Evaluation of absorption of micro-droplets on paper for creation of paper-based microstructures
This study clarifies the absorption behavior of micro-droplets of water on treated paper to support the design of functional microstructures, such as electronics and micro-fluid channels, on paper. The period of time between when a micro-droplet of water ejected from an ink-jet head lands on the paper’s surface and its complete disappearance by absorption was defined as the micro-sizing degree (MSD), and an MSD measurement method was established. The MSD was evaluated using microscopic high-speed video images of the side view recorded every millisecond. Several grades of commercially available ink-jet paper media and laboratory sheets having different levels of water repellency prepared from a pulp and a sizing agent were examined. The MSD of commercial ink-jet papers, which are known to absorb water very quickly, was 3–6 ms. Weakly sized laboratory sheets exhibited a lower MSD of 2–3 ms. The absorption behavior was analyzed in terms of the capillary pressure with and without the Laplace pressure; the theoretical and experimental results agreed moderately well. The results indicated that the Laplace pressure cannot be neglected in the analysis. The MSD of a wet surface where a preceding micro-droplet had already landed was higher than that on a dry or partially wet surface, presumably because water remains inside pores for an unexpectedly long time.
KeywordsContact Angle Capillary Pressure Water Repellency Pulp Fiber Laplace Pressure
The authors express special thanks to Mr. Kazumasa Matsumoto, Konica Minolta, for giving us the opportunity to use the KIE-2 system. They also thank the Japan Society for the Promotion of Science (JSPS) and the Japanese Society of Printing Science and Technology (JSPST) for their financial support: Grant-in-Aid for Scientific Research (B) No. 22380092 (2009) and Research Fund for Printing Technology (2009 and 2011), respectively.
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