Comment pubs.acs.org/Langmuir
Reply to Comment on “Influence of Microwaves on the Water Surface Tension” n the comment on our publication “Influence of Microwaves on the Water Surface Tension”,1 Amiri and Amiri commented on nonequilibrium conditions, fluid motion, and contamination from the Teflon during MW radiation. Here, we reply to the issues raised:
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1. NONEQUILIBRIUM CONDITIONS Equilibrium conditions are not required for many of the surface tension measurement methods.2 For example, the maximum bubble pressure method involves rapid expansion and movement of the air/water surface area. The two force methods, the Wilhelmy plate and De Noüy ring, also have moving surfaces, albeit at extremely slow speed. The ADSA method has been used to measure surface tension during different dynamic processes at air/water surfaces, e.g. surfactant adsorption,2 protein unfolding,3 and surface elasticity.4 The setup of all of these methods involves some surface evaporation.
Figure 1. Microwave-induced convection flow within the pendant drop.
2. POSSIBLE MATERIAL LEACHING FROM THE TEFLON
4. OTHER FACTORS DURING MICROWAVE IRRADIATION The commenters examined some of the abnormalities in our data (temperature/surface tension) during MW irradiation. It is possible that those factors can influence the “measured surface tension”. Nevertheless, these effects ceased to exist once MW irradiation was stopped. Hence, these effects cannot be used to explain the low surface tension after MW irradiation.
The measurement of pure water at 21 °C was within 98% of the literature value. Similarly, the measurement with ethylene glycol, after MW heating to 160 °C, was within 98% of the literature value. It was assumed, therefore, that no material from the Teflon caused contamination of the water at 80 °C.
3. LIQUID MOTION DURING MW From PIV measurements during MW irradiation,5 the maximum angular motion for a 1-mm-radius droplet was 25 rad/s. This motion reduced to zero once MW irradiation was stopped. The influence of the motion can be qualitatively (semiquantitative) characterized by the following analysis. The dynamic pressure, which is caused by liquid motion, and static pressure are given by Pd = ρ
Ps =
ϖ2rc 2 2
5. MECHANISM At 420 s after MW irradiation ceased, it is reported that while the droplets had returned to room temperature and despite the fact that there was no longer any fluid motion, the computed surface tension values were different from that of pure water reported in the literature at that temperature. The difference between the computed and reported surface tension was a function of the MW intensity. We simply propose that those differences could be contributed by changes in H-bonding as described in the paper. There is also the possibility that the change might be due to the presence of nanobubbles.6 For instance, assuming that nanobubbles were formed during the microwave radiation process, they might adsorb at the air/water interface and reduce the surface tension. These bubbles would most likely disappear; however, due to coalescence with the air/water interface. Eventually the surface tension would reach the normally reported equilibrium value.
(1)
2γ rc
(2)
where Pd and Ps are the dynamic and static pressure, respectively, ω is the angular velocity, rc is the droplet radius (Figure 1), and ρ is the water density. For this study the droplet radius, rc, was approximately 1 mm, and a typical value for surface tension, γ, was around 60 mN/m. For these conditions the corresponding Pd/Ps ratio is less than 0.3%. While there will be some dynamic influence on the radius of curvature, the effect will be on the order of 1%, and when the motion ceases the dynamic influence will disappear completely. © 2015 American Chemical Society
6. LONGER MEASUREMENT After approximately 10 min the droplet was too small to cover the whole thermal probe, and the surface tension measurement Received: June 29, 2015 Published: September 11, 2015 10933
DOI: 10.1021/acs.langmuir.5b02378 Langmuir 2015, 31, 10933−10934
Comment
Langmuir had to be terminated at this time. Ideally, it would be good to be able to continue the measurement for a longer time, especially to explore the possibility of the presence of nanobubbles.
Harisinh Parmar† Masahiro Asada‡ Yushin Kanazawa‡ Yusuke Asakuma‡ Chi M. Phan*,† Vishnu Pareek† Geoffrey M. Evans§
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† Centre for Process Systems Computations, Curtin University, Perth, WA 6845, Australia ‡ Department of Mechanical and Systems Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan § Department of Chemical Engineering, University of Newcastle, Callaghan NSW 2308, Australia
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Parmar, H.; Asada, M.; Kanazawa, Y.; Asakuma, Y.; Phan, C. M.; Pareek, V.; Evans, G. M. Influence of Microwaves on the Water Surface Tension. Langmuir 2014, 30, 9875−9879. (2) Chang, C. H.; Franses, E. I. Adsorption dynamics of surfactants at the air/water interface: a critical review of mathematical models, data, and mechanisms. Colloids Surf., A 1995, 100, 1−45. (3) Phan, C. M.; Nguyen, A. V.; Evans, G. M. Dynamic adsorption of beta-casein at the gas-liquid interface. Food Hydrocolloids 2006, 20, 299−304. (4) Loglio, G.; Pandolfini, P.; Miller, R.; Makievski, A. V.; Krägel, J.; Ravera, F.; Noskov, B. A. Perturbation−response relationship in liquid interfacial systems: non-linearity assessment by frequency−domain analysis. Colloids Surf., A 2005, 261, 57−63. (5) Asada, M.; Kanazawa, Y.; Asakuma, Y.; Honda, I.; Phan, C.; Parmar, H.; Pareek, V.; Evans, G. In In-situ observation of convection in droplet under microwave radiation by PIV, International Conference on Optical Particle Characterization, 2014; pp 92320F−92320F-7. (6) Yang, S.; Dammer, S. M.; Bremond, N.; Zandvliet, H. J. W.; Kooij, E. S.; Lohse, D. Characterization of Nanobubbles on Hydrophobic Surfaces in Water. Langmuir 2007, 23, 7072−7077.
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DOI: 10.1021/acs.langmuir.5b02378 Langmuir 2015, 31, 10933−10934
