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Powder Technology 208 (2011) 582 589 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec Adhesion properties of nanoparticle-coated emulsion aggregation toner Huan Zhang a, Weiqiang Ding a, Kock-Yee Law b, Cetin Cetinkaya a, a Nanomechanics and Nanomaterials Laboratory, Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY 13699-5725, USA b Xerox Corporation, 800 Phillips Rd., Webster, NY 14580, USA article info abstract Article history: Received 11 October 2009 Received in revised form 7 December 2010 Accepted 18 December 2010 Available online 24 December 2010 Keywords: Polymerized emulsion aggregation (EA) toner Nanoparticle Surface coating Work of adhesion Rolling resistance moment Toner adhesion Toner is the key material in printing and copying processes. Fundamental understanding of toner detachment and adhesion during the printing process is critical to improve both the efficiency of toner usage and the quality of print. To control their adhesion property, toner particles can be surface-coated with nanoparticle additives to modify their surface roughness, and consequently, to tune their adhesion properties. In this study, a technique based on the rolling resistance moment of the particle substrate adhesion bond is used to quantify the effect of nanoparticle surface area coverage (SAC) on the effective work of adhesion of individual toner particles. Nanoparticle-coated model emulsion aggregation (EA) toner microparticles with the specified SAC levels of 0%, 10%, 50% and 100% were studied and the corresponding particle substrate work of adhesion values were determined and compared. It is quantitatively demonstrated that the work of adhesion between a surface-coated toner particle and a flat silicon substrate decreases significantly with increasing nanoparticle SAC, which provides an effective means to tailor the adhesion performance of the EA toner. Also, based on the experimental data, for a nanoparticle-coated microparticle on a flat substrate, two possible modes of contact formation were identified and discussed. 2011 Elsevier B.V. All rights reserved. 1. Introduction In electrophotographic printing, charged micro-sized toner particles are transferred from the magnetic brush inside a toner cartridge to the photoreceptor drum, then from there to the transfer belt, and finally to the paper for fusing and binding. Fundamental understanding of toner detachment and adhesion during the printing process is, therefore, critical to improve both the toner usage efficiency and the print quality. Due to the technological significance, using various techniques, the adhesion of micro-scale particles has been studied extensively and reported in the literature [1 12]. Electrical field and centrifugal force detachment methods have been two of the popular techniques [1,2,4,5,7,8,11]. The former technique emulates detachment of charged toner particles from the magnetic brush to the photoreceptor and from the photoreceptor to paper. Under the specified experimental conditions, toner particles detach when the applied detachment force exceeds the particle substrate adhesion force. Statistical adhesion data is obtained by analyzing the amount of toner detached as a function of the electric field. However, adhesion originating from both electrostatic and non-electrostatic forces are operating, and, thus, such measurements provide only limited insight for the type of adhesion force that is dominant in the detachment process. The same shortfall also exists for the centrifugal force Corresponding author. Tel.: +1 315 268 6514; fax: +1 315 268 6695. E-mail address: cetin@clarkson.edu (C. Cetinkaya). detachment technique as well as the recently-proposed vibration assisted detachment technique [13]. The use of a calibrated atomic force microscope (AFM) cantilever to directly measure the adhesion force between a toner particle and a substrate represents a significant advance in toner adhesion measurement [1,6,14,15]. The adhesion force is measured directly since the force constant of the cantilever can be calibrated accurately. However, since the measurement process involves physically fixing (e.g., gluing) a toner particle onto the end of the cantilever, each measurement can only determine the adhesion of one area of a toner particle in single measurement. Therefore, the cantilever measurement does not represent an average adhesion of a toner particle, nor does it provide any statistical information regarding the adhesion of the toner sample. In view of this backdrop, Lee [16] in his 2008 letter to the editor of the Journal of Imaging Science and Technology discussed the existence of discrepancies in the literature and the inaccuracy of these earlier adhesion data in detail. Part of the difficulty in the adhesion characterization of toner is the lack of qualified adhesion measurement techniques that can be used to convincingly characterize and differentiate the electrostatic and non-electrostatic components of the adhesion force. Another complication in qualifying the experimental data has been the type of toner samples being studied and the techniques used to prepare the toner samples. For example, while in earlier works irregular conventional toners were used, in recent studies spherical chemical toners are employed more often. Furthermore, the type of additives and the aging of the toner samples and associated complications were seldom systematically studied and/or reported 0032-5910/$ see front matter 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.12.022

H. Zhang et al. / Powder Technology 208 (2011) 582 589 583 in earlier works. These factors complicate the comparison and interpretation of available adhesion and detachment data. Considering these experimental issues and accompanying modeling difficulties, the existence of the current controversy as to the nature of the force components in adhesion should be no surprise. The particle substrate adhesion has been theoretically investigated since early 70s. Several contact mechanics-based adhesion models have been developed, such as the Hertz model, JKR (Johnson, Kendall and Robert) model [17], DMT (Derjaguin, Muller and Toporov) model [18] and MD (Maugis and Dugdale) mode [19]. Later, Johnson and Greenwood [20] proposed a unifying framework for existing models, and established the transition between these theories and their applicability zones for ranges of external loads and an elasticity parameter. The adhesion models and experimental techniques discussed above all focus on the one-dimensional (axial) adhesion and assume a symmetric pressure field at the particle substrate contact. Recently, two-dimensional adhesion model has been proposed to analyze the rolling motion of an adhered particle, where the stress distribution at the particle substrate contact becomes asymmetric during pre-rolling and rolling. For example, adhesion and frictional forces between micro-spherical particles were studied by Heim and Blum [21] on the basis of rolling resistance moment using an AFM. The rolling resistance moment is generated by the adhesion bond between the particle and substrate against the rolling motion of the particle, which can be estimated based on the adhesion model of Dominik and Tielens [22]. Recently, Cetinkaya and co-workers experimentally confirmed the existence of the rolling resistance moment of an adhered polystyrene latex (PSL) microsphere with a non-contact acoustic excitation technique [23,24]. In this work, we report the use of a recently developed rolling resistance moment technique [25,26] in determining the adhesion between model emulsion aggregation (EA) base and surface-coated toner particles and silicon substrate to quantify the effect of nanoparticle coating on the adhesion properties of these toner particles. A lateral force acting on a spherical particle in obtaining a detachment criterion has been used to predict the initiation of rolling-based detachment of the particle. This technique was previously used to investigate the adhesion between the PSL microspheres and silicon substrate in the vacuum chamber of a scanning electron microscope (SEM) [26]. In current study, the technique is employed to characterize the adhesion properties of the toner particles in the ambient under an inverted optical microscope. While the spatial resolution of the optical microscope is substantially inferior to that of the SEM, performing measurements in the ambient can avoid the possible sources of inaccuracies associated with the charging of the nonconductive toner particles by the SEM electron beam. Moreover, working in the ambient provides the flexibility to explore the effects of certain realistic operational parameters such as the temperature and humidity on particle substrate adhesion, which is important for toner adhesion characterization but difficult to achieve in high vacuum. Since the base toner particles are randomly dusted (dry-deposited) onto the silicon substrate and the silicon substrate is doped to be electrically conductive, no toner charging is anticipated in the experiments, and the adhesion forces measured in the reported experiments should be purely van der Waals in nature. In addition, in order to determine the distribution of the adhesion properties, multiple rolling resistance moment measurements on the base toner particles have been performed. It is known that the surface roughness of the particle has a significant effect on the particle substrate adhesion [2,4,6]. In recent decades, the influence of surface roughness on particle substrate adhesion has attracted considerable attention, and several adhesion models have been proposed for taking the surface roughness of the particle and/or substrate into account to better predict the particle substrate adhesion [27 30]. Generally, for a particle with a rough surface settled on a flat substrate, the contact area between the particle and the substrate is much smaller than that between a smooth particle of the same diameter and the substrate. Therefore, in general significant reduction in the adhesion force is expected with increasing particle surface roughness. In this work, the effect of additive nanoparticle coating on the toner adhesion has been systematically explored by measuring the adhesion of a series of micron-scale model toner particles surface-coated with silica nanoparticle additive at various surface area coverage (SAC) levels. 2. Materials To examine the effect of additive coating on toner adhesion, a series of model toners of 6.0 μm nominal diameters were surface-coated with fumed silica nanoparticles (R805, Degussa AG, Frankfurt, Germany) at various specified SAC levels, namely 10%, 50%, and 100%. The SEM images of such surface-coated near-spherical model coated toner particles are shown in Fig. 1. The outer layer of the base toner particle consists of a polyester resin, cyan pigment and wax. The nominal size of the silica nanoparticle is about 12 nm in diameter, but the nanoparticles occasionally can form aggregates as large as 100 nm in effective diameter (Fig. 1). The model toner particles employed in the experiments were prepared by the EA process [31], and, subsequently, surface-coated with silica nanoparticles using a toner blender at the Xerox Research Center (Webster, New York, USA). The toner particles were used in the reported experiments as-received with no additional aging and/or chemical treatment and were dry-deposited on a plasma cleaned single-crystal silicon (p-type doped (100) oriented) substrate immediately prior to the pushing experiments. 3. Experimental setup The schematic of the rolling resistance moment-based lateral pushing experimental setup is depicted in Fig. 2. When a lateral pushing force is applied to an adhered particle, the stress distribution in the particle substrate contact area becomes non-uniform, which creates a moment (rolling resistance moment) to resist the free rolling motion of the particle. This rolling resistance moment is proportional to the angle of rotation of the particle [23,25]. With increasing lateral force, eventually the rolling resistance moment is unable to withstand the external rolling moment, and the particle begins rolling (at early stages, almost certainly without slip) on the substrate surface. The experimental setup developed for the reported work is composed of two opposing xyz linear positioning stages (122-1135/ 1155, OptoSigma Inc., Santa Ana, California, USA) mounted on the top of an inverted optical microscope (Epiphot 200, Nikon, Japan). These positioning stages are driven by six piezoelectric actuators (MRA 8351, New Focus, Inc., San Jose, California, USA) that can provide linear motion with a displacement resolution of approximately 30 nm. A piezoelectric bender (CMBP 05, Noliac A/S, Denmark) that provides fine positioning at a sub-nanometer resolution is mounted on one of the axes of the xyz-linear stage. The positioning and particle pushing processes can be monitored through a 100 objective lens using a high-resolution digital camera (DXM 1200, Nikon, Japan) attached to the optical microscope. For pushing tests, a tipless AFM cantilever with a length of 350 μm and a nominal force constant of 0.03 N/m (CSC 12, MikroMasch, Inc., Wilsonville, Oregon, USA) was attached to the free end of the piezoelectric bender, and the silicon substrate with toner particles deposited was mounted on the opposing linear positioning stage (Fig. 3). During the lateral pushing test, a dc voltage was applied to the piezoelectric bender to actuate the bender. The tipless AFM cantilever that attached to the free-end of the piezoelectric bender was thus actuated to push a chosen particle adhered on the substrate. The dc voltage was increased in discrete steps, and the corresponding lateral pushing force was thus also increased discretely. By acquiring a series of digital images, the entire pushing test was recorded with a time interval of approximately 30 s per each pushing step (for image

584 H. Zhang et al. / Powder Technology 208 (2011) 582 589 Fig. 2. Schematics of an adhered particle subjected to a lateral applied force (F) generating a resisting moment (M y ) to free-rolling at a lateral translation of (ξ) (contact area is not to scale). edge of the particle in each digitally recorded image with an image analysis software (ImageJ, the U.S. National Institute of Health, Bethesda, Maryland, USA) [35]. With the pushing force and particle displacement information at each pushing step, a lateral force vs. particle displacement (F Δx) curve was constructed for each particle tested. The work of adhesion between the particle and substrate was then extracted from this curve as follow [25]. For the lateral pushing configuration shown in Fig. 2, assuming no sliding at contact, the slope of the force displacement curve (k) can be approximated in a displacement range corresponding to the pre-rolling phase of motion as k = F Δx = M = ðd = 2Þ θðd = 2Þ = 4M θd 2 ; ð1þ where M is the moment generated by the pushing force with respect to the particle substrate contact, D the diameter of the spherical particle, and θ the angle of rotation of the particle with respect to the center of particle substrate contact. According to Dominik and Tielens [22], the rolling resistance moment (M y ) as a function of the angle of rotation θ can be approximated as M y 6πW A ðd=2þ 2 θ; ð2þ where W A is the work of adhesion between the particle and substrate. From Eqs. (1) and (2), when the rolling resistance moment balances Fig. 1. SEM images of nanoparticle-coated model toner particles on silicon substrates with a 10 nm gold layer coating for charge dissipation during SEM observation: (a) 10%; (b) 50%; and (c) 100% specified SAC levels (side view at 10,000 magnification). recording and voltage increase). The AFM cantilever served as the force-sensing element, and the applied lateral pushing force (F) was calculated from the relative cantilever deflection using the bending stiffness constant of the cantilever beam. The relative deflection of the AFM cantilever beam at each pushing step was obtained with a piezoelectric bender response calibration procedure described in detail elsewhere [32,33]; and the stiffness constant of the AFM cantilever beam was calibrated in the ambient prior to the test with a resonance method developed by Sader et al. [34] The lateral displacement of the particle (Δx) was obtained from the processing of a set of recorded images by tracking the pixel positions of the left Fig. 3. Close-up view of the lateral pushing test configuration.

H. Zhang et al. / Powder Technology 208 (2011) 582 589 585 Fig. 4. (a) The lateral force vs. particle displacement curves of four representative uncoated EA toner particles with a nominal diameter of 6.0 μm; (b) the close-up of the initial portion of the curves. the applied pushing moment, the work of adhesion is directly proportional to slope of the force displacement curve, i.e., W A = k 6π : Note that no knowledge of the particle diameter is required for extracting the work of adhesion between the particle and substrate using this approach. ð3þ Fig. 6. (a) The lateral force vs. particle displacement curves for four representative surface-coated EA toner particles (6.0 μm nominal diameter, 10% specified SAC); (b) A close-up of the initial portions of the curves. 4. Results and discussions The lateral pushing experimental procedure detailed above was applied to the uncoated base model toner particles and the coated model toner particles with three levels of specified SACs. The representative force displacement relationships of the uncoated particles (0% SAC) are presented in Fig. 4. Similar force displacement relationships of a set of the particles with a specified SAC of 10% are presented in Figs. 5 and 6, and those of the particles with 50% and 100% nominal SAC are shown in Figs. 7 and 8, respectively. As depicted Fig. 5. (a) The lateral force vs. particle displacement curves for four representative surface-coated EA toner particles (6.0 μm nominal diameter, 10% specified SAC); (b) A close-up of the initial portions of the curves. Fig. 7. (a) The lateral force vs. particle displacement curves of four representative surface-coated EA toner particles (6.0 μm nominal diameter, 50% specified SAC); (b) A close-up of the initial portions of the curves.

586 H. Zhang et al. / Powder Technology 208 (2011) 582 589 Fig. 8. (a) The lateral force vs. particle displacement curves of four representative surface-coated EA toner particles (6.0 μm nominal diameter, 100% specified SAC); (b) A close-up of the initial portions of the curves. in these graphs, while the displacements of the particles are kept increasing with increasing lateral pushing force, the slopes of the force displacement curves decrease significantly after the first few pushing steps. Such behavior is consistent with our previously reported observations [25,26]. Based on this data, we believe that at the first few loading steps there exist a rolling resistance moment against the (free-) rolling initiation of the particle, and the initial prerolling displacement of the particle is due to the pre-rolling motion of the particle. Following the first few pushing steps, the slope decreases significantly, indicating that the adhesion bond between the particle and the substrate is broken and the particle begins rolling (possibly without slip) or sliding on the substrate. The work of adhesion between the polymer particle and silicon substrate is calculated from the pre-rolling slope of each force displacement curve by fitting a least-square linear trendline of the pre-rolling section. The experimentally determined work of adhesion values for the particles of same nominal diameters with two different specified SAC levels (i.e., 50% and 100%) are summarized in Table 1. The average work of adhesion values between the surface-coated polymer particles with 50% and 100% SAC levels and the silicon substrate are approximated as 2.8 mj/m 2 and 1.1 mj/m 2, respectively. Such values are almost an order of magnitude lower than that between the uncoated base toner particle and silicon substrate (~23 mj/m 2 ) [26]. A one-sided Student's t test at α=0.05 level suggests that the work of adhesion data for 50% SAC particles are significantly larger than the data for 100% SAC particles. For the 10% SAC toner particles, it is observed that the distribution of measured work of adhesion values is broad (Table 2). The distribution of the experimentally determined work of adhesion values for toner particles with four levels of SAC are presented in Fig. 9. Firstly, the results indicate that, even with the base toner particles, there exists a wide distribution of van der Waals adhesion. One possible contribution to the wide distribution of experimentally determined work of adhesion data is the pushing test technique. We have previously performed repeated pushing tests on the same particle for five poly (N-vinyl-2-pyrrolidone) (PVP) particles, and observed that the standard deviation of the results from the same particle is much smaller than the standard deviation of the results from a group of five particles. Therefore, the measurement technique is unlikely to be the major cause of the wide distribution of the work of adhesion data. Since EA base toner particle consists of a polyester resin, cyan pigment and wax and the estimated contact radius between a particle and substrate is only 100 300 nm, the composite nature of the EA base toner particle and the local structure and roughness of its surface may contribute significantly to the distribution of the experimental results. Other researchers have also observed distribution of the measured adhesion for uncoated toner particles [4,6]. For instance, Mize et al. simulated the adhesion between a smooth toner particle and a carrier bead and between a rough toner particle and a carrier bead for a large number of contacts at various orientations and reported histograms of simulated adhesion [6]. When compared to the expected adhesion between a perfect sphere and a carrier bead, the simulated adhesion for smooth toner particles showed some degree of deviation, while that of rough toner particle shows a much larger deviation. Moreover, because of the small number of experimental data points, the outlier data points also contribute to the wide distribution of adhesion. As shown in Tables 1 and 2, the outlier data point (12 mj/m 2 ) in 50% SAC data set and the two outlier data points (95.5 mj/m 2 ) in 10% SAC data set significantly skew the mean and standard deviation of the work of adhesion results. It is concluded that the silica additive substantially reduces the adhesion between the toner particle and the silicon substrate due to its additive spacer effect. At 100% SAC, the base toner particle is almost fully covered with silica additive. Therefore, Table 1 Summary of the experimental results of the surface-coated toner particles with 50% and 100% specified SAC levels on the silicon substrate. Particle type 50% SAC coated 100% SAC coated a b Particle # Outlier data point. Outlier data point included. Particle diameter (μm) Average particle diameter (μm) Pre-rolling stiffness (N/m) Average pre-rolling stiffness (N/m) Work of adhesion (mj/m 2 ) Average work of adhesion (mj/m 2 ) 12 a 2.8±1.87 1 6.1 6.0±0.3 0.23 a 0.052±0.035 2 6.1 0.044 (0.075 ±0.070) b 2.3 (4.0 ±3.7) b 3 5.6 0.066 3.5 4 6.0 0.066 3.5 5 6.3 0.022 1.2 6 5.7 0.024 1.3 7 6.2 0.027 1.4 8 5.6 0.12 6.4 1 6.3 6.3±0.5 0.018 0.020±0.015 0.96 1.1±0.78 2 6.2 0.018 0.96 3 5.8 0.030 1.6 4 5.8 0.045 2.4 5 7.1 0.007 0.37 6 6.5 0.0065 0.35

H. Zhang et al. / Powder Technology 208 (2011) 582 589 587 Table 2 Summary of the experimental results of the base and surface-coated toner particles with 10% specified SAC on the silicon substrate. Particle type 0% SAC uncoated 10% SAC coated 10% SAC coated a b Particle # Particle diameter (μm) Average particle diameter (μm) Pre-rolling stiffness (N/m) Average pre-rolling stiffness (N/m) Work of adhesion (mj/m 2 ) Average work of adhesion (mj/m 2 ) 1 6.7 6.0±0.4 0.586 0.429±0.171 31.1 22.8 ±9.1 2 5.8 0.512 27.2 3 6.0 0.368 19.5 4 6.4 0.368 19.5 5 5.7 0.172 9.1 6 5.4 0.702 37.3 7 6.2 0.269 14.3 8 5.7 0.455 24.2 1 6.7 7.3±0.6 0.80 0.464±0.258 (0.75±0.68) b 42.5 25.3 ±13.0 (40 ±36) b 2 7.2 0.49 26.0 3 7.2 0.59 31.0 4 6.8 0.24 12.7 5 7.5 0.37 19.6 6 6.6 1.8 a 95.5 a 7 8.3 1.8 a 95.5 a 8 7.1 0.91 48.3 9 7.6 0.22 11.7 10 6.7 0.39 20.7 11 8.2 0.29 15.4 1 6.5 7.4±0.7 0.074 0.095±0.031 3.9 5.0±1.7 2 8.5 0.068 3.6 3 6.6 0.097 5.1 4 7.9 0.059 3.1 5 7.4 0.12 6.4 6 7.1 0.15 8.0 7 7.5 0.095 5.0 Outlier data points. Outlier data points included. the adhesion between the 100% SAC toner particle and the silicon substrate is low and its statistical distribution becomes considerably narrower. At 10% SAC, the adhesion ranges from low due possibly to the additive spacer and to very high due to the incomplete surface coverage that allows direct contact of microparticle with the substrate. As depicted in Table 2, the experimentally determined work of adhesion results for 10% SAC particles can be divided into two groups: one in the range of 12 48 mj/m 2 (excluding two outlier data points) that are close to those of the base toner particles (with a SAC of 0%), and the other in the range of 3.1 8.0 mj/m 2 that are close to those for the 50% SAC toner particles. Statistical analysis is performed on the experimentally determined work of adhesion data of the 10% SAC Fig. 9. Summary of the distribution of experimentally determined work of adhesion values of the uncoated and three types of surface-coated toner particles. toner particles, the base toner particles and the 50% SAC toner particles. For the work of adhesion data of the two groups of 10% SAC toner particles presented in Table 2, a one-sided Student's t test at α=0.05 level suggests that the first data group are significantly larger than the second data group. For the work of adhesion data of the first group of 10% SAC toner particles (12 48 mj/m 2 ) and the base toner particles (9.1 37.3 mj/m 2 ), a two-sided Student's t test at α=0.10 level reveals no significant difference between these two set of data. For the work of adhesion data of the second group of 10% SAC toner particles (3.1 8.0 mj/m 2 ), a one-sided Student's t test at α=0.05 level suggests that they are significantly larger than the data of 50% SAC toner particles (1.2 6.4 mj/m 2 ). We hypothesize that such bi-normal distribution of the experimentally determined work of adhesion data of 10% SAC toner particles is due to the nature of particle substrate contact conditions. As can be seen from the SEM image of the 10% SAC particle (Fig. 1a), the nanoparticle coverage on this type of toner particle is low, and the characteristic distance between the particle groups is well above the contact diameter. Therefore, when a 10% SAC polymer particle is deposited onto a flat substrate, there could be two possible modes of contact (Fig. 10): (1) the microparticle surface in direct contact with the substrate with no or insignificant amount of nanoparticles in the contact area; and (2) a substantial number of coating nanoparticles on the microparticle surface arrested in the contact area. For toner particles with higher SAC levels such as 50% and 100%, the contact mode 1 is unlikely to occur due to the relatively high spatial nanoparticle density on the toner particle surface. Note that for mode 2 contact, the actual contact is between the nanoparticles and the substrate. Therefore, strictly speaking, the rolling-resistance model (Fig. 2) is not directly applicable for such analysis since it is based on the smooth particle substrate contact. Here we consider work of adhesion values for those surface-coated toner particles determined with the rolling resistance moment-based technique as the effective work of adhesion, which allows us to directly compare the adhesion performance of the same type of microparticles with different levels of nanoparticle coating. Such information is valuable for the toner design for optimizing its adhesion and printing

588 H. Zhang et al. / Powder Technology 208 (2011) 582 589 a b Silica Nanoparticle Toner Surface Substrate Silica Nanoparticle Toner Surface Substrate The dependence of the adhesion properties of surface-coated polymer microparticles on the nanoparticle SAC levels was systematically studied with a recently developed rolling resistance momentbased adhesion characterization technique in the ambient environment. Uncoated base polymer particles (0% SAC) and surface-coated polymer particles with three different SAC levels (i.e., 10%, 50% and 100%) were studied, and the corresponding particle substrate adhesion properties were determined. Nanoparticle coating was found to significantly reduce the microparticle substrate adhesion. According to the study, the work of adhesion between base toner particles (0% SAC) and silicon substrates is around 23 mj/m 2. The work of adhesion values between toner particles with 50% and 100% specified SAC levels and silicon substrates are around 2.8 mj/m 2 and 1.1 mj/m 2, respectively. For the 10% SAC toner particles, the distribution of the experimentally determined work of adhesion values have two peaks, one close to that of the uncoated particles and the other close to that of the 50% SAC particles. Two possible modes of contact are proposed for the nanoparticle-coated polymer microparticles to explain the bi-normal distribution of the experimentally determined work of adhesion data of the 10% SAC toner particles: (1) the microparticle in direct contact with the substrate (with no interfering nanoparticles in the contact area); and (2) the nanoparticle coating arrested in the contact area with the substrate. For toner particles with a low SAC (i.e., 10%), the experimental data with binormal adhesion property distribution indicate that both contact modes are simultaneously present. For toner particles with high SAC levels (e.g., 50% and 100%), the contact mode 2 appears to be dominant. The controllable reduction of particle substrate adhesion with nanoparticle surface coating provides a mean to optimize the toner design to achieve higher transfer efficiency and improved removability as well as more predictable printing performance. Compared with other particle adhesion characterization techniques, the current method has several advantages: (i) it provides the adhesion property of individual particles rather than the average properties of a group of particles; (ii) it requires no permanent attachment of a particle to the tip of a probe and is thus nondestructive to the particle; and (iii) no particle diameter information is required for determining the work of adhesion between the particle and the substrate. Acknowledgements Authors thank to Dr. Santokh S. Badesha and Dr. Grazyna Kmiecik- Lawrynowicz of Xerox Corporation for fruitful discussions and sample toners, and Dr. Feng Yang of West Virginia University for helpful discussion on statistical analysis of the experimental data. This research project was partially supported by grants from Xerox Corporation (Xerox University Affairs Committee Award, 2007-2010) and the New York State Energy Research and Development Authority (NYSERDA) for the Nanomechanics and Nanomaterials Laboratory at Clarkson University. The authors also acknowledge the New York State Foundation for Science, Technology and Innovation (NYSTAR) and the Center for Advanced Materials Processing (CAMP) at Clarkson University for their partial financial supports. Fig. 10. Schematics of two possible contact configurations between a surface-coated microparticle with low SAC level and a flat substrate: (a) microparticle in direct contact with the substrate; (b) nanoparticle coating arrested in the contact area. performance. 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