Bidirectional Control of Flow in Thin Polymer Films by Photochemically Manipulating Surface Tension

Abstract

The Marangoni effect causes liquids to flow toward localized regions of higher surface tension. In a thin film, such flow results in smooth thickness variations and may represent a practically useful route to manufacture topographically patterned surfaces. An especially versatile material for this application should be able to be spatially programmed to possess regions of higher or lower relative surface tension so that the direction of flow into or out of those areas could be directed with precision. To this end, we describe here a photopolymer whose melt-state surface tension can be selectively raised or lowered in the light exposed regions depending on the wavelength and dose of applied light. The direction of Marangoni flow into or out of the irradiated areas agreed with expected surface tension changes for photochemical transformations characterized by a variety of spectroscopic techniques and chromatographic experiments. The maximum film thickness variations achieved in this work are over 200 nm, which developed after only 5 min of thermal annealing. Both types of flow patterns can even be programmed sequentially into the same film and developed in a single thermal annealing step, which to our knowledge represents the first example of harnessing photochemical stimuli to bidirectionally control flow. ■ INTRODUCTIONLiving organisms have evolved in remarkable ways to exploit surface tension and interfacial phenomena. 1−4 For example, water striders of the genus Microvelia possess an emergency ability to propel themselves forward by directing surface tension gradients on the water surface. By excreting organic, surfactant-like molecules to their rear, they create localized regions where the surface tension is 23 dyn/cm lower than its surroundings. 5 Microvelia are pulled, in the direction of higher surface tension via the Marangoni effect, which describes fluid transport due to surface tension gradients. 6,7 The Marangonidriven propulsion yields speeds of 17 cm/s, twice Microvelia's normal water walking rate. 5 Inspired by Microvelia, we seek to direct chemical changes in materials that rapidly alter the liquid surface tension in desired regions. To build upon observations of nature, we describe here a solid film that can be patterned with light to increase or decrease its melt-state surface tension in the light exposed regions. Light is an ideal stimulus to direct these changes because a wide array of chemical transformations can be applied with spatial precision 8,9 using photomasks. Control of the directionality of melt-state surface tension changes is obtained by selecting irradiating wavelengths and doses that activate different photochemistries in the solid state. To confirm directionality, the solid is heated to a liquid state, at which point Marangoni-flow occurs spontaneously from low-to-high surface tension regions. As a consequence, film thickness variations develop, which can be monitored in situ by optical microscopy or on cooled, vitrified films by atomic force microscopy (AFM) and profilometry. P...

Thermocapillary approaches to the deliberate patterning of polymers

“…These are shown schematically in Figure and will now be treated in turn. As alluded to, other Marangoni methods for polymer patterning, such as photomodification of surface tension or breath figures, while highly promising, are beyond the scope of this review, unless coupled to thermocapillary forces.…”

“…One example is the diffusion‐induced solvocapillary flow up thermal gradients that has been observed in FLaSk dewetting of metal alloys discussed above—while such mechanisms are less common polymers, they may be deliberately incorporated. For example, several groups have been exploring the UV‐induced chemical alteration of polymeric systems to trigger subsequent dewetting during melting . Currently, these effects are limited by the homogenization of the film at lengthy heating, which clears the surface pattern.…”

Application of Plasmon-Induced Lithography for Creation of a Residual-Free Pattern and Simple Surface Modifications

“…8−12 Because the surface tension is a function of several parameters, the introduction of its gradient can be performed through several technological approaches, including the use of vertical or horizontal constant thermal treatment, transverse zone annealing, and so on. 4,13−17…”

“…The lithographic methods should provide rapid, high-resolution patterning, with high throughput, maximal structure resolution, and an opportunity to control the created structure. , The use of thin polymer film instability represents an elegant way for polymer pattering, which satisfies the above-mentioned requirements. − The physical background of thin film instability and related polymer mass redistribution is typically explained within the framework of Marangoni flow, assuming that under the action of a surface tension gradient, a system tends to evolve toward a stable state by minimizing its surface area through the material flow. , The Marangoni-driven mass redistribution was successfully applied to create self-assembled polymer structures and the structures with arbitrary symmetry and, more recently, for the nanoscale removal of a protective thin film by resistive heating of carbon nanotubes. − Because the surface tension is a function of several parameters, the introduction of its gradient can be performed through several technological approaches, including the use of vertical or horizontal constant thermal treatment, transverse zone annealing, and so on. ,− …”

Monitoring Photochemical Reactions Using Marangoni Flows

“…The authors identified decarboxylation and dehydrogenation reactions as the chemical processes driving the flow. 75 …”