McHale G, Käb NA, Newton MI, Rowan SM
Department of Chemistry and Physics, The Nottingham Trent University, Clifton Lane, Nottingham, NG11 8NS, United Kingdom
J Colloid Interface Sci. 1997 Feb 15;186(2):453-61. doi: 10.1006/jcis.1996.4665.
The measurement of the equilibrium contact angle of a small droplet of fluid partially wetting a flat solid surface provides information on the solid-liquid interfacial energy. However, if the spreading power, S = gammaSV - (gammaSL + gammaLV), of the surface is positive the liquid spreads completely, no equilibrium contact angle exists, and the resulting thin film has an ultimate thickness determined by Van der Waal's forces. On a chemically identical solid surface with only the geometry changed to a cylinder the same droplet of fluid which completely wets the flat surface can provide an equilibrium conformation. The indefinite spreading tendency is inhibited and the equilibrium is not necessarily a thin sheathing film about the fiber, but can have a macroscopic profile. On a high energy cylindrical surface a barrelling type droplet is only approximately spherical in cross section. Near the three phase contact line the curvature can change sign and measurement of the contact angle becomes difficult. In this work we consider the theoretical profile for such droplets and calculate the extent to which decreasing the fiber radius changes the surface energy and the maximum slope of the profile. We suggest that measurements of the inflection angle in addition to the reduced thickness and reduced length of the droplet provide an improved means of characterizing droplet on fiber systems. Experiments are reported showing the changes in contact length, droplet height, and inflection angle for poly(dimethyl)siloxane oils on copper cylinders of different diameters. These cylinders are produced from the same initial copper wire by etching in sodium hydroxide to produce controlled diameters ranging from 0.07 to 0.49 mm. As the curvature increases with reducing diameter the influence of gravity diminishes and the shape increasingly conforms to a symmetric barreling droplet type. Furthermore, as the reduced volume of fluid increases the inflection angle increases from 7° to 30° while the contact angle remains at 0°. Consistency between measured values of equilibrium parameters are compared to the theoretical values which we compute numerically and the suggested radius and volume dependence of the inflection angle is confirmed.
测量部分润湿平坦固体表面的小液滴的平衡接触角可提供有关固 - 液界面能的信息。然而，如果表面的铺展力S = γSV - (γSL + γLV)为正，则液体完全铺展，不存在平衡接触角，并且形成的薄膜具有由范德华力决定的最终厚度。在化学性质相同但几何形状变为圆柱体的固体表面上，完全润湿平坦表面的同一液滴可呈现平衡形态。无限的铺展趋势受到抑制，且平衡状态不一定是围绕纤维的薄护套膜，而是可以具有宏观轮廓。在高能圆柱表面上，桶形液滴的横截面仅近似为球形。在三相接触线附近，曲率可能会改变符号，接触角的测量变得困难。在这项工作中，我们考虑此类液滴的理论轮廓，并计算纤维半径减小对表面能和轮廓最大斜率的影响程度。我们建议，除了测量液滴的减小厚度和减小长度外，测量拐点角度可提供一种更好地表征纤维上液滴系统的方法。报告了关于不同直径铜圆柱体上聚（二甲基）硅氧烷油的接触长度、液滴高度和拐点角度变化的实验。这些圆柱体由相同的初始铜线通过在氢氧化钠中蚀刻制成，以产生范围从0.07至0.49毫米的可控直径。随着直径减小曲率增加，重力的影响减小，形状越来越符合对称桶形液滴类型。此外，随着流体减小体积增加，拐点角度从7°增加到30°，而接触角保持在0°。将平衡参数测量值之间的一致性与我们通过数值计算得到的理论值进行比较，并确认了所建议的拐点角度与半径和体积的依赖关系。
