2. Wettability and interfacial phenomena—implications for barium sulfate particle precipitated using a lobed inner cylinder Taylor-Couette flow reactor

The role of surface phenomena and contacts can play significant roles in the transitions of particle morphologies during precipitation. From the preceding section, it can be seen that the formation of different types of particles is related to interfacial phenomenon between liquid and particles. The effects of fluid behavior on particles are embodied mainly from two aspects. On one hand, fluid flow in the reactor contributes to the interfacial ion concentration distribution, thus affecting the mass transfer to the aggregated crystal nuclei for formation of barium sulfate particles. Concentration gradient between nuclei surface (so referred to as equilibrium concentration) and bulk solution (so called bulk concentration) is the driving force for particle growth. During this process, in spite of molecular diffusion, convection dispersion resulted from fluid flow has a dominant effects. Many previous studies (e.g., [25–27]) have adopted Sherwood number, defined as the ratio of convective mass transfer to diffusive mass transport, to characterize the mass transfer contributed from turbulence on fluid-particle interface for particle size ranging from macroparticles to micro-particles. This contribution comes from eddy fluctuation, thus indicating that the shear caused by turbulent eddies may play an important role in the formation of particles in the precipitation. Armenante and Kiwan [28] proposed the correlation with Sherwood number for estimation of the mass transfer coefficient for description of the dissolution process of AgCl crystal. They have reaffirmed the effect of turbulence by applying different power input to the mixing tank reactor, and obtained an improved correlation between the Sherwood number and Reynolds number. Furthermore, Jung et al. [3] have obtained different calcium carbonate morphologies by changing the wall shear stress of the Taylor-Couette flow reactor used in their study. They interpreted this phenomenon from the point of view that ion adsorption takes place on the particle surface while the concentration gradient of adsorbed ions is also caused by shear stress. Thus, interfacial turbulence induced shear has an impact on aggregation. Bubakova et al. [29] proposed an equation to describe the relationship between aggregate size and average shear rate of turbulent eddies formed in the mixing process. Similarly, for the process of Ni-rich hydroxide preparation, Mayra and Kim [20] have also confirmed the parameters that are strongly associated with the fluid shear can have significant influence on agglomeration process. They have indicated that individual crystals can stack together to form irregular aggregates with fluid flow (mainly characterized by turbulent eddy motion), and then shear force generated by turbulent eddies can facilitate these aggregates to bind together to form regular agglomerates. Thus, it may be concluded that interfacial ion concentration and interfacial fluid shear between the solution and particles have significant influences on particle growth and aggregation.
