Acoustic monitoring of the gelation of a colloidal suspension

Because they are sensitive to mechanical properties of materials and can propagate even in opaque systems, acoustic waves provides us with a powerful tool for local rheological characterization of various systems. While most common acoustic techniques rely on time-of-flight measurements, acoustic spectra of speed and attenuation contain rich information on the propagation medium which led to the development of acoustic spectroscopy techniques. They however remain underused in the field of complex fluids because of the difficulty to interpret quantitatively acoustic signals. In this note, we use a simple ultrasound spectroscopy set up to investigate the gelation dynamics by precipitation of a silica colloidal gel. First, we show that a simple analysis of acoustic attenuation allows to define a gelation time, which is proportional to this obtained with rheological measurements. This validates the possibility to use acoustic spectroscopy for monitoring of gelation process, and our setup also has the possibility to perform mappings, showing that the formed gels display some heterogeneity. By studying in more detail acoustic spectra, we finally attempt to relate more precisely acoustic measurements with mechanical parameters of the material.