Abstract:
Reducing drag force saves enormous resources spent to overcome it. In wall-bounded flows, small changes in interfacial properties such as viscosity and density may have crucial effects on the fluid flow field [1]. Drag force on the golden boundary surface in flowing aqueous solutions can be reduced by positive shift of the electrode potential of this boundary [2]. Measured potential dependencies of interfacial water viscosity at the gold surface allowed to conclude that the observed drag reduction was related to the decrease in interfacial water viscosity. These findings show that control of the applied potential enhances the ability to actively manipulate a wall-bounded liquid flow field to effect desired changes.
Stainless steel used instead of gold does not show any pronounced trend in the dependence of the drag force on an applied potential. A more viable alternative to gold appears to be the use of polymer-coated surfaces, which we have recently confirmed for polyvinyl butyral (PVB)-coatings [3]. In the present work, we continue the earlier investigation [4] with a purpose to find polymeric coatings exhibiting better properties for technological applications of this effect. We have started with the commercial one-component polyurethane-alkyd (PUA) resin coating URETHANE 71 (CRC Industries) which, as stated in the technical data sheet, has an outstanding adhesion performance and can also be used in humid or aggressive environments.
Measurement and analysis of the potential electrode effect upon the drag force on PUA-coated spheres is described like previously [2]. For measuring solution properties at a PUA surface, the earlier used the quartz resonator admittance technique [2] which was supplemented by measurements at higher odd harmonics.
The PUA coating provides higher protection for coated metal surfaces. Bode plots for PUA and PVB-coated (~5µ thick) electrodes exposed to electrolyte solutions show that the ac impedance value measured at 0.1 Hz does not noticeably change during admittance measurements. For PUA it is equal to 3.5×109 Ω cm-2 whereas for PVB it is 108 Ω cm-2. Drag force and interfacial solution viscosity decrease with a positive shift of the electrode potential. Quartz resonator admittance data obtained at several harmonics allowed determining the character of change of interfacial viscosity with the distance from the electrode surface.
The findings confirm that polymer-coated surfaces are effective alternatives to metal surfaces when applying electrode potential in order to reduce the drag force. Further investigations will be directed towards a more exhaustive understanding of this effect and will elaborate on appropriate conditions for the technological application of this effect.
References:[1] M. Gad-el-Hak, Flow Control: Passive, Active, and Reactive Flow Management (Cambridge University Press) 2006.
[2] D. Plausinaitis, A. Pulmanas, V. Kubilius, R. Raudonis and V. Daujotis, Electrochim. Acta 121, (2014) 278-284.
[3] F. Ambrulevicius, A. Pulmanas, D. Plausinaitis, V. Daujotis, Int. J. Electrochem. Sci. 14 (2019) 441-452.
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