We have investigated the birefringence in packed films of binary spherical colloidal particles. Particulate films were obtained by drying a mixed suspension of colloidal particles with two different diameters. We observed positive and negative birefringence depending on the diameters and volume ratios of the large and small particles. When the diameters of the large and small particles were similar, the films showed positive birefringence. However, negative birefringence or weakening of positive birefringence was observed in films with a large diameter ratio and an optimal volume fraction of large particles. The large particles were embedded in packed small particles in the negative and weakened positive birefringent films. We propose a packing structure in which a single shell layer of small particles formed around a large particle. Using this model, we estimated the required volume ratio of large particles, and it was in good agreement with the optimal volume fraction. The relation between the packing structure of the binary colloidal particles and the birefringence is discussed.

中文翻译：

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二元球形胶体颗粒填充薄膜的正负双折射

我们研究了二元球形胶体颗粒填充薄膜中的双折射。通过干燥具有两种不同直径的胶体颗粒的混合悬浮液获得颗粒膜。我们观察到正负双折射取决于大小颗粒的直径和体积比。当大颗粒和小颗粒的直径相近时，薄膜显示出正双折射。然而，在具有大直径比和大颗粒最佳体积分数的薄膜中观察到负双折射或正双折射减弱。大颗粒嵌入负片和弱化正片双折射薄膜中的小颗粒中。我们提出了一种填充结构，其中小颗粒的单个壳层围绕大颗粒形成。使用该模型，我们估计了大颗粒所需的体积比，并且与最佳体积分数非常吻合。讨论了二元胶体颗粒的堆积结构与双折射的关系。