Hydrophobic/hydrophilic static powder beds : competing horizontal spreading and vertical imbibition mechanisms of a single droplet

There are two competing mechanisms of a single liquid droplet impacting on static powder beds. The dh spreading describes the length the liquid droplet travels horizontally across the powder bed, whereas the dv imbibition is the length the liquid droplet travels vertically into the powder bed.

In this work, results are presented from an experimental study of single liquid droplets of varying viscosities impacting static hydrophilic and hydrophobic powder beds. By using a high-speed video camera, the dynamics of the droplet spreading behaviour on static powder bed compacts was captured. The resultant nuclei were then analysed. Results show that the measured maximum dh spreading diameter is less than the diameter of the resultant nuclei. This indicates that the dh spreading of the liquid droplet continues to spread after complete imbibition in a late-diffusive intra-spreading process. This process refers to liquid movement driven by capillary forces in the powder beds and showed a higher degree of dependency on the liquid binder viscosity. The dv imbibition length was found to be dependent on the interfacial tension between the solid and the liquid droplet. This transition in the liquid spreading and imbibition behaviour is explained based on the transportation of a liquid droplet occurring increasingly via site percolation theory with increase in powder bed hydrophobicity. The liquid droplet imbibition rates are described in terms of Darcy's law scaling behaviour. Understanding of single droplet spreading mechanisms provides a basis to select the optimal binder for granulation to form more uniform and robust granules when using hydrophilic and hydrophobic formulations.