M. Girard, J.R. Tavares, M.-C. Heuzey
Keywords: cellulose nanocrystals, dispersion, ultrasonication, modelling
Summary:Cellulose nanocrystals (CNCs) are bio-based nanoparticles with high mechanical strength, making them a promising candidate for polymer reinforcement. However, their high surface area induces strong interparticle forces, making the dispersion difficult. Probe ultrasonication is being widely used for this purpose, providing enough energy to break these interparticle bonds of particles in suspension. Ultrasonication induces an acoustic wave in the medium leading to series of compression and rarefaction. Air bubbles are created by this mechanism, their size varying with the pressure. At one point, bubbles are not stable anymore and collapse: this is the cavitation phenomenon, which allows a uniform particle dispersion by generating a high energy level. This process is complex, and the role of each parameter is not fully understood. It seems important to fill this gap to implement a reproducible method for optimized dispersion results. Thus, experiments and modelling are compared in this work to shed light on the inhomogeneity of the mixing with presence of non-mixing zones. This inhomogeneity can be related to the not full efficiency of the dispersion and is influenced by the medium, the container geometry and the probe position. The modelling part is carried out using a finite element software and these parameters, as well as the ultrasonication process, can be easily changed to study their influence on the local velocity field and on the particle dispersion. First results evidence that for the same power, the mixing velocity is lower for a less dense and less viscous liquid, meaning that the power should be increased to reach the same dispersion state. Container geometry impacts the mixing too, and larger non-mixing zones are obtained for larger beakers. Again, in this case, a higher power may be needed. In addition, if the probe is not close enough to the surface, there is a larger zone where there is no mixing. Thus, for each tested condition, it is possible to evaluate the homogeneity of the dispersion by comparing the non-mixing area with the whole beaker size. The dispersion efficiency is then related to this information and the values of the velocity obtained with this process. Thus, through these experiments and modelling, it is possible to identify what should be the optimized parameters for a uniform and reproducible dispersion. An operating window could be then determined to define the set of parameters to be used to get a reproducible homogeneous dispersion and aims to be applicable for any operators at any scale.