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Capillary flow and drying of polymer solutions in open microchannels are explored over time scales spanning seven orders of magnitude: from capillary filling (10-3-10 s) to the formation of a dry thin film (a "capillary coating" 102-103 s). During capillary filling, drying-induced changes (increased solids content and viscosity) generate microscale pinning events that impede contact line motion. Three unique types of pinning are identified and characterized, each defined by the specific location(s) along the contact line at which pinning is induced. Drying is shown to ultimately pin the contact line permanently, and the associated total flow distances and times are revealed to be strong functions of channel width and drying rate. In general, lower drying rates coupled with intermediate channel widths are found to be most conducive to longer flow distances and times. After the advancing contact line permanently pins, internal flows driven by uneven evaporation rates continue to drive polymer to the contact line. This phenomenon promotes a local accumulation of solids and persists until all motion is arrested by drying. The effects of channel width and drying rate are investigated at each stage of this capillary coating process. These results are then applied to case studies of two functional inks commonly used in printed electronics fabrication: a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)) ink and a graphene ink. Although drying is shown to permanently arrest flow in both inks, both systems exhibit an increased resistance to pinning unexplained by mechanisms identified in aqueous polymer systems. Instead, arguments based on chemistry, particle size, and rheology are used to explain their novel behavior. These case studies provide insight into how functional inks can be better designed to optimize flow distances and maximize overall dry film uniformity in capillary coatings.
Bibliographical noteFunding Information:
The authors would like to thank the industrial supporters of the Coating Process Fundamentals Program (CPFP) of the Industrial Partnership for Research in Interfacial and Materials Engineering (IPRIME) for supporting this research. R.K.L. was further supported by the University of Minnesota Graduate School under the Doctoral Dissertation Fellowship (DDF) program. L.F.F. and K.S.J. gratefully acknowledge support from the National Science Foundation through Grant number CMMI-1634263. K.S.J. further acknowledges support from the National Science Foundation (NSF) Graduate Research Fellowship Program under grant no. 00039202. The authors thank Wieslaw Suszynski for high-speed video assistance, help designing and constructing the humidity chamber, and for other helpful discussions. The authors would also like to thank Satish Kumar for stimulating discussions, Ethan Secor and Professor Mark Hersam for providing the graphene ink, Panayiotis Kolliopoulos for helpful suggestions, and Nitika Thakral, whose diligent efforts in the lab during the summer of 2016 inspired some of this work. Lastly, the authors would like to extend their gratitude to colleagues working with printed electronics at the University of Minnesota, including C. Daniel Frisbie, Woo Jin Hyun, Ankit Mahajan, Donghoon Song, Fazel Bidoky, and Motao Cao for providing invaluable feedback throughout this work, especially with respect to its application to printed electronics. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the MRSEC program, and the Minnesota Nano Center, which receives partial support from the NSF through the NNCI program.
© 2018 American Chemical Society.
How much support was provided by MRSEC?
Reporting period for MRSEC
- Period 5
PubMed: MeSH publication types
- Journal Article
- Research Support, U.S. Gov't, Non-P.H.S.
- Research Support, Non-U.S. Gov't
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