Article pubs.acs.org/Langmuir
Dynamic Adhesion Forces between Microparticles and Substrates in Water Quan Xu,†,‡ Mingtao Li,‡,∥ Lipeng Zhang,‡ Jianbing Niu,‡ and Zhenhai Xia*,‡,§ †
Institute of New Energy, China University of Petroleum (Beijing), Beijing 102249, P. R. China Department of Materials Science and Engineering and §Department of Chemistry, University of North Texas, Denton, Texas 76203, United States ∥ International Research Center for “Solar-Hydrogen” Renewable and Clean Energy, State key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China ‡
ABSTRACT: The interactions between micrometer-sized particles and substrates in aqueous environment are fundamental to numerous natural phenomena and industrial processes. Here we report a dynamically induced enhancement in adhesion interactions between microparticles and substrates immerged in water, air, and hexane. The dynamic adhesion force was measured by pulling microsized spheres off various substrate (hydrophilic/hydrophobic) surfaces at different retracting velocities. It was observed that when the pull-off velocity varies from 0.02 to 1500 μm/s, there is 100−200% increase in adhesion force in water while it has a 100% increase in nitrogen and hexane. The dynamic adhesion enhancement reduces with increasing effective contact angle defined by the average cosine of wetting angles of the substrates and the particles, and approaches the values measured in dry nitrogen and hexane as the effective contact angle is larger than 90o. A dynamic model was developed to predict the adhesion forces resulting from this dynamic effect, and the predictions correlate well with the experimental results. The stronger dynamic adhesion enhancement in water is mainly attributed to electrical double layers and the restructuring of water in the contact area between particles and substrates. substrates under dry and wet conditions,17−19 The interaction force between liquid droplets in water was shown to strongly depend on the velocity of droplet separation.20 Therefore, understanding of the dynamic interactions between the particle and substrate in water may advance a wide range of technologies including surface cleaning, paper making, printing and coating. In present work, we systemically study the dynamic adhesion of polystyrene (PS), SiO2, and Al2O3 microparticles on various hydrophilic/hydrophobic substrates in deionized water, dry nitrogen, and hexane. The adhesion was measured under different preloading forces, loading time, and loading velocity changing in a range of five orders of magnitude. It was found that the adhesion force was drastically increased by the retraction velocity. Factors such as applied load, contact time, and hydration force were evaluated. Models were developed to explain the dynamic effects observed in the experiment.
1. INTRODUCTION The interactions between microparticles and surfaces in aqueous environment are of great significance in a number of industrial processes such as paper coating,1,2 drug delivery,3 pharmacology, and chemical development.4 Extensive work has been done on the attachment/detachment of the particles from substrate in aqueous environments.5−7 Similar experiments have been performed on the adhesion forces between two polystyrene (PS) particles,8 two deformable oil droplets,9 two titania surfaces,10 an air bubble and a hydrophilic spherical particle,11 and nanobubbles on a solid surface12,13 in water and/ or electrolyte solutions. Solid surfaces in aqueous environment become charged due to the adsorption of ions or dissociation of surface groups, resulting in more complex interfacial interactions between particles and substrate surfaces. The adhesion forces are dominated by electrical double layers and van der Waals attraction as predicted by Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory.14 Most studies on the adhesion of small particles concern the static or quasi-static interactions. However, in industrial processes such as food,15 pharmaceutics,4 and centrifuge technology,16 the detachment process usually occurs within a microsecond or faster. The detachment forces may be strongly influenced by detachment velocity in these situations. Recent experiments have shown a dynamically induced enhancement in interfacial adhesion between microsized particles and © 2014 American Chemical Society
2. EXPERIMENTAL SECTION Microspheres (http://microspheres-nanospheres.com) with a diameter of 10 ± 0.25 μm made of PS, SiO2, and Al2O3 were carefully glued at the end of a tipless probe (ACTA-TL-50, AppleNano Inc.) by a 30 Received: July 10, 2014 Revised: August 25, 2014 Published: August 27, 2014 11103
dx.doi.org/10.1021/la502735w | Langmuir 2014, 30, 11103−11109
Langmuir
Article
Figure 1. (a) Typical pull-off curves for PS particle glued on AFM tip (inset, SEM image) approaching and retreating from fused silica substrate in water for a preload of 1 μN and a loading time of 10 s. The stiffness of the cantilever K is 35.2 N/m. (b) Schematic of AFM measuring system in aqueous environment.
Table 1. Surface Roughness and Contact Angles of Substrates material average roughness Ra (nm) contact angle (deg)a a
mica 0.43 ± 0.04 5° ( π/2), a cavity between particle and substrate will form as the particle approaches and contacts the substrate, as schematically shown in Figure 8b and d. Even though the Laplace pressure states that the pressure inside the nanobubbles should be too high for them to be stable, numerous studies have shown their existence as well as their apparent stability for hours33−37 Because of the existence of the cavity around the contact area, there is no water confined in the contact area and thus no icelike structures bridging the particle and substrate. Hence, the condition of surface contact in this case will resemble the surface contact in air. Consequently, the dynamic enhancement in adhesion force in water will be similar to that in air because of the similar contact condition and crack propagation. Other factors, such as van der Waals attraction and electrical double layers, may also enhance the dynamic interactions between the particles and substrates. The van der Waals attraction generates short-range attractive force that is determined by the Hamaker constant. The constants largely vary for these particle−substrate systems. For example, the Hamaker constant for the FS−SiO2 system in air is 6.5 × 10−20 J,38 over 3 times larger than that for the PS−PS system. However, the enhancement factors calculated based on the experiments in a dry nitrogen environment are nearly constant with the variation of 0.12, as shown in Figure 7. In an aqueous environment, the Hamaker constant for the FS−SiO2 system is 0.63 × 10‑20 J,39 about 2 times smaller than that for the PS−PS system, but the enhancement factor for the former system is larger than that for the latter system (Figure 7). Thus, the van der Waals attraction is not a major factor in determining the
It has been shown in Figure 5 that the dynamic effect is closely related to the surface properties of the substrate and particles. To reveal the effect of surface wetting ability of both particles and substrates on the dynamic effect, we define a parameter θ as an effective contact angle to represent hydrophobicity of the interface between a particle and a substrate as calculated below28 θ = a cos{[cos(θp) + cos(θs)]/2}
(2)
In this equation, θp and θs represent the contact angle of the particle and substrate, respectively. Here, we emphasize that θ does not represent the real contact angle of the system and is just a parameter related to wetting angles of substrate and particle although its unit is the same as that of substrate or particle. The adhesion force F1 at V = 156 μm/s, normalized by the adhesion force F0 at V = 0.02 μm/s, also called adhesion force enhancement factor, ϕ = F1/F0 − 1, is plotted against the effective contact angles for the experiments in water, nitrogen, and hexane, and shown in Figure 7. Although the data are somewhat scattering, basically, the enhancement factor ϕ measured in dry nitrogen keeps nearly constant, but in water it nearly linearly reduces with increasing θ and approaches the level of adhesion under the dry condition at θ > π/2. The adhesion force enhancement of the particles in water can be attributed to the restructuring of water in particle− substrate contact area. For hydrophilic surfaces and particles, there may be small pocks of water isolated by the roughness in the particle-substrate contact area, as schematically shown in Figure 8a and c. In this experiment, although the surface roughness is in the atomic level (
