On the bridging mechanism in vibration controlled dispensing of pharmaceutical powders from a micro hopper

• Vibration can start/stop dispensing of pharmaceutical scale powder precisely.
• DEM simulation validated by mass flow in experimental vibration dispensing.
• Visualisation identifies mechanism that rapidly halts the flow after vibration stops.

Accurate batch dispensing of pharmaceutical powders, on the scale of hundreds of microns, in small doses is a challenging task. A novel dispensing technique has been developed by Yang et al. using high-frequency vibration to control powder flow out of a narrow hopper. This method removes the need for mechanical valves because the powder, very quickly, forms a bridge-like structure across the passive outlet preventing outflow. Activation of the vibration has been found to destabilise the bridging structure enabling the powder to flow, when vibration stops the bridge structure quickly rebuilds and dispensing stops. In this work the Discrete Element Method (DEM) was used to simulate this novel dispensing control method in order to identify the internal mechanism that allows the flow to be controlled so precisely. A simulated conical hopper was filled with particles then oscillated vertically at high frequency (≈ 10 kHz), amplitude and frequency were scaled from the experimental system. Two orifice sizes, a variety of DEM parameters and two vibration modes were simulated. The parametric study of DEM parameters was based around a case that provided flow rates within a factor of 2 of the experimental flow rates. Dispensing after vibration was found to stop very quickly as in experiments. Visualisation of internal structures during fill, vibration and immediately after vibration revealed a central mass of slow moving particles floating above the nozzle outlet. When the vibration stops the central mass of particles drops into contact with the walls and quickly plugs the flow.

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