Tunable Interaction Potential in Charge-Stabilized Colloidal Suspensions

[7074 . 76] Larsen提出的研究将开发一种连续调节带电胶体球之间相互作用的技术。旋转电场的应用将在介电球中引起偶极矩，从而产生净吸引相互作用。这种有吸引力的相互作用可以通过改变产生电场的电压来连续调节。将使用闪烁光镊测量诱导相互作用的定量函数形式。这种可调的相互作用将为研究胶体球模型系统中的相变提供独特的机会。值得注意的是，该技术可以用来帮助辨别二维相变的基本性质。一些实验表明有一个连续的转变；另一些则表现出突然的转变，让人想起三维的转变。最近的研究（Marcus和Rice， PRE 55 p.637, 1997）表明，相互作用势的范围和性质将允许直接测试相互作用对所发生的相变类型的影响。提出的研究将开发一种连续调节微小带电球之间相互作用的方法。这种相互作用可以通过利用球体的电学特性，即通过施加电场来不断调整。这种球体是一种流行的模型系统，用于研究融化和冻结的转变，如冰融化成水。例如，对于大量的水样品，在特定的温度下会突然发生冻结转变，然而，对于非常薄的薄膜，这种转变的性质尚未得到解决。一些实验表明，薄膜的转变与散装样品一样是突然的；其他研究表明，这种转变是渐进的，就像果冻凝固一样——它会不断地变得越来越硬。最近的研究表明，薄膜的转变非常敏感地取决于球体之间相互作用的形式。通过开发一种连续调节球体之间相互作用的技术，可以直接测试这种相互作用对薄膜转变的影响。＊＊＊

9707476 Larsen The proposed research would develop a technique for continuously adjusting the interaction between charged colloidal spheres. The application of a rotating electric field will induce a dipole moment in the dielectric spheres, resulting in a net attractive interaction. This attractive interaction would be continuously tunable by varying the voltage used to create the electric field. The quantitative functional form of the induced interaction will be measured using blinking optical tweezers. This tunable interaction will then provide unique opportunities for studying phase transitions in the model system of colloidal spheres. Notably, the technique could be used to help discern the fundamental nature of two dimensional phase transitions. Some experiments have indicated a continuous transition; others demonstrate an abrupt transition reminiscent of three dimensional transitions. Recent studies (Marcus and Rice, PRE 55 p.637, 1997) have suggested that the range and nature of the interaction potential would allow for direct tests of the influence of the interaction on the type of phase transition which occurs. %%% The proposed research would develop a method for continuously adjusting the interaction between tiny charged spheres. This interaction can be continually tuned by exploiting the electrical properties of the spheres, namely through the application of an electric field. Such spheres are a popular model system for studying melting and freezing transitions such as ice melting to water. The freezing transition occurs abruptly at a specific temperature for bulk samples of water for example, however, the nature of the transition for a very thin film has not yet been resolved. Some experiments indicate that the transition for the thin film is abrupt as in the bulk sample; others have demonstrated that the transition is gradual, like jello solidifying-it continually gets stiffer and stiffer. Recent studies have suggested that the thin fi lm transition depends very sensitively on the form of the interaction between the spheres. By developing a technique to continuously tune the interaction between the spheres, the influence of this interaction on the thin film transition could be directly tested. ***