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Visualization of flow inside a small evaporating droplet Kwan
Hyoung Kang, Sang Joon
Lee, Choung Mook Lee, and In Seok Kang Department of Mechanical Engineering, Pohang University of
Science and Technology, San 31, Hyoja-dong, Pohang 790-784, Republic of Korea (Selected for publication in the special issue of the Measurement
Science & Technology for PIV¡¯03) Droplets have
many interesting applications associated with microfluidic problems, e.g.,
DNA molecules imaging, micro-pumps, and ink-jet printing. The details of
droplet-related phenomena in micro and nanoscales such as evaporation
process, Marangoni effects, contact angles with solid substrates, and
electrowetting are not well known. There is a consensus of
opinion that the fluid flow inside a droplet may play an important role on
the overall transport phenomena. Compared to its importance of fluid flows, only a few systematic investigations have been performed in the past, toward understanding of overall transport phenomena inside a droplet.
Fig. 1. Verification of the image-restoring method. (a) Plexiglas lens; (b)
original image; (c) restored image. The accuracy of the present image
restoration method which incorporates the ray-tracing method is checked by
using a hemi-spherical Plexiglas lens
(see Fig. 1a). In Fig. 1b, the distorted image of 30¡¿15 meshes due
to the presence of the hemi-spherical Plexiglas lens is shown. The restored
image by using the ray-tracing method is shown in Fig. 1c.
The center region is well restored, while the accuracy of image restoration
is not so good for the region of r/Ro > 0.75.
Fig. 2 Flow inside evaporating droplets for different concentration of ethanol
at 20 sec exposure except for (d) at 2 sec exposure. (a) 1%; (b) 5%; (c) 20%; (d) 20%. The flow is upward at the center region for
all the cases. For the case of 5% mixture, the flow is initially rather
unstable at the periphery of the droplet as shown in Fig. 2b. In all the
cases considered here, the regular flow patterns such as shown in Figs. 2a
and 2c are established after moderate times. A similar flow pattern is
also observed when KCl (sodium chloride) mixed with
deionized water is used. It is very intriguing
phenomenon that very small amount of additives generates such a change in the
fluid motion. It is conjectured that the flow shown in Fig. 2 is either driven by the density gradient inside a droplet or by
viscous diffusion, rather than by the Marangoni instability.
Fig. 3 Restoration of
velocity vectors for the case of 1% mixture. (a) before restoration; (b) after restoration. The
most important difference between the two results is that the magnitude of
upward velocity component at the center region of the droplet is reduced
after the image restoration. Additionally, the center of vorticity is moved a
little towards the origin. Lastly
Modified in October 13, 2003 |
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