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Study on adhesion of asphalt using AFM tip modified with mineral particles
Construction and Building Materials 207 (2019) 422–430
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Study on adhesion of asphalt using AFM tip modified with mineral particles Xiaobo Lv a, Weiyu Fan a,⇑, Jiqian Wang a, Ming Liang a, Chengduo Qian a, Hui Luo a, Guozhi Nan a, Bingjian Yao b, Pinhui Zhao c a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, China College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, China c School of Transportation Engineering, Shandong Jianzhu University, Jinan 250101, Shandong, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Microspheres of mineral particle were
We report, the first of its kind, modified AFM probe tip of mineral particles could effectively measure the adhesion differences between mineral particle and asphalt binders which provide a new approach to investigate adhesion properties of asphalt binder in microscale.
prepared to modify AFM tips. Adhesion differences between mineral-asphalt interfaces could be measured by modified mineral probe tips. Adhesive force strength enhances as the content of SiO2 increases. Physical adsorption plays a greater role in interfaciale adhesion than chemical adsorption.
a r t i c l e
i n f o
Article history: Received 8 May 2018 Received in revised form 17 February 2019 Accepted 18 February 2019
Keywords: Asphalt Aggregate Adhesive force Atomic force microscopy Modification of probe tip
a b s t r a c t Adhesion between asphalt binders and aggregates is important to the performance of pavement. Atomic force microscopy is commonly used to measure the adhesive forces of asphalt binders. In this paper, AFM probe tips were modified with four types of spherical mineral particles: limestone, basalt, granite and sandstone. These were prepared by planetary ball mill grinding and their compositions investigated by X-ray diffraction. The modified tips were used to investigate the adhesive forces at mineral particleasphalt substrate interfaces. Adhesive forces increased obviously with SiO2 content. This indicates that physical adsorption plays a greater role in interfaciale adhesion than chemical adsorption. Linear model, single-factor analysis of variance, and interquartile analyses were also applied to investigatinge the distributions of adhesive force. It was found that different degrees of distributions might be attributable to the different microstructures of the mineral particle surfaces. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
⇑ Corresponding author. E-mail address: [email protected] (W. Fan). https://doi.org/10.1016/j.conbuildmat.2019.02.115 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
Asphalt binders have long been used as components of road pavement [1], as they bind aggregates together. Their mechanical properties, particularly their adhesive characteristics, contribute
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significantly to the asphalt’s overall performance. In fact, both chemical and physical adsorption contribute to asphalt binder adhesion [2,3]. Although progress has been made in understanding the compositions and properties of asphalt binders, the performance of asphalt sometimes fails to meet expectations. Therefore, more fundamental materials science research is needed to better characterize and predict the service performance of asphalt binders. The adhesion force of asphalt is one of the fundamental properties that affect asphalt pavement’s moisture-induced damage performance [4]. Atomic force microscopy (AFM) is a powerful tool that is used not only to view surface morphology, but also to probe electrical, magnetic, van der Waals, adhesion, and chemical interactions. AFM is one of the few methods that are capable of measuring adhesion of asphalt at the micro-scale. AFM has been utilised to measure the force-distance curve, which is a plot of cantilever deflection as a function of sample position. By analysing this curve, one can measure the tackiness or adhesive properties of an asphalt binder [4]. Currently, AFM is used to measure the adhesive forces of asphalt binders as it provides high-resolutions of displacement and force. An AFM uses a cantilever spring with a sharp tip to measure the interactions between the tip and an asphalt sample. It can use various tip materials and modifications, most of which are made of silicon or silicon nitride [5], which cannot observe the adhesion behaviors and interactions between asphalt binders and aggregates. Another problem is that the use of such tips to study asphalt causes tip contamination, which reduces the image resolution and generates errors [6]. Therefore, it is highly desirable to develop techniques to modify Si/SiN tips with functional materials that are used in road pavement. AFM was initially developed to complement scanning tunneling microscopy by observing topography at the nanoscale or even atomic scale [7,8]. Gradually, the application of AFM has been greatly expanded to realize functional characterization. This has made it possible to modify the probe tip on the forearm of an AFM, which changes or augments the interaction mechanism between the tip and the target surface [9]. Growth of a selfassembled monolayer of thiolates on gold-coated AFM tips is a common type of chemical modification [10]. However, thinner gold films cannot fully cover the surface, and the large tip size decreases the resolution of AFM imaging. Ali and Lim [11] trimmed AFM probe tips to minimize measurement and imaging errors. However, higher ion doses caused over-milling and easily damaged the tip at its base. Moreover, modified tips were fragile and could only be used at low scanning speeds. Tarefder and Zaman [12] reported that the use of silicon nitride tips and chemically functionalized tips could minimize the adhesion force between the tip and asphalt surface. Furthermore, the intermittent-contact mode was considered inappropriate for determining the adhesion differences of asphalt binders because the phase contrasts relate not only to the effects of adhesive force and stiffness, but also to the damping and morphological features of the sample [2,13]. Comparatively, techniques for direct coating of Si/SiN tips with a robust monolayer of functional molecules were reported by Lee and Laibinis [14]. Nevertheless, it is difficult to control the conditions needed to prevent the deposition of multiple or large particles on the tip surface. Conventional techniques for probe modification are favorable to the attachment of particles, such as a single object to the probe tip, since the geometry and composition of the particle can be well controlled. Attaching a microparticle, especially a micro-sized sphere to an AFM cantilever for force measurements has become routine in AFM labs [15,16]. In past years, techniques have been developed for attaching even smaller objects, such as single nanoparticles and nanotubes to tips used for AFM applications [17]. Yang [18] modified an AFM probe cantilever by attaching a micro- sized colloidal particle to the foremost part of it. Li [19]
attached microspheres of alumina, silica and calcium carbonate to cantilever ends with this technique, and evaluated the adhesion between the probe and binder substrates under ambient conditions. Despite the development of a variety of techniques for attaching particles to AFM probe tips, none have modified Si/SiN tips with functional material particles used in road pavement. Besides, AFM-based adhesion tests on soft and sticky asphalt binders have proven difficult [20–22]. On the other hand, adhesion forces vary significantly with operational conditions and sample treatments. Therefore, probe tips modified using microscale limestone, basalt, granite, and sandstone are investigated in this paper to study the adhesive forces between the modified tips and asphalt binders. We develop a systematic, AFM-based, tip modification and adhesion testing procedure to fill the knowledge gaps existing in this field. Our approach develops a mineral probe tip and reduces the effect of operational procedures on the adhesion measurement as much as possible. The modified AFM probe tips effectively measure the adhesion differences between mineral particles and asphalt binders. This thorough, AFM-based, tip modification and adhesion testing procedure is used to directly quantify the adhesion differences between asphalt binders and four primary mineral tip types. This will be helpful for discovering other compliant mineral materials with which to characterize the adhesion properties of mineral particle-substrate interfaces by AFM.
2. Materials and methods 2.1. Materials 2.1.1. Asphalt Asphalt binder (AH-70, obtained from Qinhuangdao Refinery, China) with a penetration grade of 70 was used to evaluate the interactions between mineral particle-substrate interfaces and mineral particle materials. Table 1 shows the physical properties and chemical compositions of the asphalt used. The binder samples used for adhesion measurements were prepared via a solution-cast approach [23], which produces more flat and less sticky surfaces compared to the heat-cast approach [2]. Heat-cast thin film was found when the AFM tip be contaminated by sticky binder or when the sample surface was damaged by the formation of humps.
2.1.2. Sphere particles and cantilevers Four types of mineral microspheres were prepared in the laboratory and used for tip modification: basalt (from Liaoning, China), limestone, granite and sandstone (from Shandong, China). Alkali testing (T0347-2000, JTG E42-2005) was carried out to evaluate the properties of the mineral particles. Table 2 shows the alkali values of the four mineral materials. Their mean particle diameter was 5 lm. The cantilevers used in this study were triangular (DNP-S10, 4 cantilevers, nonconductive silicon nitride) with spring constants ranging from 0.06 to 0.35 N/m.
2.2. Sample preparation Mineral microspheres of limestone, basalt, granite and sandstone were prepared by a ball milling machine. The particles were added to a ball milling pot, then large and agglomerated particles were rendered into microspheres by crushing
Table 1 The properties and compositions of the asphalt binder. Items
AH-70
Penetration (25 °C, 0.1 mm) Ductility (15 °C, cm) Softening point (R&B, °C) Viscosity (135 °C, Pas) Saturates (wt%) Aromatics (wt%) Resins (wt%) Asphaltenes (wt%) Ic = (At + S)/(A + R) (%)
73 >100 49.4 0.388 22.67 42.15 26.64 8.53 0.45
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Table 2 Alkali value of minerals. Items
Limestone
Basalt
Granite
Sandstone
Alkali value
0.96
0.091
0.034
0.068
(shearing and extrusion forces) as illustrated in Fig. 1(a). Dry grinding of mineral particles was carried out with a ball filling fraction of 0.7 and rotation speed of 40 rpm for 30 min for application to probe modification. Asphalt binder samples were prepared by melting them in toluene solutions for further characterization. A 0.5 mm-thick silicon wafer was cut to an approximately 10 10 mm square with a diamond cutter and cleaned with Piranha solution in a terrarium [6]. After cleaning, sample solutions were spread on clean silicon wafer substrates. Then, the samples were stored in a vacuum drying oven at 100 °C for 24 h. This temperature allowed the asphalt binders to smoothly cover the wafer surfaces and removed the toluene from the samples. After that, all specimens were cooled to ambient temperature and stored in a silica-gel drying container until adhesion measurement.
2.3. Methods 2.3.1. AFM probe tips modification technique of AFM probe tips with attached micro-particles have been extensively used for interfacial force measurements. A probe tip modified with a spherical particle with a diameter larger than 1 lm gains an advantage over a normal tip. The sphere has a well-defined geometry and can obtain force measurements with a higher signal-tonoise ratio, since more effective information can be derived from the curves [15,16]. As only particles larger than 1 lm can be easily observed under the optical mode of AFM, an optical microscope and an AFM head are the standard tools for attaching a micro particle to the probe tip [9]. Most importantly, to measure the average adhesive force across an asphalt sample’s surfaces, a normal probe should contact every point of the sample. However, this can be easily derived using probe tips modified with a spherical particle. Asphalt is mainly a complex mixture of hydrocarbons whose structure is well described by the colloidal model, where solid particles with a radius of a few nanometers are dispersed in an oily liquid matrix [24]. Topographic images of asphalt binder samples display different domains (including Catanaphase, Salphase, Paraphase and Periphase [25,26]) with height differences of a few nanometers between them. The surface structure of the domain does not share the same features as other phases, such as force behavior, stiffness, and morphology [27,28]. Since only a few nanometers of the normal tip actually contact the sample, even minute forces are distributed over an exceedingly small area, which add up quickly. Many materials are easily dented by the tip under such conditions. Therefore, a
probe tip with a well-defined spherical particle can better solve the problem and determine the average adhesion force between the particle and the asphalt binder surface with finite contact points. 2.3.2. Modification of AFM probe tips with mineral spherical particles In this study, limestone, basalt, granite and sandstone microsphere particles were used to modify the probe tips. These were used to measure the adhesive forces at the mineral particle-asphalt interface on a Bruker Multimode 8 AFM in Nanomechanical Mapping-Peakforce QNM in air conditions. While observing with the optical mode of AFM, a probe was fixed on a cantilever holder with the cantilevers facing up and blocking the holder on the AFM head. The cantilever was moved around first to pick up a tiny amount of optical adhesive (Norland, USA), then used to pick up a particle spread onto a silicon wafer preliminarily with capillary force. The final particle attachment was completed as seen by the optical microscope. After this procedure, the modified probe was allowed to stay under an ultraviolet lamp for 30 min to let the optical adhesive cure. This technique is especially suitable for dealing with triangular cantilevers with spring constant k < 1 N/m. The main reason is that small-amplitude thermal vibrations induce larger systematic measurement errors. 2.3.3. Calibration of the cantilever deflection sensitivity and spring constant The deflection sensitivity allows conversion from the raw photodiode to deflection of the cantilever, and is normally set from the force calibration mode. Force calibration mode allows minimization of the contact force of the cantilever on the sample surface. Since the force curve clearly illustrates the relationship between the set-point and the cantilever deflection voltage when the cantilever is off the sample surface, the sensitivity must be calibrated before accurate deflection data can be obtained. The sensitivity is equal to the inverse of the slope of the force curve while the cantilever is in contact with a hard sample surface (sapphire, silicon wafer, etc.). Since the set-point defines the value of the deflection signal maintained by the feedback loop, the force curve can be used to calculate the constant force of the tip on the sample if the spring constant k of the modified probe cantilever is known. By knowing the spring constant of the cantilever, it is possible to measure the attractive forces of tip-sample interactions with good precision. The spring constant was calibrated with the thermal tune method. This method is based on the equipartition theorem from fundamental thermodynamic theory [29]. It is possible to determine the spring constant of a contact mode cantilever quickly and easily. 2.3.4. Measurement of adhesive force Mineral limestone, basalt, granite, and sandstone microsphere particles have never been used for AFM probe tip modification and investigation of the adhesion properties of asphalt binders. In this study, the AFM force curve data were obtained for limestone-, basalt-, granite- and sandstone- modified tips on an AFM in Nanomechanical Mapping-Peakforce QNM in air. In the index sets, three curves
Fig. 1. Schematic representation of (a) mineral crushing in the planetary ball mill and (b) the procedure of tip modifying.
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3. Results and discussion 3.1. Mineral microsphere particles Many kinds of machines are available for fabrication of spherical particles, depending on the material properties and required product sizes. The present study prepared mineral particles that were as fine as possible by grinding process in a planetary ball mill, which was expected to efficiently reduce the size to the micro range [30–32]. Powder grinding was selected as it is a simple and well-established large-scale manufacturing process compared with liquid-phase methods, and also avoids possible contamination by ball and pot materials in wet grinding. The initial grinding rate in the planetary ball mill was reported to have an approximate maximum of a 0.3 ball filling fraction, where the maximum force of the mass of the balls could be applied to the mill’s wall [31]. Meanwhile, the median diameter was negatively correlated with the rotation speed [30]. This was the reason that a 0.7 ball filling fraction and 40 rpm rotation speed for 30 min was adopted in this study. After preparation, the X-ray diffraction (XRD) was used to investigate the composition and surface morphology of the four mineral particles. The results are shown in Fig. 2 and Table 3 respectively. From the analysis of the XRD patterns of the prepared mineral particles, the main chemical components were obtained, and normalization processing was made for contrast, as shown in Table 3. For limestone, CaO was the largest proportion (93.79%), so it exhibited a higher alkali value. While for basalt, the main components were SiO2, Al2O3, and CaO at proportions of 48.36%, 33.34%, and
Fig. 2. XRD patterns of limestone, basalt, granite, and sandstone.
15.85%, respectively. Granite has a complex composition, containing SiO2 (67.99%), Al2O3 (16.15%), K2O (10.53%), MgO (3.06%), and Na2O (2.27%). Sandstone was comprised of SiO2 (72.36%), followed by 16.92% Al2O3, and 9.81% of Na2O. As the content of acidic oxides (SiO2) increased in basalt, granite, and sandstone, the alkali values decreased, respectively. Sandstone, however, also contained Na2O at a proportion of 9.81%, which caused a higher alkali value than granite but less than that of basalt. 3.2. Modification of probe tips with mineral particle Spherical-particle probe tips were created by attaching individual mineral particles to a triangular cantilever. The resulting mineral particle probe tips were checked using a fluorescence microscope to ensure good fabrication quality, as exhibited in Fig3(b). Figs. 1(b) and 3(a) show the setup and procedure for attaching a basalt particle to the probe tip of an AFM triangular cantilever, respectively. The spherical particle ensures suitable contact between the binder surface and the modified probe tip. The calibrated cantilever deflection sensitivities of the probe tips modified with limestone, basalt, granite, and sandstone were 54.51 nm/V, 43.96 nm/V, 27.83 nm/V, and 39.48 nm/V, while the spring constants were 0.5747 N/m, 0.5499 N/m, 0.6145 N/m, and 0.6706 N/m, respectively. 3.3. Measurement of adhesion of asphalt binder The performance of asphalt binder is mainly dependent on its mechanical properties, especially its adhesive characteristics, which are impacted significantly by mineral aggregates and asphalt binder. Adhesion characteristics between aggregate and asphalt was related to many factors, which were mainly affected by the materials themselves, such as SARA fractions of the asphalt and surface components of aggregate [33]. Many attempts have been made to evaluate and quantify the adhesion properties between asphalt binder and mineral compounds such as alumina, silica and calcium carbonate [19]. It is well-known that there are differences in adhesion between samples, within samples, and in the degree of variation between samples. However, it is generally recognized that adhesive forces vary with different set-points of the samples, which results in poor repeatability and reproducibility in AFM testing, but the details remain unclear. Regarding chemical adsorption, it is well-known that carboxylic acids in asphalt strongly adsorb on mineral surfaces, then chemically adsorb with the basic compounds of the mineral surfaces [3,34]. In order to achieve the desired performance of pavement, it is critical to measure the adhesive forces at mineral particle and asphalt binder substrate interfaces at the microscale. The adhesive forces at mineral particle-substrate interfaces are illustrated in Fig. 4. There were differences in the adhesive forces among mineral particles. Considerable discrepancies can be observed for adhesive forces at the particle-substrate interfaces of different mineral materials. It is noteworthy that the adhesive force increases obviously as the content of SiO2 increases in Fig. 5. As exhibited in Fig. 4, the sandstone-substrate group exhibited a larger adhesion value than the other three mineral-substrate groups, while the granite and basalt substrate group exhibited rel-
Table 3 Main composition content of the four prepared mineral particles/% Items
SiO2
Al2O3
Limestone Basalt Granite Sandstone
48.36 67.99 72.36
33.34 16.15 16.92
CaO 93.79 15.85 0.39
Na2O
K2O
MgO
FeO
6.21 2.45 2.27 9.81
10.53
3.06 0.19
0.33
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Fig. 3. (a) Spherical probe preparation, (b) probe tips modified with spherical mineral particles.
atively small adhesive forces, and the limestone-substrate group shows the lowest adhesion value. This phenomenon was hardly noticed for asphalt samples in the AFM experiment because of the different structures and domains of the asphalt surface [19]. Such a critical difference in adhesive force might be mainly attributable to differences in the chemical compositions and surface microstructures of the four minerals. Among these minerals, CaO (alkaline oxide) comprised 93.79% of limestone, so limestone exhibited a high alkali value. While in the other three minerals, the alkali value decreased as the composition of SiO2 (acidic oxide) increased. This indicates that physical adsorption plays a stronger role in the interfacial adhesive properties than chemical adsorption. This is because, in asphalt concrete, both chemical and physical adsorption occurs between the asphalt and aggregate, which together form an adhesive bond. Chemical adsorption usually refers to a reaction between an acid and alkali. Asphalt is considered to be acidic, so the aggregate should be alkaline to make its adhesion to asphalt stronger. CaO (alkaline oxide) comprises 93.79% of limestone, so limestone exhibited a high alkali value. While in the other three minerals, the alkali value decreased as the composition of SiO2 (acidic oxide) increased. It is noteworthy that the adhesive force increased obviously as the SiO2 content increased (Fig. 5). However, if chemical adsorption plays a more
Fig. 4. Box chart of adhesive force distributions of the four types of mineral particle-substrates.
important role, the adhesive force should decrease as the content of SiO2 increases. Therefore, physical adsorption plays a stronger role in interfacial adhesion than chemical adsorption.
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3.4. Analysis of adhesive force distribution behavior
Fig. 5. Adhesive forces versus SiO2 content for different mineral particles.
To compare the differences between particle-substrate interfaces, linear models were applied to their adhesive force data, which are displayed in Figs. 6 and 4, respectively. Fig. 6 shows the linear correlations of adhesive forces between particlesubstrate interfaces and row number. As can be observed, the method produces essentially perfect results, even though it seems to be quite difficult when a random starting point is used. In Fig. 6, the complex adhesive force distributions of the studied asphalt samples are sensitive to the modified tip materials. Table. 4 presents the slopes and F values of the adhesive forces distributions at the sample surfaces. F is a ratio of two estimators of r2 (variance). A slope or F value obtained from the linear fitting can reflect the distribution and degree of dispersion of the adhesive forces at the particle-substrate interfaces. As can be seen in Table 4, the granite-asphalt group exhibited the minimum slope value (absolute value) and F value of the samples, which means that the distribution of adhesive forces tested by AFM was more concentrated. This is consistent with the results presented in Fig. 4.
Fig. 6. Linear correlation of adhesive forces at the interfaces of the substrate and particles of (a) limestone, (b) basalt, (c) granite, and (d) sandstone.
Table 4 Slope and F value of the adhesive forces distribution of the sample surfaces. Index
Limestone-substrate
Basalt-substrate
Granite-substrate
Sandstone-substrate
Slope value F value
0.3290 10.0642
0.1112 2.9622
0.0157 0.2202
1.7270 116.9921
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Fig. 7. Comparative distributions of adhesive forces at particle-substrate interfaces.
In addition to linear fitting algorithm, analysis of variance in single-factor was also applied for investigating the distribution behavior of various adhesive force values of the particlesubstrate interfaces [35]. The adhesive forces at the particlesubstrate interfaces could be considered as a single-factor ANOVA experiment involving I = 4 types of mineral particle-modified probe tips (the sample means and standard deviations were in good agreement with values obtained from the AFM tests). Here, li denotes the true average adhesive force for tips modified with mineral type i (i = limestone, basalt, granite, sandstone). The null hypothesis is H0: llimestone = lbasalt = lgranite = lsandstone. The test statistic is the ratio F = MSTr/MSE. The numerator (the betweensamples estimator), MSTr, is unbiased when H0 is true but tends to overestimate r2 when H0 is false, whereas the denominator (the within-samples estimator), MSE, is unbiased regardless of the status of H0. Thus if H0 is true the F ratio should be reasonably close to 1, but if the li values differ considerably from one another, F should greatly exceed 1. Thus, a value of F considerably exceeding 1 argues for rejection of H0. Fig. 4 shows a comparative boxplot for the four samples. There is hardly any overlap in the ranges of values for the four types of mineral particle probe tips. As demonstrated in Table 4, the F value of the granite-substrate group is closer to 1 than those of the other three groups, while that of the basalt and limestone substrate groups slightly exceed 1 and that of the sandstone-substrate group considerably exceed 1. This means that H0 is not true, which suggests that llimestone – lbasalt – lgranite – lsandstone. Apparently, the adhesive forces varied among the different mineral samples, and had a good repeatability and reproducibility in the tests. It can be seen that this method is very applicable to all samples used in this study.
In order to clearly reveal the distinctions in the distribution behaviors of the adhesive forces between the particle-substrate interfaces, Fig. 7 displays comparative plots of the distributions of adhesive forces for the four modified probe tip and asphalt substrate groups. It shows the interquartile ranges (IQRs), which are measures of variability based on dividing the dataset into quartiles (four equal parts separated by the first, second, and third quartiles, denoted Q1, Q2, and Q3, respectively). The interquartile range (IQR), or middle 50%, is a measure of statistical dispersion and can be calculated by integrating the probability density function: IQR = Q3–Q1 [36]. It is a trimmed estimator, defined as the 25% trimmed range, and is the most measure of scale. Fig. 7 shows that the confidence intervals (from 10% to 90%) and means of the adhesive force distributions differ significantly between each particle-substrate group. Furthermore, the IQRs of the adhesive forces between modified probe tips and asphalt substrates (calculated from Fig. 7) are 20.9 for limestone, 37.1 for basalt, 4.7 for granite, and 98.0 for sandstone. The quartile deviation may be taken as a measure of dispersion. Hence, the distribution of adhesive forces at the granite-asphalt substrate interface is more concentrated than those of the limestone- and basalt-asphalt interfaces, while that at the sandstone-asphalt interface is more dispersed. This might be attributed to the different microstructures of the mineral particle surfaces. The dispersion decreases when the modified probe tips are stressed uniformly. Subsequently, the adhesive forces at the modified probe tip-asphalt substrate interfaces are reasonably stable. Digital images taken by SEM (scanning electron microscope) demonstrate the microstructural difference between the mineral particles (Fig. 8). How the surface microstructure affects adhesion will be explored in our future work.
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Fig. 8. SEM images of the surfaces of limestone, basalt, granite, and sandstone particles.
4. Conclusions
Acknowledgement
In summary, we developed four types of mineral spherical mineral particle probe tips (limestone, basalt, granite, and sandstone) using the optical mode of AFM. Microspheres of mineral particles were prepared by grinding in a planetary ball mill in the laboratory, then characterized by XRD. For limestone, CaO (alkaline oxide) comprised 93.79%, while for basalt, granite, and sandstone, the alkali value decreased as the composition of SiO2 (acidic oxide) increased. The AFM probe tips were modified with prepared mineral particles to investigate the adhesive forces between the probe tips and asphalt substrates. The results show that adhesive forces increase obviously as the SiO2 content increases. This indicates that physical adsorption has a greater influence than chemical adsorption on the interfacial adhesion properties. Besides, linear models, single-factor ANOVA and IQR analyses were also applied for evaluating the distributions of adhesive force values. The adhesive forces differed between minerals, and had good repeatability and reproducibility in the AFM tests. The IQR is a measure of dispersion and was 4.7 for granite, which means that the main section of the mineral-asphalt adhesive force distribution was more concentrated for granite than for limestone (20.9), basalt (37.1), and sandstone (98.0). These different levels of dispersion might be attributable to the different microstructures of the mineral particle surfaces. The results of this study provide a new approach to the investigation of adhesion between mineral particles and asphalt at the microscale. Physical adsorption was demonstrated to be the primary adhesive mechanism between asphalt and aggregate, which is useful information for establishing mechanical models of asphalt binders.
We are grateful for financial support from the National Natural Science Foundation of China (No. 51608511).
Conflict of interest None.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Study on adhesion of asphalt using AFM tip modified with mineral particles Xiaobo Lv a, Weiyu Fan a,⇑, Jiqian Wang a, Ming Liang a, Chengduo Qian a, Hui Luo a, Guozhi Nan a, Bingjian Yao b, Pinhui Zhao c a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, China College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, China c School of Transportation Engineering, Shandong Jianzhu University, Jinan 250101, Shandong, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Microspheres of mineral particle were
We report, the first of its kind, modified AFM probe tip of mineral particles could effectively measure the adhesion differences between mineral particle and asphalt binders which provide a new approach to investigate adhesion properties of asphalt binder in microscale.
prepared to modify AFM tips. Adhesion differences between mineral-asphalt interfaces could be measured by modified mineral probe tips. Adhesive force strength enhances as the content of SiO2 increases. Physical adsorption plays a greater role in interfaciale adhesion than chemical adsorption.
a r t i c l e
i n f o
Article history: Received 8 May 2018 Received in revised form 17 February 2019 Accepted 18 February 2019
Keywords: Asphalt Aggregate Adhesive force Atomic force microscopy Modification of probe tip
a b s t r a c t Adhesion between asphalt binders and aggregates is important to the performance of pavement. Atomic force microscopy is commonly used to measure the adhesive forces of asphalt binders. In this paper, AFM probe tips were modified with four types of spherical mineral particles: limestone, basalt, granite and sandstone. These were prepared by planetary ball mill grinding and their compositions investigated by X-ray diffraction. The modified tips were used to investigate the adhesive forces at mineral particleasphalt substrate interfaces. Adhesive forces increased obviously with SiO2 content. This indicates that physical adsorption plays a greater role in interfaciale adhesion than chemical adsorption. Linear model, single-factor analysis of variance, and interquartile analyses were also applied to investigatinge the distributions of adhesive force. It was found that different degrees of distributions might be attributable to the different microstructures of the mineral particle surfaces. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
⇑ Corresponding author. E-mail address: [email protected] (W. Fan). https://doi.org/10.1016/j.conbuildmat.2019.02.115 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
Asphalt binders have long been used as components of road pavement [1], as they bind aggregates together. Their mechanical properties, particularly their adhesive characteristics, contribute
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significantly to the asphalt’s overall performance. In fact, both chemical and physical adsorption contribute to asphalt binder adhesion [2,3]. Although progress has been made in understanding the compositions and properties of asphalt binders, the performance of asphalt sometimes fails to meet expectations. Therefore, more fundamental materials science research is needed to better characterize and predict the service performance of asphalt binders. The adhesion force of asphalt is one of the fundamental properties that affect asphalt pavement’s moisture-induced damage performance [4]. Atomic force microscopy (AFM) is a powerful tool that is used not only to view surface morphology, but also to probe electrical, magnetic, van der Waals, adhesion, and chemical interactions. AFM is one of the few methods that are capable of measuring adhesion of asphalt at the micro-scale. AFM has been utilised to measure the force-distance curve, which is a plot of cantilever deflection as a function of sample position. By analysing this curve, one can measure the tackiness or adhesive properties of an asphalt binder [4]. Currently, AFM is used to measure the adhesive forces of asphalt binders as it provides high-resolutions of displacement and force. An AFM uses a cantilever spring with a sharp tip to measure the interactions between the tip and an asphalt sample. It can use various tip materials and modifications, most of which are made of silicon or silicon nitride [5], which cannot observe the adhesion behaviors and interactions between asphalt binders and aggregates. Another problem is that the use of such tips to study asphalt causes tip contamination, which reduces the image resolution and generates errors [6]. Therefore, it is highly desirable to develop techniques to modify Si/SiN tips with functional materials that are used in road pavement. AFM was initially developed to complement scanning tunneling microscopy by observing topography at the nanoscale or even atomic scale [7,8]. Gradually, the application of AFM has been greatly expanded to realize functional characterization. This has made it possible to modify the probe tip on the forearm of an AFM, which changes or augments the interaction mechanism between the tip and the target surface [9]. Growth of a selfassembled monolayer of thiolates on gold-coated AFM tips is a common type of chemical modification [10]. However, thinner gold films cannot fully cover the surface, and the large tip size decreases the resolution of AFM imaging. Ali and Lim [11] trimmed AFM probe tips to minimize measurement and imaging errors. However, higher ion doses caused over-milling and easily damaged the tip at its base. Moreover, modified tips were fragile and could only be used at low scanning speeds. Tarefder and Zaman [12] reported that the use of silicon nitride tips and chemically functionalized tips could minimize the adhesion force between the tip and asphalt surface. Furthermore, the intermittent-contact mode was considered inappropriate for determining the adhesion differences of asphalt binders because the phase contrasts relate not only to the effects of adhesive force and stiffness, but also to the damping and morphological features of the sample [2,13]. Comparatively, techniques for direct coating of Si/SiN tips with a robust monolayer of functional molecules were reported by Lee and Laibinis [14]. Nevertheless, it is difficult to control the conditions needed to prevent the deposition of multiple or large particles on the tip surface. Conventional techniques for probe modification are favorable to the attachment of particles, such as a single object to the probe tip, since the geometry and composition of the particle can be well controlled. Attaching a microparticle, especially a micro-sized sphere to an AFM cantilever for force measurements has become routine in AFM labs [15,16]. In past years, techniques have been developed for attaching even smaller objects, such as single nanoparticles and nanotubes to tips used for AFM applications [17]. Yang [18] modified an AFM probe cantilever by attaching a micro- sized colloidal particle to the foremost part of it. Li [19]
attached microspheres of alumina, silica and calcium carbonate to cantilever ends with this technique, and evaluated the adhesion between the probe and binder substrates under ambient conditions. Despite the development of a variety of techniques for attaching particles to AFM probe tips, none have modified Si/SiN tips with functional material particles used in road pavement. Besides, AFM-based adhesion tests on soft and sticky asphalt binders have proven difficult [20–22]. On the other hand, adhesion forces vary significantly with operational conditions and sample treatments. Therefore, probe tips modified using microscale limestone, basalt, granite, and sandstone are investigated in this paper to study the adhesive forces between the modified tips and asphalt binders. We develop a systematic, AFM-based, tip modification and adhesion testing procedure to fill the knowledge gaps existing in this field. Our approach develops a mineral probe tip and reduces the effect of operational procedures on the adhesion measurement as much as possible. The modified AFM probe tips effectively measure the adhesion differences between mineral particles and asphalt binders. This thorough, AFM-based, tip modification and adhesion testing procedure is used to directly quantify the adhesion differences between asphalt binders and four primary mineral tip types. This will be helpful for discovering other compliant mineral materials with which to characterize the adhesion properties of mineral particle-substrate interfaces by AFM.
2. Materials and methods 2.1. Materials 2.1.1. Asphalt Asphalt binder (AH-70, obtained from Qinhuangdao Refinery, China) with a penetration grade of 70 was used to evaluate the interactions between mineral particle-substrate interfaces and mineral particle materials. Table 1 shows the physical properties and chemical compositions of the asphalt used. The binder samples used for adhesion measurements were prepared via a solution-cast approach [23], which produces more flat and less sticky surfaces compared to the heat-cast approach [2]. Heat-cast thin film was found when the AFM tip be contaminated by sticky binder or when the sample surface was damaged by the formation of humps.
2.1.2. Sphere particles and cantilevers Four types of mineral microspheres were prepared in the laboratory and used for tip modification: basalt (from Liaoning, China), limestone, granite and sandstone (from Shandong, China). Alkali testing (T0347-2000, JTG E42-2005) was carried out to evaluate the properties of the mineral particles. Table 2 shows the alkali values of the four mineral materials. Their mean particle diameter was 5 lm. The cantilevers used in this study were triangular (DNP-S10, 4 cantilevers, nonconductive silicon nitride) with spring constants ranging from 0.06 to 0.35 N/m.
2.2. Sample preparation Mineral microspheres of limestone, basalt, granite and sandstone were prepared by a ball milling machine. The particles were added to a ball milling pot, then large and agglomerated particles were rendered into microspheres by crushing
Table 1 The properties and compositions of the asphalt binder. Items
AH-70
Penetration (25 °C, 0.1 mm) Ductility (15 °C, cm) Softening point (R&B, °C) Viscosity (135 °C, Pas) Saturates (wt%) Aromatics (wt%) Resins (wt%) Asphaltenes (wt%) Ic = (At + S)/(A + R) (%)
73 >100 49.4 0.388 22.67 42.15 26.64 8.53 0.45
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Table 2 Alkali value of minerals. Items
Limestone
Basalt
Granite
Sandstone
Alkali value
0.96
0.091
0.034
0.068
(shearing and extrusion forces) as illustrated in Fig. 1(a). Dry grinding of mineral particles was carried out with a ball filling fraction of 0.7 and rotation speed of 40 rpm for 30 min for application to probe modification. Asphalt binder samples were prepared by melting them in toluene solutions for further characterization. A 0.5 mm-thick silicon wafer was cut to an approximately 10 10 mm square with a diamond cutter and cleaned with Piranha solution in a terrarium [6]. After cleaning, sample solutions were spread on clean silicon wafer substrates. Then, the samples were stored in a vacuum drying oven at 100 °C for 24 h. This temperature allowed the asphalt binders to smoothly cover the wafer surfaces and removed the toluene from the samples. After that, all specimens were cooled to ambient temperature and stored in a silica-gel drying container until adhesion measurement.
2.3. Methods 2.3.1. AFM probe tips modification technique of AFM probe tips with attached micro-particles have been extensively used for interfacial force measurements. A probe tip modified with a spherical particle with a diameter larger than 1 lm gains an advantage over a normal tip. The sphere has a well-defined geometry and can obtain force measurements with a higher signal-tonoise ratio, since more effective information can be derived from the curves [15,16]. As only particles larger than 1 lm can be easily observed under the optical mode of AFM, an optical microscope and an AFM head are the standard tools for attaching a micro particle to the probe tip [9]. Most importantly, to measure the average adhesive force across an asphalt sample’s surfaces, a normal probe should contact every point of the sample. However, this can be easily derived using probe tips modified with a spherical particle. Asphalt is mainly a complex mixture of hydrocarbons whose structure is well described by the colloidal model, where solid particles with a radius of a few nanometers are dispersed in an oily liquid matrix [24]. Topographic images of asphalt binder samples display different domains (including Catanaphase, Salphase, Paraphase and Periphase [25,26]) with height differences of a few nanometers between them. The surface structure of the domain does not share the same features as other phases, such as force behavior, stiffness, and morphology [27,28]. Since only a few nanometers of the normal tip actually contact the sample, even minute forces are distributed over an exceedingly small area, which add up quickly. Many materials are easily dented by the tip under such conditions. Therefore, a
probe tip with a well-defined spherical particle can better solve the problem and determine the average adhesion force between the particle and the asphalt binder surface with finite contact points. 2.3.2. Modification of AFM probe tips with mineral spherical particles In this study, limestone, basalt, granite and sandstone microsphere particles were used to modify the probe tips. These were used to measure the adhesive forces at the mineral particle-asphalt interface on a Bruker Multimode 8 AFM in Nanomechanical Mapping-Peakforce QNM in air conditions. While observing with the optical mode of AFM, a probe was fixed on a cantilever holder with the cantilevers facing up and blocking the holder on the AFM head. The cantilever was moved around first to pick up a tiny amount of optical adhesive (Norland, USA), then used to pick up a particle spread onto a silicon wafer preliminarily with capillary force. The final particle attachment was completed as seen by the optical microscope. After this procedure, the modified probe was allowed to stay under an ultraviolet lamp for 30 min to let the optical adhesive cure. This technique is especially suitable for dealing with triangular cantilevers with spring constant k < 1 N/m. The main reason is that small-amplitude thermal vibrations induce larger systematic measurement errors. 2.3.3. Calibration of the cantilever deflection sensitivity and spring constant The deflection sensitivity allows conversion from the raw photodiode to deflection of the cantilever, and is normally set from the force calibration mode. Force calibration mode allows minimization of the contact force of the cantilever on the sample surface. Since the force curve clearly illustrates the relationship between the set-point and the cantilever deflection voltage when the cantilever is off the sample surface, the sensitivity must be calibrated before accurate deflection data can be obtained. The sensitivity is equal to the inverse of the slope of the force curve while the cantilever is in contact with a hard sample surface (sapphire, silicon wafer, etc.). Since the set-point defines the value of the deflection signal maintained by the feedback loop, the force curve can be used to calculate the constant force of the tip on the sample if the spring constant k of the modified probe cantilever is known. By knowing the spring constant of the cantilever, it is possible to measure the attractive forces of tip-sample interactions with good precision. The spring constant was calibrated with the thermal tune method. This method is based on the equipartition theorem from fundamental thermodynamic theory [29]. It is possible to determine the spring constant of a contact mode cantilever quickly and easily. 2.3.4. Measurement of adhesive force Mineral limestone, basalt, granite, and sandstone microsphere particles have never been used for AFM probe tip modification and investigation of the adhesion properties of asphalt binders. In this study, the AFM force curve data were obtained for limestone-, basalt-, granite- and sandstone- modified tips on an AFM in Nanomechanical Mapping-Peakforce QNM in air. In the index sets, three curves
Fig. 1. Schematic representation of (a) mineral crushing in the planetary ball mill and (b) the procedure of tip modifying.
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3. Results and discussion 3.1. Mineral microsphere particles Many kinds of machines are available for fabrication of spherical particles, depending on the material properties and required product sizes. The present study prepared mineral particles that were as fine as possible by grinding process in a planetary ball mill, which was expected to efficiently reduce the size to the micro range [30–32]. Powder grinding was selected as it is a simple and well-established large-scale manufacturing process compared with liquid-phase methods, and also avoids possible contamination by ball and pot materials in wet grinding. The initial grinding rate in the planetary ball mill was reported to have an approximate maximum of a 0.3 ball filling fraction, where the maximum force of the mass of the balls could be applied to the mill’s wall [31]. Meanwhile, the median diameter was negatively correlated with the rotation speed [30]. This was the reason that a 0.7 ball filling fraction and 40 rpm rotation speed for 30 min was adopted in this study. After preparation, the X-ray diffraction (XRD) was used to investigate the composition and surface morphology of the four mineral particles. The results are shown in Fig. 2 and Table 3 respectively. From the analysis of the XRD patterns of the prepared mineral particles, the main chemical components were obtained, and normalization processing was made for contrast, as shown in Table 3. For limestone, CaO was the largest proportion (93.79%), so it exhibited a higher alkali value. While for basalt, the main components were SiO2, Al2O3, and CaO at proportions of 48.36%, 33.34%, and
Fig. 2. XRD patterns of limestone, basalt, granite, and sandstone.
15.85%, respectively. Granite has a complex composition, containing SiO2 (67.99%), Al2O3 (16.15%), K2O (10.53%), MgO (3.06%), and Na2O (2.27%). Sandstone was comprised of SiO2 (72.36%), followed by 16.92% Al2O3, and 9.81% of Na2O. As the content of acidic oxides (SiO2) increased in basalt, granite, and sandstone, the alkali values decreased, respectively. Sandstone, however, also contained Na2O at a proportion of 9.81%, which caused a higher alkali value than granite but less than that of basalt. 3.2. Modification of probe tips with mineral particle Spherical-particle probe tips were created by attaching individual mineral particles to a triangular cantilever. The resulting mineral particle probe tips were checked using a fluorescence microscope to ensure good fabrication quality, as exhibited in Fig3(b). Figs. 1(b) and 3(a) show the setup and procedure for attaching a basalt particle to the probe tip of an AFM triangular cantilever, respectively. The spherical particle ensures suitable contact between the binder surface and the modified probe tip. The calibrated cantilever deflection sensitivities of the probe tips modified with limestone, basalt, granite, and sandstone were 54.51 nm/V, 43.96 nm/V, 27.83 nm/V, and 39.48 nm/V, while the spring constants were 0.5747 N/m, 0.5499 N/m, 0.6145 N/m, and 0.6706 N/m, respectively. 3.3. Measurement of adhesion of asphalt binder The performance of asphalt binder is mainly dependent on its mechanical properties, especially its adhesive characteristics, which are impacted significantly by mineral aggregates and asphalt binder. Adhesion characteristics between aggregate and asphalt was related to many factors, which were mainly affected by the materials themselves, such as SARA fractions of the asphalt and surface components of aggregate [33]. Many attempts have been made to evaluate and quantify the adhesion properties between asphalt binder and mineral compounds such as alumina, silica and calcium carbonate [19]. It is well-known that there are differences in adhesion between samples, within samples, and in the degree of variation between samples. However, it is generally recognized that adhesive forces vary with different set-points of the samples, which results in poor repeatability and reproducibility in AFM testing, but the details remain unclear. Regarding chemical adsorption, it is well-known that carboxylic acids in asphalt strongly adsorb on mineral surfaces, then chemically adsorb with the basic compounds of the mineral surfaces [3,34]. In order to achieve the desired performance of pavement, it is critical to measure the adhesive forces at mineral particle and asphalt binder substrate interfaces at the microscale. The adhesive forces at mineral particle-substrate interfaces are illustrated in Fig. 4. There were differences in the adhesive forces among mineral particles. Considerable discrepancies can be observed for adhesive forces at the particle-substrate interfaces of different mineral materials. It is noteworthy that the adhesive force increases obviously as the content of SiO2 increases in Fig. 5. As exhibited in Fig. 4, the sandstone-substrate group exhibited a larger adhesion value than the other three mineral-substrate groups, while the granite and basalt substrate group exhibited rel-
Table 3 Main composition content of the four prepared mineral particles/% Items
SiO2
Al2O3
Limestone Basalt Granite Sandstone
48.36 67.99 72.36
33.34 16.15 16.92
CaO 93.79 15.85 0.39
Na2O
K2O
MgO
FeO
6.21 2.45 2.27 9.81
10.53
3.06 0.19
0.33
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Fig. 3. (a) Spherical probe preparation, (b) probe tips modified with spherical mineral particles.
atively small adhesive forces, and the limestone-substrate group shows the lowest adhesion value. This phenomenon was hardly noticed for asphalt samples in the AFM experiment because of the different structures and domains of the asphalt surface [19]. Such a critical difference in adhesive force might be mainly attributable to differences in the chemical compositions and surface microstructures of the four minerals. Among these minerals, CaO (alkaline oxide) comprised 93.79% of limestone, so limestone exhibited a high alkali value. While in the other three minerals, the alkali value decreased as the composition of SiO2 (acidic oxide) increased. This indicates that physical adsorption plays a stronger role in the interfacial adhesive properties than chemical adsorption. This is because, in asphalt concrete, both chemical and physical adsorption occurs between the asphalt and aggregate, which together form an adhesive bond. Chemical adsorption usually refers to a reaction between an acid and alkali. Asphalt is considered to be acidic, so the aggregate should be alkaline to make its adhesion to asphalt stronger. CaO (alkaline oxide) comprises 93.79% of limestone, so limestone exhibited a high alkali value. While in the other three minerals, the alkali value decreased as the composition of SiO2 (acidic oxide) increased. It is noteworthy that the adhesive force increased obviously as the SiO2 content increased (Fig. 5). However, if chemical adsorption plays a more
Fig. 4. Box chart of adhesive force distributions of the four types of mineral particle-substrates.
important role, the adhesive force should decrease as the content of SiO2 increases. Therefore, physical adsorption plays a stronger role in interfacial adhesion than chemical adsorption.
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3.4. Analysis of adhesive force distribution behavior
Fig. 5. Adhesive forces versus SiO2 content for different mineral particles.
To compare the differences between particle-substrate interfaces, linear models were applied to their adhesive force data, which are displayed in Figs. 6 and 4, respectively. Fig. 6 shows the linear correlations of adhesive forces between particlesubstrate interfaces and row number. As can be observed, the method produces essentially perfect results, even though it seems to be quite difficult when a random starting point is used. In Fig. 6, the complex adhesive force distributions of the studied asphalt samples are sensitive to the modified tip materials. Table. 4 presents the slopes and F values of the adhesive forces distributions at the sample surfaces. F is a ratio of two estimators of r2 (variance). A slope or F value obtained from the linear fitting can reflect the distribution and degree of dispersion of the adhesive forces at the particle-substrate interfaces. As can be seen in Table 4, the granite-asphalt group exhibited the minimum slope value (absolute value) and F value of the samples, which means that the distribution of adhesive forces tested by AFM was more concentrated. This is consistent with the results presented in Fig. 4.
Fig. 6. Linear correlation of adhesive forces at the interfaces of the substrate and particles of (a) limestone, (b) basalt, (c) granite, and (d) sandstone.
Table 4 Slope and F value of the adhesive forces distribution of the sample surfaces. Index
Limestone-substrate
Basalt-substrate
Granite-substrate
Sandstone-substrate
Slope value F value
0.3290 10.0642
0.1112 2.9622
0.0157 0.2202
1.7270 116.9921
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Fig. 7. Comparative distributions of adhesive forces at particle-substrate interfaces.
In addition to linear fitting algorithm, analysis of variance in single-factor was also applied for investigating the distribution behavior of various adhesive force values of the particlesubstrate interfaces [35]. The adhesive forces at the particlesubstrate interfaces could be considered as a single-factor ANOVA experiment involving I = 4 types of mineral particle-modified probe tips (the sample means and standard deviations were in good agreement with values obtained from the AFM tests). Here, li denotes the true average adhesive force for tips modified with mineral type i (i = limestone, basalt, granite, sandstone). The null hypothesis is H0: llimestone = lbasalt = lgranite = lsandstone. The test statistic is the ratio F = MSTr/MSE. The numerator (the betweensamples estimator), MSTr, is unbiased when H0 is true but tends to overestimate r2 when H0 is false, whereas the denominator (the within-samples estimator), MSE, is unbiased regardless of the status of H0. Thus if H0 is true the F ratio should be reasonably close to 1, but if the li values differ considerably from one another, F should greatly exceed 1. Thus, a value of F considerably exceeding 1 argues for rejection of H0. Fig. 4 shows a comparative boxplot for the four samples. There is hardly any overlap in the ranges of values for the four types of mineral particle probe tips. As demonstrated in Table 4, the F value of the granite-substrate group is closer to 1 than those of the other three groups, while that of the basalt and limestone substrate groups slightly exceed 1 and that of the sandstone-substrate group considerably exceed 1. This means that H0 is not true, which suggests that llimestone – lbasalt – lgranite – lsandstone. Apparently, the adhesive forces varied among the different mineral samples, and had a good repeatability and reproducibility in the tests. It can be seen that this method is very applicable to all samples used in this study.
In order to clearly reveal the distinctions in the distribution behaviors of the adhesive forces between the particle-substrate interfaces, Fig. 7 displays comparative plots of the distributions of adhesive forces for the four modified probe tip and asphalt substrate groups. It shows the interquartile ranges (IQRs), which are measures of variability based on dividing the dataset into quartiles (four equal parts separated by the first, second, and third quartiles, denoted Q1, Q2, and Q3, respectively). The interquartile range (IQR), or middle 50%, is a measure of statistical dispersion and can be calculated by integrating the probability density function: IQR = Q3–Q1 [36]. It is a trimmed estimator, defined as the 25% trimmed range, and is the most measure of scale. Fig. 7 shows that the confidence intervals (from 10% to 90%) and means of the adhesive force distributions differ significantly between each particle-substrate group. Furthermore, the IQRs of the adhesive forces between modified probe tips and asphalt substrates (calculated from Fig. 7) are 20.9 for limestone, 37.1 for basalt, 4.7 for granite, and 98.0 for sandstone. The quartile deviation may be taken as a measure of dispersion. Hence, the distribution of adhesive forces at the granite-asphalt substrate interface is more concentrated than those of the limestone- and basalt-asphalt interfaces, while that at the sandstone-asphalt interface is more dispersed. This might be attributed to the different microstructures of the mineral particle surfaces. The dispersion decreases when the modified probe tips are stressed uniformly. Subsequently, the adhesive forces at the modified probe tip-asphalt substrate interfaces are reasonably stable. Digital images taken by SEM (scanning electron microscope) demonstrate the microstructural difference between the mineral particles (Fig. 8). How the surface microstructure affects adhesion will be explored in our future work.
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Fig. 8. SEM images of the surfaces of limestone, basalt, granite, and sandstone particles.
4. Conclusions
Acknowledgement
In summary, we developed four types of mineral spherical mineral particle probe tips (limestone, basalt, granite, and sandstone) using the optical mode of AFM. Microspheres of mineral particles were prepared by grinding in a planetary ball mill in the laboratory, then characterized by XRD. For limestone, CaO (alkaline oxide) comprised 93.79%, while for basalt, granite, and sandstone, the alkali value decreased as the composition of SiO2 (acidic oxide) increased. The AFM probe tips were modified with prepared mineral particles to investigate the adhesive forces between the probe tips and asphalt substrates. The results show that adhesive forces increase obviously as the SiO2 content increases. This indicates that physical adsorption has a greater influence than chemical adsorption on the interfacial adhesion properties. Besides, linear models, single-factor ANOVA and IQR analyses were also applied for evaluating the distributions of adhesive force values. The adhesive forces differed between minerals, and had good repeatability and reproducibility in the AFM tests. The IQR is a measure of dispersion and was 4.7 for granite, which means that the main section of the mineral-asphalt adhesive force distribution was more concentrated for granite than for limestone (20.9), basalt (37.1), and sandstone (98.0). These different levels of dispersion might be attributable to the different microstructures of the mineral particle surfaces. The results of this study provide a new approach to the investigation of adhesion between mineral particles and asphalt at the microscale. Physical adsorption was demonstrated to be the primary adhesive mechanism between asphalt and aggregate, which is useful information for establishing mechanical models of asphalt binders.
We are grateful for financial support from the National Natural Science Foundation of China (No. 51608511).
Conflict of interest None.
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