Accepted Manuscripts

Sourabh K. Saha, Chuck Divin, Jefferson A. Cuadra and Robert M. Panas
J. Micro Nano-Manuf   doi: 10.1115/1.4036445
Two-photon polymerization (TPP) is a laser writing process that enables fabrication of millimeter scale 3D structures with submicron features. In TPP, writing is achieved via nonlinear two-photon absorption that occurs at high laser intensities. Thus, it is essential to carefully select the incident power to prevent laser damage during polymerization. Currently, the feasible range of laser power is identified by writing small test patterns at varying power levels. Herein, we demonstrate that the results of these tests cannot be generalized because the damage threshold power depends on the proximity of features and reduces by as much as 47% for overlapping features. We have identified that this reduction occurs primarily due to an increase in the single-photon absorptivity of the resin after curing. We have captured the damage from proximity effects via X-ray 3D computed tomography images of a non-homogenous part that has varying feature density. Part damage manifests as internal spherical voids that arise due to boiling of the resist. We have empirically quantified this proximity effect by identifying the damage threshold power at different writing speeds and feature overlap spacings. In addition, we present a first-order analytical model that captures the scaling of this proximity effect. Based on this model and the experiments, we have identified that the proximity effect is more significant at high writing speeds; therefore it adversely affects scalability of manufacturing. The scaling laws and the empirical data generated here can be used to select the appropriate TPP writing parameters.
TOPICS: Photons, Polymerization, Additive manufacturing, Damage, Lasers, Manufacturing, Scaling laws (Mathematical physics), Hardening (Curing), Boiling, Computerized tomography, Absorption, Density, X-rays, Resins
Nicholas Clegg, Krishna Kota, Xin He and Sean Ross
J. Micro Nano-Manuf   doi: 10.1115/1.4036446
Altering the wetting characteristics of copper will positively impact numerous practical applications. The contact angle (CA) of a water droplet on the polished copper surface is usually between 70° and 80°. This paper discusses a facile, scalable, tuned bulk micro-manufacturing approach for altering the surface topology of copper concomitantly at the micro- and nano- length scales, and thus significantly influence its wetting characteristics. The resultant copper surfaces were found to be robust, non-toxic, and exhibited ultra-omniphilicity to various industrial liquids. This extreme wetting ability akin to a paper towel (CA of zero for multiple liquids) was achieved by tuning the bulk micro-manufacturing process to generate connected hierarchical micro- and nano-roughness with nano-cavities within the embryos of micro-cavities. With an adsorbed coating of ester, the same ultra-omniphilic copper surfaces were found to exhibit robust super-hydrophobicity (CA ~ 152° for water).
TOPICS: Copper, Micromanufacturing, Wetting, Cavities, Water, Ester, Hydrophobicity, Topology, Coating processes, Coatings, Surface roughness, Polishing, Toweling, Drops

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