Hexadecane Contact Angles Research Articles

Tailoring surface properties for controlling droplet formation at microsieve membranes

The preparation of water-in-oil emulsions with silicon nitride microfluidic nozzle arrays requires a surface, which is not wetted by the continuous water phase. Hence, we deposited several hydrophobic coatings on silicon nitride and studied the wetting behavior of water in pure and emulsifier containing hexadecane. In pure medias we could demonstrate that an octyl-chain terminated is the optimal hydrophobic coating. It has a high affinity to hexadecane due to its similar chemical nature expressed by a very low solid/liquid interfacial tension and a low affinity to water resulting in the highest measured water in hexadecane contact angle of 164°. Unmodified silicon nitride in contrast has the highest affinity to water and the lowest to hexadecane resulting in a contact angle of 97°. Based on the determination of the solid/liquid interfacial tensions in pure liquids we found for several hydrophobic coatings and for unmodified silicon nitride a deviation from Young's equation, which can be related to negative Hamaker constants of the studied systems. With the soybean-based emulsifier BolecMT we observed for several coatings no negative influence of the adsorbed emulsifier on the hydrophobicity. In fact, the water/hexadecane contact angles increased in time. In case of unmodified silicon nitride the surface was rendered into a more hydrophobic state. For low surface energy coatings, we observed a dramatic time-dependent change of the wetting characteristics. After a certain period of time water drops collapsed and wetted the surface. The time scales of this observed wetting behavior is several magnitudes larger than the drop formation process itself. Therefore, its relevance for the drop formation process could be neglected. With the unmodified silicon nitride surface successful drop formation was not possible and the low surface energy coating performed successfully in membrane emulsification experiments.

Superhydrophobic Surfaces Prepared by Microstructuring of Silicon Using a Femtosecond Laser

We present a simple method for fabricating superhydrophobic silicon surfaces. The method consists of irradiating silicon wafers with femtosecond laser pulses and then coating the surfaces with a layer of fluoroalkylsilane molecules. The laser irradiation creates a surface morphology that exhibits structure on the micro- and nanoscale. By varying the laser fluence, we can tune the surface morphology and the wetting properties. We measured the static and dynamic contact angles for water and hexadecane on these surfaces. For water, the microstructured silicon surfaces yield contact angles higher than 160 degrees and negligible hysteresis. For hexadecane, the microstructuring leads to a transition from nonwetting to wetting.

Odd−Even Variations in the Wettability of n-Alkanethiolate Monolayers on Gold by Water and Hexadecane: A Molecular Dynamics Simulation Study

Molecular dynamics (MD) simulations were performed to investigate odd-even chain length dependencies in the wetting properties of self-assembled monolayers (SAMs) of n-alkanethiols [CH3(CH2)n-1SH] on gold by water and hexadecane. Experimentally, the contact angle of hexadecane on the SAMs depends on whether n is odd or even, while contact angles for water show no odd-even dependence. Our MD simulations of this system included a microscopic droplet of either 256 water molecules or 60 hexadecane molecules localized on an n-alkanethiolate SAM on gold with either an even or odd chain length. Contact angles calculated for these nanoscopic droplets were consistent with experimentally observed macroscopic trends in wettability, namely, that hexadecane is sensitive to structural differences between odd- and even-chained SAMs while water is not. Structural properties for the SAMs (including features such as chain tilt, chain twist, and terminal methyl group tilt) were calculated during the MD simulations and used to generate IR spectra of these films that compared favorably with experimental spectra. MD simulations of SAMs in contact with slabs of water and hexadecane revealed that the effects of these solvents on the structure of the SAM was restricted to the chain terminus and had no effect on the inner structure of the SAM. The density profiles for water and hexadecane on the SAMs were different in that water displayed a significant depletion in its density at the liquid/SAM interface from its bulk value, while no such depletion occurred for hexadecane. This difference in contact may explain the lack of an odd-even variation in the wetting characteristics of water on these surfaces, because the water molecules are positioned further away from the surface and, therefore, are not sensitive to the structural differences in the average orientations for the terminal methyl groups in odd- and even-chained SAMs. In contrast, the differences in the wetting properties of hexadecane on the odd- and even-chained SAMs may reflect the closer proximity of these molecules to the SAM surface and a resulting greater sensitivity to the differences in the terminal methyl group orientations in the SAMs. SAM-solvent interaction energies were calculated during the MD simulations, yielding interaction energies that differed on the even- and odd-chained surfaces by approximately 10% for hexadecane and negligibly for water, in accord with estimates using experimental wetting results.

Synthesis and characterization of copolymers based on styrene and partially fluorinated acrylates

Copolymers of styrene and fluorinated acrylate monomers with F-octylalkyl, F ( CF 2 ) 8 ( CH 2 ) n ′ side groups were prepared by free radical polymerization. Thermal behaviour of the resulting polymers was investigated by DSC and TGA. Even if at the macroscopic scale the polymers surfaces are homogeneous and clear, the analysis indicates that all samples exhibit two glass transitions temperatures. This discontinuity may be regarded as an indication for microphase separation of fluorine-rich and polystyrene-rich microphases. Water and hexadecane contact angles measurements show that these polymers are quite surface active in the solid state. Surface and bulk organizations were investigated by XPS analysis. A strong correlation between bulk organization and surface properties of the polymers could be established. Preferential adsorption of fluorinated segments at the material surface were more pronounced than expected in the bulk.

Vibrational spectroscopic studies of vinyltriethoxysilane sol–gel and its coating

Infrared and Raman spectra of vinyltriethoxysilane (VTES), the VTES sol–gel, and the sol–gel coated aluminum have been collected. The assignments of the vibrational modes for the silane, the sol–gel, and the sol–gel films have been made based on the group frequencies and the spectral variation collected at different physical and chemical states of the sol–gel samples. Applying the sol–gels onto the metal grids allowed the drying and the high temperature treatment of the samples for the collection of infrared transmission spectra. From the variation of the sol–gel and the sol–gel coated aluminum IR spectra with temperature, it was noticed that the samples partially decomposed when the temperature was higher than 100 °C. Electrochemical experiments have demonstrated that the anticorrosion property of the coated aluminum has significantly increased. The water and the hexadecane contact angle measurements showed that the surface modified metal had a much higher hydrophobic property than the untreated metal.

Adsorption of Lipid-Functionalized Poly(ethylene glycol) to Gold Surfaces as a Cushion for Polymer-Supported Lipid Bilayers

Inclusion of a polymer cushion between a lipid bilayer membrane and a solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, the properties of a heterofunctional, telechelic PEG lipopolymer (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N- [3-(2-(pyridyldithio)propionate]) (DSPE-PEG-PDP) adsorbed from ethanol and water solutions onto gold surfaces were studied using a variety of surface-sensitive techniques. X-ray photoelectron spectroscopy showed that the PEG molecules are tethered to the gold surface via thiolate bonds. When adsorbed from water, ethanol, or their mixtures, reflection-absorption infrared spectroscopy showed that amorphous PEG layers with disordered DSPE alkyl chains were formed, independent of adsorption time or solution concentration. On the basis of advancing and receding water and hexadecane contact angles on the lipopolymer films, the DSPE lipid groups appear to segregate from the PEG layer and become exposed at the surface of the polymer films. Swelling observed in surface plasmon resonance experiments and the large contact angle hysteresis observed indicate that highly swellable, mobile films capable of molecular rearrangements are formed. The self-assembling and amorphous properties of these PEG layers make them ideal candidates as polymer cushions for polymer-supported lipid bilayers. The DSPE surface concentration can be controlled, to a limited degree, by varying the adsorption time of DSPE-PEG-PDP from ethanol. A more effective strategy is to coadsorb DSPE-PEG-PDP with a non-lipid-functionalized PEG-PDP from an ethanol/water mixture, which allows the PEG thickness and density to remain constant while decreasing the density of DSPE groups.

Boundary Lubrication and Surface Mobility of Mixed Alkylsilane Self-Assembled Monolayers

We use a combination of experiment and theory to study the boundary lubrication characteristics of mixed self-assembled monolayers (SAMs) formed by tethering alkylsilanes with different chain lengths to planar substrates. The structure of mixed SAMs is characterized using atomic force microscopy, infrared spectroscopy, and hexadecane contact angle measurements. Lateral force microscopy is used to quantify the load and velocity dependence of friction force for one-component (pure) and two-component (mixed) SAMs. We find that if two-component SAMs and one-component SAMs are created with comparable packing density, the two-component systems possess lower friction coefficients. The friction force measured on silicon surfaces grafted with mixed SAMs increases linearly with the logarithm of sliding velocity over a broad range, whereas for pure SAMs, the friction force first increases linearly with the logarithm of velocity and then reaches a plateau. This plateau is believed to arise from the viscoelasticity of...

Structure, Wettability, and Electrochemical Barrier Properties of Self-Assembled Monolayers Prepared from Partially Fluorinated Hexadecanethiols

We report the preparation and characterization of self-assembled monolayers (SAMs) derived from the adsorption of partially fluorinated hexadecanethiols (CF3(CF2)n-1(CH2)mSH; n = 2, 4, 6, and 8; n + m = 16) onto the surface of gold. The quality of the SAMs, as measured by electrochemical impedance spectroscopy, was found to be sensitive to the solvent used as SAMs prepared in dichloromethane exhibited higher resistances and lower capacitances than those prepared in liquid CO2. The extent of fluorination was observed to influence the wettability, structure, and capacitance of the films without significantly affecting their charge-transfer resistance or their stability against exchange by a competing adsorbate. Reflectance-absorption infrared spectra showed that the fluorocarbon chains were oriented more normal to the surface for n = 8 but more parallel to the surface for n = 2. Advancing contact angles of hexadecane increased sharply with fluorination from n = 0 to 4 but slightly with additional fluorination, suggesting that hexadecane is insensitive to CH2 groups that are buried by >4 fluorinated carbon atoms. We also were able to model the dielectric properties of these partially fluorinated SAMs as two capacitors in series: one due to the outer fluorocarbon region and one due to the inner hydrocarbon region. The dielectric constants of the fluorocarbon and hydrocarbon regions of these SAMs were quantified as ∼1.7 and ∼2.3, respectively, and were comparable to the values for the pure polymer counterparts.

Low surface energy films based on partially fluorinated isocyanates: the effects of curing temperature

Self-stratification strategy can be used to prepare films in which both bulk and surface properties can be optimized. By using this approach, only a very small quantity of fluorinated species is needed to generate a surface with low surface energy. When cross-linking is involved during film formation, we are dealing with a competition behavior between the diffusion of fluorinated species and the formation of cross-linked network. In this study low surface energy polymeric films were prepared on the basis of partially fluorinated polyisocyanates, in combination with hydroxyl-end-capped three-armed solventless liquid oligoesters and modified hyperbranched polyesters. At a fluorine concentration of only 0.5 wt.%, contact angles of water and hexadecane can reach 120° and 80°, respectively. A surface energy as low as 10–15 mN/m can be obtained upon the addition of less than 1 wt.% of fluorine in the films. It was shown, from real time ATR-FTIR and contact angle measurements, that the curing temperatures demonstrated significant effects on the cross-linking rate as well as on the wettability of the films.

Ionic strength effects on hexadecane contact angles on a gold-coated glass surface in ionic surfactant solutions

Abstract As aqueous surfactant cleaning becomes increasingly prevalent in industry, an understanding behind surfactant cleaning behavior is needed to improve the systems. Improvements in these aqueous surfactant solution systems can reduce the impact of the industrial cleaning systems on the environment and improve their efficiency. Studying various parameters of the cleaning systems can provide useful insight into surfactant behavior. The contact angle of the soil to the solid surface is strongly related to cleaning system performance and is a relatively simple parameter to measure. For this study, a simplified cleaning system was studied to investigate the effects of modifications to the surfactant system. The contact angle of hexadecane on a gold-coated glass surface in aqueous surfactant solutions of sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) were determined up to and above the critical micelle concentrations. The effects of modifying the ionic strength of the aqueous surfactant solution by the addition of sodium chloride (NaCl) have been studied. Similar trends in contact angle behavior of the oil droplet to the gold-coated surface were found for both the anionic (SDS) and cationic (CTAB) surfactant systems. The addition of NaCl to the aqueous surfactant systems resulted in a significant increase in the contact angles over the surfactants concentration ranges. The increasing trend in the contact angles was observed for NaCl concentrations ranging from 0.05 to 0.5 mM, while little effect was noted for the higher NaCl concentrations. The observed effect is consistent with literature on electrostatic-charge-induced changes in surfactant adsorption at the solid surface.

Self-assembled monolayer on diamond-like carbon surface: formation and friction measurements

We have investigated self-assembled monolayers (SAMs) of heptadecafluoro-1,1,2,2-tetradecyltrietoxysilane (FTE) on diamond-like carbon (DLC) surfaces formed by a simple immersing process. SAM formation on DLC surfaces was verified by contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy (XPS). Water and hexadecane contact angles increased gradually with immersing time and saturated at about 110 and 70 degrees, respectively. Ellipsometric measurements showed that the film thickness was 1.4 to 1.6 nm, which corresponded reasonably to the thickness of FTE monolayer. XPS data showed the presence of FTE molecules on the DLC surface. These results ensured the SAM formation of FTE molecules on the DLC surface. We further measured and compared the friction of unlubricated, SAM coated and 2 nm thick perfluoropolyether (PFPE) coated DLC surfaces using lateral force microscopy (LFM) as functions of the applied load and the sliding velocity. The SAM coated DLC surfaces showed lower friction than the unlubricated DLC surfaces and the friction coefficient decreased by about 15% compared to the unlubricated DLC surfaces. Scratch tests revealed that the critical load of the DLC film increased due to the SAM deposition. These results are attributed to the hydrophobic nature of the SAM coated surface. On the other hand, even though the water contact angle of the SAM coated surface was larger than the 2 nm thick PFPE coated surface, the friction of the SAM coated surface was larger than that of the PFPE coated surface. Also, the critical load of the SAM coated DLC surface in scratch test was lower than the PFPE coated surface. These results indicate that the hydrophobic nature of the surface is not the only factor which determines the friction characteristics in the nano-lubricating system, and it is attributed to the mobile characteristic of PFPE lubricant.

Water-repellent thin films from mixtures of fluoroalkylmethoxysilane and bis-(trialkoxysilyl)alkanes of various carbon-chain lengths using the sol–gel method and the fluoroalkylmethoxysilane dispersion mechanism

Water-repellent thin films were fabricated on silicon or germanium wafers by spin-coating sol mixtures of fluoroalkyalkoxysilane (FAS) and bis-(trialkoxysilyl)alkanes with various carbon-chain lengths (Cn: (C2H5O)3Si(CH2)nSi(OC2H5)3, and Cf: (CH3O)3SiC2H4(CF2)6C2H4Si(OCH3)3; n=1, 2, and 8. C0 is defined as Si(OC2H5)4). Before heat treatment, the contact angle of hexadecane on the films increased in the order Cf, C1, C0, C2 and C8 when the FAS concentration in the sol mixture was above 5 mmol/l. With heat treatment higher than 70 °C, the contact angles on the films decreased and the longer the carbon-chain lengths of Cn and Cf the greater the decrease. We show that during spin-coating of the Cn films, FAS accumulates on the outer surface of the film. In Cn films, with n=0, separation of FAS is governed by the repulsion between the non-polar and polar chemical groups while with n≧1, separation is governed by repulsion between the fluoroalkyl-chains and the alkyl-chains. It is speculated that heat treatment causes FAS evaporation, which increases as the carbon-chain length of Cn or Cf increases, owing to the differences in their readiness to undergo hydration.

Low Surface Energy Polymeric Films from Solventless Liquid Oligoesters and Partially Fluorinated Isocyanates

Partially fluorinated isocyanates were synthesized from hexamethylene diisocyanate (HDI) and an HDI trimer (Desmodur N3300). The perfluoroalkyl group (Rf) was C6F13 or C8F17, and the ratio between Rf and the isocyanate group (NCO) was 1/99 in the case of HDI or 1/49 in the case of N3300. Polymeric films with surface energies as low as 10 mN/m were obtained from mixtures of these partially fluorinated isocyanates and a previously reported hydroxyl-end-capped solventless liquid oligoester. Contact angles of water and hexadecane reached 120° and 80°, respectively, when less than 1 wt % of fluorine was present in the films. The surface enrichment of fluorine-containing species was confirmed by X-ray photoelectron spectroscopy (XPS) investigations. At a fluorine concentration of 0.5−1.0 wt %, the surface F/C atomic ratio (at a 15° takeoff angle) was greater than 1; the surface enrichment factor of fluorine was up to 600. The topological structures of the polymeric films were recorded by an atomic force microscope under tapping mode. While the height images indicated that the surface was smooth at the nanometer scale in a 1 μm × 1 μm area, the phase images revealed that fluorine-enriched domains were present at the surface. As the fluorine concentration increased, the fluorine-enriched domains grew from tiny spots (2−3 nm) to larger round domains (15−25 nm in diameter). The low surface energies of the films could be ascribed to the strong surface segregation of fluorinated species.

Alkene based monolayer films as anti-stiction coatings for polysilicon MEMS

This paper describes a new class of anti-stiction coatings for polysilicon MEMS. This class of molecular film is based on the free radical reaction of a primary alkene (e.g. 1-octadecene C 16H 33CHCH 2) with hydrogen terminated silicon [1,2]. The new coating has several key advantages over the previously reported octadecyltrichlorosilane (OTS) and perfluorodecyltrichlorosilane (FDTS) based self-assembled monolayers (SAM) [3,4]: (1) the coating does not produce HCl at any stage in the monolayer formation whereas chlorosilane based chemistry does. (2) The coating does not require the formation of an intervening charge-trapping oxide layer. (3) The film formation procedure for alkene based monolayers is simpler than for chlorosilane based SAMs for two main reasons. First, the surface re-oxidation step is entirely eliminated. Second, the coating solution does not need to be conditioned before use, since water is not a reagent in this process. (4) The coating process is much more robust since it is essentially insensitive to relative humidity. (5) The coated structures have many fewer particulates in comparison to those coated with OTS. (6) The coating process can be made selective to coat only exposed silicon by generating radicals using a radical initiator. The coating has been evaluated in several ways, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle analysis, work of adhesion by cantilever beam array technique and coefficient of static friction using a sidewall testing device. The octadecene film is compared to the OTS SAM with respect to anti-stiction properties. XPS data confirm the absence of oxygen in both freshly prepared samples and in samples aged for more than 4 months in laboratory ambient. Water and hexadecane contact angles, and work of adhesion data are similar to those of OTS. AFM shows that the samples, which receive 1-octadecene films, accumulate far fewer particles during processing than those which receive the OTS SAM treatment. Based upon the data presented here, we find that the anti-stiction properties of films produced with the alkene chemistry are indeed comparable to those produced with the chlorosilane SAMs, but without many of the limitations imposed by the chlorosilane chemistry.

Dichlorodimethylsilane as an anti-stiction monolayer for MEMS: a comparison to the octadecyltrichlorosilane self-assembled monolayer

This paper presents a quantitative comparison of the dichlorodimethylsilane (DDMS) monolayer to the octade-cyltrichlorosilane (OTS) self-assembled monolayer (SAM) with respect to the film properties and their effectiveness as anti-stiction coatings for micromechanical structures. Both coatings have been evaluated in several ways, including atomic force microscopy (AFM), contact angle analysis (CAA), work of adhesion by cantilever beam array (CBA) technique and coefficient of static friction using a sidewall testing device. While water and hexadecane contact angles are comparable, the DDMS coated microstructures exhibit higher adhesion than OTS coated ones. Furthermore, coefficient of static friction data indicate that the DDMS films are not as effective at lubrication as the OTS SAMs are, although both exhibit much improvement over chemical oxide. However, AFM data show that the samples which receive DDMS treatment accumulate fewer particles during processing than those which receive the OTS SAM treatment. The thermal stability of the DDMS film in air far exceeds the OTS SAM, as the DDMS remains very hydrophobic to temperatures upwards of 400/spl deg/C.

Formation of Alkanethiol Monolayer on Ge(111)

Alkanethiols, CH3(CH2)(n-1)SH, are shown to react readily with HF-treated Ge(111) surface at room temperature to form a high-quality monolayer. The resulting films are characterized by using contact angle analysis (CAA), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and high-resolution electron energy loss spectroscopy (HREELS). The octanethiol and octadecanethiol films on Ge(111) both exhibit 101 degrees and 40 degrees water and hexadecane contact angles, respectively. These values indicate that the thiol surface coverage is relatively high, and that the films possess a high degree of orientational ordering. The angle-resolved XPS analysis supports that thiols are bound to the Ge surface by Ge-S bonds at the monolayer/Ge interface. The film thickness values obtained by XPS and SE agree well with the earlier reported values on alkyl monolayers on Ge(111) prepared by Grignard reaction. On the basis of HREEL spectra taken after thermal annealing steps, the monolayers are found to be thermally stable up to 450 K. The thermal stability provides further evidence that thiols are covalently bonded to Ge(111).

Contact Angle and Electrochemical Characterization of Multicomponent Thiophene-Capped Monolayers

We have investigated the wetting behavior and electrochemical response of self-assembled monolayers made from ω-(3-thienyl)-alkyltrichlorosilanes and alkyltrichlorosilanes on indium−tin oxide coated glass slides. Contact angle measurements and cyclic voltammetry show that the properties of the monolayers change as the relative loading of the two components is changed by altering the concentration of the deposition solution. The water contact angles indicate that a fully loaded hydrophobic layer is formed, and both water and hexadecane contact angle measurements show that the tethered thiophenes are at the periphery of the monolayer. However, the hexadecane contact angles also imply that the surface roughness exposes the methylene groups of the monolayer chains. Cyclic voltammograms obtained from mixtures of 11-(3-thienyl)undecyltrichlorosilane and decyltrichlorosilane and mixtures of 16-(3-thienyl)hexadecyltrichlorosilane and hexadecyltrichlorosilane show that as the relative amount of thienyl-bearing chains in the monolayer decreases, the oxidation peak position shifts to higher potential and the peak area, which is related to the amount of charge transferred, decreases. Integration of the oxidation peak area from the cyclic voltammetry experiments yields a coverage that is consistent with a monolayer. Preliminary evidence for formation of a surface-tethered redox-active polymer is observed by cycling the self-assembled monolayers at low potentials.

Photoinduced Amphiphilic Surface on Polycrystalline Anatase TiO2 Thin Films

The photoinduced wettabilities of both water and hexadecane on a polycrystalline anatase TiO2 thin film were investigated. An amphiphilic surface was produced on the polycrystalline TiO2 thin film by UV illumination as on the rutile single crystal. Further illumination of the amphiphilic surface increased the hexadecane contact angle gradually and produced a hydrophilic−oleophobic surface. The total energies of the illuminated UV light to produce and terminate the amphiphilic state were independent of the UV light intensity. This wettability change is attributable to the variations of hydrophilicizing rate of TiO2 grains, and the resultant hydrophilic−oleophobic property may contribute to overall self-cleaning performance of the TiO2 thin films.

Contact Angles on Surfaces with Mesoscopic Chemical Heterogeneity

The contact angle of water was measured on surfaces composed of random hydrophilic and hydrophobic patches with typical length scales of 10−100 nm. By quenching self-assembled monolayers at various stages of growth, the fractional surface coverage of the hydrophobic patches was varied in the range 0.04−0.97 as determined by atomic force microscopy. The cosine of the contact angle of water cos θ was systematically lower than the prediction of the mean field Cassie equation cos θC. The deviation from this prediction cos θC − cos θ had an approximately linear dependence on the total contour length between hydrophobic and hydrophilic patches (a measure of the degree of heterogeneity). The contact angle was insensitive to droplet size, suggesting that line tension effects were minimal. Also, contact angles of hexadecane were in good agreement with the Cassie prediction. We propose two possible explanations for the observed behavior. Long-range (approximately 5 nm) hydrophobic interactions may result in a relatively hydrophobic boundary region around each hydrophobic patch which effectively increases the coverage of the hydrophobic phase altering the equilibrium contact angle. Alternatively, an increased density of “pinning” sites may prevent the contact angle from relaxing to the equilibrium value.

Effect of hydrophobicity on in vitro streptococcal adhesion to dental alloys.

Non-specific interactions such as electrostatic interactions, and surface free energy are of importance in bacterial adhesion to dental surfaces as they determine whether or not bacteria are attracted to the surface. The relationship between adherence of Streptococcus mitis, S. mutans, S. oralis and S. sanguinis on precious and non-precious dental alloys, and the bacterial and alloy surface hydrophobicities (a measure of the surface free energy) was studied. The number of adhering bacteria was determined by fluorescence microscopy counts. The hydrophobicity of the bacteria and alloy surfaces were evaluated by adhesion to hexadecane and water contact angles, respectively. Our results showed that (i) the surfaces of the tested alloys were hydrophobic, (ii) S. sanguinis, S. mutans and S. oralis were hydrophobic, and (iii) S. mitis was hydrophilic. S. oralis, the more hydrophobic strain, demonstrated the highest adherence on the tested materials, whereas S. mitis adhered least on the hydrophobic surfaces. For the tested alloys, bacterial adherence was highest for the high gold content alloy, and lowest for the non-precious alloy. Our results showed that for the tested bacterial strains, there was a significant correlation between bacterial adhesion and substratum hydrophobicity: hydrophobic metal surfaces favor adhesion of hydrophobic bacteria.