Effect of Tribocharging on Powder Packing

Author Last Name: 
Antonio Castellanos
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During this initial year of the project we have worked in developing a setup to charge a sample of powder. Ideally, this set-up must achieve a reproducible specific charge for a sample weighting several grams, to allow the charged sample to be used in packing experiments. We completed work on an initial design based on dry powder inhalers. The sample weight delivered each time in this initial design was up to 15 mg. We measured the specific charge by means of a Faraday cup, and the particle charge distribution by particle tracking image analysis. For this design we found them to be compatible within the accuracy of the experiments. Since a sample weight of 15 mg was placed manually in the powder dispenser, this setup is not practical to make packing experiments.

We moved to a setup based on venturi injection of the powder sample in a gas stream.  This new setup can charge larger samples (several grams in a continuous way), and it is suitable for packing experiments. We are currently testing that the charge acquired by the particles is homogeneously distributed in the final packing. To this aim, we have resorted to the same procedures of measuring the specific charge and charge distribution that we used for the inhaler-like tribocharger. The measurement of the charge distribution has been improved to discriminate the sign of the charge of each particle. We have also set controlled humidity enclosures to keep the samples in order to investigate the effect of humidity on the charge and particle aggregation.

However, at the moment of writing the present report, we have not been able to reconcile the results of these two types of measurements for this new tribocharger (venturi based). The reasons for the discrepancy are that most of the sample comes out of the venturi tribocharger with larger velocities and with a larger total charge than in the inhaler-like tribocharger. This fact implies that: a) the particle charge distribution is made from a small subset of the whole sample, i.e. those particles at the final part of the injected sample that move at smaller speeds, and may not be representative of the powder, and b) a sample weighing some milligrams, although much smaller than the weight that we can pass through the tribocharger, saturates the electrometer measuring the total charge of the sample in the Faraday cup setup. Work is currently in progress to overcome these difficulties.

We have also started experiments to measure the charge distribution on the surface of particles. We have used Kelvin Force Micros copy (KFM) as we have found than it is easier to interpret than Electrostatic Force Microscopy, which was proposed for the initial stages of the project. We have developed a procedure to fix the particles for Atomic Force Microscopy imaging. We have been able to measure both the topography of a particle and its contact potential with a conductive AFM's cantilever. Further analysis is now in progress to separate the different contributions to the contact potential measured by the microscope in order to determine the charge distribution and its polarity on the particle's surface.

In the next pages, we describe in detail the techniques used as well as the evolution of experiments as setups and analysis were improved. We discuss the results obtained with the tested materials. Finally, in the conclusion we summarize the main findings.