Emulsion droplets on the colloidal length scale are a model system of frictionless compliant spheres. Direct imaging studies of the microscopic structure and dynamics of emulsions offer valuable insights into fundamental processes, such as gelation, jamming, and self-assembly. A microscope, however, can only resolve the individual droplets in a densely packed emulsion if the droplets are closely index-matched to their fluid medium. Mitigating perturbations due to gravity additionally requires the droplets to be density-matched to the medium. Creating droplets that are simultaneously index-matched and density-matched has been a long-standing challenge for the soft-matter community. The present study introduces a method for synthesizing monodisperse micrometer-sized siloxane droplets whose density and refractive index can be precisely and independently tuned by adjusting the volume fraction of three silane precursors. A systematic optimization protocol yields fluorescently labeled ternary droplets whose densities and refractive indexes match, to the fourth decimal place, those of aqueous solutions of glycerol or dimethylsiloxane. Because all of the materials in this system are biocompatible, we functionalize the droplets with DNA strands to endow them with programmed inter-droplet interactions. Confocal microscopy then reveals both the three-dimensional structure and the network of droplet–droplet contacts in a class of self-assembled droplet gels, free from gravitational effects. This experimental toolbox creates opportunities for studying the microscopic mechanisms that govern viscoelastic properties and self-assembly in soft materials.

中文翻译：

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具有可编程 DNA 相互作用的折射率和密度匹配乳液

胶体长度尺度上的乳液液滴是无摩擦柔顺球体的模型系统。对乳液微观结构和动力学的直接成像研究为了解凝胶、干扰和自组装等基本过程提供了宝贵的见解。然而，如果液滴与其流体介质的折射率紧密匹配，则显微镜只能分辨密集乳液中的单个液滴。减轻重力引起的扰动还需要液滴与介质的密度匹配。创建同时折射率匹配和密度匹配的液滴一直是软物质界面临的长期挑战。本研究介绍了一种合成单分散微米尺寸硅氧烷液滴的方法，其密度和折射率可以通过调节三种硅烷前体的体积分数来精确且独立地调节。系统优化方案产生荧光标记的三元液滴，其密度和折射率与甘油或二甲基硅氧烷水溶液的密度和折射率相匹配，精确到小数点后第四位。由于该系统中的所有材料都具有生物相容性，因此我们用 DNA 链对液滴进行功能化，赋予它们程序化的液滴间相互作用。然后，共焦显微镜揭示了一类自组装液滴凝胶中的三维结构和液滴-液滴接触网络，不受重力影响。该实验工具箱为研究控制软材料粘弹性和自组装的微观机制创造了机会。