Vivek Skanda, Weeks Eric R
Department of Physics, Emory University, Atlanta, Georgia, United States of America.
PLoS One. 2015 Mar 30;10(3):e0121981. doi: 10.1371/journal.pone.0121981. eCollection 2015.
We observe tracer particles diffusing in soap films to measure the two-dimensional (2D) viscous properties of the films. Saffman-Delbrück type models relate the single-particle diffusivity to parameters of the film (such as thickness h) for thin films, but the relation breaks down for thicker films. Notably, the diffusivity is faster than expected for thicker films, with the crossover at h/d = 5.2 ± 0.9 using the tracer particle diameter d. This indicates a crossover from purely 2D diffusion to diffusion that is more three-dimensional. We demonstrate that measuring the correlations of particle pairs as a function of their separation overcomes the limitations of the Saffman-Delbrück model and allows one to measure the viscosity of a soap film for any thickness.
我们观察示踪粒子在肥皂膜中的扩散，以测量肥皂膜的二维粘性特性。对于薄膜，萨夫曼 - 德尔布吕克类型的模型将单粒子扩散率与膜的参数（如厚度h）联系起来，但对于较厚的膜，这种关系就不成立了。值得注意的是，对于较厚的膜，扩散率比预期的要快，使用示踪粒子直径d时，在h/d = 5.2 ± 0.9处出现交叉。这表明从纯二维扩散转变为更三维的扩散。我们证明，测量粒子对的相关性作为它们间距的函数，可以克服萨夫曼 - 德尔布吕克模型的局限性，并能够测量任何厚度肥皂膜的粘度。
