Urine Advancing Contact Angle on Several Surfaces

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Journal of Adhesion Science and Technology 23 (2009) 1917–1923 www.brill.nl/ ... Urine, capillarity, contact angle, wetting, microgravity fluid management. 1.
Journal of Adhesion Science and Technology 23 (2009) 1917–1923 www.brill.nl/jast

Urine Advancing Contact Angle on Several Surfaces Evan A. Thomas ∗ , Darwin H. Poritz and Dean L. Muirhead NASA-Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA Received in final form 24 May 2009; revised 24 July 2009; accepted 25 July 2009

Abstract Urine wetting properties may influence the design and performance of catheters, urinalysis instruments, and lab-on-a-chip technologies. In this study the advancing contact angle θadv of urine on several materials is characterized. Material type and surface tension have a significant effect on θadv , while pretreatment and aging do not. Mean urine θadv are between ≈78◦ and ≈89◦ on hydrophilic surfaces, and up to over ≈105◦ on hydrophobic surfaces. Expected urine contact angles will decrease from the DI water contact angles by on average 10◦ , and up to 20◦ , while urine surface tension will be lower than DI water by 12.12 mN/m and 18.53 mN/m. A unit change (mN/m) in surface tension results in a 0.75◦ change in θadv . These results indicate that systems attempting to exploit urine wetting must account for highly variable conditions. © Koninklijke Brill NV, Leiden, 2009 Keywords Urine, capillarity, contact angle, wetting, microgravity fluid management

1. Introduction Surface tension is due to intermolecular forces resulting in liquid free surfaces acting in a state of tension. Fluids attempt to have the lowest surface free energy, by having the smallest surface area possible. In microgravity, liquids achieve this by forming spheres. Additionally, fluids will minimize surface free energy by bonding with other materials. This behavior defines wetting. The particular surface free energy of liquids, as well as gasses and solids, determines the adhesion characteristics between these three phases [1]. A hydrophilic system consists of a liquid with more affinity for an adjacent solid surface than for itself. In a hydrophobic system, the liquid has more affinity for itself than for the solid surface. In general three general wetting regimes arise (spreading, partial wetting and non-wetting), that are characterized primarily by the contact angle, θ . In many wetting systems, spontaneous capillary driven flows occurs as a natural consequence of the system geometry. This is known as capillary *

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© Koninklijke Brill NV, Leiden, 2009

DOI:10.1163/016942409X12508517390879

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action [2]. Capillary systems often consider advancing contact angles, θadv , which are observed when liquids move across rough or heterogeneous surfaces. Lower θadv values generally produce better performing capillary systems. The contact angle is a function of surface tension forces as given by Young’s equation, where σGL , σSL and σSG are the surface tension forces between the gas–liquid, the solid–liquid and the solid–gas, respectively [2]:   −1 σSG − σSL . (1) θ = cos σGL This paper presents the results of a study examining θadv for several urine types on several surfaces. The experiment is conducted in support of a spacecraft microgravity wastewater management system. The results are generally applicable to other terrestrial studies. 2. Materials and Methods This study examines the extent to which urine solutions differ in their wetting characteristics on a range of smooth, clean hydrophilic and hydrophobic surfaces. The study also examines the impact of urine surface tension, aging, and use of oxidizing pretreatment on urine wetting properties. Three surface tension categories, low, medium and high, plus the categories of pretreatment absence or presence, and the categories of fresh versus 7-day-old room temperature aged urine, and a control of DI water, led to 13 total fluid types employed in the tests. The wastewater used in this study is primarily composite human urine from several male and female donors. For some test points, the urine is pretreated with a standard 5 g/l dose of Dupont Oxone (predominately potassium monopersulfate), an oxidizing and disinfecting compound used by NASA for urine pretreatment aboard spacecraft. The pretreatment is designed to lower the pH of the urine to as low as 3 and prevent bacterial growth from proliferating and producing the enzyme urease which contributes to the formation of ammonia gas and ammonium through the hydrolysis of urea [3]. The hydrolysis process raises the pH of the urine, which reacts with other chemicals in the urine to form precipitates, primarily struvite [4]. Turbidity increases due to bacterial growth, crystal formation and precipitation. All of these processes pose potential problems to wastewater management systems [3]. Several fluid properties of urine are important to consider when designing urine interacting with hardware. The surface tension of human urine is important for capillary-based urine management systems. In one study, the surface tension properties of human urine were examined with donations from six volunteers. The results indicated a surface tension mean of 58.7 mN/m, with a standard deviation of 2.16 mN/m. The surface tension of major constituents in this study included water at 73.0 mN/m, sodium chloride at 73.1 mN/m and urea at 71.0 mN/m. The study concluded that there is a linearly inverse relationship between log urine concentrations

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and surface tension, and an inverse relationship between bile salt concentration and surface tension [5]. The solid materials used in this experiment include Titanium-64, Stainless Steel316, Aluminum-6061, Aluminum-7075, Polyethylene, Teflon 7A and ULTEM. Each of the solid coupons used are approximately 2 × 2 by 1/8 . Each of the metal materials is polished to a visually shiny finish. The non-metallic materials are not polished. The coupons are cleaned with a mildly alkaline detergent, a mildly acid cleaner, and then rinsed with methanol and DI water. 2.1. Experimental Design The five experimental treatment factors and their levels include two liquid types (urine and DI water), three levels of surface tension, pretreatment presence or absence, fresh or aged urine, and seven materials leading to 91 test points. By matching each combination of treatment factor and level and randomizing the test points, the experimental design allows for analysis of the major effects and interactions of the treatments and levels on θadv . Surface tension is determined with a Kruss Tensiometer K100 and the Wilhelmy plate method at room temperature of ∼25◦ C. Ranges for low, medium and high surface tension categories were determined based on preliminary surface tension results. The surface tension of DI water in this preliminary test averaged 72.478 mN/m, while the average surface tension of fresh, untreated urine was 57.278. The highest surface tension of urine observed was 65.264 mN/m, while the lowest was 50.379 mN/m. The category ranges were therefore determined to be less than 53, between 53 and 63, and greater than 63 mN/m. However, categorized surface tension was subsequently not considered because the addition of pretreatment caused a change in the surface tension of the low, medium and high urine batches such that the previously ‘medium’ surface tension in fact switched placement with the ‘high’ surface tension category. Likewise, the aged ‘medium’ surface tension category switched with the ‘low’ category. Therefore, only the actual measured surface tension results were analyzed. The cleaning process is a critical factor in obtaining good contact angle measurements. During handling, storage, and measurement, care was taken not to breathe on the surfaces. For this experiment, the contact angle instrument was not in a clean room, so care was taken to ensure clean conditions. The solid surfaces were cleaned with 4 g/l of Neodisher MA, a mildly alkaline detergent, and 2 ml/l of Neodisher Z, a mildly acid cleaner. The coupons were then rinsed in Ultra-Pure DI water, rinsed in 99.8% Methanol, rinsed again in DI water, and allowed to dry in sterile containers. Providing a direct measure of surface wettability, advancing contact angle data θadv are collected using a Dataphysics Optical Contact Angle 15 (OCA 15) Goniometer and the sessile drop method at room temperature of ∼25◦ C. For advancing and receding contact angles, an 8-microliter drop is used with a dosing rate of 1 mi-

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croliter per second. The Laplace–Young fitting method is nominally used. Video files recorded at 2.5 frames per second. Because of varying focus on the optics, surface imperfections and other wetting characteristics, the number of data points collected by the Laplace–Young fitting varied between test points. In order to be consistent when applying data reduction and selection, a Matlab program was written to apply rules selecting advancing contact angle data points to the collected data and output contact angle results and standard deviations. The program selected data points within a maximum fitting error and rate of contact angle change. For data points that did not directly output reliable results, the data files were modified to run within the constraints of the program. These modifications included occasionally using an ellipse fitting method, or removing clearly erroneous points. For any test point where modifications were made to allow the program to run, each of these test points were carefully reviewed visually, and the resultant output correlates well to observation, static contact angle results, and advancing contact angle results with the same liquids on similar materials. Since contact angle is an optically derived characteristic, this is a reasonable approach for a high number of test points with such a variety of liquids and solids. The slip/stick pinning behavior occasionally observed on one side of the drop, as well as other interpretation challenges observed, are consistent with the nature of contact angle determinations [6], therefore interpretation standards held constant by the researcher is an imperfect but currently appropriate method of data collection. 3. Results An analysis of variance (ANOVA) is conducted on the contact angle results. An inspection of the normal probability plot of the residuals indicates that they are normally distributed, validating the probability values in the ANOVA. p-values less than or equal to 0.05 indicate that the effect is statistically significant at the 5% level. The analysis investigates if there are significant differences between θadv across the experimental parameters, including surface tension category, pretreatment use, age, and material type. Table 1 presents the major effects and interactions in order of significance, with the degrees of freedom (d.o.f.) for effect and error, and the p-value. The solid material substrate appears to have a significant effect on θadv , as evidenced by the small p-value (