Multiscale Roughness Research Articles

ZnO superhydrophobic coating via convenient spraying and its biofouling resistance

The attachment of microbial biomass on solid surfaces, also referred to as biofouling, is a universal phenomenon that occurs in natural and engineering systems. However, traditional antibiofouling surfaces based on either the release of biocidal compounds or surface chemical/physical design have some drawbacks, such as the high cost, the complicated process, the low accuracy, and the limitation to achieve coatings over large area. Herein, to overcome these problems, a superhydrophobic coating is fabricated via spraying the mixture of hydrophobized zinc oxide nanoparticles and epoxy resin. The zinc oxide nanoparticles form a multiscale roughness, and the epoxy resin promotes the robustness of the coating. The so‐formed superhydrophobic coating resists the attachment of protein, bacteria, and marine algae. It is expected that the so‐developed superhydrophobic coating can be applied in the fields of biomedical instruments, antimicrobial material, marine platform, and ships.

Combining TEM, AFM, and Profilometry for Quantitative Topography Characterization Across All Scales.

Surface roughness affects the functional properties of surfaces, including adhesion, friction, hydrophobicity, biological response, and electrical and thermal transport properties. However, experimental investigations to quantify these links are often inconclusive because surfaces are fractal-like, and the values of measured roughness parameters depend on measurement size. Here, we demonstrate the characterization of topography of an ultrananocrystalline diamond (UNCD) surface at the angstrom scale using transmission electron microscopy (TEM), as well as its combination with conventional techniques to achieve a comprehensive surface description spanning 8 orders of magnitude in size. We performed more than 100 individual measurements of the nanodiamond film using both TEM and conventional techniques (stylus profilometry and atomic force microscopy). While individual measurements of root-mean-square (RMS) height, RMS slope, and RMS curvature vary by orders of magnitude, we combine the various techniques using the power spectral density and use this to compute scale-independent parameters. This analysis reveals that "smooth" UNCD surfaces have an RMS slope greater than 1, even larger than the slope of the Austrian Alps when measured on the scale of a human step. This approach of comprehensive multiscale roughness characterization, measured with angstrom-scale detail, will enable the systematic evaluation and optimization of other technologically relevant surfaces, as well as systematic testing of the many analytical and numerical models for the behavior of rough surfaces.

Femtosecond laser-induced porosity on poly(ethylene) surfaces—A crystallographic and rheological study

In recent years, femtosecond (fs) laser irradiation of polymer surfaces has been shown to produce novel porous topologies that present a multi-scale roughness. In our study, grazing incidence x-ray diffractometry results showed the appearance of a monoclinic crystalline phase on fs laser machined high density polyethylene (HDPE) which is a deformation of the orthorhombic phase typically observed in pristine HDPE. This was accompanied by a local decrease in crystallinity. These findings confirmed that the induced porosity was a consequence of the rapid quenching of a superheated melt layer undergoing phase explosion. In addition, several poly(ethylene) samples with different average molecular weights were machined under the same conditions. Scanning electron micrography, along with small angle oscillatory shear tests, indicated that the final pore size decreases with increasing average molecular weight or increasing melt viscosity since these parameters act against bubble growth during phase explosion. In addition, through computed tomography, the internal structure of the porous layer was investigated. Through this technique, the average pore size and pore size distribution can be computed by chord length analysis. In conclusion, in this study, the mechanisms behind the formation of femtosecond laser-induced porous structures and the parameters that control their dimensions have been established.

Improved Multiscale Roughness Algorithm for Lunar Surface

The statistical analysis of surface roughness has been applied based on the roughness parameters calculated by the algorithms in spatial domain (SDAs). The morphological surface roughness (MSR), one of the SDAs based on the mathematical morphology, has been succeeded in mapping the lunar topographic roughness at multiscales. However, some typical roughness characteristics, such as the small sharp rock distribution and the inner irregular materials of craters, cannot be indicated on the multiscale MSR maps. In this paper, we improve the MSR method by utilizing the Fourier power spectra (FPS), which is applied to characterize these typical topographic variations by transforming them into their frequency forms. A synthesized parameter called improved morphological roughness (IMR) is designed for quantifying multiscale roughness based on the MSR and FPS values. Compared with the other methods (MSR, root mean square, multiscale morphological roughness, Fourier roughness, and kurtosis), IMR identifies more complicated roughness characteristics on the roughness map based on the Lunar Orbiter Laser Altimeter-derived digital elevation models (DEMs). For the multiscale analysis of different roughness maps, IMR can provide a stable scale-dependent distribution for the classifications of different terrains.

Reflection of an ultrasonic wave on the bone-implant interface: A numerical study of the effect of the multiscale roughness.

Quantitative ultrasound is used to characterize and stimulate osseointegration processes at the bone-implant interface (BII). However, the interaction between an ultrasonic wave and the implant remains poorly understood. This study aims at investigating the sensitivity of the ultrasonic response to the microscopic and macroscopic properties of the BII and to osseointegration processes. The reflection coefficient R of the BII was modeled for different frequencies using a two-dimensional finite element model. The implant surface roughness was modeled by a sinusoidal function with varying amplitude h and spatial frequency L. A soft tissue layer of thickness W was considered between bone tissue and the implant in order to model non-mineralized fibrous tissue. For microscopic roughness, R is shown to increase from around 0.55 until 0.9 when kW increases from 0 to 1 and to be constant for kW > 1, where k is the wavenumber in the implant. These results allow us to show that R depends on the properties of bone tissue located at a distance comprised between 1 and 25 μm from the implant surface. For macroscopic roughness, R is highly dependent on h and this dependence may be explained by phase cancellation and multiple scattering effects for high roughness parameters.

Influence of particle size in fluorine free corrosion resistance superhydrophobic coating - Optimization and stabilization of interface by multiscale roughness

Fluorine free superhydrophobic coatings with stabilized air-water interface had been fabricated by utilising different particle sized silica. Coatings were formulated with various composition of different sized nanosilica's along with nanotitania and aluminium stearate embedded in a matrix of silicone resin of Mn ∼1,65,000. The as-prepared coating exhibited higher contact angle (CA) 152.5° along with high mechanical properties like abrasion, adhesion and impact resistance. As particles with different sizes were incorporated in the coating, the interface was stabilized during immersion in water uptake study. In addition most of these coatings also exhibited exceptional corrosion resistance withstanding upto 1000 h of salt spray.

Nanoparticle Structures with (Un-)Hydrogenated Castor Oil as Hydrophobic Paper Coating.

The encapsulation of vegetable oils within an aqueous dispersion of polymer nanoparticles provides an alternative route to create functional paper coatings from renewable resources, by combining the presentation of hydrophobic moieties together with variations in roughness at the paper surface. The effects of two selected vegetable oil types, i.e., castor oil and hydrogenated castor oil (wax), are compared in terms of nanoparticle synthesis, coating hydrophobicity and surface gloss. The nanoparticles were synthesized by adding 50 wt.-% oil during imidization of poly(styrene-co-maleic anhydride) with ammonium hydroxide. From evaluation of the thermal properties, the nanoparticles have a high glass transition temperature that is suppressed in presence of oil. The nanoparticles with hydrogenated castor oil have higher imide content and better thermal stability compared to castor oil, in parallel with lower chemical reactivity of the hydrogenated oil and less interference with the imidization reaction. After deposition as a coating on paper, the physical coating properties are discussed in parallel with the coating chemistry and morphology or roughness at different scale lengths. The nanoparticle coatings with hydrogenated oil provides a multi-scale roughness with an open, porous nanoparticles structures and presentation of some amount free oil augmenting hydrophobicity towards a water contact angle of 128° (static contact angle) or 138° (advancing contact angle). The differences in surface coverage of coated papers in terms of imide and oil contents are confirmed by chemical Raman mapping. The differences in surface roughness are confirmed by non-contact profilometry, laser interferometry and atomic force microscopy.

Fabricating superhydrophobic and oleophobic surface with silica nanoparticles modified by silanes and environment-friendly fluorinated chemicals

Fluorinated polymers or fluorinated chemicals are often used to enhance the hydrophobicity and oleophobicity of surfaces. In this work, amphiphobic coating surfaces of silica nanoparticles modified by methyltrimethoxysilane (MTMS), ethyltriethoxysilane (ETES) or/and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (HFOTES) were fabricated via a sol-gel process. The results demonstrated that the surface wettability of coatings was greatly influenced by the type of silanes which were used to modify silica, and superhydrophobic and oil-repellent coating surface was obtained by using silica co-modified by MTMS and HFOTES (MF-SiO2). The water contact angles of dip-coated MF-SiO2 coatings could reach to 150.2° with the lowest angle of water hysteresis (3.3°) among all samples. One reason was that the addition of MTMS changed the holes structures of F-SiO2, and the co-modified surface of MF-SiO2 formed the multi-scale rough structures. Moreover, the percentage of fluorine at the surfaces for sample MF-SiO2 was greatly increased due to the packing of CF3 groups on surface. In other words, the addition of MTMS influenced the outmost layer of coating and changed the distribution of fluorinated chemical groups for sample MF-SiO2. Therefore, the coating for sample MF-SiO2 could exhibit superhydrophobic and oil-repellent properties owing to the multi-scale surface roughness as well as the packing of CF3 groups on the outmost layer of surface.

Roughness scale dependence of the relationship between tracer longitudinal dispersion and Peclet number in variable‐aperture fractures

AbstractThe relationship between the longitudinal dispersion (DL) and Peclet number (Pe) is crucial for predicting and simulating tracer through the variable‐aperture fracture. In this study, the roughness of the self‐affine fracture wall was decomposed into primary roughness (relatively large‐scale waviness) and secondary roughness (relatively small‐scale waviness) by a multiscaled wavelet analysis technique. Based on the complete dispersion mechanism (diffusion, macrodispersion, and Taylor dispersion) in the variable‐aperture fracture, three relationships (second‐order, power‐law, and linear relationships) between the DL and Pe were investigated at large and small scales, respectively. Our results showed that the primary roughness mostly controlled the Taylor dispersion mechanism, whereas the secondary roughness was a dominant factor for the macrodispersion mechanism. Increasing the Hurst exponent and removing the secondary roughness led to the decreasing range of Pe where macrodispersion mechanism dominated the solute transport. It was found that estimating the DL from the power‐law relationship based on Taylor dispersion theory resulted in considerable errors, even in the range of Pe where the Taylor dispersion mechanism dominated. The exponent of the power‐law relationship increased as the secondary roughness was removed. Analysing the linear relationship between the DL and Pe revealed that the longitudinal dispersivity αL increased linearly. However, this linear increase became weak as the Taylor dispersion mechanism dominated. In the range of Pe where the macrodispersion mechanism dominated, increasing the Hurst exponent caused the increase of αL and the secondary roughness played a significant role in enhancing the αL. As the Taylor dispersion mechanism dominated, the αL was insensitive to the influence of multiscale roughness in variable‐aperture fractures.

Characterization and simulation methods applied to the study of fretting wear in Zircaloy-4

Zr-based alloys have been used in the commercial nuclear industry for decades. The wear behavior of such alloys has also attracted attention. However, the extreme operating environment that exists a nuclear reactor presents considerable experimental difficulties for investigating aspects of wear. Therefore, a robust computational model of wear, based on experimental data, could provide a useful tool to examine wear for different variables. Towards this end, specimens of Zircaloy-4 were subjected to wear testing within the aqueous environment of a high-temperature, high-pressure autoclave for a range of amplitudes at a fixed pressure. The wear surfaces before and after testing were investigated using microscopy techniques. The change in surface roughness that resulted from wear was measured and evidence of plastic deformation and wear debris was observed. Characterization of surfaces prior to testing revealed some degree of self-affinity over multiple length scales which diminishes as the evaluation length approaches tens of microns. The experimental roughness profiles of the starting surfaces were comparable to computer simulations of multiscale roughness using separate fractal and Monte Carlo techniques and demonstrate that these methods could be used to generate surfaces. Further, we demonstrate that isotropic methods of roughness simulation can be tuned to reproduce the observed departure from self-affinity, but must be modified to capture directional features. Lastly, results of FEM analysis of surfaces with RMS roughness of 1.25–94 µm showed a significant spatial variation in starting stress state with indications of localized plastic deformation at surface asperities for the applied loads used in this study, which qualitatively agreed with the observations of localized wear and plastic deformation of the worn surfaces. This work demonstrates a path by which experimental results can be coupled with computational methods to construct a means to investigate the effects of surface roughness on wear.

Quantification method for parameters affecting multi-scale roughness-induced fatigue crack closure

Our challenge is to clarify the relationship between crack roughness and microstructure. Roughness-induced crack closure (RICC) is known to be one of the main factors decelerating fatigue crack growth. Two factors trigger RICC: 1) nanometer-scale roughness on the crack surface (nano-roughness) and 2) degree of crack deflection (micro-roughness). These factors affect the friction stress acting on the crack planes and the stress intensity factor range for crack closure. For instance, S. Suresh and R. O. Ritchie discussed the effects of geometrical mismatch between fatigue crack planes on crack closure. We further attempt to measure multi-scale crack roughness to quantitatively estimate RICC with respect to both friction and crack closure effects. In this study, we examine the multi-scale crack roughness of a lamellar-structured Fe-9Mn-3Ni-1.4Al-0.01C steel sample as a case study. We verify the effect of crack surface friction. Here, we assume that the basic effect of nano-roughness on friction is significant when the inclination angle of micro-roughness against the loading direction is less than the angle of nano-roughness. If the inclination angle of micro-roughness against the loading direction is larger than the angle of nano-roughness, the nano-roughness effect does not occur because the crack surfaces do no contact with each other. To further discuss the underlying effects of multi-scale roughness, we will present more details on the microstructure- and mechanical condition-related roughness parameters and their quantification techniques.

A Generalized Johnson Parameter for Pull-Off Decay in the Adhesion of Rough Surfaces

There is no simple theory at present to predict accurately the decay of pull-off in the adhesion of randomly rough surfaces. The asperity model of Fuller and Tabor has shown significant error in recent numerical investigations by Pastewka and Robbins of self-affine random roughness from micrometer to atomic scale which corresponds to low values of Tabor parameter. For sinusoidal contact, the Johnson parameter, originally introduced for the JKR regime (from Johnson-Kendall-Roberts) is the dominant parameter ruling the pull-off at intermediate Tabor values. Hence, we define a generalized Johnson parameter as the ratio between the adhesive energy to the elastic strain energy to flatten the surface in the case of multiscale roughness and find that it correlates very well with the data of Pastewka and Robbins spanning almost five orders of magnitude of reduction from theoretical strength, improving significantly with respect to other possible single parameter criteria. For the most important case in practice, that of low fractal dimensions, this suggests the product of amplitude and slope of the largest wavelength components of roughness dominate pull-off decay, and not small scales features like slopes and curvatures, as suggested by Pastewka and Robbins.

Multi-scale surface topography to minimize adherence and viability of nosocomial drug-resistant bacteria

Toward minimizing bacterial colonization of surfaces, we present a one-step etching technique that renders aluminum alloys with micro- and nano-scale roughness. Such a multi-scale surface topography exhibited enhanced antibacterial effect against a wide range of pathogens. Multi-scale topography of commercially grade pure aluminum killed 97% of Escherichia coli and 28% of Staphylococcus aureus cells in comparison to 7% and 3%, respectively, on the smooth surfaces. Multi-scale topography on Al 5052 surface was shown to kill 94% of adhered E. coli cells. The microscale features on the etched Al 1200 alloy were not found to be significantly bactericidal, but shown to decrease the adherence of S. aureus cells by one-third. The fabrication method is easily scalable for industrial applications. Analysis of roughness parameters determined by atomic force microscopy revealed a set of significant parameters that can yield a highly bactericidal surface; thereby providing the design to make any surface bactericidal irrespective of the method of fabrication. The multi-scale roughness of Al 5052 alloy was also highly bactericidal to nosocomial isolates of E. coli, K. pneumoniae and P. aeruginosa. We envisage the potential application of engineered surfaces with multi-scale topography to minimize the spread of nosocomial infections.

Robust and underwater superoleophobic coating with excellent corrosion and biofouling resistance in harsh environments

Underwater superoleophobic surfaces are based on the surface with micro-/nanoscale roughness and hydration layer. But the self-cleaning surfaces are usually mechanically weak and will lose their underwater superoleophobicity when the surfaces are corroded or damaged. In this paper, to overcome these problems, the robust underwater superoleophobic coating (HN/ER-coating) has been fabricated successfully through MPS (methacryloxy propyl trimethoxyl silane)-SiO2/PNIPAM (N-isopropylacryamide) hybrid nanoparticles and epoxy resin (ER) via a simple solution-casting method. The SiO2/PNIPAM hybrid nanoparticles can enhance multiscale roughness and excellent abrasion-resistant property, and the epoxy resin can be used as an interlayer between hybrid nanoparticles and substrates to promote the robustness and corrosion resistance of the coating. The obtained coatings have excellent underwater superoleophobicity, and exhibit highly stability in harsh environments (including acid-base, strong ionic strength, mechanical abrasion). Moreover, this coating can provide protective effect on the substrate in corrosive solution, and may also resist bacterial attachment and subsequent biofilm formation because of the presence of high density PNIPAM polymers. Herein, the developed underwater superoleophobic coating can be applied as an effective platform for the applications in underwater instruments, underwater oil transport, marine oil platform and ships.

Adaptation of a dry metalorganic chemical vapor deposition metallization process to a wet chemical pretreatment of polymers

A wet chemical pretreatment is applied on a carbon fiber reinforced polymer composite in order to provide a metalorganic chemical vapor deposition metallization layer with satisfactory adhesion. The pretreatment modifies the surface composition and the surface energy by forming a high concentration of surface polar groups. It also creates a multiscale roughness which results in a nonuniform topography; the etching efficiency being dependent on the considered surface area. Hence, previously underlying C fibers become exposed at the free surface and this modifies the reactivity toward metallization in these surface regions. The adhesion and resistivity of the Cu films are then impacted by such heterogeneity. This is exemplified by the visual inspection of adhesive tapes used for testing adhesion where Cu is exclusively removed at the location of bare C fibers. Addition of water vapor during the first step of the chemical vapor deposition process results in the formation of a copper oxide buffer layer. The latter perfectly wets the entire substrate surface, making the copper layer uniform while preserving adhesion and improving the electrical resistivity.

A Multiscale Morphological Insight into Graphene Based Coatings for Pool Boiling Applications

ABSTRACTThe application of graphene for pool boiling is an attractive option to facilitate compaction and promote efficient heat removal from high power density devices. In this context, chemical characterization of the depositions achieved through commonly employed coating techniques are an important topic of discussion. A detailed structure-property relationship between the morphologies obtained on the mono and multilayered graphene coatings and their corresponding pool boiling performance quantified by the experimental critical heat fluxes is presented. Three different types of graphene (G) and graphene oxide (GO) substrates are characterized: (i) nanoscale: mono and multilayer samples developed through chemical vapor deposition, (ii) Rochester Institute of Technology (RIT)-G/GO colloid generated through an oxygen embrittlement electrochemical process, and (iii) commercially available chemical vapor deposited (CVD)-G/GO colloid. The morphological features were characterized with scanning electron microscope while X-Ray Diffractometer analysis and Raman spectroscopy were used to examine the ordering and stacking of the sheets that result in the unique structural features. Fourier transform infrared and energy dispersive X-ray spectroscopy were employed to identify the overall compositional characteristics of the coated surfaces. The wettability changes and additional nucleation sites for nanoscale coatings, and multiscale roughness features and ridge microstructures for microscale coatings were identified as enhancement mechanisms.

Enhanced water repellency of surfaces coated with multiscale carbon structures

Abstract Low cost and well characterized superhydrophobic surfaces are frequently required for industrial applications. Materials are commonly structured at the micro or nano scale. Surfaces decorated with nanotube derivatives synthesized by plasma enhanced chemical vapor deposition (PECVD) are of particular interest, since suitable modifications in the growth parameters can lead to numerous designs. In this article, we present surfaces that are selected for their specific wetting features with patterns ranging from dense forests to jungles with concave (re-entrant) surface such as flake-like multiscale roughness. Once these surfaces are functionalized adequately, their wetting properties are investigated. Their ability to sustain a superhydrophobic state for sessile water drops is examined. Finally, we propose a design to achieve a robust so-called “Fakir” state, even for micrometer-sized drops, whereas with classic nanotubes forests it is not achievable. Thus, the drop remains on the apex of the protrusions with a high contact angle and a low contact angle hysteresis, while the surface features demonstrate good mechanical resistance against capillary forces.

Hydrophobic Coatings by Thiol-Ene Click Functionalization of Silsesquioxanes with Tunable Architecture.

The hydrolysis-condensation of trialkoxysilanes under strictly controlled conditions allows the production of silsesquioxanes (SSQs) with tunable size and architecture ranging from ladder to cage-like structures. These nano-objects can serve as building blocks for the preparation of hybrid organic/inorganic materials with selected properties. The SSQs growth can be tuned by simply controlling the reaction duration in the in situ water production route (ISWP), where the kinetics of the esterification reaction between carboxylic acids and alcohols rules out the extent of organosilane hydrolysis-condensation. Tunable SSQs with thiol functionalities (SH-NBBs) are suitable for further modification by exploiting the simple thiol-ene click reaction, thus allowing for modifying the wettability properties of derived coatings. In this paper, coatings were prepared from SH-NBBs with different architecture onto cotton fabrics and paper, and further functionalized with long alkyl chains by means of initiator-free UV-induced thiol-ene coupling with 1-decene (C10) and 1-tetradecene (C14). The coatings appeared to homogeneously cover the natural fibers and imparted a multi-scale roughness that was not affected by the click functionalization step. The two-step functionalization of cotton and paper warrants a stable highly hydrophobic character to the surface of natural materials that, in perspective, suggests a possible application in filtration devices for oil-water separation. Furthermore, the purification of SH-NBBs from ISWP by-products was possible during the coating process, and this step allowed for the fast, initiator-free, click-coupling of purified NBBs with C10 and C14 in solution with a nearly quantitative yield. Therefore, this approach is an alternative route to get sol-gel-derived, ladder-like, and cage-like SSQs functionalized with long alkyl chains.

Synthesis and hydrophobic evaluation of the electro-spun nano-TiO2/PET fibrous bats after treatment with an alkaline solution and fluorocarbon material

Superhydrophobicity of a substrate is the result of a low surface energy and proper surface roughness. In the present research, the potential roughening effect of using nano-TiO2 (NT) in a PET electro-spinning solution followed by an alkaline modification of the synthesized bats was investigated to create appropriate surface roughness while a low surface energy was achieved in owing to a layer of fluorocarbon coating. The results were evaluated via employing FTIR, SEM, digital microscopy, 3 M water repellency, water contact angle, and sliding angle. The best result of hydrophobicity was achieved by the alkaline modified-fluorocarbon coated NT (5.5%)/PET fibrous bat with the 3 M water repellency of 10, contact angle of more than 150° (155.9 ± 0.1°), and sliding angle of less than 10° (6.1 ± 1.9°) due to a multiscale surface roughness obtained by the fibrous bat structure itself, and roughening effect of the NT as well as alkaline solution surface etching.

Theory of Work Function and Potential of Zero Charge for Metal Nanostructured and Rough Electrodes

We developed a general theory for the work function and potential of zero charge of arbitrary shaped nanostructured and rough metal. Theory accounts for the influence of adsorbed solvent layer, i.e., partial to full solvent coverage, on work function (WF). We show that the change in local WF is due to redistribution of surface charge caused by local surface curvature and adsorbing dipoles. Special cases of spherical and ellipsoidal geometries are obtained from the generic curvature-dependent WF equation to illustrate the size and shape effects. Theory predicts increase in WF of immersed nanoparticles with decrease in their size, while change in shape causes nonuniform WF over the surface. Theory shows anomalous fluctuations in WF for a surface with multiscale roughness, viz., the Weierstrass–Mandelbrot function as a surface. Ensemble or surface averaged work function of randomly curved rough surface is predicted to depend on average mean and Gaussian curvatures. The intrinsic field of various metals regul...