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