Automated Axisymmetric Drop Shape Analysis-profile Research Articles

Dynamic Cycling Contact Angle Measurements: Study of Advancing and Receding Contact Angles

Dynamic cycling contact angle (DCCA) measurements of six liquids from two homologous series (i.e., alkanes and alcohols) on FC-732-coated silicon wafer surfaces were performed using automated axisymmetric drop shape analysis-profile (ADSA-P). Unlike the previous one-cycle measurements that have been made in a number of studies, these cycling contact angle measurements provide more information on the mechanisms of contact angle hysteresis θhyst. Both the advancing contact angles θa (except for the one measured from the first cycle) and the receding contact angles θr obtained from different cycles were found to be time-dependent. By comparing the results between cycles, were obtained θa and θr values at some specific drop radii. It was found that both θa and θr decreased with increasing number of cycles. Furthermore, both θa and θr values obtained at the larger contact radius were larger than those obtained at the smaller radius. The result is plausible in terms of liquid sorption and/or retention by the solid surface: the solid surface modification by the liquid increases with longer solid/liquid contact, leading to smaller values of θa and θr. It was also found that contact angle hysteresis θhyst, the difference between θa and θr at each radius, increased initially and then leveled off with increasing number of cycles. The result suggests that processes which occurred on the polymer surface during the experiment, such as liquid sorption and evaporation, will eventually approach a steady state and hence lead to constant hysteresis of the contact angle. This supports the contention that liquid sorption and/or retention is a likely cause of the time dependence of contact angle hysteresis (as well as advancing and receding contact angles). All θa data obtained beyond the first cycle and all θr data reflect liquid sorption and/or retention by the solid and are therefore not a property of the solid alone. Therefore, only θa obtained in the first cycle (on the dry solid) should be used in the calculation of the surface energetics of solids.

The effect of liquid properties to contact angle hysteresis

Low-rate dynamic advancing and receding contact angles of 21 liquids from two homologous series (i.e. n-alkanes and 1-alcohols) and octamethylcyclotetrasiloxane (OMCTS) on FC-732-coated silicon wafer surfaces were measured by an automated Axisymmetric Drop Shape Analysis-Profile (ADSA-P). The receding contact angle was found to decrease with time, suggesting surface swelling and sorption. An initial receding angle ( θ ri), which theoretically represents the receding angle before sorption starts, was extrapolated back to time of zero contact ( t 0) by least-squares regression in each experiment. The contact angle hysteresis, i.e. the difference between advancing contact angle and θ ri, was found to decrease with increasing chain length of the liquid molecules for both the alkane and alcohol series. Contact angle hysteresis was found to vanish when the chain length was extrapolated to infinity. These results are plausible in terms of liquid sorption by the solid surface: very large molecules are unlikely to penetrate into the solid film. In order to reconfirm that contact angle hysteresis depends on molecular size of the liquid, contact angles of OMCTS, which is a bulky and quasi-spherical molecule, were measured. The hysteresis was found to be 4.1°, which is smaller than for any alkane or alcohol system. The molecular size-dependent hysteresis suggests that contact angle hysteresis phenomena are mainly due to liquid penetration and surface swelling, or at least liquid retention, even on this very hydrophobic surface. The fact that receding contact angles reflect liquid retention by the solid and are therefore not a property of the solid alone, supports the widespread practice of using only the advancing contact angles in studies of surface energetics of solids and disregarding the receding contact angles.

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

Contact angle measurements and interpretation: wetting behavior and solid surface tensions for poly(alkyl methacrylate) polymers

Low-rate dynamic contact angles of a large number of liquids were measured on a poly(ethyl methacrylate) (PEMA) polymer using an automated axisymmetric drop shape analysis profile (ADSA-P). The results suggested that not all experimental contact angles can be used for the interpretation in terms of solid surface tensions: eight liquids yielded non-constant contact angles and/or dissolved the polymer on contact. From the experimental contact angles of the remaining four liquids, we found that the liquid-vapor surface tension times the cosine of the contact angle changes smoothly with the liquid-vapor surface tension, i.e. γlv cos ζ depends only on γlv for a given solid surface (or solid surface tension). This contact angle pattern is again in harmony with those from other methacrylate polymer surfaces of different compositions and side-chains. The solid-vapor surface tension of PEMA calculated from the equation-of-state approach for solid-liquid interfacial tensions was found to be 33.6 ± 0.5 mJ/m2 from the experimental contact angles of the four liquids. The experimental results also suggested that surface tension component approaches do not reflect physical reality. In particular, experimental contact angles of polar and nonpolar liquids on polar methacrylate polymers were employed to determine solid surface tension and solid surface tension components. Contrary to the results obtained from the equation-of-state approach, we obtained inconsistent values from the Lifshitz-van der Waals/acid-base (van Oss and Good) approach using the same sets of experimental contact angles.

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.

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 polystyrene and the determination of solid surface tensions

AbstractLow‐rate dynamic contact angles of 13 liquids on a polystyrene polymer are measured by an automated axisymmetric drop shape analysis – profile (ADSA‐P). It is found that 7 liquids yielded non‐constant contact angles, and/or dissolved the polymer on contact. From the experimental contact angles of the other 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. γlvcosθ 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 (7–13, 24–26). The solid‐vapor surface tension calculated from the equation‐of‐state approach for solid‐liquid interfacial tensions (33) is found to be 29.8 mJ/m2, with a 95% confidence limit of ±0.5 mJ/m2 from the experimental contact angles of 6 liquids.

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.

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.