The effects of urine and temperature on the physicochemical surface properties and adhesion behaviour of uropathogenic bacteria

Bacterial adhesion in relation to urinary-tract infections has gained importance in the last years because of the increasing catheterization in hospitals to assist post-surgery flow of urine. Since the initial adhesion of bacteria to biomaterials is governed by physicochemical forces emerging from the physicochemical properties of both interacting phases, we have investigated the physicochemical surface changes of uropathogen Enterococcus faecalis ATCC29212 bacteria due to the presence of urine in its growth medium and to the differences in the environmental temperature. Urine-grown cells were found to be less hydrophobic based on water contact angles at 22°C, while no changes were detected at 37°C. In addition, they exhibited higher acid-base surface energy component than urine-free cultured cells. These changes in surface properties were also reflected in thermodynamic predictions of the adhesion to glass and silicone, which were experimentally compared with the in vitro adhesion curves obtained in a parallel plate flow chamber. The shapes of the adhesion graphs indicated that interaction free energies should be used to describe only the initial adhesion stages. Adhesion to silicone was always enhanced by urine-grown cells, while the adhesion to glass did not seem to be affected by the urine constituents. Despite the fact that the interaction free energies were not able to explain the adhesion process in some cases, changes in the electron-donor and electron-acceptor parameters of their surface free energy due to urine addition seemed to have a relation with initial adhesion rates.     

The effect of liquid-induced adhesion changes on the interfacial shear strength between self-assembled monolayers

Friction between chemically-modified tips and surfaces has been studied with chemical force microscopy (CFM) to evaluate the effect of changing solid/liquid free energy on energy dissipation in sliding tip-surface contact. Well-controlled conditions were necessary to attain a single asperity contact in these experiments. We found that in a series of methanol- water mixtures the interfacial shear strength between CH3-terminated surfaces of the siloxane self-assembled monolayers (SAMs) was independent of the adhesion force. The shear strength value of 10.2 ± 1.0 MPa found for this interface under methanol-water media is consistent with the previous studies of similar systems under dry gas conditions. A comparison to available data on interfacial shear strengths demonstrated that siloxane monolayers were much more effective in reducing friction than various carbon coatings.     

Surface characteristics and adhesion performance of black oxide coated copper substrates with epoxy resins

Black oxide is a conversion coating applied onto the copper substrate to improve its interfacial adhesion with polymeric adhesives. A comprehensive study is made to characterize the black oxide coating using various characterization techniques, including SEM, XPS, AFM, XRD, Auger electron spectroscopy, TEM, D-SIMS, RBS and contact angle measurements. It was found that the oxide coating consisted of cupric and cuprous oxide layers from the top surface to inside. The cuprous oxide layer was formed on the copper crystal surface, on which densely-packed fibrillar cupric oxide grew continuously until saturation. The cupric oxide had a fibrillar structure with high roughness at the nanoscopic scale, whereas the cuprous oxide was rather flat and granular. There was a continuous change in oxide composition with no distinct boundary between the two oxide layers. The bond strength between the epoxy resin and the oxide coated copper substrate increased rapidly at a low level of oxide thickness, and became saturated at thicknesses greater than about 800 nm. There were similar dependences of bond strength on surface roughness, oxide thickness especially of cupric oxide and surface energy, reflecting the importance of these surface characteristics in controlling the interfacial adhesion.     

Temperature influence on the physicochemical surface properties and adhesion behaviour of Enterococcus faecalis to glass and silicone

The interest in studies on the physicochemical surface properties of bacteria has increased because they are related to the causes of the initial adhesion of microorganisms to biomaterials and the subsequent biofilm formation on indwelling medical devices. The determination of physicochemical parameters such as hydrophobicity or surface tension is usually done at room temperature, not taking into account the real temperature at which bacteria cause infection inside the human body. In this work, the influence of the experimental temperature on the surface physicochemical characteristics and adhesion behaviour of Enterococcus faecalis ATCC29212 to glass and silicone has been studied. Water, formamide and diiodomethane contact angles on bacterial lawns changed when the experimental temperature was increased from 22°C to 37°C. Moreover, hydrophobicity, as determined by water contact angle, increases with temperature, in agreement with the higher and lower adhesion to silicone and glass, respectively, observed at 37°C, with respect to the results at 22°C. Also, when the formamide and diiodomethane contact angles are considered, the changes in the adhesion behaviour to glass and silicone are predicted by the sum of Lifshitz-van der Waals and acid-base interaction free energies if the measurement temperature is the same as the bacterial growth temperature, i.e. 37°C.     

Surface energy characterization of modified carbon blacks and its relationship to composites tearing properties

The effects of three types of chemical treatments, i.e. as polar acidic, polar basic, and nonpolar oxidations, on virgin carbon blacks have been studied in terms of pH, acid-base surface values, specific surface area, X-ray diffraction analysis, and surface free energy. The acidic chemical treatment leads to significant changes in surface and adsorption properties, surface free energy, and microstructures. The increased acidic surface functional groups on carbon blacks result from reaction between the basic carbon and the acidic chemical solution. Also, based on the determination of surface free energy from contact angle measurements, a good correlation between the London dispersive component or apolar (γ^d _s) of surface free energy and specific surface area (S_BET) (or crystalline size S along the c-axis, L_C) is shown in this work. Particularly, it is found that the γ^d _s of the carbon blacks studied is highly correlated with the mechanical tearing test results based on hydrocarbon rubber compound composites.     

Bacterial Adhesion to Low Energy Solid Surfaces: A Surface Thermodynamics Approach

A desired approach to reduce bacterial adhesion to ship hull, heat exchanger and medical device surfaces is to make them less attractive for bacteria by applying anti-fouling or foul-release surface coatings. However, the selection of a useful anti-fouling coating is a difficult problem and surface thermodynamics may guide us in this respect. In this work, we investigated the independent contributions of substrate–water, γ _SW ^Tot, substrate–bacteria, γ _SB ^Tot, and bacteria–water, γ _BW ^Tot, interfacial free energies to the total free energy of adhesion, ΔG _SWB ^Tot, of Pseudomonas fluorescens bacteria on the Si- and SiN-doped DLC coated glass slide surfaces using the Lifshitz–van der Waals and acid–base surface free energy components theory. It was found that mostly acid–base interactions determine the bacterial removal properties. The repulsion between bacteria and the solid surface in water increases if γ _SB ^AB is large and γ _SW ^AB is small, when they are both positive. When Lifshitz–van der Waals and acid–base components of free energy of adhesion are considered, it was found that the effect of ΔG _SWB ^LW was very small and the main effect on bacterial removal was found to depend on the ΔG _SWB ^AB parameter and bacterial % removal increased linearly with the increase of both ΔG _SWB ^AB and ΔG _SWB ^Tot parameters for all the samples.     

Investigation of optimal surface treatments for carbon/epoxy composite adhesive joints

Although an adhesive joint can distribute the load over a larger area than a mechanical joint, requires no holes, adds very little weight to the structure and has superior fatigue resistance, but it not only requires a careful surface preparation of the adherends but also is affected by service environments. In this paper, suitable conditions for surface treatments such as plasma surface treatment, mechanical abrasion, and sandblast treatment were investigated to enhance the mechanical load capabilities of carbon/epoxy composite adhesive joints. A capacitively coupled radiofrequency plasma system was used for the plasma surface treatment of carbon/epoxy composites and suitable surface treatment conditions were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time by measuring the surface free energies of treated specimens. The optimal mechanical abrasion conditions with sandpapers were investigated with respect to the mesh number of sandpaper, and optimal sandblast conditions were investigated with respect to sandblast pressure and particle size by observing geometric shape changes of adherends during sandblast process. Also the failure modes of composite adhesive joints were investigated with respect to surface treatment. From the peel tests on plasma treated composite adhesive joints, it was found that all composite adhesive joints failed cohesively in the adhesive layer when the surface free energy was higher than about 40 mJ/m², because of high adhesion strength between the plasma treated surface and the adhesive. From the peel tests on mechanically abraded composite adhesive joints, it was also found that the optimal surface roughness and adhesive thickness increased as the failure load increased.     

Adhesion characteristics of plasma surface treated carbon/epoxy composite

The load transmission capability of adhesive joints can be improved by increasing the surface free energy of the adherends with surface treatments. In this paper, suitable plasma surface treatment conditions for carbon/epoxy composite adherend were investigated to enhance the strength of carbon/epoxy composite adhesive joints using a capacitively coupled radio-frequency plasma system. Effects of plasma surface treatment parameters on the surface free energy and adhesion strength of carbon/epoxy composite were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time. Quantitative chemical bonding analysis determined with XPS (X-ray photoelectron spectroscopy) was also performed to understand the load transmission capabilities of composite adhesive joints with respect to surface treatment time.     

Effect of the microstructure of copper oxide on the adhesion behavior of epoxy/copper leadframe joints

The thermal oxidation of copper leadframe was carried out at 175°C and the adhesion behavior of the epoxy/copper leadframe joint was analyzed by investigating the microstructure changes of copper oxide with the thermal oxidation time of copper. The peel strength increased sharply at an early stage of oxidation (~20 min) followed by a slight increase. After further oxidation (120 min), the peel strength showed a slight decrease. The contact angles of water and diiodomethane decreased sharply at an early stage of oxidation with negligible change afterwards. As the oxidation time increased, X-ray photoelectron spectroscopy (XPS) results revealed that the chemical composition of copper oxide had changed (Cu/Cu₂O → Cu₂O → CuO); this change improved the wettability of the copper surface, which affected the peel strength. Increase of the surface roughness of copper oxide, investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM), causes the epoxy resin and copper oxide to undergo mechanical interlocking, which increases the peel strength. Failure analysis by SEM and XPS indicated that failure was largely in the copper oxide, and the amount of copper oxide on the peeled epoxy increased as the oxidation time increased, due to the weak mechanical strength of the oxide layer. However, a small portion of the epoxy resin was also fractured during the failure process, regardless of the oxidation time. Consequently, fracture proceeded mainly in the copper oxide close to the epoxy resin/copper oxide interface.     

Surface and adhesion changes of atmospheric barrier discharge-treated polypropylene in air and nitrogen

Oriented polypropylene treated by atmospheric barrier discharges in air and nitrogen was investigated using several techniques: contact angle measurements, ATR-FT-IR spectroscopy and two adhesion tests based on the stripping of an applied ink layer. The activation in an air discharge was found to be much weaker compared to the activation in industrial grade nitrogen, particularly with respect to adhesion. The adhesion was found to be much better in nitrogen in spite of the common use of air in industrial 'corona discharges'. A new 'abrasive shear-stripping' (AS) test for ink coating adhesion was designed and performed. It was shown that the AS test was much more sensitive than the classical adhesive tape test and was sensitive enough to monitor ageing and overtreatment. The contact angle measurements did not correlate completely with the adhesion properties and could not monitor the overtreatment, while the ATR-FT-IR technique indicated changes just for overtreated foils.     

Effect of the physical and mechanical properties of epoxy resins on the adhesion behavior of epoxy/copper leadframe joints

In this study, the adhesion strength of three epoxy resins, which are used as basic materials for epoxy molding compound (EMC) in microelectronics, to copper leadframe was determined using the peel test. The epoxy resins used were O-cresol Novolac (OCN), dicyclopentadiene (DCPD), and biphenyl sulfide (BIPHS) epoxy resins. It was found that DCPD showed the highest peel strength and OCN had the lowest value. The difference in the peel strength was explained by investigating the physical and mechanical properties, as well as the surface properties of the epoxy resins. These properties included the surface energy, viscosity and gelation time, fracture toughness, and the coefficient of thermal expansion. As a result of the lower viscosity of BIPHS and DCPD than OCN epoxy resin, BIPHS and DCPD have a better peel strength than OCN. The DCPD resin has a better peel strength than BIPHS because of its higher fracture toughness.     

Effects in Surface Free Energy of Sputter-Deposited VNx Films

Vanadium nitride (VN _x ) thin films have attracted much attention for semiconductor integrated circuit (IC) packaging molding dies, and forming tools due to their excellent hardness and, thermal stability. VN _x thin films with VN_0.45, VN_0.83, VN_1.22, VN_1.73, VN_2.06 were prepared using a radio frequency (RF) sputter technique. The experimental results showed that the contact angle at 20°C increases with increasing nitrogen content of the VN _x films, to 101.4° corresponding to VN_1.73 and then decreased. In addition, the contact angles decreased with increasing surface temperature, because an increase of the surface temperature disrupts the hydrogen bonds between water and the films and the water gradually vaporizes. The total surface fee energy (SFE) at 20°C decreased with nitrogen content of the VN _x films to 29.8 mN/m (VN_1.73) and then increased. This is because a larger contact angle means weaker hydrogen bonding which results in a lower SFE. The polar SFE component had the same trend as the total SFE, but the dispersive SFE component had the opposite trend. The polar SFE component is also lower than the dispersive SFE component. This is because hydrogen bonds are polar. The total SFE, dispersive SFE and polar SFE of the VN _x films all decrease with increasing surface temperature. This is because with increasing temperature, water evaporates from the surface, disrupting hydrogen bonds and hence increasing surface entropy. The film roughness has an obvious effect on the SFE and there is tendency for the SFE to increase with increasing film surface roughness. As a result the SFE and surface roughness can be expressed in terms of a simple ratio function.     

Modification of epoxy resins for improvement of adhesion: a critical review

Modification of epoxy resins for improvement of adhesion has been the subject of intense research throughout the world. Unlike for thermoplastics, physical blending is not successful for improvement of bond strength and impact strength of epoxy resins. The bond strength of an epoxy resin can be improved only by chemical modification with a suitable flexible modifier. Such chemical modification may either plasticize the epoxy matrix or lead to a two-phase microstructure. Both methods of chemical modifications are discussed critically in the present review.     

Wettability and surface free energy of corona-treated biaxially-oriented polypropylene film

Several methods for the determination of both the surface free energy of polymer materials and the conditions necessary to perform contact angle measurements are discussed. The effects of the corona-treatment energy on the surface free energy and on the adhesion of acrylic adhesive were studied using a biaxially-oriented polypropylene film. The surface free energy was determined by the Owens-Wendt, and van Oss-Chaudhury-Good approaches, as well as with the wettability method, using different liquids. The presented results confirm that the surface free energy value depended on both the method used and the nature of probe liquids. Thus, it cannot be considered as a parameter characterizing unambiguously the surface layer of a corona-treated film. The values of the surface free energy for different film samples can be compared with one another only if determined using the same method and the same liquids. The variations of particular components of the surface free energy with the corona-treatment energy depend on e.g. the nature of probe liquids, which makes interpretation of the observed effects difficult.     

Influence of silane coupling agents on the surface energetics of glass fibers and mechanical interfacial properties of glass fiber-reinforced composites

The effect of various silane coupling agents on glass fiber surfaces has been studied in terms of the surface energetics of fibers and the mechanical interfacial properties of composites. γ-Methacryloxypropyltrimethoxysilane (MPS), γ-aminopropyltriethoxysilane (APS), and γ-glycidoxypropyltrimethoxysilane (GPS) were used for the surface treatment of glass fibers. From contact angle measurements based on the wicking rate of a test liquid, it was observed that silane treatment of glass fiber led to an increase in the surface free energy, mainly due to the increase of its specific (or polar) component. Also, for the glass fiber-reinforced unsaturated polyester matrix system, a constant linear relationship was observed in both the interlaminar shear strength (ILSS) and the critical stress intensity factor (K_IC) with the specific component, γ_S ^SP, of the surface free energy. This shows that the hydrogen bonding, which is one of the specific components of the surface free energy, between the glass fibers and coupling agents plays an important role in improving the degree of adhesion at the interfaces of composites.     

Adhesion characteristics of carbon/epoxy composites treated with low- and atmospheric pressure plasmas

Although an adhesive joint can distribute load over a larger area than a mechanical joint, requires no holes, adds very little weight to structures and has superior fatigue resistance, it requires careful surface preparation of adherends for reliable joining and low susceptibility to service environments. The load transmission capability of adhesive joints can be improved by increasing the surface free energy of the adherends with suitable surface treatments. In this study, two types of surface treatment, namely the low pressure and the atmospheric pressure plasma treatment, were performed to enhance the mechanical load transmission capabilities of carbon/epoxy composite adhesive joints. The suitable surface treatment conditions for carbon/epoxy composite adhesive joints for both low and atmospheric pressure plasma systems were experimentally investigated with respect to chamber pressure, power intensity and surface treatment time by measuring the surface free energies of the specimens. The change in surface topography of carbon/epoxy composites was measured with AFM (Atomic Force Microscopy) and quantitative surface atomic concentrations were determined with XPS (X-ray Photoelectron Spectroscopy) to investigate the failure modes of composite adhesive joints with respect to surface treatment time. From the XPS investigation of carbon/epoxy composites, it was found that the ratio of oxygen concentration to carbon concentration for both low and atmospheric pressure plasma-treated carbon/epoxy composite surfaces was maximum after about 30 s treatment time, which corresponded with the maximum load transmission capability of the composite adhesive joint.     

Surface characterization and adhesion of black-oxide-coated copper substrate: effect of surface hardening processes

The effects of surface-hardening processes on the changes in surface characteristics and adhesion of black copper oxide substrate with epoxy resins are studied. Various techniques, namely SEM, XPS, AFM, XRD, Auger electron spectroscopy, contact angle goniometry, D-SIMS and RBS, were used to identify the changes in surface characteristics. Dense, fibrillar cupric oxide crystals characterized the as-deposited oxide coating with high surface roughness. The surface-hardening process flattened and consolidated the fibrils without changing the compositional and thermodynamic characteristics of the coated surface. The surface-hardening process reduced the total thickness of copper oxide by approximately 50–150 nm. The reduction in oxide thickness was not a predominant factor for the reduced bond strength of the surface-hardened coating. The bond strengths of both the as-deposited and surface-hardened black oxide coatings increased with oxidation time, until saturation at about 120–150 s. For the as-deposited oxide coating, mechanical interlocking, high wettability and resistance to surface contamination are the three major sources for improved adhesion, amongst which the enhanced mechanical interlocking provided by the fibrillar cupric oxide is the most important. Surface hardening reduced the efficiency of mechanical interlocking mechanism. There was close functional dependence between the button-shear strength and surface characteristics, such as surface roughness, coating thickness and surface free energy.     

Characterization of epoxy resin surface energetics

This study has characterized the energetics of both the liquid state and the solid state of two commercially available epoxy resins: a DGEBA- and a TGMDA-based epoxy system. The surface properties of the liquid epoxies were evaluated by wetting measurements using a dynamic contact angle analysis (DCA). The Lifshitz-van der Waals components of the surface tension were found to be similar for both epoxy systems, while the acid-base components were found to be slightly different. Two different techniques were used to characterize the cured epoxy surface properties: wetting measurements and vapor adsorption measurements by means of inverse gas chromatography (IGC). The Lifshitz-van der Waals components of the surface energy were observed to be nearly the same for both epoxies, confirming that both resins have the same potential for non-specific interactions, in both liquid and solid states. Evaluations of the acid-base components of the work of adhesion by DCA and the Gibbs free energy change by IGC suggest that both cured epoxies show non-negligible specific interactions with both acidic and basic probes. However, computations of the accepticity and donicity parameters showed that both cured epoxies are predominantly basic, but also possess non-negligible acidity. It is likely that the presence of water on the solid surface contributes to the acidic character of the cured epoxies. The temperature dependence of the liquid surface tension for both epoxy systems was investigated. The same temperature dependence was observed: the surface tension decreased with temperature, following a linear regression. Corrections for viscous-drag effects on the liquid surface tension measurements were also made.     

The critical humidity effect in the adhesion of epoxy to glass: role of hydrogen bonding

Adhesion between an epoxy [diglycidyl ether of bisphenol F (DGEBF) cured with diethylene triamine] and glass was lowered abruptly when the epoxy was equilibrated with air whose relative humidity (RH) exceeded a critical value of approximately 70% RH. The critical humidity marking the onset of adhesion loss was also associated with a sudden increase of water uptake by the epoxy. In earlier work, it was shown that this 'transition' was not due to capillary condensation, osmotic cell formation or a decrease in the T _g of the material. Instead, it was speculated that the critical humidity effect was due to the trapping of water by hydroxyl groups which become available as inter-chain hydrogen-bonded structures are broken. To verify the above hypothesis, two model compounds were synthesized. One closely mimicked the cured DGEBF resin and the other had all of its hydroxyl groups replaced by hydrogen. Comparison of the water sorption isotherms of these two model compounds clearly suggested that hydroxyl groups played a key role in the critical humidity effect. Using molecular simulation software, hydrogen bonding between the various polar sites of the hydroxylated model compound was also studied. In the dry state or at low water concentrations, simulations predicted the formation of hydrogen bonds between polar sites. These hydrogen bonds always involved one or more hydroxyl groups. At higher water concentrations, molecular simulations showed that water tended to displace the hydrogen bond network of the epoxy, and in the process, water-mediated 'bridges' between polar groups were formed. The large decrease in entropy associated with the formation of such macrocyclic conformers is thought to be offset by the decreased enthalpy of condensation of water made possible by multiple hydrogen bonding. This suggests that the critical humidity effect might be an 'order-disorder' transition associated with the formation of ring structures closed by hydrogen-bonded water linkages between polar groups. The first-order energetics of this type of transition is consistent with the abrupt nature of the critical humidity effect.     

A study of adhesion forces by atomic force microscopy

An atomic force microscope (AFM) was used to investigate Si₃N₄ tip interactions with various materials in four different liquid media (water, ethanol, ethylene glycol, and formamide). The adhesion forces calculated using surface energies and the values measured experimentally were compared. For all materials, the calculated adhesion force closely correlated with AFM measurements, except in water. In the case of water, the AFM experiments showed strong adhesion, whereas theoretically (van Oss-Chaudhury-Good model) repulsion is predicted. The difference observed is discussed in terms of the chemical interactions between Si₃N₄ and water.