Micromechanical Aspects of Contact Printing
Sanjay Lakshmanan PhD Defense, Advised by Professor John Lambropoulos
Wednesday, February 2, 2022
Micro-contact printing (μCP), a soft lithography technique, uses a soft elastomeric stamp to initiate intimate contact between the protrusions of the stamp feature and a substrate to print intricate patterns on the latter. With the advent of smart materials the scope of stamp materials has expanded considerably. In this thesis, we study the micro-mechanical aspects of contact printing using a shape-memory polymer stamp. We develop a framework of numerical analyses and analytical solutions to analyze the various aspects of contact printing: backing layer, adhesion, stamp viscoelasticity and pattern collapse mechanisms.
The effect of backing layer thickness was modeled using a linear elastic spring in conjunction with numerical simulation using ABAQUS for various stamp feature geometries. Thicker backing layers were approximated well by linear elastic springs while thinner backing layers introduced localized non-uniform strain in the stamp feature across all stamp feature geometries tested thereby invalidating the linear spring approximation.
Adhesion between the stamp feature and substrate was modeled using an interfacial traction-separation law on ABAQUS. The traction-separation parameters extracted from experimental pull-off force curves could accurately estimate the interfacial fracture energy providing a means of isolating the interfacial adhesion from the overall elastic response of the stamp and backing layer.
Experimental stress relaxation data of PUA (polyurethaneacrylate), analyzed using a DMA (dynamic mechanical analyzer), was modeled using rheological models. A linear prony series model with averaged parameters across strains failed to provide concrete information about the dependence of viscoelasticity on temperature and strain. Modeling the material using a nonlinear strain-rate dependent model provided more insight into the softening of the material at higher temperatures.
Stamp pattern collapse mechanisms such as roof collapse and buckling were analyzed as potential failure mechanisms for various configurations of pattern and aspect ratio of stamp features. A map of dimensionless pressure required to cause roof collapse and buckling as a function of aspect ratio shows the existence of an empirical relationship between them uncovering a potential scaling law that can be extended to smaller feature dimensions.