Low-rate Dynamic Contact Angles Research Articles

Measurement of contact angle under different gravity generated by a long-arm centrifuge

The contact angle, as a vital measured parameter of wettability of a material surface, is pertinent to adhesion, nucleation, capillary phenomena, and liquid management in space. With the development of space exploration, the relationship between the contact angle and gravity has attracted increasing attention. In this study, we designed and manufactured a long-arm centrifuge (d =6 m) and a matched low-rate dynamic contact angle measurement system, which has the capacity to measure the advancing and receding contact angles of sessile drops under 1G-8G. The advancing and receding contact angles of water on smooth and rough surfaces were measured, and the results showed that 1) this apparatus is reliable for measuring the advancing and receding contact angles of sessile drops under 1G-8G, 2) the contact angle is not changed by gravity, while the measured apparent contact angles are changed by gravity owing to they are slope angles of liquid-vapor interface away from the solid surface due to limitation of the resolution of measurement system, and 3) wetting state of the rough surface can be changed by gravity.

Interfacial energy of heavy naphthenic bitumen in aqueous medium

The impact of the interfacial properties of naphthenic bitumen is very relevant for road paving. In this work, we propose an approach for predicting the interfacial energy of bitumen in aqueous media based on contact angle hysteresis measurements. We measured the surface tension of naphthenic bitumen at high temperature with the pendant drop method. We estimated the surface energy of bitumen as the value of surface tension extrapolated to room temperature. The acid–base properties of naphthenic bitumen films were investigated by contact angle titration with buffered water. Due to the occurrence of contact angle hysteresis, we performed advancing–receding contact angle measurements on bitumen films at different values of pH with the low-rate dynamic contact angle technique. We observed that the receding angle was dependent on pH. Instead, advancing contact angle just reflected the nature of bitumen in dry conditions. We chose the average contact angle of advancing and receding contact angles as an estimate of the thermodynamically meaningful contact angle. Finally, the bitumen-water interfacial energy at room temperature was calculated using the Young equation. The low interfacial energy of bitumen at high pH confirmed the existence of native surfactants.

The effect of contact line dynamics and drop formation on measured values of receding contact angle at very low capillary numbers

Owing to contact angle hysteresis, the contact angle that a sessile drop adopts on a surface can be quite different depending on the process of drop formation. This is particularly noticeable for receding contact angle because it is most susceptible to the details of the measuring method. We performed low-rate dynamic contact angle experiments with steadily moving contact lines and millimeter-size drops in the very low-capillary number regime. We employed a quadratic volumetric flow rate (V∝t3) to achieve the steady motion of contact lines such as happens in the Wilhelmy balance. The values of receding contact angle provided with the quadratic flow rate were stable and time-independent over a larger area of the polymer surfaces studied. Next, we monitored receding sessile drops with equal initial volume but different static contact angle on the same surface. This procedure allowed us to scan the contact angle hysteresis range up to the minimum observable value of contact angle. We probed with distilled water the different response of surfaces of polymer (PMMA, PC and PTFE) and metal oxide (titanium). We found two behaviors according to the substrate employed: a constant value of receding contact angle regardless of the static contact angle and a decrease of receding contact angle as the static contact angle decreased.

Effects of Surface Defects, Polycrystallinity, and Nanostructure of Self-Assembled Monolayers for Octadecanethiol Adsorbed onto Au on Wetting and Its Surface Energetic Interpretation

We report low-rate dynamic contact angle data of various liquids on self-assembled monolayers (SAMs) of octadecanethiol on annealed and nonannealed gold. It was found that interpretation of solid surface tensions using contact angle data on less well-prepared polycrystalline nonannealed gold surfaces can be misleading. Our findings were supported by reflectance infrared spectra and atomic force microscopy data that the surface nanostructure, defects, and polycrystallinity can be important factors for a systematic study of wettability on SAMs in terms of surface energetics. We found that the contact angle and adhesion patterns of various liquids on SAMs of octadecanethiol adsorbed onto annealed gold substrates are consistent with recent experimental data for the relatively thick polymer-coated surfaces. The variation of surface structure in terms of surface energetics can be estimated only when a fundamental understanding of contact angles and surface tensions is known. We estimated an increase in the soli...

Dynamic contact angle and spreading rate measurements for the characterization of the effect of dentin surface treatments

A new methodology capable of providing reliable and reproducible contact angle ( θ) data has been employed to study the effect of clinical treatments grinding, acid etching, and deproteinization on medial dentin tissue. It is based on the application of the ADSA-CD algorithm to the determination of low-rate dynamic contact angles, obtained from slowly growing drops, and on contact angle measurement, as well as spreading behavior analysis, during the relaxation of the system (water on treated dentin) after initial drop growth. The θ data obtained were substantially more reproducible than those obtained with classical methods. A net effect of the treatment on θ was found, increasing dentin wettability: θ (polished) > θ (etched) > θ (deproteinized). The spreading rates correlate with the angles and are adequate for the dentin surface characterization. ANOVA and SNK tests show that for advancing contact angles the means corresponding to all treatments are significantly different. In the relaxing phase, mean angle and spreading rates on polished dentin differ significantly from those on etched and deproteinized dentin, but the latter do not differ significantly from each other.

Influence of Aqueous Electrolytes on the Wetting Behavior of Hydrophobic Solid Polymers—Low-Rate Dynamic Liquid/Fluid Contact Angle Measurements Using Axisymmetric Drop Shape Analysis

The interaction of inorganic ions with low-energy hydrophobic surfaces was examined using model systems of solid polymers without ionizable functional surface groups in aqueous electrolyte solutions. Low-rate dynamic contact angle measurements with captive bubbles in conjunction with axisymmetric drop shape analysis (ADSA) were performed to study the influence of electrolyte ions (in the aqueous test solutions) on the wettability of the polymers. When various types of ions were used, no significant change in advancing and receding contact angles was observed. The contact angle hysteresis was small. The zeta potential of the model polymers in aqueous electrolyte solutions was determined from streaming potential measurements. The variation of the zeta potential at different pH levels indicates preferential adsorption of hydroxyl ions at this interface. However, the presence of electrolytes at the interface between water and the different model polymers did not influence the macroscopic contact angle. The results may suggest the absence of any specific interaction between the ions and the solid polymer, as this should result in changes of hydrophobicity. Similar to the air/water interface, the composition and the potential of the polymer/water interface are obviously determined predominantly by the aqueous phase with only slight influence from the solid phase.

Contact angle hysteresis on dentin surfaces measured with ADSA on drops and bubbles

Contact angle measurement on biological surfaces is difficult due to substrate hydration, porosity and heterogeneity. The former promotes very low contact angles, and the latter leads to irregular drop shapes and contact angle hysteresis, being meaningful in terms of surface energetics only low rate dynamic contact angles, i.e. the extreme angles corresponding to a slowly advancing and receding contact line. ADSA-CD has been developed to take into account the drop shape irregularity. It requires as input the liquid surface tension and density, the co-ordinates of some contact line points and the drop volume, yielding a mean contact angle. Due to the latter requirement ADSA-CD has been used until now only for static contact angle measurements, i.e. at constant drop volumes. We present an upgrade of the method that permits to control very precisely the drop volume, and to capture simultaneously drop images for contact angle determination, permitting this way to use ADSA-CD for hysteresis measurements. Low contact angles lead to difficulties in their measurement, being the captive bubble technique a known method to overcome this shortcoming. As our system is also capable to measure captive bubble low rate dynamic contact angles, we performed an experimental study with Teflon and bitumen model surfaces, which permits to identify the advancing bubble angles with the receding drop angles. We were therefore able to measure hysteresis on dentin tissue, which has a too low receding drop angle to be measured by ADSA-CD.

Wettability of biodegradable surfaces

The study of the interfacial characteristics of biodegradable polymers/copolymers is of importance from the point of view of both surface science and pharmaceutical/cosmetic applications. Films formed from biodegradable polymers allow systematic wettability studies on surfaces with a wide range of copolymer (chemical) compositions. The possibility of interchanging these drug carrier polymers, if their wetting characteristics are similar, could be beneficial to diverse applications. Low-rate dynamic contact angles on films (solvent cast on polar substrates, i.e. on silicon wafer) of poly(lactic acid), and its copolymers with poly(glycolic acid), (with four different copolymer ratios of 85/15, 75/25, 65/35 and 50/50) were measured by axisymmetric drop shape analysis-profile (ADSA-P) with four liquids: water, formamide, 2,2′-thiodiethanol and 3-pyridylcarbinol. The solid surface tensions, γsv, were calculated from the advancing contact angles, θA. The surface topography of the polymer films was investigated by atomic force microscopy (AFM). The surface composition of the polymer layers was analyzed by X-ray photoelectron spectroscopy (XPS). The advancing contact angles were found to be independent of the composition of the copolymers, while the receding angles, θR, did decrease with increasing ratio of the polar component [poly(glycolic acid)] in the copolymers. The solid surface tensions calculated from the advancing contact angles of the liquids for all homo- and copolymers were the same within the error limit; the mean value being γsv=35.6 ± 0.2 mJ/m2. The surface roughness, which was obtained from AFM images, increased with increasing poly(glycolic acid) ratio, without affecting the advancing contact angles. The constancy of γsv is attributed to the effect of the surface activity of the nonpolar segments of the polymer chains, which oriented to form the outermost layer of the film. This was confirmed by XPS analysis.

Wetting of heterogeneous surfaces of block copolymers containing fluorinated segments

The wetting of well-characterized heterogeneous surfaces of block copolymers has been studied by low-rate dynamic contact angle measurements using axisymmetric drop-shape analysis. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to investigate the roughness, the heterogeneity and the chemical composition of the surfaces. By changing the block length of polysulfone and semifluorinated polyester segments in the block copolymers, the surface heterogeneity of thin films prepared on silicon wafers could be controlled. Tapping-mode AFM measurements showed that soft, hydrophobic domains of varying size on the submicrometer length scale were obtained on these surfaces (60–250 nm). The mean roughness was of the order of several nanometers. The results of the contact angle measurements showed that neither roughness nor heterogeneity had a significant effect on the advancing contact angle of water, at the scale of the features present; however, the contact angle hysteresis increased with increasing percentage of the soft domains. We assume that liquid retention by the solid upon retraction of the three-phase line is the main cause for the observed increase in contact angle hysteresis. Concerning the molecular composition of these block copolymer surfaces, angle-resolved XPS analysis showed a surface segregation of fluorine within the surface region. A direct correlation was found between the fluorine content of the block copolymer surfaces and the advancing contact angle of water.

Dependence of the contact angle on the molecular structure of poly(propene-alt-N-maleimide) and poly(styrene-alt-N-maleimide) copolymers

Dynamic contact angle measurements on poly(propene-alt-N-maleimide) and poly(styrene-alt-N-maleimide) copolymers are presented. The copolymers were altered by side chains of saturated hydrocarbons of increasing length. Films of these copolymers were formed on silica wafers by casting from polymer solutions. Axisymmetric Drop Shape Analysis for Profiles (ADSA-P) was used for low rate dynamic contact angle measurements. The solid surface tension γSV was calculated using an “Equation of State”. For poly(propene-alt-N-maleimide) copolymers increasing length of the side chains lead to an increasing contact angle. The solid surface tension γSV varied in the range from 22.6 to 41.5 mJ/m2. For a side chain with 12 carbon atoms the surface tension γSV is at the value reported in the literature for surfaces consisting mainly of CH3-groups. The wetting behavior for poly(styrene-alt-N-maleimide) copolymers, however, showed a very strange behavior for water and glycerol, respectively. No trend of the contact angle versus the length of the side chain could be found. Instead, two different ranges of the contact angle appeared. At three carbon atoms it is inconsistent. Molecular modeling suggested a conformation for poly(styrene-alt-N-maleimides) to explain the unexpected behavior. This model was supported by wide angle X-ray diffraction (WAXS). A similar conformation was also suggested by Natta. From this it can be concluded that contact angle measurements are very sensitive to structural changes and represent a simple, yet powerful technique to characterize solid surfaces.

Influence of different alkyl side chains on solid surface tension of polymethacrylates

Low-rate dynamic contact angles on poly(t-butyl methacrylate) (PtBMA) were measured by an automated axisymmetric drop shape analysis profile (ADSA-P). The solid surface tension of PtBMA is calculated to be 18.1 mJ/m2, with a 95% confidence limit of ±0.6 mJ/m2. This value was compared to previous results with different homopolymeric polymethacrylates [poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), and poly(n-butyl methacrylate) (PnBMA)] and with copolymeric polymethacrylates {poly(methyl methacrylate/ethyl methacrylate, 30/70) [P(MMA/EMA, 30/70)] and poly(methyl methacrylate/n-butyl methacrylate) [P(MMA/nBMA)]}. It was found that increasing length and size of the alkyl side chain decrease the solid surface tension, as expected. Comparison with pure alkyl surfaces suggests that the surface tension of PtBMA is dominated by the very hydrophobic t-butyl group. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2493–2504, 2000

Characterization of adsorbed protein layers by low-rate dynamic liquid–fluid contact angle measurements using axisymmetric drop shape analysis (part II)

Inverse low-rate dynamic contact angles were measured using captive air bubbles in conjunction with axisymmetric drop shape analysis (ADSA) to study the wettability of adsorbed protein layers in their highly hydrated state at the stationary end point of the adsorption process. The initial surface state of smooth fluorohydrocarbon polymer films, used as substrates for the adsorption of two human plasma proteins (albumin, fibrinogen) from phosphate buffered saline, has also been studied by low-rate dynamic contact angle measurements using sessile liquid drops or captive air bubbles. The solid–vapour surface tension of the dry polymer film calculated from the equation-of-state approach for solid–liquid interfacial tensions was found to be 14.1 mJ m −2, from the mean advancing water contact angle of 114.9±0.9°. Concerning the receding contact angles of the sessile liquid drops, a time dependence was revealed probably due to the modification of the polymer surface by the liquids. After the protein adsorption the physicochemical state of the solid–liquid interface is changed from a hydrophobic to a hydrophilic state observed by the decrease of the inverse advancing and receding contact angles. Differences in the overall hydrophobicity/hydrophilicity of the adsorbed protein layers could be revealed depending on the type of protein and on the concentration of the protein solution.

Low-rate dynamic contact angles on poly(methyl methacrylate/ethyl methacrylate, 30/70) and the determination of solid surface tensions

Low-rate dynamic contact angles of 12 liquids on a poly(methyl methacrylate/ethyl methacrylate, 30/70) P(MMA/EMA, 30/70) copolymer were measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). It was found that five liquids yield nonconstant contact angles, and/or dissolve the polymer on contact. From the experimental contact angles of the remaining seven liquids, it is found that the liquid-vapor surface tension times cosine of the contact angle changes smoothly with the liquid-vapor surface tension (i.e., γlυ cos θ depends only on γlυ for a given solid surface or solid surface tension). This contact angle pattern is in harmony with those from other methacrylate polymer surfaces previously studied. 45,50 The solid-vapor surface tension calculated from the equation-of-state approach for solid-liquid interfacial tensions 14 is found to be 35.1 mJ/m 2 , with a 95% confidence limit of ± 0.3 mJ/m 2 , from the experimental contact angles of the seven liquids.

Low-rate dynamic contact angles on poly[styrene-alt-(hexyl/10-carboxydecyl(90/10)maleimide)] and the determination of solid surface tensions

Low-rate dynamic contact angles of 14 liquids on a poly[styrene-alt-(hexyl/10-carboxydecyl(90/10)maleimide)] P[S-(H/CM)] were measured by means of an automated axisymmetric drop shape analysis-profile (ADSA-P). It was found that 9 liquids yield non-constant contact angles, and/or dissolve the polymer. From the experimental contact angles of the remaining 5 liquids, it was found that the product of the liquid-vapour surface tension and the cosine of the contact angle changes smoothly with the liquid-vapour surface tension, i.e. γ lv cosθ depends only on γ lv for a given solid surface (or solid surface tension). This contact angle pattern is in harmony with those from other inert and non-inert (polar and non-polar) surfaces. The solid-vapour surface tension calculated from the equation-of-state approach for solid-liquid interfacial tensions was found to be 31.0 mJ/m 2 , with a 95% confidence limit of ±0.6 mJ/m 2 from the experimental contact angles of the 5 liquids in Laboratory 1, and γ sv = 29.9 mJ/m 2 from measurements with two liquids in Laboratory 2. The difference is essentially due to actual physical differences between the polymer films prepared in the two laboratories.

Contact angle measurements by axisymmetric drop shape analysis and an automated polynomial fit program

Low-rate dynamic contact angles of eight liquids on an inert (non-polar) FC-722 surface and a non-inert (polar) poly(propene- alt- N-methylmaleimide) copolymer are reported using two different contact angle methods: axisymmetric drop shape analysis (ADSA) and an automated polynomial fit (APF) scheme. The latter technique was found to be more sensitive to minute surface heterogeneity and/or roughness of the surface. For the non-inert solid–liquid systems, very complex contact angle responses were observed with both methods. If one omits these inconclusive contact angle measurements, the values of γ lv cos θ change smoothly with γ lv for these polymers, independent of which of the two methods is used. When comparing with previously studied maleimide copolymer surfaces, a consistent change in the wettability was observed as the length of the side chains decreases from hexyl to methyl groups.

Measuring and interpreting contact angles: a complex issue

Low-rate dynamic contact angles of 30 liquids on a FC-725-coated wafer surface were measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). Surprisingly, results indicate that FC-725 behaves differently in some respects from what one would expect for non-polar surfaces: only nine liquids yield essentially constant contact angles whereas the others show slip/stick contact angle behaviour. In the worst case, the contact angle increases from ca 50 to 160° at essentially constant three-phase contact radius. These angles should be disregarded for the interpretation in terms of surface energetics since there is no guarantee that Young's equation is applicable. If one employs a conventional goniometer-sessile drop technique, such contact angle behaviour cannot be easily seen in all cases. These results indicate that the claim from van Oss et al. [Langmuir 4 (1988) 884] that liquids with the same contact angles do not have the same surface tensions is misleading. If the meaningful contact angles are plotted as the liquid–vapour surface tension times cosine of the contact angle versus the liquid–vapour surface tension, that is, γ lv cos θ versus γ lv, a smooth curve emerges. Thus, intermolecular forces (or surface tension components) do not have an additional and independent effect on the contact angles, in good agreement with the results from other studies on non-polar and polar polymers.

Low-Rate Dynamic Contact Angles on Poly(methyl methacrylate) and the Determination of Solid Surface Tensions

Low-rate dynamic contact angles of nine liquids on a poly(methyl methacrylate) (PMMA) polymer are measured by an automated axisymmetric drop shape analysis—profile (ADSA-P). It is found that two liquids dissolved the polymer on contact. From the experimental contact angles of the other seven polar and nonpolar liquids, it is found that the liquid–vapor surface tension times cosine of the contact angle changes smoothly with the liquid–vapor surface tension (i.e., γlνcos θ depends only on γlνfor a given solid surface). The dependence of γlνcos θ on γsνis explicitly illustrated by replacing the solid surface from the PMMA to other methacrylate polymers: such a procedure shifts the curves in a very regular manner. Thus, because of Young's equation, γsldepends only on γlνand γsν. This contact angle pattern is in harmony with those from other inert and noninert (polar and nonpolar) surfaces. The solid–vapor surface tension of PMMA calculated from the equation of state approach for solid–liquid interfacial tensions is found to be 38.5 mJ/m2, with a 95% confidence limit of ±0.5 mJ/m2from the experimental contact angles of the seven liquids.

Low-rate Dynamic Contact Angles on Poly(methyl methacrylate/n-butyl methacrylate) and the Determination of Solid Surface Tensions

Low-rate dynamic contact angles of 12 liquids on a poly(methyl methacrylate/n-butyl methacrylate) P(MMA/nBMA) copolymer are measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). It is found that 6 liquids yield non-constant contact angles, and/or dissolve the polymer on contact. From the experimental contact angles of the remaining 6 liquids, it is found that the liquid- vapour surface tension times the cosine of the contact angle changes smoothly with the liquid-vapour surface tension, i.e., γiv cos θ depends only on γiv for a given solid surface (or solid surface tension). This contact angle pattern is in harmony with those from other inert and noninert (polar and non-polar) surfaces [34-42, 51 -53]. The solid-vapour surface tension calculated from the equation-of-state approach for solid -liquid interfacial tensions [14] is found to be 34.4 mJ/m2, with a 95% confidence limit of \\pm 0.8mJ/m2, from the experimental contact angles of the 6 liquids.

Determination of contact angles and solid surface tensions of poly(4-X-styrene) films

Low-rate dynamic contact angles of water, glycerol and methylene iodide on polystyrene and poly(4-(X = CH3, (CH3)3, Cl, OH)-styrene) films were measured by axisymmetric drop shape analysis-profile (ADSA-P) for sessile drops. It was found that glycerol in the case of poly(4-chlorostyrene) and methylene iodide on all investigated surfaces did not yield constant contact angles: dissolving of the polymer during the contact with the liquid, penetration effects and stick/slip behavior were found. Water and glycerol yielded meaningful constant contact angles (except for glycerol on poly(4-chlorostyrene), for which we used formamide additionally). From these meaningful contact angles the solid surface tensions of the modified polymers were calculated using the equation of state approach [6]. The following values of γsv were determined: polystyrene γsv = 28.3 mJ/m2, poly(4-methylstyrene) γsv = 25.8 mJ/m2, poly(4-tert-butylstyrene) γsv = 22.0 mJ/m2, poly(4-hydroxystyrene) γsv = 44.1 mJ/m2, poly(4-chlorostyrene) γsv = 27.2 mJ/m2.

Low-rate dynamic contact angles on poly( n -butyl methacrylate) and the determination of solid surface tensions

Low-rate dynamic contact angles of 22 liquids on a poly(n-butyl methacrylate) (PnBMA) polymer are measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). It is found that 16 liquids yielded non-constant contact angles, and/or dissolved the polymer on contact. From the experimental contact angles of the remaining 6 liquids, it is found that the liquid–vapor surface tension times cosine of the contact angle changes smoothly with the liquid–vapor surface tension, i.e. γlv cos θ depends only on γlv for a given solid surface (or solid surface tension). This contact angle pattern is in harmony with those from other inert and non-inert (polar and non-polar) surfaces [34–37, 45–47]. The solid–vapor surface tension calculated from the equation-of-state approach for solid-liquid interfacial tensions [14] is found to be 28.8 mJ/m2, with a 95% confidence limit of ±0.5 mJ/m2, from the experimental contact angles of the 6 liquids.