Looking at the micro-topography of polished and blasted Ti-based biomaterials using atomic force microscopy and contact angle goniometry

Surface topography of polished and blasted samples of a Ti6Al4V biomaterial has been studied using an atomic force microscope. Surface RMS roughness and surface area have been measured at different scales, from 1 to 50 μm, while at distances below 10 μm the surface RMS roughness in both kinds of samples is not very different, this difference becomes significant at larger scanning sizes. This means that the surface roughness scale that could have a main role in cell adhesion varies depending on the size, shape and flexibility of participating cells. This consideration suggests that in cell–material interaction studies, surface roughness should not be considered as an absolute and independent property of the material, but should be measured at scales in the order of the cell sizes, at least if a microscopic interpretation of the influence of roughness on the adhesion is intended. The microscopic information is contrasted with that coming from a macroscopic approach obtained by contact angle measurements for polar and non-polar liquids whose surface tension is comprised in a broad range. Despite the very large differences of contact angles among liquids for each surface condition, a similar increase for the blasted surface with respect to the polished has been found. Interpretation of these results are in accordance with the microscopic analysis done through the use of a functional roughness parameter, namely the valley fluid retention index, evaluated from the AFM images, which has been shown not to correlate with the RMS roughness, one of the most commonly used roughness parameter.     

Microspreading studies on rough surfaces by scanning electron microscopy

The use of scanning electron microscopy for direct observation of the effects of surface roughness on the spreading of liquids is described, making it possible to view moving liquid drops at distances less than 1 μm from the advancing contact line. Various surfaces were examined including several with simple forms of roughness which can assist in explaining the behavior of more complex surfaces. Spreading is shown to be highly dependent on the orientation and texture of the roughness; in particular, the presence of sharp edges of step height 0.05 μm are shown to influence spreading significantly. These observations reinforce our previously stated doubts of the significance of conventionally measured macroscopic contact angles.     

Dual quartz crystal microbalance compensation using a submerged reference crystal. Effect of surface roughness and liquid properties

Absolute resonant frequency measurements were made on gold-coated AT-cut quartz crystals with one face in contact with a series of liquids. The effect of surface roughness and liquid properties (viscosity and density) was analyzed in terms of a “trapped liquid” model. In this model, liquid present in surface imperfections is viewed as rigidly coupled mass. In some of the literature this density-dependent, but not viscosity-dependent, term is viewed as being additive to the hydrodynamic shift seen for a smooth surface. Data obtained using 1 μm and 5 μm surface finish crystals are inconsistent with the predictions of the trapped liquid model. This suggests hydrodynamic coupling between liquid internal and external to the crevices. Despite the lack of a theoretical model for the liquid motion, it is possible to compensate for frequency variations resulting from changing liquid properties and for roughness effects by making direct measurements of the resonant frequency difference between two crystals exposed to the same solution. This novel procedure works to the extent that the two crystals have similar surface topographies. Compensation is excellent for 1 μm finish crystals and good for 5 μm finish crystals.     

Adhesion as an interplay between particle size and surface roughness

Surface roughness plays an important role in the adhesion of small particles. In this paper we have investigated adhesion as a geometrical effect taking into account both the particle size and the size of the surface features. Adhesion is studied using blunt model particles on surfaces up to 10 nm root-mean-square (RMS) roughness. Measurements with particles both smaller and larger than surface features are presented. Results indicate different behavior in these areas. Adhesion of particles smaller than or similar in size to the asperities depend mainly on the size and shape of the asperities and only weakly on the size of the particle. For large particles also the particle size has a significant effect on the adhesion. A new model, which takes the relative size of particles and asperities into account, is also derived and compared to the experimental data. The proposed model predicts adhesion well over a wide range of particle/asperity length scales.     

Correction of random surface roughness on colloidal probes in measuring adhesion

Atomic force microscopes (AFM) are commonly used to measure adhesion at nanoscale between two surfaces. To avoid uncertainties in the contact areas between the tip and the surface, colloidal probes have been used for adhesion measurements. We measured adhesion between glass spheres and silicon (100) surface using colloidal probes of different radii under controlled conditions (relative humidity of < 3%, temperature of 25 +/- 1 degrees C). Results showed that the adhesion forces did not correlate with the radii of the spheres as suggested by elastic contact mechanics theories. Surface roughness and random surface features were found on the surfaces of the colloidal probes. We evaluated various roughness parameters, Rumpf and Rabinovich models, and a load-bearing area correction model in an attempt to correct for the roughness effects on adhesion, but the results were unsatisfactory. We developed a new multiscale contact model taking into account elastic as well as plastic deformation in a successive contacting mode. The new model was able to correct for most of the surface roughness features except for surface ridges with sharp angular features, limited by the spherical asperity assumption made in the model.     

The deformation and adhesion of randomly rough and patterned surfaces

Using a surface forces apparatus (SFA) and an atomic force microscope (AFM) we have studied the effects of surface roughness (root-mean-square (RMS) roughness between 0.3 and 220 nm) on the "contact mechanics", which describes the deformations and loading and unloading adhesion forces, of various polymeric surfaces. For randomly rough, moderately stiff, elastomeric surfaces, the force-distance curves on approach and separation are nearly reversible and almost perfectly exponentially repulsive, with an adhesion on separation that decreases only slightly with increasing RMS. Additionally, the magnitude of the preload force is seen to play a large role in determining the measured adhesion. The exponential repulsion likely arises from the local compressions (fine-grained nano- or submicron-scale deformations) of the surface asperities. The resulting characteristic decay lengths of the repulsion scale with the RMS roughness and correlate very well with a simple finite element method (FEM) analysis based on actual AFM topographical images of the surfaces. For "patterned" surfaces, with a nonrandom terraced structure, no similar exponential repulsion is observed, suggesting that asperity height variability or random roughness is required for the exponential behavior. However, the adhesion force or energy between two "patterned" surfaces fell off dramatically and roughly exponentially as the RMS increased, likely owing to a significant decrease in the contact area which in turn determines their adhesion. For both types of rough surfaces, random and patterned, the coarse-grained (global, meso- or macroscopic) deformations of the initially curved surfaces appear to be Hertzian.     

Importance of surface tension for physical paint properties

Surface tension is a parameter of decisive importance for characterizing painted and unpainted surfaces related to wetting and adhesion phenomena. This article presents measurements of the surface tension of solids by means of an automatic contact angle measurement device. The evaluation of the surface tension is based on a separation into polar and disperse components. In addition, this paper briefly touches on other, more far-reaching approaches (acid/base) and discusses a method for the determination of dynamic surface tension of liquids.     

Roughness assessment and wetting behavior of fluorocarbon surfaces

The wetting behavior of fluorocarbon materials has been studied with the aim of assessing the influence of the surface chemical composition and surface roughness on the water advancing and receding contact angles. Diamond like carbon and two fluorocarbon materials with different fluorine content have been prepared by plasma enhanced chemical vapor deposition and characterized by X-ray photoemission, Raman and FT-IR spectroscopies. Very rough surfaces have been obtained by deposition of thin films of these materials on polymer substrates previously subjected to plasma etching to increase their roughness. A direct correlation has been found between roughness and water contact angles while a superhydrophobic behavior (i.e., water contact angles higher than 150° and relatively low adhesion energy) was found for the films with the highest fluorine content deposited on very rough substrates. A critical evaluation of the methods currently used to assess the roughness of these surfaces by atomic force microscopy (AFM) has evidenced that calculated RMS roughness values and actual surface areas are quite dependent on both the scale of observation and image resolution. A critical discussion is carried out about the application of the Wenzel model to account for the wetting behavior of this type of surfaces.     

Surface modification of low density polyethylene (LDPE) film by low pressure O2 plasma treatment

In this work, low pressure glow discharge O₂ plasma has been used to increase wettability in a LDPE film in order to improve adhesion properties and make it useful for technical applications. Surface energy values have been estimated using contact angle measurements for different exposure times and different test liquids. In addition, plasma-treated samples have been subjected to an aging process to determine the durability of the plasma treatment. Characterization of the surface changes due to the plasma treatment has been carried out by means of Fourier transformed infrared spectroscopy (FTIR) to determine the presence of polar species such as carbonyl, carboxyl and hydroxyl groups. In addition to this, atomic force microscopy (AFM) analysis has been used to evaluate changes in surface morphology and roughness. Furthermore, and considering the semicrystalline nature of the LDPE film, a calorimetric study using differential scanning calorimetry (DSC) has been carried out to determine changes in crystallinity and degradation temperatures induced by the plasma treatment. The results show that low pressure O₂ plasma improves wettability in LDPE films and no significant changes can be observed at longer exposure times. Nevertheless, we can observe that short exposure times to low pressure O₂ plasma promote the formation of some polar species on the exposed surface and longer exposure times cause slight abrasion on LDPE films as observed by the little increase in surface roughness.     

A review of factors that affect contact angle and implications for flotation practice

Contact angle and the wetting behaviour of solid particles are influenced by many physical and chemical factors such as surface roughness and heterogeneity as well as particle shape and size. A significant amount of effort has been invested in order to probe the correlation between these factors and surface wettability. Some of the key investigations reported in the literature are reviewed here.It is clear from the papers reviewed that, depending on many experimental conditions such as the size of the surface heterogeneities and asperities, surface cleanliness, and the resolution of measuring equipment and data interpretation, obtaining meaningful contact angle values is extremely difficult and such values are reliant on careful experimental control. Surface wetting behaviour depends on not only surface texture (roughness and particle shape), and surface chemistry (heterogeneity) but also on hydrodynamic conditions in the preparation route. The inability to distinguish the effects of each factor may be due to the interplay and/or overlap of two or more factors in each system. From this review, it was concluded that:

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Surface geometry (and surface roughness of different scales) can be used to tune the contact angle; with increasing surface roughness the apparent contact angle decreases for hydrophilic materials and increases for hydrophobic materials.

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For non-ideal surfaces, such as mineral surfaces in the flotation process, kinetics plays a more important role than thermodynamics in dictating wettability.

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Particle size encountered in flotation (10-200 μm) showed no significant effect on contact angle but has a strong effect on flotation rate constant.

There is a lack of a rigid quantitative correlation between factors affecting wetting, wetting behaviour and contact angle on minerals; and hence their implication for flotation process. Specifically, universal correlation of contact angle to flotation recovery is still difficult to predict from first principles. Other advanced techniques and measures complementary to contact angle will be essential to establish the link between research and practice in flotation.     

Influence of Chemical Heterogeneity and Nanoscale Roughness on the DLVO Energy Interaction by Spherical Coordinates

In this work, the influence of chemical heterogeneity on the stability of nanocolloidal systems is surveyed with a new method. Zone of influence as a very important parameter for chemical patch surveying is modeled for sphere and flat surface. Surface chemical heterogeneity with specified properties, size, and position are created by spherical coordinate integration method. Rippled sphere model is used to create roughness and the flat surface is created by changing two sphere radius ratio. Using the spherical coordinate system for modeling of surface roughness and chemical heterogeneity is very accurate and fast. Results show that the patches could destabilize the colloidal system at very small sizes. Surface roughness reduces the effect of chemical patches for destabilizing the colloidal system, and with increasing the size of roughness the total DLVO energy interaction increases.     

The role of few-asperity contacts in adhesion

The surface roughness of a few asperities and their influence on the work of adhesion is of scientific interest. Macroscale and nanoscale adhesion data have seemingly given inconsistent results. Despite the importance of bridging the gap between the two regimes, little experimental work has been done, presumably due to the difficulty of the experiment needed to determine how small amounts of surface roughness might influence adhesion data lying in between the two scales. To investigate the role of few-asperity contacts in adhesion, the pull-off force was measured between different sized atomic-force microscope (AFM) tips (with different roughnesses) and sample surfaces that had well-controlled material properties. There were seventeen tips of four different types, with radii from 200 nm to 60 microm. The samples were unpatterned single crystal silicon with a chemical silicon dioxide surface resulting from a standard silicon wafer clean. Some of the samples were treated with a few angstroms of vapor deposited diphenylsiloxane. We observed that the uncorrected (for surface roughness) pull-off force was independent of the radius of the AFM tip, which was contrary to all continuum-mechanics model predictions. To explain this behavior, we assumed that the interactions between the AFM tip and sample were additive, material properties were constant, and that the AFM tip, asperities, and sample surfaces were of uniform density. Based on these assumptions, we calculated a simple correction due to the measured root mean square (RMS) surface roughness of the AFM tips. The simple correction for the RMS surface roughness resulted in the expected dependence of the pull-off force on radius, but the magnitudes were higher than expected. Commercial and heat-treated AFM tips have minimal surface roughness and result in magnitudes that are more reliable. The relative uncertainty for the pull-off force was estimated to be 10%. In this paper, we derive how the cantilever and tip parameters contribute to the measured pull-off force and show how the corrected results compare with theory. Although much work is still needed, the work presented here should advance the understanding of adhesion between the macroscale and nanoscale regimes.     

The total surface free energy and the contact angle in the case of low energetic solids

Using the literature data of the refractive index, the structural unit molar volume of polymers and their dipole moment, as well as the literature data of the polarizability, ionization potential, and dipole moment of many liquids, values of the Φ parameter for paraffin—liquid and polymer—liquid interfaces were calculated. Next, introducing these values of Φ and the earlier measured values of the contact angle for many liquids to the Young equation, values of the surface free energy (γ_S) of paraffin, polytetrafluoroethylene (PTFE), polyethylene (PE), polyethylene terephthalate (PET), and polymethacrylate (PMMA), were determined. It was found that the average values of γ_S for these solids were in agreement with those calculated on the basis of geometric, harmonic, or harmonic—geometric mean approaches. The values of the surface free energy of paraffin, PTFE, PE, PET, and PMMA were also calculated from the Young equation modified by Neumann et al. and, using the earlier measured values of the contact angle for many liquids, they were compared with the values obtained by other methods. Next, employing the mean value of the surface free energy, values of the contact angles for many liquids were calculated and compared with those measured earlier for the same liquids. It was found that for paraffin, PTFE, and PE there were big differences among the values of their surface free energies calculated from the contact angles for some liquids; however, the average values were in agreement with those obtained by other methods. The average values of the surface free energies of PET and PMMA were also in the range of the results obtained by other authors. It was also found that the average deviations of the contact angles calculated from the Young equation modified by Neumann et al. from the measured ones were slightly larger than those of the contact angles calculated from equations employing the geometric and harmonic means of the surface free energy components; the method of Neumann et al. may also be used to predict the wettability in some systems.     

Enhancement of surface wettability via the modification of microtextured titanium implant surfaces with polyelectrolytes

Micrometer- and submicrometer-scale surface roughness enhances osteoblast differentiation on titanium (Ti) substrates and increases bone-to-implant contact in vivo. However, the low surface wettability induced by surface roughness can retard initial interactions with the physiological environment. We examined chemical modifications of Ti surfaces [pretreated (PT), R(a) ≤ 0.3 μm; sand blasted/acid etched (SLA), R(a) ≥ 3.0 μm] in order to modify surface hydrophilicity. We designed coating layers of polyelectrolytes that did not alter the surface microstructure but increased surface ionic character, including chitosan (CHI), poly(L-glutamic acid) (PGA), and poly(L-lysine) (PLL). Ti disks were cleaned and sterilized. Surface chemical composition, roughness, wettability, and morphology of surfaces before and after polyelectrolyte coating were examined by X-ray photoelectron spectroscopy (XPS), contact mode profilometry, contact angle measurement, and scanning electron microscopy (SEM). High-resolution XPS spectra data validated the formation of polyelectrolyte layers on top of the Ti surface. The surface coverage of the polyelectrolyte adsorbed on Ti surfaces was evaluated with the pertinent SEM images and XPS peak intensity as a function of polyelectrolyte adsorption time on the Ti surface. PLL was coated in a uniform thin layer on the PT surface. CHI and PGA were coated evenly on PT, albeit in an incomplete monolayer. CHI, PGA, and PLL were coated on the SLA surface with complete coverage. The selected polyelectrolytes enhanced surface wettability without modifying surface roughness. These chemically modified surfaces on implant devices can contribute to the enhancement of osteoblast differentiation.     

Topography and surface energy dependent calcium phosphate formation on Sol-Gel derived TiO2 coatings

Heterogeneous nucleation and growth of calcium phosphate (CaP) on sol-gel derived TiO(2) coatings was investigated in terms of surface topography and surface energy. The topography of the coatings was derived from AFM measurements, while the surface energy was determined with contact angle measurements. The degree of precipitation was examined with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The precipitation of CaP was found to be dependent on both topography and surface energy. A high roughness value when combining the RMS roughness parameter S(q) with the number of local maxima per unit area parameter S(ds) enhances CaP formation. The hydrophilicity of the coating was also found to be of importance for CaP formation. We suggest that the water contact angle, which is a direct measure of the hydrophilicity of the surface, may be used to evaluate the surface energy dependent precipitation kinetics rather than using the often applied Lewis base parameter.     

Electropolishing of medical-grade stainless steel in preparation for surface nano-texturing

The purpose of this work is to investigate the electropolishing of medical-grade 316 L stainless steel to obtain a clean, smooth, and defect-free surface in preparation for surface nano-texturing. Electropolishing of steel was conducted under stationary conditions in four electrolyte mixtures: (A) 4.5 M H₂SO₄ + 11 M H₃PO₄, (B) 7.2 M H₂SO₄ + 6.5 M H₃PO₄, (C) 6.4 M glycerol + 6.1 M H₃PO₄, and (D) 6.1 M H₃PO₄. The influence of electrolyte composition and concentration, temperature, and electropolishing time, in conjunction with linear sweep voltammetry and chronoamperometry, on the stainless steel surface was studied. The resulting surfaces of unpolished and optimally polished stainless steel were characterized in terms of contamination, defects, topography, roughness, hydrophilicity, and chemical composition by optical and atomic force microscopies, contact angle goniometry, and X-ray photoelectron spectroscopy. It was found that the optimally polished surfaces were obtained with the following parameters: electrolyte mixture A at 2.1 V of applied potential at 80 °C for 10 min. This corresponded to the diffusion-limited dissolution of the surface. The root mean square surface roughness of the electropolished surface achieved was 0.4 nm over 2 × 2 μm². Surface analysis showed that electropolishing led to ultraclean surfaces with reduced roughness and contamination thickness and with Cr, P, S, Mo, Ni, and O enrichment compared to untreated surfaces.     

Poly (ethylene-co-vinyl acetate) films prepared by solution blow spinning: Surface characterization and its relation with E. coli adhesion

Solution blow spinning, SBS, a quite novel processing method, was used to obtain poly (ethylene-co-vinyl acetate), EVA, films with controlled surface properties. The influence of the surface characteristics of EVA films on the adhesion of DH5α Escherichia coli was studied. In particular, the initial concentration of the EVA solution to be blow spun was varied in order to get different surface topographies. Considering the potential use of EVA based materials in applications such as food packaging or scaffolds for tissue engineering all factors affecting proliferation of microorganisms on their surfaces should be studied and understood. Structural, morphological and surface characterizations based on the use of infrared spectroscopy, FTIR, scanning electron microscopy, SEM, and contact angle measurements were performed to ascertain the main factor influencing the E. coli adhesion on the EVA films. Roughness data were determined at different scales from 3D surfaces obtained using a stereoscopic reconstruction of SEM images. It was concluded that, depending on the conditions of the SBS process, only variations of topography were found on the EVA films, being therefore the unique cause of different adhesion capacity of E. coli cells. A correlation between roughness and the number of attached E. coli cells showed that the higher the roughness at microscale level the higher the biofilm development, demonstrating that, apart from specific interactions at nanoscale surface, heterogeneity at microscale can greatly modify the antibacterial action.     

An analysis of the adhesion strength between polyethylene and paper

Composites of polyethylene with paper were studied in order to determine the influence of the surface energy of the paper on their cohesive strength. The papers used in this study had the same roughness, but fifferent surface energy owing to treatment with different sizing agents. The surface energies were determined by the contact angle method with various liquids, and the variations of the cohesive strength were followed by the energy of peeling at 180 °C. It was shown that the peeling energy decreases linearly when the reversible energy of adhesion decreases, and becomes negligible when the paper surface energy is approximately equal the surface energy of polyethylene.     

Effect of Spontaneous and Water-Based Passivation on Components and Parameters of Ti6Al4V (ELI Grade) Surface Tension and Its Wettability by an Aqueous Solution of Sucrose Ester Surfactants

Solid wettability is especially important for biomaterials and implants in the context of microbial adhesion to their surfaces. This adhesion can be inhibited by changes in biomaterial surface roughness and/or its hydrophilic–hydrophobic balance. The surface hydrophilic–hydrophobic balance can be changed by the specifics of the surface treatment (proper conditions of surface preparation) or adsorption of different substances. From the practical point of view, in systems that include biomaterials and implants, the adsorption of compounds characterized by bacteriostatic or bactericidal properties is especially desirable. Substances that are able to change the surface properties of a given solid as a result of their adsorption and possess at least bacteriostatic properties include sucrose ester surfactants. Thus, in our studies the analysis of a specific surface treatment effect (proper passivation conditions) on a biomaterial alloy’s (Ti6Al4V ELI, Grade 23) properties was performed based on measurements of the contact angles of water, formamide and diiodomethane. In addition, the changes in the studied solid surface’s properties resulting from the sucrose monodecanoate (SMD) and sucrose monolaurate (SML) molecules’ adsorption at the solid–water interface were also analyzed. For the analysis, the values of the contact angles of aqueous solutions of SMD and SML were measured at 293 K, and the surface tensions of the aqueous solutions of studied surfactants measured earlier were tested. From the above-mentioned tests, it was found that water environment significantly influences the components and parameters of Ti6Al4V ELI’s surface tension. It also occurred that the addition of both SMD and SML to water (separately) caused a drop in the water contact angle on Ti6Al4V ELI’s surface. However, the sucrose monolaurate surfactant is characterized by a slightly better tendency towards adsorption at the solid–water interface in the studied system compared to sucrose monodecanoate. Additionally, based on the components and parameters of Ti6Al4V ELI’s surface tension calculated from the proper values of components and parameters of model liquids, it was possible to predict the wettability of Ti6Al4V ELI using the aqueous solutions of SMD and SML at various concentrations in the solution.     

On the Problem of the Relationship between the Contact Angle and Surface Roughness Coefficient: Quartz Wettability with Melted Germanium

Equilibrium contact angles of melted geranium have been measured at fused quartz surfaces. The surfaces were preliminarily grinded, polished, and, in some cases, etched. Then, the roughness coefficients are determined for the relief profile and the surface itself by optical interferometry using a NanoMap 1000 WLI profilometer. The contact angle has been found to vary in a range of 147°–164° depending on the method of surface pretreatment. The measured values of the contact angles agree with the data of other researchers. At the same time, the analysis of the obtained data has led to the conclusion that Wenzel’s equation, which relates the contact angles at smooth and rough surfaces, is not valid for germanium droplets on quartz surface.