2, Argonne National Laboratory, Lemont, Illinois, United States
3, Johns Hopkins University, Baltimore, Maryland, United States
Gels are formed when particles in solution aggregate to form a mechanically rigid system spanning network upon varying the concentration and/ or the strength of attraction between the particles. Subtle changes in these parameters can alter the formation times and the mechanical properties of the resultant gel by orders of magnitude. This sensitivity creates both a scientific challenge central to the field of soft matter but also an opportunity for designing suspensions tailored for specific applications. Such control is crucial to technologies in a diverse range of areas including ceramics, food processing, pharmaceuticals, etc. We examine the fluid to solid transition for a model system composed of nanometer scale octadecyl silica particles in decahydronaphthalene (82 nm and 110 nm, volume fraction = 0.2) that undergoes thermoreversible gelation. Taking advantage of newly developed x-ray scattering capabilities and the ability to tune precisely the strength of the particle attractions, we track the evolution in the microscopic organization and mobility of the particles and correlate them with the time-dependent macroscopic mechanical behavior of the suspensions. We find that the suspensions proceed through identical intermediate states of microscopic and macroscopic behavior even as the gel formation times vary by orders of magnitude upon changing the temperature (or, equivalently, strength of attraction) 0 – 2 K below the gel point. We propose a model of gel formation in the regime of weak attraction in which network formation is a hierarchical process whose initiation depends on the creation and stability of small clusters in which the particles arrange in locally favorable configurations. Finally, we introduce a scaling parameter that captures the similarity in the evolution of the gel as it forms at different strengths of attraction.