Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Tallahassee, Florida 32310, USA.
X-Ray Science Division, Argonne National Laboratory, Lemont, Illinois 60439, USA.
J Chem Phys. 2019 Sep 14;151(10):104902. doi: 10.1063/1.5111521.
A combined X-ray photon correlation spectroscopy and rheology study is carried out to capture the evolution of structure, fast particle-scale dynamics, and moduli (elastic and loss) at early times of gel formation near the fluid-gel boundary of a suspension of nanoparticles. The system is comprised of moderately concentrated suspensions of octadecyl silica in decalin (ϕ = 0.2) undergoing thermoreversible gelation. Near the gel boundary, the rate of gel formation is very sensitive to changes in attraction strength. However, we find that at different attraction strengths, the system goes through identical intermediate states of microscopic and macroscopic behavior, even though the absolute time needed to form a gel varies by orders of magnitude. We identify a single dimensionless time parameter, t/t, where t is the wait time following the quench and t is the rheologically determined gel time, that captures the similarity in gel formation at a range of attraction strengths. Following a temperature quench below the gel boundary, the system is initially fluidlike and forms diffusive clusters (∼8.5 times the particle diameter). After a lag-time, t, clusters aggregate to form a network like structure which is characterized by the onset of mechanical rigidity and a rapid growth in microscopic relaxation times. At t, the Baxter parameter obtained from adhesive hard sphere fits of the structure factor attains a constant value corresponding to the theoretical percolation boundary, thus demonstrating that gelation is percolation driven.
我们采用 X 射线光子相关光谱学和流变学相结合的方法，研究了纳米颗粒悬浮液在靠近流体-凝胶边界的凝胶形成早期，结构、快速粒子尺度动力学以及模量（弹性和损耗）的演变。该体系由十八烷氧基硅烷在十氢萘中的中等浓度悬浮液组成，经历热可逆凝胶化。在凝胶边界附近，凝胶形成的速率对吸引力强度的变化非常敏感。然而，我们发现，在不同的吸引力强度下，尽管形成凝胶所需的绝对时间相差几个数量级，但系统都经历了微观和宏观行为相同的中间状态。我们确定了一个单一的无量纲时间参数 t/t，其中 t 是淬火后的等待时间，t 是流变学确定的凝胶时间，它捕获了在一系列吸引力强度下凝胶形成的相似性。在低于凝胶边界的温度淬火后，系统最初呈流体状，并形成扩散性簇（约为颗粒直径的 8.5 倍）。在滞后时间 t 之后，簇聚合成具有机械刚性和微观松弛时间快速增长的网络状结构。在 t 时，从结构因子的粘附硬球拟合得到的 Baxter 参数达到对应于理论逾渗边界的恒定值，从而表明凝胶化是由逾渗驱动的。
