Investigation of time dependence of dryness for a tiny ink dot by using Digital Holographic Microscopy

 The conventional setup of digital holography cannot achieve monitoring the dryness of a tiny ink dot discharged by an ink-jet nozzle. Therefore, we propose a novel method to study the drying process of a tiny ink dot by using the configuration of digital holographic microscopy.

 The experimental setup is shown in Fig. 1. The water-based ink was applied to a glass plate by an ink-jet nozzle, and ink dot is shown in Fig. 2. The transmitted beam from the ink dot was expanded by an objective lens, and interfered with a reference beam reflected from a tilted mirror for off-axis configuration. For monitoring a drying process, the recording of the off-axis holograms was conducted.

Fig. 1 Experimental setup.

Fig. 2 Microscopic image of ink dot after the experiment.

 To investigate the change of image depending on the drying status, the intensity images at various lapses were obtained and shown in Fig. 3. We also took the difference of the reconstructed phase of 200 s intervals and the results are shown in Fig. 4. From Fig. 3, the time variation of dryness of the ink dot surface due to the drying can be confirmed. For the detection of the stationary state of the ink dot surface, we calculated the standard deviation σ of phase difference in the area of ink dot surface. The temporal variation of value of σ are shown in Fig. 5. The value of σ at the time of 20000 s has been decreasing monotonically. It is shown that the change of the ink dot surface due to the drying continued even after 20000 s.


Fig. 3 Intensity images for ink dot.

Fig. 4 Phase difference images for ink dot.

Fig. 5 Time dependence of σ for phase difference image.