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Applications of Polymer, Composite, and Coating Materials
Acoustic bubble suppression by constructing a hydrophilic coating on HDPE surface Yuemei Ye, Stanislav Klimchuk, Mingwei Shang, Kenneth McDonald, and Junjie Niu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04038 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019
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ACS Applied Materials & Interfaces
Acoustic bubble suppression by constructing a hydrophilic coating on HDPE surface
Yuemei Ye1, Stanislav Klimchuk1, Mingwei Shang1, Kenneth McDonald2, Junjie Niu1* 1Department
of Materials Science and Engineering, CEAS, University of Wisconsin-Milwaukee Milwaukee, WI 53211 2Controls
Technologies Division, SSI Technologies, Inc. Plymouth, MI 48170
*Corresponding author e-mail:
[email protected] (J Niu)
Abstract The ultrasonic bubbles on the solid surface of various sonochemical devices largely affect the signal resolution due to the serious reflection/scattering of sound waves. The Laplace pressure of the cavitation bubble can be tuned by constructing an ultra-thin hydrophilic layer, which leads to the solvation or pinching off of the bubbles from the surface. In this article, we successfully coated a polydopamine polymer layer on the high-density polyethylene (HDPE) surface. The formed hydrophilic layer with contact angle less than 45 degree almost completely eliminates the bubbles in both water and 32.5 vol% diesel exhaust fluid (DEF) solutions upon sonication, which results in the operation of the piezoelectric sensor over 500 hours while the sensor with pure HDPE only ran less than 2 hours. Further, the coated sensors showed a high stability under the temperatures of 60-80 oC. An improved mechanical property was confirmed via abrasion test, enabling a long-term stability in hash environments including acidic urine and ultrasonic agitation. The acoustic bubble suppression via the hydrophilic polymer coating on HDPE surface displays broad applications particularly with acoustic sensors, sonobuoys and nondestructive surface detection in sonochemistry. Keywords: Acoustic bubbles; Suppression; Hydrophilicity; HDPE; Sonochemistry
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Cavitation bubbles are created by the pressure difference and flow rate in liquids near to the solid surface, particularly under ultrasonic agitation1. The subsequent collapse of bubbles generates locally extreme pressures, temperatures, and shock waves to impinge the surface. The grown bubbles in liquid are detrimental to the resolution of ultrasonic sensors and sonobuoys by scattering and reflecting the ultrasound waves. The ultrasonic bubble nucleation and growth at solid surfaces have become a critical issue in sonochemistry2-5. Homogeneous nucleation of the bubbles originates from the cavitation of a pure liquid arising from microscopic voids due to thermal fluctuations while heterogeneous nucleation forms from the presence of sub-microscopic air pockets at the solid-liquid interfaces6-8. The growth of bubbles under ultrasonic irradiation in a liquid can occur through two mechanisms: inertia effects with highintensity, low-frequency and rectified diffusion with low-intensity, high-frequency ultrasounds9-11. Two major parameters affect the ultrasonic cavitation generation: one is the sonication condition such as temperature and the liquid property12-13 and another is the solid surface property, particularly with the wettability14. The liquid-solid interface energy can be controlled via coating a solid surface with suitable polymers15-16. Bremond et al reported that gas bubbles prefer to stay at hydrophobic surfaces17-20. Upon the immersion in water, the hydrophobic nanopits can serve as bubble nucleation sites and thus reduce the water density near to the interface21. The hydrophobic property with high contact angles causes a lower energy barrier, as a result cavitation dominates on the surface. In contrast, hydrophilic surface with low contact angles possesses a relatively high-energy barrier to the cavitation, which leads to repel the grown bubbles on the surface. The hydrophilic coatings can be made by polyethylene glycol, titanium dioxide and nanoporous silica with a complicated process22-25. On the other side, a large variety of HDPE polymers are extensively used in different devices particularly with sensors due to the high chemical and mechanical stability26. This outstanding stability also poses a challenge on modifying the surface with designed functional groups. The original HDPE surface shows weak hydrophilicity with a contact angle larger than 60o, which generates plenty of bubbles when it suffers the ultrasound agitation. In order to prevent the acoustic bubble generation, here we report a strategy of creating a strong hydrophilic layer of polydopamine on the HDPE surface of piezoelectric sensors. In contrast to the large influence on device resolution due to the high thickness of other coatings, the formed ultra-thin polydopamine layer with strong adhesion reduces the cavitation nucleation and repels the bubbles, thus to remarkably improving the sensor resolution upon long-time ultrasonic agitation. It is known that the hydrophilic function groups give high wetting property with low contact angle. Similar to hydrophobicity, hydrophilic coatings have been widely studied because they clean a substrate when the contact angle is lower than 90˚27. Surface energy of the liquid in parallel planes will be positive value of capillary force. The capillary force of the liquid will be raised until parallel height with
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ACS Applied Materials & Interfaces
hydrostatic force. Multifunctional polymer coatings on different surfaces using dopamine have been recently studied28-29. The similar polarity of hydroxyl and ammonium groups of dopamine contributes to a hydrophilic property to water molecules. Here we created a strong hydrophilic surface via polymerization of dopamine monomers on the HDPE substrate (Figure 1a-b). The existing functional groups contribute to a hydrophilicity of the surface and this polydopamine layer can be coated in a wet-chemical process (Figure 2c,e). Without the coating, the pure HDPE has a contact angle of >66o (Figure 1c). It displays plenty of bubbles in deionized (DI) water after ultrasonic agitation for 0.5 hours (Figure 1e). The Laplace pressure Δp = kγ/r, was decreased due to the surface hydrophobicity, thus less neighboring bubbles were dissolved, leading to an enhanced cavitation bubble nucleation and growth (Figure 2a). However, the HDPE surface coated with a homogeneous polydopamine layer exhibits a hydrophilic property with a low contact angle of 23o (Figure 1d) due to the existing hydroxyl and NH groups (Figure 2e). A clean surface without any bubbles was observed after ultrasonic agitation for 0.5 hours (Figure 1f). A clear bubble morphology comparison of the uncoated (Figure 1g) and coated (Figure 1h) sensors under stronger agitation with 4 hours and 60 °C demonstrates an excellent bubble suppression of the hydrophilic surface. In a typical experiment, we coated a thin layer of polydopamine on two locations of a piezoelectric sensor (ultrasonic temperature level and concentration (UTLC) or ultrasonic temperature and concentration (UTC) sensor, SSI Technologies, Inc.): the HDPE surface of the tombstone (piezoelectric ultrasound emitter located) and the metal surface of the reflector (Figure 2d). According to the crevice model of bubble nucleation30-31, a smaller spherical radius r is reached when the surface becomes more hydrophilic, which leads to an increase of the Laplace pressure Δp (Figure 2b). The surface tension is induced due to the increased internal gas pressure Pg, thus more bubbles dissolve into the surrounding liquid under sonication or expand, pinch off by buoyancy/radiation. In parallel, the interface energy between the bubbles and HDPE surface is lower than the interface energy between the bubbles and the polymer hydrophilic coating. In other words, the surface energy on the HDPE is lower than the surface energy on the hydrophilic coating, which leads to the bubbles being repelled on the surface of hydrophilic coating rather than on the pure HDPE surface. Due to the higher surface energy (high surface tension), the polymer surface will repel the gas bubbles depending on the competition force between the Laplace pressure from the bubble and the hydrostatic force on the top. As a result, both the acoustic bubbles from homogeneous and heterogeneous nucleation are removed, as shown in Figure 2b. The ultrasonic cavitation bubbles are affected by temperature, time, impurities, liquid, etc. As negative pressure is reduced due to the passing of the ultrasound energy (rarefaction) and atmospheric pressure is reached, the cavitation bubble starts to collapse. During the compression of the sound wave, any gas that diffuses into the bubbles is compressed and finally starts to diffuse across the boundary again to re-enter the liquid. Also, once the bubble is compressed, the available boundary surface for diffusion is reduced.
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As a result, cavitation bubbles do not always collapse to implosion but rather well in a small pocket of compressed gas in the liquid. The small gas bubbles group together until they have enough buoyancy to come to the surface. In order to check the stability of the coated surface, the bubble removal evolution on the tombstone location of the sensor was recorded with different times upon the operation in fresh DI water at 60 °C. No bubbles were observed after 1, 2, and 4 hours, which indicates an excellent bubble suppression with the coated HDPE surface. The abundant OH groups on the polydopamine polymer (Figure 2e) via a series of reactions such as oxidation and nucleophilic reaction (Figure 2c) makes it feasible to bond with un-saturated solid surfaces such as plastics, polymer and metals32, which can contribute to a strong mechanical adhesion to the substrates33. The existing benzene rings and C=C bonds ensure a high mechanical strength (Figure 2c), even with a very thin thickness and micrometer roughness (Figures S1 and S4). In order to further evaluate the mechanical property of the coating, an abrasion of the polydopamine coated HDPE surface using sandpaper was conducted (Figure 3). As shown in the Figure 3a, a metal ingot with 9.8 kPa pressure was placed on the top of the reverse side of the sandpaper to abrase the coated surface. Each cycle the abrasion was run a distance of 10 cm back and forth. The contact angle limit to generate bubbles was confirmed by testing the anti-aeration in DI water at 60 °C for 0.5 hours after each cycle. It was found that the minimum contact angle with bubbles is ~45° (Figure 3b insets). As displayed in Figure 3b, the contact angle reached 45° after the abrasion with 21 cycles, which indicates a strong connection between the polydopamine layer and the HDPE substrate. In addition to the HDPE surface, we also coated the polydopamine layer on the metal alloy surface of the reflector, which shows a greatly decreased contact angle from 53° to 32°, as shown in Figure S2a. The good bubble prevention on metal surface upon ultrasonic agitation in Figure S2c-d indicates diverse applications of polydopamine with acoustic bubble suppression, e.g., polymers, plastics and metals. To maintain the minimum bubbles, the abrasion tests on the coated reflector show a contact angle less than 39° is needed (Figure S2b). To verify the sensor performance with the coated surface, the sensor was measured in DI water and DEF liquid under different temperatures and time, respectively. For the bubble nucleation hot soak test, the performance of the polydopamine coated and uncoated UTLC sensors was compared in DI water at 60 °C during 24 hours. The analog-to-digital converter (ADC) is a numerical measure of power to the piezoelectric element to maximize the signal amplitude. The sensor software sets the value between 40 and 90. Lower ADC value is more desirable since higher value can lead to noisy signals. The UTLC sensor will adjust the ultrasound emission strength by changing the value of ADC to maintain certain resolution. In this case, if bubbles are formed in the liquid that will scatter and mitigate the ultrasound
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ACS Applied Materials & Interfaces
intensity, the sensor will increase the ADC value to enhance the signal. Therefore, we may use the variation of ADC as an indicator to evaluate the bubble influence during the test. In other words, stable and small ADC value means less bubbles are generated. As shown in Figure 4b,e, the dopamine-coated sensor exhibited a much lower level of 40, while the un-coated sensor presented a high ADC level of 90 after the initial heating process (from 20 °C to 45 °C). The coated sensor only showed a slight jump at the heating process, which indicates a negligible effect of the bubbles on the sensor function. In parallel, the fluid level with coated sensor showed a slight decrease that was caused by evaporation in the open vessel system during the test while the curve with un-coated sensor exhibited obvious fluctuation. During the acoustic to electrical signal conversion, the sensor is designed to detect the first lobe of the ultrasonic signal passing through a threshold. The first lobe sets the timer counts, which measure the travel time of sound wave between the piezoelectric element and target. Thus the concentration output is established from an empirical look-up table. In the case of a low amplitude signal, the lobe will detect the ‘shift’ to the next highest sound wave (called ‘lobe shifting’). Bubbles on the surface can cause low signal amplitude, resulting in an increase of timer counts and hereby a decrease of the concentration output by approximately 10 to 12%. As shown in Figure 4b, the corresponding temperature compensated concentration (derived from temperature and timer count) with the coated sensor was stabilized at -8% during the whole heating process, indicating a zero ‘lobe shifting’ due to the excellent surface antiaeration. As comparison, a steep drop of the concentration with un-coated sensor were observed when the temperature reached 45 °C (Figure 4e), owing to the severe aeration during the heating process. The surface morphologies after the 24-hours operation further confirm the bubble suppression of the coated sensor. No bubbles appear on the coated sensor (Figure 4a) while large amount of bubbles are observed on the un-coated sensor (Figure 4d). Thermal cycle of the sensors was done under the temperature from -40 to 70°C in DI water. During the tests, the sensors in the closed vessel started to run at room temperature and then was quickly heated up to 64.5°C (Figure 4c) and 70°C (Figure 4f), respectively. After cooling down to room temperature, this heating process was repeated. It is observed that the variation of the liquid level centered at ~48 mm and concentration at ~-8% upon higher temperatures with both coated and un-coated sensors are similar in the closed testing chamber (Figure 4c,f). However, the ADC with the un-coated sensor was drastically increased from 40 to 82.2 when the temperature was raised from -40 to 70 °C, while the value with the coated one only showed a slight increase from 40 to 47 within a very short time
