# Fluidization Dynamics of Hydrophobic Nanosilica with Velocity Step Changes
## 1 Introduction  
Intimate fluid-solid contact and good interphase mixing are essential to ensure the efficiency of processes that require the interaction of the solid and fluid phases. While small particles inevitably provide high interfacial area, thereby enhancing the surface-based rate processes, their effectiveness could however be compromised due to strong interparticle forces. The role of the interparticle forces has long been recognized in the context of the fluidization of powders [1]. Fine powders, composed of small particles (<20 \(\upmu\)m), are mostly cohesive due to interparticle forces, which make their fluidization challenging as well as unpredictable [2, 3]. Strong hysteresis effect and severe bed non-homogeneities are observed in their fluidization behavior. Interestingly, nanopowders, despite being composed of ultrafine particles, mainly exhibit two distinct kinds of fluidization behavior, viz. agglomerate bubbling fluidization (ABF) and agglomerate particulate fluidization (APF) [4; 5]. As the name suggests, the constituent nanoparticles of ultrafine powders exist as large agglomerates, which are sometimes as much as a thousandfold larger than the primary size of the individual nanoparticle. Therefore, the fluidization hydrodynamics of ultrafine powders, whether bubbling or particulate, are characterized by the fluidization behavior of their constituent agglomerates. Nano-silica is extensively used in various applications, e.g., concrete, agriculture, rubber, plastics, electronics, cosmetics, gypsum, battery, pharmaceuticals, and food [6; 7; 8; 9]. The estimated global market of nanosilica in 2015 was 3.3 million tons, which is projected to reach over USD 5 billion by 2025 [10]. Depending upon the application requirement, silica nanoparticles' surface treatment can significantly alter the interparticle force equilibrium, thereby affecting their mutual interaction. For example, hydrophilic nanosilica shows ABF, while its hydrophobic counterpart mostly undergoes APF, even if their primary dimension and density are comparable [5]. The bubbling in ABF limits the bed's expansion and leads to a size-based stratification of constituent agglomerates along the bed height [11; 12]. On the contrary, APF shows a smooth and homogeneous expansion of the bed [13]. The height of the expanded bed can reach several times the height of the original bed. The agglomerates of the hydrophobic nanosilica are morphologically soft and fluffy. The U\({}_{\text{mf}}\) in APF could sometimes be as much as an order of magnitude lower than the one in ABF. As a result, the fluidization hydrodynamics of the ABF have attracted a lot of attention. Efforts were mainly oriented towards understanding ABF hydrodynamics and using assisted fluidization techniques to improve their fluidization behavior [14; 15; 16; 17; 18; 19; 20]. On the other hand, any rigorous study on the hydrodynamics of the APF appears to be still lacking the literature. Therefore, a detailed investigation of the hydrophobic nanosilica undergoing APF has been carried out to thoroughly characterize its hydrodynamics. The information obtained from this study can be readily used during the processing of such nanopowders in large-scale industrial applications, where critical process conditions, such as velocity, could change during the operation. In this study, the nanosilica bed was subjected to velocity step changes (of fixed duration), while the transients in various bed regions were monitored using sensitive pressure transducers located at different heights in the fluidized bed. Analog voltage signals were acquired using an AD converter (18-bit) of a data acquisition system (DAQ). We used the sampling frequency of 100 Hz to ensure accurate monitoring of different events occurring in the fluidized bed. The velocity was first gradually increased during the fluidization cycle, followed by a gradual reduction during the defluidization cycle to delineate the effect of the hysteresis on the APF hydrodynamics.