Patterning by Photochemically Directing the Marangoni Effect

Abstract

Polystyrene (PS) that has been exposed to ultraviolet light (UV) undergoes partial dehydrogenation of the alkane polymer backbone which increases its surface energy. Exploiting this photochemistry, we exposed polystyrene films to UV light using a photomask to induce a patterned photochemical reaction producing regions in the film with differing surface energy. Upon heating the solid polymer film with the preprogrammed surface energy pattern to a liquid state, the polymer flows from the low surface energy unexposed regions to high surface energy exposed regions. This flow creates three-dimensional topography by the Marangoni Effect, which describes convective mass transfer due to surface energy gradients. The topographical features can be permanently preserved by quenching the film below its glass to liquid transition temperature. Their shape and organization are only limited by the pattern on the photomask.

Directing convection to pattern thin polymer films

“…In addition, it is responsible for often undesirable striation defects in spin coated films,81 among other phenomena. As our group demonstrated in recent experiments,21 by patterning surface energy gradients into a solid thin polymer film, the Marangoni effect can be harnessed to direct flow in the melt state. It is important to note that previous theoretical studies82–84 have explored similar ideas, although some of the details are different.…”

“…(b) An optical micrograph and overlaid height profile of the pattern formed by employing a mask with a regular array of lines and spaces. Reproduced from Ref 21. with permission from the American Chemical Society.…”

“…Perhaps the most well‐known method to direct convection for patterning surfaces is by so called “coffee ring patterning.”18 Although this and other evaporative‐based methods are briefly discussed, we focus especially on methods that employ polymer melts, specifically in thin films, such as lithographically induced self‐assembly (LISA),19 and electrohydrodynamic patterning (EHP) 20. This manuscript concludes with a description and discussion of a method recently explored in our group for generating patterned thin films by photochemically directing Marangoni flow with subtle gradients in surface energy 21…”

Low-cost replication of self-organized sub-micron structures into polymer films

“…Utilization of the self-organized patterns in various applications is of high interest [12] and it has been applied for example in the case of curved photonic crystals by Kolaric et al [13]. It has been found out that very different kinds of materials and methods can be employed for selforganizing coatings [14][15][16][17][18], or the whole process set-up can be designed from end-to-start, to obtain structures with desired shapes or sizes, as a result [19,20]. The self-organizing pattern formation to be occurred, the structure of the layered system and materials employed must be appropriate for the process and fulfill certain requirements.…”

Marangoni flows induced by atmospheric-pressure plasma jets

“…Due to the plasma-liquid interaction, the chemical composition of the liquid is altered, most likely via partial oxidation or the addition of nitrogen-containing functional groups. The liquid surface tension changes with composition and becomes spatially non-uniform, which induces solutocapillary Marangoni flows [35][36][37][38][39][40][41] and redistributes the liquid film. We developed a model that qualitatively reproduces many of the morphological and dynamical features observed in the experiments.…”