The Marangoni effect describes fluid flow near an interface in response to a surface tension gradient. Here, we demonstrate that the Marangoni effect is the underlying mechanism for flow driven feature formation in an azobenzene-containing polymer film; features formed in azobenzene-containing polymers are often referred to as surface relief gratings or SRGs. An amorphous poly(4-(acryloyloxyhexyloxy)-4′-pentylazobenzene) was synthesized and studied as a model polymer. To isolate the surface tension driven flow from the surface tension pattern inscription step, the surface tension gradient was preprogrammed via photoisomerization of azobenzene in a glassy polymer film without forming topographical features. Subsequently, the latent image was developed in the absence of light by annealing above the glass transition temperature where the polymer is a liquid. The polymer flow direction was controlled with precision by inducing different surface tension changes in the exposed regions, in accordance with expectation based on the Marangoni effect. Finally, the height of the formed features decreased upon extensive thermal annealing due to capillary leveling with two distinct rates. A scaling analysis revealed that those rates originated from dissimilar capillary velocities associated with different azobenzene isomers.
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The authors thank Professor C. Grant Willson for allowing use of his facilities and Professor Roger Bonnecaze, Talha Arshad, and Mark Ilton for helpful discussions. C.B.K. thanks the Cockrell School of Engineering Fellowship for partial financial support. H.H. thanks the Kwanjeong Educational Foundation for financial support. S.X.Z. thanks the Virginia & Earnest Cockrell, Jr., Fellowship in Engineering for partial financial support. R.K. thanks the Takenaka Scholarship Foundation and Dr. Thomas F. Edgar Endowed Graduate Fellowship in Chemical Engineering for financial support. K.M.M. thanks Murray State University's Committee for Institutional Studies and Research (Presidential Research Fellowship) and the Department of Chemistry for partial financial support. C.J.E. gratefully acknowledges partial financial support for this work from the Robert A. Welch Foundation (Grant no. F-1709), the National Science Foundation CAREER Award (Grant no. DMR-1053293), the 3M Nontenured Faculty Award, and the DuPont Young Professor Award.