Saint-Michel Brice, Petekidis George, Garbin Valeria
Department of Chemical Engineering, Delft University of Technology, Delft 2629 HZ, The Netherlands.
IESL - FORTH and Department of Material Science and Technology, University of Crete, GR - 71110, Heraklion, Greece.
Soft Matter. 2022 Mar 9;18(10):2092-2103. doi: 10.1039/d1sm01771c.
Colloidal gels possess a memory of previous shear events, both steady and oscillatory. This memory, embedded in the microstructure, affects the mechanical response of the gel, and therefore enables precise tuning of the material properties under careful preparation. Here we demonstrate how the dynamics of a deformable inclusion, namely a bubble, can be used to locally tune the microstructure of a colloidal gel. We examine two different phenomena of bubble dynamics that apply a local strain to the surrounding material: dissolution due to gas diffusion, with a characteristic strain rate of ∼10 s; and volumetric oscillations driven by ultrasound, with a characteristic frequency of ∼10 s. We characterise experimentally the microstructure of a model colloidal gel around bubbles in a Hele-Shaw geometry using confocal microscopy and particle tracking. In bubble dissolution experiments, we observe the formation of a pocket of solvent next to the bubble surface, but marginal changes to the microstructure. In experiments with ultrasound-induced bubble oscillations, we observe a striking rearrangement of the gel particles into a microstructure with increased local ordering. High-speed bright-field microscopy reveals the occurrence of both high-frequency bubble oscillations and steady microstreaming flow; both are expected to contribute to the emergence of the local order in the microstructure. These observations open the way to local tuning of colloidal gels based on deformable inclusions controlled by external pressure fields.
胶体凝胶具有对先前剪切事件（包括稳态和振荡剪切）的记忆。这种记忆嵌入在微观结构中，会影响凝胶的力学响应，因此在精心制备的情况下能够精确调整材料特性。在此，我们展示了可变形内含物（即气泡）的动力学如何用于局部调整胶体凝胶的微观结构。我们研究了气泡动力学的两种不同现象，它们会对周围材料施加局部应变：由于气体扩散导致的溶解，其特征应变率约为10 s；以及由超声驱动的体积振荡，其特征频率约为10 s。我们使用共聚焦显微镜和粒子追踪技术，通过实验表征了Hele-Shaw几何结构中气泡周围模型胶体凝胶的微观结构。在气泡溶解实验中，我们观察到气泡表面旁边形成了一个溶剂袋，但微观结构变化不大。在超声诱导气泡振荡的实验中，我们观察到凝胶颗粒显著重排为局部有序性增加的微观结构。高速明场显微镜揭示了高频气泡振荡和稳定微流的发生；两者都有望促成微观结构中局部有序性的出现。这些观察结果为基于由外部压力场控制的可变形内含物对胶体凝胶进行局部调整开辟了道路。
