Particle and Aerosol Generation Technology

Particle Technology

In order to measure and control particles found in emissions and the environment, scientists and technologists need to be able to generate their own well defined, repeatable and easily manufacturable solid and liquid particles. These are required to allow the evaluation of removal mechanisms, delivery mechanisms in the medical and other scientific fields and to calibrate instruments used to detect and characterise the particles seen in the environment.

This requirement has been met by a wide range of standardised particles which have been available from certified sources for years. These particles are generally available to purchase from suppliers for subsequent dispersion or can be generated on-demand in solid or liquid form.

Solid Particles

Test Dust Standards

Test dusts have been required by technologists interested in their impact on machine wear, the atmospheric environment or personal respiratory health and are used in the testing of filter performance against typical ingested particles.

Graded natural sources were used initially, for example, Arizona Test Dust as defined in the 1940's:

The material shall be dust that settles out of the air behind or around tractors or implements operating in the Salt River Valley, Arizona. It is recommended that this dust be caught on a canvas cloth.

  1. Dry raw dust in oven
  2. Sift dust through 200 mesh screen (0.0029 in. width of openings)
  3. Discard dust retained on 200 mesh screen
  4. Sift dust obtained in section 2 through a 270 mesh screen (0.0021 in. width opening) until no more will go through.

Screen Analysis (by Weight)

  • 80% through 270 mesh screen.
  • 20% through 200 mesh screen, but shall be retained on 270 mesh screen.

Lack of supply and variation in the particle size lead to the development of custom designed specifics as required e.g. AC Test Dust, AC standing for the manufacturer AC Spark Plugs, used ball mills to grind the same Arizona sand to the required size characteristics but validated with new particle size assessment techniques.

ASHRAE 52 Test dust was developed for HEPA filtration testing where carbon black and cotton linters (cotton wool) are added to a test dust mix to make a more typical filter challenge dust.

Each standard has historically been defined by it's own size definition and chemical make-up to suit the individual aims. There are now several standard test dusts available with many relating back to old and withdrawn standards still being made available. Some current examples are:

  • BS 4552 Grade 2 / ISO 4020 (Mira 2) Aluminium Oxide (3-11µm) Automotive Fuel Filter Test Dust
  • ISO 5011 Quartz Fine (0-75┬Ám) Coarse (0-125µm) Combustion Engine Inlet Filter Test Dust
  • ASHRAE 52.2 (Fine test dust; Carbon Black; Cotton Linters), HVAC Filter Test Dust
  • ISO 12103-1 A1 Ultrafine (0-30µm), A2 Fine (0-80µm), A3 Medium (0-125µm) and A4 Coarse (0-180µm). Road Vehicles Test Dust for filter application

The common factor with these dusts is that they need to be dispersed in the fluid that will contain them, be it in liquids, for example in lubricating oils, or in gases such as air. The distribution of the generated suspension must remain constant, be controlable and mass delivery must be repeatable.

For aerosol dispersion, several devices have been developed using a range of principles to achieve these features, but some of the most successful have been the rotating brush and belt disperses such as the RBG and BEG series offered by Filter Integrity.

Soots

The development of our knowledge of health related concerns with particles such as soot, generated by combustion in either automotive or industrial application, has led to the requirement to generate controlled analogues for testing.

Combustion soot generators are now available to generate particles through the defined combustion of fuels. Generators such as the DSP range, can produce combustion soot particles in large quantity over a size range from 10nm to simulate those particles generated by current combustion soot sources.

Nano Particles

With the recent developments in nanotechnology and nanoparticles, reliable sources of stable particles in the nano range have become required.

In this range, spark based generators have become identified as a good source of carbon or metal based particles in the sub 100nm range. Particles can be generated by vapourisation and subsequent condenstion / coagulation produced when a high voltage discharge arcs between two electrodes made of the material required. Generators such as the GFG and DNP ranges from Filter Integrity operate using this principle, under well controlled conditions, to generate particles in the 10 to 150nm range.

Liquid Particles

Liquid based aerosols have been generated typically either by using shear forces to break up the bulk liquid in nozzles and sprays, or by evaporation and subsequent condensation onto condensation 'cores' to form the liquid particles.

Nozzle and spray generators are often referred to as nebulisers or atomisers.

Nozzle based precision aerosol generators, such as the AGF, use pneumatic ( binary) nozzles and a high velocity of air to atomise the liquid combined with downstream separation systems to ensure removal of large particles from the generated aerosol.

Laskin GeneratorThe Laskin nozzle based generator design, named after S. Laskin's and his original design in 1948, uses a submerged nozzle arrangement to generate high volumes of sub-micron particles and forms the basis of many generators today, including the PLG range

The prosess of generating a liquid aerosol has also been used for many years to generate crystaline solid aerosols from salts, NaCl and KCl, by nebulising an aqueous salt solution and evaporating the water, with the concentration of the salt in solution impacting on the size of particle generated.

The dispersion of well characterised Polystyrene Latex (PSL) beads into an aerosol form, for the calibration of particle counting equipment for example, is also achieved using nebulisers.

More recently electo-spray and inkjet based technologies have become available for particle generation down into the nano range.

Liquids

The liquids specified to generate technical aerosols are many and varied, from plain water to complex medicines.

The test fluid specified in the filter testing industry was originally the plasticiser DOP, bis (2-ethylhexyl) phthalate or Di iso Octyl Phthalate, hence the references in the filter testing industry to the DOP test. DOP was unfortunately identified as a potential carcinogen, particularly following thermal breakdown when used to generate condensation based aerosols and has been superceded.

DOP has changed as an acronym, now referring to Dispersed Oil Particle and the current industry standard is DEHS, bis (2-ethylhexyl) decanedioate, or Di-(2-EthylHexyl) Sebacate, although highly refined aromatic-free paraffinic or naphthenic white mineral oils are also being used.

Some current filter test standards that reference a liquid aerosol are:

  • ISO 14644-3:2005 Cleanrooms and associated controlled environments - Part 3: Test Methods
  • EN 1822-2:2009 High efficiency air filters (EPA, HEPA and ULPA). Aerosol production, measuring equipment, particle counting statistics
  • ISO 12500-1:2007Filters for Compressed Air - Test Methods

The condensation method of liquid aerosol generation, first described by Sinclair and La Mer in 1943, was developed to produce monodisperse particles and the basic principle is still used today, although much modified, as for example in the MAG 3000.

Classification methods, based on selection using the electrical mobility of particles (a function of charge v mass) are also available to generate a monodisperse aerosol output from a polydisperse aerosol feed.

Please contact us for more detailed information.