Molecular Visualization: Spreading Kinematics and Dynamics

Sergei S. Sheiko, UNC Chapel Hill, DMR-0306787

Our research program develops a molecular-level understanding of spreading phenomena in thin liquid films by visualizing individual molecules as they spread to cover a solid substrate.1 This impacts microfluidics, lubrication, and surface coatings.2

Recently, a new type of flow fingering instability has been discovered in polymer films as they spread on a solid substrate. 3 Tracing the movement of individual molecules by AFM enabled us to follow the development of the instability on the molecular level and to understand the underlying physical mechanism. The fingering instability is triggered by conformational changes of brush-like macromolecules in response to variations in the pressure gradient driving the flow.

1 Phys. Rev. Lett. 93, 206103/1-4 (2004)2 http://pubs.acs.org/cen/news/8246/8246molecules.html 3 Phys. Rev. Lett. 94, 237801/1-4 (2005)

(a) After 7 hours of spreading, a drop reveals a precursor film with a dendritic structure, characteristic for the flow instability. (b-e) Higher magnification AFM images of the precursor film show two conformationally different phases of the monolayer (c, d) separated by a sharp phase boundary (e). The square boxes in a and b labeled with italic letters show approximate areas where images (b-e) were captured. (f) The schematics of the conformational phase transition in the precursor film.

Molecular Visualization: Spreading Kinematics and DynamicsSergei S. Sheiko, UNC Chapel Hill, DMR-0306787

We encourage broad participation of underrepresented groups in scientific research through Summer Pre-Graduate Research Experience (SPGRE), a highly successful program (http://www.ibiblio.org/res/) that attracts undergraduate students nationwide for a nine-week summer research experience. This program provides extensive scientific experience to the students and exposes them to mentors who have careers in science and engineering. As a part of the summer internship, students are required to make a joint presentation with her or his summer advisor at their home university or school.

During his 9-week summer research experience in 06-07/2005, Jonathan has successfully accomplished his project focused on molding of single polymer molecules and block-copolymer micelles. The figure below shows a master of 30nm-sized spheres on a solid substrate (left) that were imprinted into a perfluoropolyether (PFPE) mold.

PFPE MoldMaster

Jonathan Alford (undergraduate student from the North Carolina A&T State University) learning AFM with his mentor Sherryl Yu (2nd year graduate student at UNC-CH)