Droplet science: a short introduction

IPCadmin | May 4, 2018

Years of experience and the increasing complexity of the industry have led IGEBA to take a closer look at the individual aspects of space treatment. The close collaboration with institutions like the International Pesticide Application Research Centre (IPARC, Imperial College, London) and the Navy Entomology Centre of Excellence (NECE, USA) in the field of droplet size measurement have enabled the company to ensure an optimal droplet spectrum.

Droplet science is a much more complex topic than some of us might think. From Vapour to Rain, drops are around us every day. So why is the right droplet size so decisive for successful space treatment, especially against vector borne diseases and talking about fogging, how do we define fog; where does it start – where does it end?

Literature defines the droplet spectrum for fog to range between 10 and 100 microns. Depending on the applied carrier liquid (oil or water) the droplets should range between 5 and 30 microns VMD (Volume Median Diameter) for an effective thermal fog and ULV aerosol application. Droplets smaller than 5 μm do not really get in contact with the vector due to evaporation and air turbulence caused by flight activity of the insects. Droplets larger than 30 μm do not remain airborne long enough to be effective.¹

As an example, a droplet of 20 microns needs about 14 minutes to fall 10 metres and a droplet of 50 microns only about 2 minutes.²

Both the IGEBA thermal foggers as well as the ULV aerosol generators will produce a certain range of droplet sizes, the main goal being to achieve the maximum number of droplets within the optimal droplet spectrum.

Table 1 shows, at a smaller output, the knockdown effect after 24 hours was higher than at a higher output.
This is due to the smaller droplets staying airborne for a longer period of time.

In the end, several factors influence the droplet size and effectiveness. The type of carrier liquid and spray concentration, the used dosage nozzles, but also external factors such as wind speed and temperature will always affect the droplet distribution.

Both carrier liquids, oil and water have their advantages and disadvantages. In general, it can be said, that the fogging qualities of water are not as good as those of oil, one reason why in many countries it is still uncommon to use water-based insecticides with thermal fog generators. The advantages such as the lower environmental impact and cost should however not be underestimated.

It is important to mention that nearly all IGEBA thermal foggers can be used for oil and water-based formulations by making only little modifications. Moreover, there are certain additives that can improve its fogging quality.

One option to enhance the fogging qualities of water as a carrier liquid is the use of Nebol fog enhancer. NEBOL stabilises the produced droplets and creates a narrower droplet spectrum. This ensures a more even distribution of the droplets and extends the contact time with the vector.

Table 2 shows the Insecticide efficacy of the product Detmol-VAP based on Vapothrin and Pyrethrum, applied with fog additive Nebol by IGEBA Unipro 5 Cold Fog Generator in 670 m³ test rooms, against Oriental cockroaches Blatta Orientalis, adults, males.

The results show: The effectiveness of the water-based chemical is considerably increased by adding the fog enhancer.

Looking at the costs and the efficacy of space treatment, the impact of the temperature on the active ingredient is a further issue that should be taken into account. The results of a study carried out, show that using some thermal foggers, can have a negative effect on the active ingredient.

IGEBA carried out tests with two different foggers applying 4 active ingredients (tetramethrin, fenitrothion, deltamethrin, and cypermethrin); the aim was to measure the concentration of the fogging solution before and after fogging. The fogger with a temperature of more than 1000°C at the point of injection was burning between 11 and 31% of the active ingredient. On the IGEBA TF-35 it was only 0 to 5%.

The special design of the foggers considerably reduces the risk of fire and thus minimises the effect of the heat on the active ingredient. When a fogger is burning up to 30 % of the active ingredient it is not only a cost factor, but also an increased risk of resistances as the chemical is not applied at the recommended concentration.

1,2 source: Communicable Disease Control – Prevention and Eradication. Who Pesticide Evaluation scheme WhoPEs Who/CDs/WhoPEs/GCDPP/2003.5

Published in International Pest Control – March/April 2018 issue