Effect Of Nanoscale Roughness Research Articles

Effect of nanoscale roughness on four different atomic force microscopy probes in aqueous solutions using adhesion force measurement

Despite substantial theoretical approaches explaining the effect of nanoscale roughness (NR) on particle deposition, experiments are rarely performed. Fabricating the sophisticated rough surfaces and obtaining the experimental data for interaction between the colloids and surfaces are crucial to understand colloidal adsorption onto the rough surfaces. We investigated the adhesion of different particle types of atomic force microscope (AFM) probes (different shapes, i.e., sphere and plateau; diameters, i.e., 2 μm–15 μm) onto smooth and rough fabricated surfaces. We successfully fabricated well-organized rough surfaces on Silicon (Si) using colloidal lithography and metal-assisted chemical etching using a cylindrical shape of constant nanoscale height and diameter. The properties of roughness (i.e., height, diameter, and fraction) were quantitatively analyzed, and the corresponding. The force-distance curves measured with the AFM revealed that, for the four types of probes, contact area varied with the surface roughness, and their adhesion forces differed accordingly. Expanding the contact area tended to increase the repulsive force possibly because of the hydration force. This study effectively devised a simplified experimental system that explains the influence of NR on particle deposition using previous theoretical data. The findings should be considered in designing NR for subsequent colloidal deposition.

Virus-like Silica Nanoparticles Improve Permeability of Macromolecules across the Blood-Brain Barrier In Vitro.

The presence of the blood-brain barrier (BBB) limits the delivery of therapies into the brain. There has been significant interest in overcoming the BBB for the effective delivery of therapies to the brain. Inorganic nanomaterials, especially silica nanoparticles with varying surface chemistry and surface topology, have been recently used as permeation enhancers for oral protein delivery. In this context, nanoparticles with varying sizes and surface chemistries have been employed to overcome this barrier; however, there is no report examining the effect of nanoscale roughness on BBB permeability. This paper reports the influence of nanoscale surface roughness on the integrity and permeability of the BBB in vitro, using smooth surface Stöber silica nanoparticles (60 nm) compared to rough surface virus-like silica nanoparticles (VSNP, 60 nm). Our findings reveal that VSNP (1 mg/mL) with virus-mimicking-topology spiky surface have a greater effect on transiently opening endothelial tight junctions of the BBB than the same dose of Stöber silica nanoparticles (1 mg/mL) by increasing the FITC-Dextran (70 kDa) permeability 1.9-fold and by decreasing the trans-endothelial electrical resistance (TEER) by 2.7-fold. This proof-of-concept research paves the way for future studies to develop next-generation tailored surface-modified silica nanoparticles, enabling safe and efficient macromolecule transport across the BBB.

Multiscale models of plasmonic structural colors with nanoscale surface roughness.

Plasmonic coloration arises from resonant interaction between visible light and metallic nanostructures, which causes wavelength-selective absorption or scattering of light. This effect is sensitive to surface roughness that can perturb these resonant interactions and cause observed coloration to deviate from coloration predicted by simulations. We present a computational visualization approach that incorporates electrodynamic simulations and physically based rendering (PBR) to investigate the effect of nanoscale roughness on the structural coloration from thin, planar silver films decorated with nanohole arrays. Nanoscale roughness is modeled mathematically by a surface correlation function and parameterized in terms of roughness that is either out of or into the plane of the film. Our results provide photorealistic visualization of the influence of nanoscale roughness on the coloration from silver nanohole arrays in both reflectance and transmittance. Out-of-plane roughness has a significantly greater effect on coloration than in-plane roughness. The methodology introduced in this work is useful for modeling artificial coloration phenomena.

The Effect of Surface Nano and Microstructures on Hydrogen Gas Evolution Behavior on Ni Patterned Electrodes

Highly efficient water electrolysis process is one of the key technologies for hydrogen economy suppressing global warming. Now, alkaline water electrolysis (AWE) and Proton exchange membrane (PEM) electrolysis are regarded as the representative industrial process. The advanced electrode without precious metal or oxide catalysts for hydrogen evolution reaction (HER) in these processes plays an important role to design HER efficiently and economically. Tremendous efforts focusing to unique characteristic catalysts exploration have been engaged world-widely not only tediously and steadily but also with materials informatic-science utilization driven by AI technology. Roughly speaking based on electrochemical engineering aspects, the energy loss in water electrolysis is composed of electrode reaction kinetics governed by catalytic activity, concentration overpotential and ohmic drop in gas-liquid two phase dispersion layer. The concentration overpotential and ohmic resistance are sometimes result in significant energy loss in large scale industrial process. From the sustainable and economical views toward Hydrogen Economy Society, AWE with less precious catalysts is considered as hydrogen production candidate with cheaper cost and longer durability. Many papers on gas electrode behavior have focused on macroscopic observation, but more fundamental understanding on nucleation and growth behavior of gas bubbles is rather limited due to the experimental difficulties. It is believed that the electrode surface structure tailoring may also provide one of promising methods to improve the catalytic performance as well as advance catalytic HER activity. Especially, nano/micro-scale structures on the electrode surface are suggested to affect the catalyst performance by controlling the nucleation and growth behaviors of bubbles. However, the detailed mechanisms of the effect of surface nano/micro-scale structures on HER and bubble behaviors have not been elucidated. Therefore, to analyze the influence of microstructure and surface wettability of catalytic electrode on bubble behaviors and HER, this study focuses on bubble behaviors on the electrode surface using Ni micro-patterned electrode fabricated by electrodeposition and Pt single crystalline electrodes. Ni micro-patterned electrodes were electrodeposited in the electrolyte containing 0.05 mol dm-3 NiCl2∙6H2O (pH 2.5) on patterned Cu substrate. Pt and Ag/AgCl electrode were employed as CE and RE, respectively. Surface structures and wettability of prepared electrodes were evaluated by scanning electron microscopy (SEM) and contact angle measurement. Water electrolysis was performed in 1 M KOH and Pt mesh and Hg/HgO. The effect of surface roughness on bubble behaviors was additionally examined in 0.1 M H2SO4 with three electrode system; Pt single crystalline electrode prepared by the Clavier method as WE, carbon rod as CE, and reversible hydrogen electrode as RE, respectively. Microdot structures, cylinder-, semisphere- and square-like were prepared. Ni coverage ratio of Ni electrodeposited area to Cu substrate, was calculated to correlate surface wettability measured by sessile drop method and Ni microstructure. It was suggested that electrode surface wettability could be controllable by changing Ni coverage and each Ni microdot structure exhibited an apparent catalytic electrode by surface tailoring. The effect of microstructures and surface wettability on HER performance was then studied during galvanostatic electrolysis at -20 mA cm-2 for 30 min. Total overpotential measured from 200 s to 1800 s seemed to proceed water electrolysis stably. It increased with decreasing surface wettability. This may indicate lower surface wettability derived from Ni microarray structures increases overpotential by accumulating newly evolving H2 bubbles. To further analyze the phenomena, high-speed CCD camera cinematography was applied. The ratio of larger diameter bubble tended to increase with increasing the contact angle. It indicates lower surface wettability would enhance bubble growth behaviors on cathode surface. Moreover, larger diameter bubbles were generated on cylinder-like Ni microdots. These results suggest increasing contact area between bubbles and Ni micropatterned electrode would enhance bubble growth behaviors. The effect of Ni microdot height on bubble nucleation behaviors was also observed. It apparently demonstrated that taller Ni dot height rather increased the nucleation site of bubbles. In order to additionally investigate the effect of nano-scale roughness tailored by single crystalline growth technique on bubble behaviors, H2 bubble evolution behaviors on Pt single crystalline electrode were cinematographically recorded. No H2 bubbles were generated on smooth surface fabricated by Pt (111) microfacet, suggesting lower nano-scale surface roughness considerably suppressed bubble nucleation on Pt electrode surface. These results suggested micro and nano structures and surface wettability influenced on bubble nucleation and growth phenomena frequently encountered in industrial electrolysis. Further unknown room still remains to be challenged even in such a rather matured AWE. This work was partly carried out in Case Western Reserve University. Prof. Daniel Scherson’s supervisions are highly appreciated.

Effects of Nanoscale Roughness on the Lubricious Behavior of an Ionic Liquid

AbstractWhile many mechanistic studies have focused on the lubricious properties of ionic liquids (ILs) on ideally smooth surfaces, little is known about the mechanisms by which ILs lubricate contacts with nanoscale roughness. Here, substrates with controlled density of nanoparticles are prepared to examine the influence of nanoscale roughness on the lubrication by 1‐hexyl‐3‐methyl imidazolium bis(trifluoromethylsulfonyl)imide. Atomic force microscopy is employed to investigate adhesion, hydrodynamic slip, and friction at the lubricated contact as a function of surface topography for the first time. This study reveals that nanoscale roughness has a significant influence on the slip along the surface and leads to a maximum slip length on the substrates with intermediate nanoparticle density. This coincides with the minimum friction coefficient at sufficiently small contact stresses, likely due to the lower resistance of the IL film to shear. However, at the higher pressures applied with a sharp tip, friction increases with nanoparticle density, indicating that the IL is not able to alleviate the increased dissipation due to roughness. The results of this work point toward a complex influence of the surface topology on friction. This study can help design ILs and nanopatterned substrates for tribological applications and nano‐ and microfluidics.

Effect of Nanoscale Roughness on Adhesion between Glassy Silica and Polyimides: A Molecular Dynamics Study

The effect of nanoscale roughness on the adhesion between glassy silica and polyimides is examined by molecular dynamics simulation. Different silica surfaces, with varying degrees of roughness, were generated by cleaving bulk structures with a predefined surface and a desired average roughness, with different roughness periods and hydroxylation densities in an effort to study the influence of these surface characteristics on adhesion at the silica–polyimide interface. The calculated results reveal that average roughness Ra is the primary controlling factor within the considered conditions. Further, an energy decomposition analysis of the pulling process suggests that hydrogen bonding contributes to the adhesion on all the rough surfaces, while the Coulombic energy contribution becomes significant at higher Ra. From a structural analysis of the vacant volume and surface area, it is shown that the periodicity of roughness provides a rather interesting trend for the adhesion energy. Adhesion can increase wi...

Near-field surface plasmon field enhancement induced by rippled surfaces.

The occurrence of plasmon resonances on metallic nanometer-scale structures is an intrinsically nanoscale phenomenon, given that the two resonance conditions (i.e., negative dielectric permittivity and large free-space wavelength in comparison with system dimensions) are realized at the same time on the nanoscale. Resonances on surface metallic nanostructures are often experimentally found by probing the structures under investigation with radiation of various frequencies following a trial-and-error method. A general technique for the tuning of these resonances is highly desirable. In this paper we address the issue of the role of local surface patterns in the tuning of these resonances as a function of wavelength and electric field polarization. The effect of nanoscale roughness on the surface plasmon polaritons of randomly patterned gold films is numerically investigated. The field enhancement and relation to specific roughness patterns is analyzed, producing many different realizations of rippled surfaces. We demonstrate that irregular patterns act as metal–dielectric–metal local nanogaps (cavities) for the resonant plasmonic field. In turn, the numerical results are compared to experimental data obtained via aperture scanning near-field optical microscopy.

Non-linear, non-monotonic effect of nano-scale roughness on particle deposition in absence of an energy barrier: Experiments and modeling.

Deposition of colloidal- and nano-scale particles on surfaces is critical to numerous natural and engineered environmental, health, and industrial applications ranging from drinking water treatment to semi-conductor manufacturing. Nano-scale surface roughness-induced hydrodynamic impacts on particle deposition were evaluated in the absence of an energy barrier to deposition in a parallel plate system. A non-linear, non-monotonic relationship between deposition surface roughness and particle deposition flux was observed and a critical roughness size associated with minimum deposition flux or “sag effect” was identified. This effect was more significant for nanoparticles (<1 μm) than for colloids and was numerically simulated using a Convective-Diffusion model and experimentally validated. Inclusion of flow field and hydrodynamic retardation effects explained particle deposition profiles better than when only the Derjaguin-Landau-Verwey-Overbeek (DLVO) force was considered. This work provides 1) a first comprehensive framework for describing the hydrodynamic impacts of nano-scale surface roughness on particle deposition by unifying hydrodynamic forces (using the most current approaches for describing flow field profiles and hydrodynamic retardation effects) with appropriately modified expressions for DLVO interaction energies, and gravity forces in one model and 2) a foundation for further describing the impacts of more complicated scales of deposition surface roughness on particle deposition.

Early osteoblast responses to orthopedic implants: Synergy of surface roughness and chemistry of bioactive ceramic coating.

Pro-osteogenic stimulation of bone cells by bioactive ceramic-coated orthopedic implants is influenced by both surface roughness and material chemistry; however, their concomitant impact on osteoblast behavior is not well understood. The aim of this study is to investigate the effects of nano-scale roughness and chemistry of bioactive silica-calcium phosphate nanocomposite (SCPC50) coated Ti-6Al-4V on modulating early bone cell responses. Cell attachment was higher on SCPC50-coated substrates compared to the uncoated controls; however, cells on the uncoated substrate exhibited greater spreading and superior quality of F-actin filaments than cells on the SCPC50-coated substrates. The poor F-actin filament organization on SCPC50-coated substrates is thought to be due to the enhanced calcium uptake by the ceramic surface. Dissolution analyses showed that an increase in surface roughness was accompanied by increased calcium uptake, and increased phosphorous and silicon release, all of which appear to interfere with F-actin assembly and osteoblast morphology. Moreover, cell attachment onto the SCPC50-coated substrates correlated with the known adsorption of fibronectin, and was independent of surface roughness. High-throughput genome sequencing showed enhanced expression of extracellular matrix and cell differentiation related genes. These results demonstrate a synergistic relationship between bioactive ceramic coating roughness and material chemistry resulting in a phenotype that leads to early osteoblast differentiation.

Selected papers from the third European Conference on Microfluidics: µFlu’12

The third European Conference on Microfluidics (lFlu’12) was held in Heidelberg in December 2012, under the sponsorship of the Hydrotechnic Society of France (SHF), the Alma Mater Studiorum University of Bologna, the University of Toulouse and the Karlsruhe Institute of Technology. This event has gathered 252 participants from 33 countries. During 3 days, 174 communications have been presented in 16 thematic sessions including Liquid Microflows, Gas Microflows, Microsensors and In-Situ Analytics, Fluidic Microactuators and Micromixing, Microfabrication Techniques for Microfluidic Systems, Convective Micro Heat Transfer, Micro Chemical Engineering, Lab-on-a-Chip, Electrokinetic Microflows, Microdroplets Management, Microflows in Biological Systems and Bioengineering, Multi-Phase Flows in Microsystems, Microflow Visualization, and other topics. Two special sessions have been devoted to Industrial Applications of Microfluidics and to Nanofluidics. The multi-disciplinarity of Microfluidics is highlighted by the variety of works presented in this Conference which confirms also that Microfluidics nowadays finds applications in every industrial sector like biology, medicine, chemical and process engineering, transports, environment, microelectronics. This year the Scientific Committee has selected four papers presented to the Conference for publication in Microfluidics and Nanofluidics; the common topic of these selected papers is the detailed analysis of the interactions at micrometric scale between the fluid and the solid walls of a microdevice by highlighting what happens when the solid surface either presents microgrooves, or gathers electrical charges, or has been submitted to a surface chemical treatment or is simply oscillating. More in detail, the paper of Gopalan and Kandlikar is devoted to the experimental analysis of the wettability of micro-patterned surfaces. Their results show that the wettability is strongly influenced by the spacing factor of a grooved surface, defined as the ratio between the channel depth and the channel width: The spacing factor influences the contact angle, the contact angle hysteresis, and the transition characteristics between the Cassie and Wenzel states. The authors demonstrate that their experimental results can be useful for the design of the gas channels of a Proton Exchange Membrane Fuel Cell (PEMFC) in order to optimize the water drainage. The paper presented by Gentili et al. is focused on the analysis of the combined effect of nanoscale roughness and surface chemistry modification in order to achieve large liquid slippage due to entrapment of air pockets in the surface asperities. In the paper, a numerical simulation campaign based on full atoms Molecular Dynamics aimed at reproducing a realistic superhydrophobic system comprising water, solid walls, and hydrophobic coating with octadecyltrichlorosilane (OTS) is described and compared with the results obtained by following a continuum S. Colin INSA, Institut Clement Ader (ICA), Universite de Toulouse, 135, Avenue de Rangueil, 31077 Toulouse, France e-mail: stephane.colin@insa-toulouse.fr

Facile fabrication of superhydrophobic poly(methyl methacrylate) substrates using ultrasonic imprinting

Superhydrophobic surfaces (SHSs) have received increasing attention in the last decade, and have been generally developed from hydrophobic materials. In this study, a facile fabrication method based on ultrasonic imprinting is proposed to develop SHSs from a hydrophilic polymer, poly(methyl methacrylate) (PMMA). To fabricate SHSs on PMMA substrates, micro electrical discharge machining, micromachining and ultrasonic imprinting were sequentially used. The ultrasonic imprinting was performed for various channel designs and imprinting conditions, and the resulting water contact angles were measured for the replicated samples. As a result, superhydrophobic characteristics could be obtained on a hydrophilic PMMA replica without any chemical treatments. The effects of nanoscale roughness on the replicated channel as well as composition change are discussed with respect to the analyses using an atomic force microscope, x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.

Using mathematical models to understand the effect of nanoscale roughness on protein adsorption for improving medical devices

Surface roughness and energy significantly influence protein adsorption on to biomaterials, which, in turn, controls select cellular adhesion to determine the success and longevity of an implant. To understand these relationships at a fundamental level, a model was originally proposed by Khang et al to correlate nanoscale surface properties (specifically, nanoscale roughness and energy) to protein adsorption, which explained the greater cellular responses on nanostructured surfaces commonly reported in the literature today. To test this model for different surfaces from what was previously used to develop that model, in this study we synthesized highly ordered poly(lactic-co-glycolic acid) surfaces of identical chemistry but altered nanoscale surface roughness and energy using poly(dimethylsiloxane) molds of polystyrene beads. Fibronectin and collagen type IV adsorption studies showed a linear adsorption behavior as the surface nanoroughness increased. This supported the general trends observed by Khang et al. However, when fitting such data to the mathematical model established by Khang et al, a strong correlation did not result. Thus, this study demonstrated that the equation proposed by Khang et al to predict protein adsorption should be modified to accommodate for additional nanoscale surface property contributions (ie, surface charge) to make the model more accurate. In summary, results from this study provided an important step in developing future mathematical models that can correlate surface properties (such as nanoscale roughness and surface energy) to initial protein adsorption events important to promote select cellular adhesion. These criteria are critical for the fundamental understanding of the now well-documented increased tissue growth on nanoscale materials.

Nanoscale surface roughness affects low Reynolds number flow: Experiments and modeling

Most micro-channel fabrication strategies generate nano-to-micro-scale, stochastic surface roughness. This inherent stochasticity can potentially be harnessed to direct microfluidic operations such as self-cleaning behavior and localized mixing. This work investigates the effect of stochastic nanoscale roughness on low to moderate Reynolds number Newtonian flow using concurrent modeling and experiments. We fabricate a microscopic channel with tailored hydrofluoric-acid-etched rough surfaces. Optical profilometry and micro-particle-image-velocimetry (micro-PIV) are used to characterize the surface roughness and flow field and is integrated with direct numerical simulation that resolves effects of nanoscale roughness. Results indicate that nanoscale roughness causes flow perturbations that extend up to the mid-plane and is insensitive to flow-rates.

Quantized thermal transport across contacts of rough surfaces

Heat transport across interfaces is often discussed in terms of the transmission probability of the heat-carrying phonons through the contact zone. Although interface roughness influences the true contact area and affects phonon scattering within the contact zone, its effect on nanoscale heat transport remains poorly understood. Here, we report experimental data on the pressure dependence of thermal transport across polished nanoscale contacts. The data can be quantitatively explained by a model of thermal conductance across interfaces that incorporates the effect of nanoscale roughness through the quantized thermal conductance across individual atomic-scale contacts within the contact zone.

How Chemistry, Nanoscale Roughness, and the Direction of Heat Flow Affect Thermal Conductance of Solid–Water Interfaces

We quantify the Kapitza thermal conductance of solid–liquid interfaces between self-assembled monolayers (SAMs) and liquid water using nonequilibrium molecular dynamics simulations. We focus on understanding how surface chemistry, nanoscale roughness, and the direction of heat flow affect interfacial thermal conductance. In agreement with calculations by Shenogina et al. (Phys. Rev. Lett., 2009, 102, 156101) for SAMs with homogeneous headgroup chemistries, we find that for mixed −CF3/–OH SAMs, thermal conductance increases roughly linearly with the fraction of −OH groups on the surface. Increasing nanoscale roughness increases solid–water contact area, and therefore the apparent thermal conductance. However, the inherent thermal conductance, which accounts for the increased contact area, shows only small and subtle variations. These variations are consistent with expectations based on recent work on the effects of nanoscale roughness on interfacial tension (Mittal and Hummer, Faraday Disc., 2010, 146, 341). Finally, we find that SAM–water interfaces show thermal rectification. Thermal conductance is larger when heat flows from the ordered SAM phase to the disordered liquid water phase, and the magnitude of rectification increases with surface hydrophilicity.

The effects of combined micron-/submicron-scale surface roughness and nanoscale features on cell proliferation and differentiation

Titanium (Ti) osseointegration is critical for the success of dental and orthopedic implants. Previous studies have shown that surface roughness at the micro- and submicro-scales promotes osseointegration by enhancing osteoblast differentiation and local factor production. Only relatively recently have the effects of nanoscale roughness on cell response been considered. The aim of the present study was to develop a simple and scalable surface modification treatment that introduces nanoscale features to the surfaces of Ti substrates without greatly affecting other surface features, and to determine the effects of such superimposed nano-features on the differentiation and local factor production of osteoblasts. A simple oxidation treatment was developed for generating controlled nanoscale topographies on Ti surfaces, while retaining the starting micro-/submicro-scale roughness. Such nano-modified surfaces also possessed similar elemental compositions, and exhibited similar contact angles, as the original surfaces, but possessed a different surface crystal structure. MG63 cells were seeded on machined (PT), nano-modified PT (NMPT), sandblasted/acid-etched (SLA), and nano-modified SLA (NMSLA) Ti disks. The results suggested that the introduction of such nanoscale structures in combination with micro-/submicro-scale roughness improves osteoblast differentiation and local factor production, which, in turn, indicates the potential for improved implant osseointegration in vivo.

The influence of molecular-scale roughness on the surface spreading of an aqueous nanodrop

We examine the effect of nanoscale roughness on spreading and surface mobility of water nanodroplets. Using molecular dynamics, we consider model surfaces with sub-nanoscale asperities at varied surface coverage and with different distribution patterns. We test materials that are hydrophobic, and those that are hydrophilic in the absence of surface corrugations. Interestingly, on both types of surfaces, the introduction of surface asperities gives rise to a sharp increase in the apparent contact angle. The Cassie-Baxter equation is obeyed approximately on hydrophobic substrates, however, the increase in the contact angle on a hydrophilic surface differs qualitatively from the behavior on macroscopically rough surfaces described by the Wenzel equation. On the hydrophobic substrate, the superhydrophobic state with the maximal contact angle of 180 degrees is reached when the asperity coverage falls below 25%, suggesting that superhydrophobicity can also be achieved by the nanoscale roughness of a macroscopically smooth material. We further examine the effect of surface roughness on droplet mobility on the substrate. The apparent diffusion constant shows a dramatic slow down of the nanodroplet translation even for asperity coverage in the range of 1% for a hydrophilic surface, while droplets on corrugated hydrophobic surfaces retain the ability to flow around the asperities. In contrast, for smooth surfaces we find that the drop mobility on the hydrophilic surface exceeds that on the hydrophobic one.

Light scattering from spherical plasmonic nanoantennas: effects of nanoscale roughness

We have investigated the light scattering properties of smooth and roughened nanoshells with dipolar and quadrupolar plasmon resonances tuned to 830 nm. In the dipole resonant case small but measurable variations in the angle dependent light scattering (ADLS) due to the introduction of surface roughness are observed. In the quadrupole case, the distinctive side lobe scattering characteristic of quadrupolar emission is strongly quenched for roughened nanoshells.

The Effect of Nanoscale Roughness on Long Range Interaction Forces

A model was developed to calculate the long range van der Waals and electrostatic energies between a rough colloidal particle and a smooth solid plate in aqueous solutions. The particle roughness was modeled as hemispherical asperities distributed uniformly over the surface. Because of the assumption of additive potentials used in calculating the electrostatic force, the model is most accurate when the particle/plate separation is larger than several Debye screening lengths, such as near the location of the secondary minimum. The model predicts that such roughness reduces the depth of the secondary minimum and pushes it to larger separation distances. The model also predicts that the height of the primary energy barrier, which controls the dispersion stability, is substantially lowered by the asperities.