Electrostatic charges generated on aerosolisation of dispersions

In responding to the international community's agreement of phasing out chlorofluorocarbon (CFC) propellants by the year 2000, hydrofluoroalkane (HFA) has been chosen to replace CFCs. Intensive investigations related to the new propellant products have been carried out. Aerosol electrostatics is one of the topics investigated. To understand and subsequently control the charging processes is the motive of the research reported here. To help elucidate the complex charging process occurring naturally during atomization of liquids from pressurised Metered Dose Inhalers (pMDIs), it has been broken down into a sequence of related, simpler sub processes-drop charging, streaming current charging (coarse spray), splashing charging and fine spray charging. Our initial studies are of single drops forming at and breaking away from the tips of capillary tubes. The drop forming processes are so slow that any hydrodynamic effect can be dismissed. Then the charge on the drop is measured. It is found that the charge on water drops is always negative (~ 10⁻¹⁴C) at field-free condition and the magnitude of the charge increases as the drop size increases and the surrounding tube diameter decreases. With salt solutions, the charge on drops is negative at dilute solutions, decreases in magnitude as the concentration of electrolytes increases and finally reverses the sign of charge at approximately 1 M - drop charge becomes positive. All these experimental results can be explained in terms of contact potential between liquid and the inner wall of the capillary, which sets up an electric field between the pendant drop and the surrounding tube. Then computational simulation work is carried out and the data are compared with experimental results. It is found that the computer simulation data are in accord with experimental observations. This is a potential method to measure absolute potential difference between a liquid and a solid. Secondly, the hydrodynamic processes are investigated by increasing the liquid delivery speed so that a jet is formed. The streaming current is monitored in this case. It is found that the streaming current is always negative for water and increases linearly in magnitude as flow rate increases. With salt solutions, the streaming current is negative at dilute solution, decreases in magnitude as the concentration of electrolytes increases and finally reverses the sign of streaming current at approximately 1 M - streaming current becomes positive. Then similar experiments are carried out with model propellant 2H, 3H- Decafluoropentane(HPFP). The streaming current is negative and firstly increases linearly, then increases dramatically and finally reaches another linear relationship with the flow rate. Furthermore the effects of pressure inside a can and concentration of salts on jet charge density are investigated using aqueous solutions and HPFP. It is shown that the magnitude of jet charge density increases as the can pressure increases and the charge sign changes from negative to positive as the salt concentration increases from 0.01M to 0.1M. Finally jet charge density of pure HFA134a, HFA227ea and their Formoterol formulations are examined. Similar experiments are carried out with pure HFAs and their respective Formoterol MDIs. The spray charge simulation experiments are carried out for both aqueous solutions and HPFP with standard plastic actuators at different pressures inside cans and different concentrations of salt solutions. It is shown that the charge sign also changes as the concentration increases from 10"^M to IM, but the polarity is the opposite of the charge on the jet. It means that the actuator actually changes the charge polarity of the spray. The spray charge density on pure HFA 134a, HFA227ea and their Formoterol formulations MDIs with both aluminium and plastic actuators are investigated Charge that occurs when liquid splashes against different materials is measured. The effects of air blow rate and different materials on the splashing charge are investigated. This is implemented to simulate the splashing process inside the expansion chamber, which is the dead space between the actuator orifice and the metering chamber. Finally spray size distribution is measured with a Malvern size analyser for aqueous solutions, HPFP, pure HFA 134a, HFA227ea and Formoterol MDIs (HFA 134a and 227ea formulations) at different pressures inside cans to relate the effect of spray size distribution on charge density. It is found that the size distribution within the spray cloud has a close correlation with the spray charge density. A syringe pump was used to deliver liquids to capillary tubes and a digital Keithley electrometer was used to monitor charge on drops and current on jets. A selfdesigned software was used to control the whole system to allow the desired delivery patterns to be implemented and record the results achieved as well. For the jet charge simulation experiments cans were modified so that the pressure inside the cans could be controlled by a pressurised nitrogen cylinder and also the liquid inside the cans can be changed to investigate the effect of different liquids on the charge density. A dose capturer was modified to capture the drug particles and also monitor the charge on them.