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Influence of local heating on Marangoni flows and evaporation kinetics of pure water drops Alexandros Askounis, Yutaku Kita, Masamichi Kohno, Yasuyuki Takata, Vasileios Koutsos, and Khellil Sefiane Langmuir, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017
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Langmuir
Influence of local heating on Marangoni flows and evaporation kinetics of pure water drops Alexandros Askounis,*,1,2 Yutaku Kita,1,2 Masamichi Kohno,1,2,3 Yasuyuki Takata,1,2,3 Vasileios Koutsos4 and Khellil Sefiane*,4,5 1
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishiku, Fukuoka, 819-0395, Japan 2
Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan
3
CREST, Japan Science and Technology Agency, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
4
School of Engineering, The University of Edinburgh, King’s Buildings, Mayfield Road, Edinburgh EH9 3JL, United Kingdom 5
Tianjin Key Lab of Refrigeration Technology, Tianjin University of Commerce, Tianjin City, 300134, PR China
*To whom correspondence should be addressed: E-mail:
[email protected]; Tel.: +8192-802-3905 Fax: +81-92-802-3905. E-mail:
[email protected]; Tel.: +44 (0)131 650 4873; Fax: +44 (0)131 650 6551 Abstract The effect of localized heating on the evaporation of pure sessile water drops was probed experimentally by a combination of infrared thermography and optical imaging. In particular, we studied the effect of three different heating powers and two different locations, directly below the center and edge of the drop. In all cases, four distinct stages were identified according to the emerging thermal patterns. In particular, depending on heating location, recirculating vortices emerge which either remain pinned or move azimuthally within the drop. Eventually, these vortices oscillate in different modes depending on heating location. Infrared data allowed extraction of temperature distribution on each drop surface. In turn, the flow velocity in each case was calculated and was found to be higher for edge heating, due to the one-directional nature of the heating. Additionally, calculation of the dimensionless Marangoni and Rayleigh numbers yielded the prevalence of Marangoni convection. Heating the water drops also affected the evaporation kinetics by promoting the “stick-slip” regime. Moreover, both the total number of depinning events and the pinning strength was found to be highly dependent on heating location. Lastly, we report a higher than predicted relationship between evaporation rate and heating temperature, due to the added influence of the recirculating flows on temperature distribution and hence evaporation flux.
Keywords: drop evaporation, laser heating, Marangoni flows, evaporation kinetics, infrared thermography
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Introduction The evaporation of sessile, liquid drops has attracted considerable scientific attention due to its importance in a wide range of biological and technological applications. Inkjet printing,1, 2 DNA mapping,3, 4, 5
biomedical diagnosis,6, 7, 8 and surface patterning9, 10, 11 are but a few of the areas which may benefit
from advancing the understanding of the process. However, the interplay among fluid dynamics, heat and mass transfer and liquid-surface interactions complicates the complete comprehension of drop drying.
A sessile drop evaporating freely on a hydrophilic surface (water contact angle
