Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada.
Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.
J Chem Phys. 2018 Jan 28;148(4):044902. doi: 10.1063/1.5007038.
Colloidal suspensions transform between fluid and disordered solid states as parameters such as the colloid volume fraction and the strength and nature of the colloidal interactions are varied. Seemingly subtle changes in the characteristics of the colloids can markedly alter the mechanical rigidity and flow behavior of these soft composite materials. This sensitivity creates both a scientific challenge and an opportunity for designing suspensions for specific applications. In this paper, we report a novel mechanism of gel formation in mixtures of weakly attractive nanocolloids with modest size ratio. Employing a combination of x-ray photon correlation spectroscopy, rheometry, and molecular dynamics simulations, we find that gels are stable at remarkably weaker attraction in mixtures with size ratio near two than in the corresponding monodisperse suspensions. In contrast with depletion-driven gelation at larger size ratio, gel formation in the mixtures is triggered by microphase demixing of the species into dense regions of immobile smaller colloids surrounded by clusters of mobile larger colloids that is not predicted by mean-field thermodynamic considerations. These results point to a new route for tailoring nanostructured colloidal solids through judicious combination of interparticle interaction and size distribution.
胶体悬浮液在参数（如胶体体积分数以及胶体相互作用的强度和性质）变化时会在流体和无序固体状态之间转换。胶体特性的看似细微的变化可以显著改变这些软复合材料的机械刚性和流动行为。这种敏感性为设计特定应用的悬浮液既带来了科学挑战，也带来了机会。在本文中，我们报告了一种在具有适度尺寸比的弱吸引力纳米胶体混合物中形成凝胶的新机制。我们采用 X 射线光子相关光谱学、流变学和分子动力学模拟的组合，发现与相应的单分散悬浮液相比，在尺寸比接近 2 的混合物中，凝胶在显著较弱的吸引力下也能稳定存在。与在较大尺寸比下由耗散驱动的凝胶化相反，混合物中的凝胶形成是由物种微相分离触发的，形成的致密区域由固定不动的较小胶体组成，周围是由可移动的较大胶体簇包围，这种现象不能用平均场热力学考虑来预测。这些结果表明，通过明智地组合粒子间相互作用和尺寸分布，可以为定制纳米结构胶体固体提供一条新途径。
