Fluid-based micropatterning for the fabrication of biomimetic functional surfaces

Intricate micro/nanoscale structural features common on biological surfaces provide
evolutionary-optimised inspirations for the design of functional materials. Micropatterns
engender enhanced collective properties from hydrophobicity to antifouling, enabling the
exploitation of often overlooked interfacial phenomena, providing new material functionality beyond that of the intrinsic properties. However, drawbacks in conventional micro-fabrication approaches limit the widespread use of micropatterned surfaces due to the
antagonistic pheneomena of manufacturing nano and microscale features over large areas. The realisation of desired properties can be emulated through biomimetic design
approaches, in which we draw inspiration from the self-assembled designs and energy-efficient fabrication mechanisms manifested in natural systems.

This thesis looks toward fluid-based fabrication approaches, for inherently low-cost and
scalable fabrication. To this end, the spontaneous nucleation and self-assembly of con-
densation water droplets are harnessed to dynamically pattern polymer films prior to
polymer solidification and water evaporation. An adapted breath figure (BF) templating methodology is developed and characterised. In this novel approach, we de-couple
condensation onset/growth and polymer curing mechanisms by using external cooling
and photocurable polymers. Through in situ interrogation of the system physics, a phenomenological mechanistic model using the condensation kinetics describes and predicts
the pattern growth. Systematic variation of highly packed porous patterns is achieved,
attaining programmable average pore sizes ranging from 100’s of nanometres to 10’s of
micrometres.

Modulation of the breath figure pattern beyond capabilities of the classical approach are
explored. The inherent phase-change reversibility of the templating condensate is exploited, modulating the final surface architecture through regimes of condensation and
evaporation. Surfaces with spatially diverse designs and bimodal distributions are manufactured. Spatial masking and gradiented patterning is created through experimental
adaptations and inverse replica samples are fabricated in PDMS. Finally, characterisation
of the pattern properties is tested in relation to wetting behaviour, and key applications
relating to thermal efficiency, directional wetting and biofouling are introduced.