A System to Create Stable Nanoparticle Aerosols from Nanopowders

We designed and developed an effective nanopowder aerosolization setup and operating protocol. The system generated nanoparticle aerosols with stable number concentrations and size distributions for long durations, requiring only small quantities of test material (min. 200 mg).

Transcript

The overall goal of this experiment is to demonstrate how to use a fluidic-bed funnel generator to obtain a stable aerosol from nanoparticle powders. This method can help answer key questions in the field of occupational health. Such as whether handling of nanomaterial powders is likely to result in worker exposure to nanoparticles and associated health risks.

The main advantage of this technique is that it provides a robust and controllable aerosolization of nanoparticle powders as a reliable source of nano-aerosols for various applications. The setup for this experiment is inside a ventilation chamber. All of the components are in place to help orient viewers.

One principal component is the V-shaped glass aerosol generator, capable of resisting high pressures. Use conductive particle transport tubing to connect this element to a mixing compartment. The mixing compartment is a one-liter, grounded metallic bottle.

More conductive particle tubing connects the mixing compartment to the measurement chamber. A grounded, 12-liter metallic drum. Particle samples are drawn from its top.

Extra flow is directed into a filtration system. This schematic provides an overview of the complete system. Dry compressed air is filtered with a hyperfilter before it enters the system.

A high-precision flow tuner controls the flow rate into the aerosol generator. The generated aerosol meets a dilution flow in the mixing chamber. the aerosol flow then enters the measurement chamber for either aerosol or de-agglomeration testing.

Computer-controlled measurement instruments include a scanning mobility particle sizer, an optical particle counter, and a transmission electron microscopy sampler. All of the chamber walls should also be cleaned and particle-free to begin the experiment. Remove the aerosol generator from the setup to prepare the system for the experiment.

Close the system by connecting the tubing that had been connected to the inlet and outlet of the aerosol generator. Then move to start the filtered, dry-air supply for the system. At the flow-controller, slowly adjust the flow rate from zero to 10 liters per minute, and let it run.

After at least 30 minutes, use the scanning mobility particle sizer to determine the concentration. If it is below 10 per cubic centimeter for three measurements, consider the environment clean and stop the airflow. Next go to the sampling and exit tube outlets.

There, use stoppers to close the outlets. Before proceeding, make sure the measurement instruments are ready for use. To prepare the aerosol generator, it should be in a well-ventilated space, like this fume hood, along with a high-precision balance.

Have the aerosol generator fixed vertically, with its bottom opening blocked. Near the balance, there should be the test material, a glass funnel, spoons, and containers. This phial contains silica powder, which is the test material for this experiment.

Measure 250 milligrams of sample, which is sufficient for at least 30 minutes of stable aerosolization. At the top of the aerosol generator, insert a clean and dry funnel for adding the powder. Feed the powder into the funnel to load the aerosol generator.

feed portions of the powder, one at a time, while tapping them down, and then take off the funnel. Close the top opening of the generator. Gently tap the sidewalls of the generator to move the deposited powder down to the bottom.

Make sure the majority of the powder particles reach the bottom of the generator, instead of on the sloping walls. Next, remove the stick at the generator bottom and close the bottom opening with a plastic cap to avoid particle loss during transfer. Remove the generator from its support to take it to the experiment setup.

At this point, the aerosol generator is in place in the ventilation chamber. It is in a vertical, standing position, and is supported by a metallic scaffold. At the bottom, remove the block to the inlet of the generator.

Then, undo the connection between the inlet and outlet tubing of the generator. Connect the inlet tubing from the filtered air-supply to the bottom of the generator. At the top of the generator, remove the block and connect it to the tubing leading to the mixing chamber.

Next, move to the setup exit and remove the block there. At this point, prepare to start the aerosolization flow at the flow tuner. Slowly increase the rate from zero liters per minute to 0.3, to 0.5 liters per minute.

Visually confirm the desired level of aerosolization by monitoring the powder in the generator. Increase the flow rate very slowly, using a tuner, while monitoring closely the powder movement. And dutifully stop increasing the flow, once the desired aerosolization level is achieved.

Move on to start the dilution flow. Slowly increase the rate from zero liters per minute to two liters per minute. As the aerosolization and dilution flows start, begin measurements with the optical particle counter and scanning mobility particle sizer.

Once the aerosolization is stable, turn on the pump connected to the transmission electron microscopy sampler. For this pump, use a flow rate of 0.3 liters per minute. After finishing the measurements, begin shutting down the experiment.

First, switch off the dilution flow and then the aerosolization flow. Next, move to the aerosol generator. Disconnect the output tubing from the top of the generator and use a plastic cap to prevent powder leakage.

Next, disconnect the tubing from the generator input and block the bottom opening. Now, remove the generator from the ventilation chamber. Transfer the generator to a well-ventilated space with a basin where it can be cleaned.

Once again, mount the generator vertically with its base blocked to prevent loss of material. For hydrophobic powders, use a solvent such as ethanol to make a suspension of the powder residue. Add the solvent through the top opening of the generator.

Next, gently shake the generator to circulate the solvent to help suspend the powder particles. The next step is to dispose of the solvent by pouring it into a recycling container. At this point, use the basin and standard cleaning procedures for lab glassware to thoroughly clean the generator.

To dry the generator, once again mount it vertically, but this time, arrange for dry air to enter from its bottom and exit from its top. Start the airflow and maintain it for at least one hour. These data are for hydrophobic silicon dioxide tested under the protocol.

In this plot, the particle number concentration as a function of time is given by the blue circles. The red diamonds are data for the geometric mean diameter as a function of time. Both particle concentration and particle size start to rise as soon as the aerosolization flow begins.

After 30 to 35 minutes, the particle concentration and mean diameter stop varying significantly, indicating a steady state. The state lasts at least 30 minutes, using 241 milligrams of silicon dioxide powder. The same data provides individual size distributions as a function of scanning mobility particle sizer scan.

The scans are 3.5 minutes long. The peak slowly rises over time, until it reaches a stable value that is maintained for the rest of the experiment. Here, the particle size distributions from aerosolization flow rates in the range of 0.3 to 1.1 liters per minute are compared to learn their effects on aerosol generation.

The peak of the spectrum rises as the flow is increased. At the highest flow rate, a secondary peak appears, indicating micron-sized, airborne particles. Particle size distributions vary, to some extent, within each test, and between tests done with the same powder.

Here is the variation from four replicate tests of the same material, using identical flow rates. The standard deviation is about 40 percent for the total particle concentration, and seven percent for the geometric mean size. Once mastered, this technique can be done in one hour, if it is performed properly.

While attempting this procedure, it's important to remember to always create a clean background for the experiment, as this can significantly affect the test results and their interpretation. Following this procedure, other measures like sheer force creation can be performed in order to answer additional questions like the stability of nano-particle agglomerates, and it's effect on the release into the air. After its development, this technique paved the way for researchers in the field of occupational health to explore human exposure to engineered nanoparticles generated from powders and associated health risks at workplaces.

After watching this video, you should have a good understanding of how to properly conduct an aerosolization experiment, regarding material and system preparations, flow controls, and cleaning procedures. Don't forget that working with nanomaterial powders can be extremely hazardous and precautions such as personal and environmental protection measures should always be taken while performing this procedure.

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Journal (Engineering)

Journal (Engineering)

A System to Create Stable Nanoparticle Aerosols from Nanopowders

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