## 2 Experimental  
We used hydrophilic fumed SiO\({}_{2}\)(Aerosil 200; Evonik Industries, Essen, Germany) in our experiments. Its primary size was 12 nm and it had a surface area of 200 \(\pm\) 25 m\({}^{2}\)/g. However, due to agglomeration, dry-particle size analysis yielded a wide size distribution, ranging from 2 to 100 \(\upmu\)m with a Sauter mean diameter of 20 \(\upmu\)m [30]. Though its true density was 2200 kg/m\({}^{3}\), the tapped density was found to be only 50 kg/m\({}^{3}\), which indicated the highly porous nature of the bed with a void fraction of 0.98. For mixing, we used inert Geldart group A particles (sand) with a size range of 38-75 \(\upmu\)m (Sautter mean diameter = 56.5 \(\upmu\)m) and a density of 2664 kg/m\({}^{3}\). Compressed air, at room temperature, was used as fluidizing medium in the experiments.  
The experimental setup (Figure 1) consisted of a test section, which was a transparent Perspex column of height 1.5 m and internal diameter 0.07 m. It was preceded by a 0.50 m-long calming section. We used a nylon mesh-covered perforated distributor with a 4% fractional open area to ensure uniform airflow in the test section.  
The pressure transients of the nanoparticle bed were recorded using a highly sensitive fast-response bidirectional differential pressure transducer (Omega PX163-005 BD5V; Norwalk, CT, USA) between pressure taps located at distances of 110 and 230 mm above the distributor. A data acquisition system (DAQ) with LabVIEW software (National Instruments, Austin, Texas, USA) was used to record pressure transients at a rate of 100 data/s from the pressure transducers' voltage signals.  
The inlet air flow to the fluidized bed was pulsed at a frequency of 0.1 Hz using a solenoid valve (Omega SV 3310; Norwalk, CT, USA). The valve was kept open for 5 s followed by 5 s of closing time, thus completing a cycle of 0.1 Hz. As shown in Figure 2, the 0.1 Hz pulsation allowed complete settling of the bed once the flow was stopped and, moreover, led to complete fluidization when the solenoid valve was open during the flow pulsation [26; 40]. The solenoid valve was controlled using a digital I/O signal from the data acquisition system (DAQ) and LabVIEW software.  
Figure 1: Schematic of the experimental setup. DAQ: data acquisition system.  
In the case of flow pulsation, the average pressure drop was carefully computed to avoid the results being affected by the opening and closing of the solenoid. To this end, the initial 2.50 s and the final 0.25 s of the pressure transients during the input flow pulse of 5 s were eliminated, and the data from the remaining 2.25 s interval were used to compute the mean value. The data averaging procedure is discussed in detail elsewhere [27].  
The following experimental strategy was followed in the present work:  
1. Unassisted fluidization experiments were carried out by first gradually increasing the velocity followed by a gradual defluidization.
2. Before carrying out the fluidization and defluidization experiment, 4.5 vol % Geldart group A particles were added and thoroughly mixed with the nanopowder.
3. The inlet air flow to the fluidized bed was pulsed at 0.1 Hz.
4. Steps 2 and 3 were repeated after increasing the fraction of group A particles to 8.6 vol %.  
Two Gilmont flow meters of different ranges were used to control the airflow to the fluidized bed. The superficial velocity was gradually increased up to a maximum of 171.5 mm/s to avoid the elutriation of nanoparticles at higher velocities.