area that approaches the size of the equilibrium dewetting morphology, new mesoscopic features result. For thicker films, the polymer organizes into ribbons at the interface between crosslinked and uncrosslinked polymer. When the width of the uncrosslinked area is large enough, droplets form between the ribbons. As the width gets larger, droplet organization evolves from incomplete to complete polygons. In addition, the structure of the dewetting morphologies changes as the thickness of the film changes. As the thickness of the film approaches the radius of gyration of the polymer, ribbons no longer form at the interface. This technique allows one to spatially choreograph the spontaneous assembly of micrometer sized features on a surface.


ABSTRACT

Trisiloxane surfactants promote the complete and rapid wetting of aqueous droplets on very hydrophobic hydrocarbon substrates. The phenomena is so dramatic that it has been termed superspreading. However, these surfactants are not widely used in industry because they are chemically unstable and are easily hydrolyzed and photodegraded. Binary aqueous systems of polyethylene oxide surfactants and 1-dodecanol, which are stable and inert, have been shown to give equilibrium air-liquid tensions nearly as low as those of the superspreading surfactants. These binary mixtures are hoped to be used as a replacement for the trisiloxanes. The goal of this work is to understand the mechanism of superspreading and use this information as a guide to designing binary surfactant mixtures which promote the rapid and complete wetting of aqueous droplets.

Pendant drop tensiometry experiments and sessile drop contact angle measurements on model hydrophobic surfaces were conducted using the binary mixtures. Although we found that a number of combinations were capable of significantly reducing the air-liquid tension to values comparable to the superspreaders, only systems that exhibited the propensity to form extended liquid crystalline phases, as shown by the combination of cross-polarized microscopy, cryo-TEM, and light scattering experiments, were able to improve on the wetting performance of these systems. In addition we have also conducted in-situ infrared internal reflection spectroscopy experiments on the hydrophobic solid-liquid interface to observe dynamic surfactant adsorption behavior. The experimental results offer insight into the interfacial behavior necessary for superspreading behavior.

The mechanism of superspreading has also been investigated using all-atom parallel molecular dynamics simulation with full electrostatic interactions. A spherical nanodroplet containing 10,000 water molecules and a variable number of trisiloxane molecules is placed in the vicinity of graphite and allowed to spread freely. At initial surface concentrations below the maximum packing value the contact angle at equilibrium is found to be less than that of pure water. At surface concentrations above maximum packing the surfactant monolayer is found to oppose spreading. For comparative purposes a polyethylene oxide surfactant system was also studied. Because these surfactant molecules are less bulky they lead to enhanced wetting for all choices of the initial surface concentration. Larger systems on less hydrophobic substrates are currently being examined.