Direct measurement of retarded van der Waals attraction
Direct measurement of retarded van der Waals attraction
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DOI：
10.1021/la981381l
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发表时间：
1999-11-09
期刊：
影响因子：
3.9
通讯作者：
Prieve, DC
中科院分区：
文献类型：
--
作者：
Bevan, MA;Prieve, DC
全内反射显微镜用于测量6 μ m聚苯乙烯（PS）胶乳珠与裸玻璃显微镜载玻片或旋涂有1 μ m厚PS膜的载玻片之间的相互作用的总势能，当两个相互作用体被10-300 nm的离子强度为0.5 - 3 mM的水溶液分开时。这些是第一次测量PS的微观体之间的货车德瓦尔斯相互作用穿过水，其中介电谱是众所周知的。在这些条件下，珠通过双层排斥悬浮在载玻片上方。在扣除引力贡献后，势能剖面显示出由长程货车德瓦耳斯吸引和短程双层排斥形成的0.5-2.3kT的极小值。这种吸引力可以在200 nm的距离内检测到。在分离距离大于100 nm（能量< 0.5kT）时，测量结果与使用Lifshitz理论预测两个PS半空间的相互作用以及Derjaguin近似来解释球体曲率的预测吻合得很好。在所有分离中，阻滞和屏蔽对货车范德华相互作用都是非常重要的。当分离变得小于100 nm时，测量的相互作用变得比预测的弱。使用原子力显微镜（AFM），我们观察到粗糙度与高度高达10 nm的旋涂PS膜和高达30 nm的乳胶珠。如果我们的实验“零”分离对应于最大粗糙体的接触，则理论中使用的分离距离应大于测量的分离距离。移动的凹凸高度的总和的理论曲线导致理论从overpredicting的货车德瓦尔斯吸引underpredictingit. We提出了一个新的理论，其中粗糙度被视为一个扩散膜，其组成从纯PS在内表面的纯水在外表面。组成分布可以独立于用AFM测量的高度的直方图来确定。
Total internal reflection microscopy is used to measure the total potential energy of interaction between a 6 mu m polystyrene (PS) latex bead and either a bare glass microscope slide or a glass slide spin-coated with a 1 mu m thick PS film, when the two interacting bodies are separated by 10-300 nm of aqueous solution having an ionic strength between 0.5 and 3 mM. In particular, these are the first measurements of van der Waals interaction between microscopic bodies of PS across water, for which the dielectric spectra are well-known. Under these conditions the bead is levitated above the slide by double-layer repulsion. After the gravitational contribution is subtracted, the potential energy profile displays a minimum of 0.5-2.3kT formed by long-range van der Waals attraction and shorter-range double-layer repulsion. The attraction was detectable at distances up to 200 nm. At separation distances greater than 100 nm (energy < 0.5kT), the measurements agree well with predictions using Lifshitz theory to predict the interaction of two PS half spaces, coupled with Derjaguin's approximation to account for the curvature of the sphere. At all separations, both retardation and screening are very important to the van der Waals interaction. As the separation becomes smaller than 100 nm, the measured interaction becomes weaker than predicted. Using atomic force microscopy (AFM), we observed asperities with heights up to 10 nm on the spin-coated PS film and up to 30 nm on the latex bead. If our experimental "zero" separation corresponds to contact of the largest asperities, the separation distance used in the theory should be larger than that measured. Shifting the theoretical curve by the sum of the asperity heights causes the theory to shift from overpredicting the van der Waals attraction to underpredicting it. We suggest a new theory in which roughness is treated as a diffuse film whose composition varies from pure PS at the inner surface to pure water at the outer surface. The composition profile can be determined independently from the histogram of elevations measured with AFM.